Publications

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308. Effects of rare-earth magnetism on the superconducting upper critical field in infinite-layer nickelates
B. Y. Wang, T. C. Wang, Y. T. Hsu, M. Osada, K. Lee, C. Jia, C. Duffy, D. Li, J. Fowlie, M. R. Beasley, T. P. Devereaux, I. R. Fisher, N. E. Hussey, and H. Y. Hwang,
SCIENCE ADVANCES, Vol 9, Issue 20, adf6655 (2023).

The search for superconductivity in infinite-layer nickelates was motivated by analogy to the cuprates, and this perspective has framed much of the initial consideration of this material. However, a growing number of studies have highlighted the involvement of rare-earth orbitals; in that context, the consequences of varying the rare-earth element in the superconducting nickelates have been much debated. Here, we show notable differences in the magnitude and anisotropy of the superconducting upper critical field across the La-, Pr-, and Nd-nickelates. These distinctions originate from the 4f electron characteristics of the rare-earth ions in the lattice: They are absent for La³⁺, nonmagnetic for the Pr³⁺ singlet ground state, and magnetic for the Nd³⁺ Kramer’s doublet. The unique polar and azimuthal angle-dependent magnetoresistance found in the Nd-nickelates can be understood to arise from the magnetic contribution of the Nd³⁺ 4f moments. Such robust and tunable superconductivity suggests potential in future high-field applications.

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307. Nanoscale phase-slip domain walls in the charge density wave state of the Weyl semimetal candidate NbTe₄
J. A. Galvis, A. Fang, D. Jimenez-Guerrero, J. Rojas-Castillo, J. Casas, O. Herrera, A. C. Garcia-Castro, E. Bousquet, I. R. Fisher, A. Kapitulnik, and P. Giraldo-Gallo,
Physical Review B107, 045120 (2023).

The transition-metal tetrachalcogenides are a model system to explore the conjunction of correlated electronic states such as charge density waves (CDWs) with topological phases of matter. Understanding the connection between these phases requires a thorough understanding of the individual states, which for the case of the CDW in this system, is still missing. In this paper we combine phonon-structure calculations and scanning tunneling microscopy measurements of NbTe₄ in order to provide a full characterization of the CDW state. We find that, at short range, the superstructure formed by the CDW is fully commensurate with the lattice parameters. Moreover, our data reveals the presence of phase-slip domain walls separating regions of commensurate CDWs in the nanoscale, indicating that the CDW in this compound is discommensurate at long range. Our results solve a long-standing discussion about the nature of the CDW in these materials and provide a strong basis for the study of the interplay between this state and other novel quantum electronic states.

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306. Quantum critical fluctuations in an Fe-based superconductor
D. Jost, L. Peis, G. He, A. Baum, S. Gepraegs, J. C. Palmstrom, M. S. Ikeda, I. R. Fisher, T. Wolf, S. Lederer, S. A. Kivelson, R. Hackl,
Communications Physics 5, Article number: 201 (2022).

Quantum critical fluctuations may prove to play an instrumental role in the formation of unconventional superconductivity. Here, we show that the characteristic scaling of a marginal Fermi liquid is present in inelastic light scattering data of an Fe-based superconductor tuned through a quantum critical point (QCP) by chemical substitution or doping. From the doping dependence of the imaginary time dynamics we are able to distinguish regions dominated by quantum critical behavior from those having classical critical responses. This dichotomy reveals a connection between the marginal Fermi liquid behavior and quantum criticality. In particular, the overlap between regions of high superconducting transition temperatures and quantum critical scaling suggests a contribution from quantum fluctuations to the formation of superconductivity.

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305. Field-tuned ferroquadrupolar quantum phase transition in the insulator TmVO₄
Pierre Massat, Jiajia Wen, Jack M. Jiang, Alexander T. Hristov, Yaohua Liu, Rebecca W. Smaha, Robert S. Feigelson, Young S. Lee, Rafael M. Fernandes and Ian R. Fisher,
PNAS 119, No.28, e2119942119 (2022).

We report results of low-temperature heat-capacity, magnetocaloric-effect, and neutron diffraction measurements of TmVO₄, an insulator that undergoes a continuous ferroquadrupolar phase transition associated with local partially filled 4f orbitals of the thulium (Tm³⁺) ions. The ferroquadrupolar transition, a realization of Ising nematicity, can be tuned to a quantum critical point by using a magnetic field oriented along the c axis of the tetragonal crystal lattice, which acts as an effective transverse field for the Ising-nematic order. In small magnetic fields, the thermal phase transition can be well described by using a semiclassical mean-field treatment of the transverse-field Ising model. However, in higher magnetic fields, closer to the field-tuned quantum phase transition, subtle deviations from this semiclassical behavior are observed, which are consistent with expectations of quantum fluctuations. Although the phase transition is driven by the local 4f degrees of freedom, the crystal lattice still plays a crucial role, both in terms of mediating the interactions between the local quadrupoles and in determining the critical scaling exponents, even though the phase transition itself can be described via mean field. In particular, bilinear coupling of the nematic order parameter to acoustic phonons changes the spatial and temporal fluctuations of the former in a fundamental way, resulting in different critical behavior of the nematic transverse field Ising model, as compared to the usual case of the magnetic transverse-field Ising model. Our results establish TmVO₄ as a model material and electronic nematicity as a paradigmatic example for quantum criticality in insulators.

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304. Second order Zeeman interaction and ferroquadrupolar order in TmVO₄
I. Vinograd, K. R. Shirer, P. Massat, Z. Wang, T. Kissikov, D. Garcia, M. D. Bachmann, M. Horvatic, I. R. Fisher and N. J. Curro,
npj Quantum Materials 7, 68 (2022).

TmVO₄ exhibits ferroquadrupolar order of the Tm 4f electronic orbitals at low temperatures, and is a model system for Ising nematicity. A magnetic field oriented along the c-axis constitutes a transverse effective field for the quadrupolar order parameter, continuously tuning the system to a quantum phase transition as the field is increased from zero. In contrast, in-plane magnetic fields couple to the order parameter only at second order, such that orienting along the primary axes of the quadrupole order results in an effective longitudinal field, whereas orienting at 45 degrees results in a second effective transverse field. Not only do in-plane fields engender a marked in-plane anisotropy of the critical magnetic and quadrupole fluctuations above the ferroquadrupolar ordering temperature, but in-plane transverse fields initially enhance the ferroquadrupolar order, before eventually suppressing it, an effect that we attribute to admixing of the higher crystalline electric field levels.

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303. Evidence for Realignment of the Charge Density Wave State in ErTe₃ and TmTe₃ under Uniaxial Stress via Elastocaloric and Elastoresistivity Measurements
J. A. W. Straquadine, M. S. Ikeda and I. R. Fisher,
Phys. Rev. X 12, 021046 (2022).

We report the evolution of the charge density wave (CDW) states in the quasi-two-dimensional rare-earth tritellurides (RTe₃ for R = Er, Tm) under the influence of in-plane uniaxial stress. Measurements of the elastocaloric effect, resistivity, and elastoresistivity allow us to demonstrate the importance of in-plane antisymmetric strain on the CDW and to establish a phase diagram. We show that modest tensile stress parallel to the in-plane a axis can reversibly switch the direction of the ordering wave vector between the two in-plane directions, and present a free-energy expansion which reproduces the general structure of the observed phenomena. This work opens a new avenue in the study of RTe₃ in its own right, and more generally establishes RTe₃ as a promising model system for the study of strain-CDW interactions in a quasi-two-dimensional square lattice.

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302. Observation of the non-linear Meissner effect
J. A. Wilcox, M. J. Grant, L. Malone, C. Putzke, D. Kaczorowski, T. Wolf, F. Hardy, C. Meingast, J. G. Analytis, J. Chu, I. R. Fisher and A. Carrington,
Nature Communications 13, 1201 (2022).

A long-standing theoretical prediction is that in clean, nodal unconventional superconductors the magnetic penetration depth λ at zero temperature, varies linearly with magnetic field. This non-linear Meissner effect is an equally important manifestation of the nodal state as the well studied linear-in-T dependence of λ, but has never been convincingly experimentally observed. Here we present measurements of the nodal superconductors CeCoIn₅ and LaFePO which clearly show this non-linear Meissner effect. We further show how the effect of a small dc magnetic field on λ(T) can be used to distinguish gap nodes from non-nodal deep gap minima. Our measurements of KFe₂As₂ suggest that this material has such a nonnodal state.

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301. Comparison of temperature and doping dependence of elastoresistivity near a putative nematic quantum critical point
J. C. Palmstrom, P. Walmsley, J. A. W. Straquadine, M. E. Sorensen, S. T. Hannahs, D. H. Burns & I. R. Fisher,
Nature Communications 13, 1011 (2022).

Strong electronic nematic fluctuations have been discovered near optimal doping for several families of Fe-based superconductors, motivating the search for a possible link between these fluctuations, nematic quantum criticality, and high temperature superconductivity. Here we probe a key prediction of quantum criticality, namely power-law dependence of the associated nematic susceptibility as a function of composition and temperature approaching the compositionally tuned putative quantum critical point. To probe the 'bare' quantum critical point requires suppression of the superconducting state, which we achieve by using large magnetic fields, up to 45 T, while performing elastoresistivity measurements to follow the nematic susceptibility. We performed these measurements for the prototypical electron-doped pnictide, Ba(Fe_1-xCo_x)₂As₂, over a dense comb of dopings. We find that close to the putative quantum critical point, the elastoresistivity appears to obey power-law behavior as a function of composition over almost a decade of variation in composition. Paradoxically, however, we also find that the temperature dependence for compositions close to the critical value cannot be described by a single power law.

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300. Coherent Modulation of Quasiparticle Scattering Rates in a Photoexcited Charge-Density-Wave System
J. Maklar, M. Schüler, Y. W. Windsor, C. W. Nicholson, M. Puppin, P. Walmsley, I. R. Fisher, M. Wolf, R. Ernstorfer, M. A. Sentef, and L. Rettig,
Phys. Rev. Lett 128, 026406 (2022).

We present a complementary experimental and theoretical investigation of relaxation dynamics in the charge-density-wave (CDW) system TbTe₃ after ultrafast optical excitation. Using time- and angleresolved photoemission spectroscopy, we observe an unusual transient modulation of the relaxation rates of excited photocarriers. A detailed analysis of the electron self-energy based on a nonequilibrium Green’s function formalism reveals that the phase space of electron-electron scattering is critically modulated by the photoinduced collective CDW excitation, providing an intuitive microscopic understanding of the observed dynamics and revealing the impact of the electronic band structure on the self-energy.

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299. Iron pnictides and chalcogenides: a new paradigm for superconductivity
Rafael M. Fernandes, Amalia I. Coldea, Hong Ding, Ian R. Fisher, P. J. Hirschfeld & Gabriel Kotliar,
Nature 601, 35 (2022).

Superconductivity is a remarkably widespread phenomenon that is observed in most metals cooled to very low temperatures. The ubiquity of such conventional superconductors, and the wide range of associated critical temperatures, is readily understood in terms of the well-known Bardeen–Cooper–Schrieffer theory. Occasionally, however, unconventional superconductors are found, such as the iron-based materials, which extend and defy this understanding in unexpected ways. In the case of the iron-based superconductors, this includes the different ways in which the presence of multiple atomic orbitals can manifest in unconventional superconductivity, giving rise to a rich landscape of gap structures that share the same dominant pairing mechanism. In addition, these materials have also led to insights into the unusual metallic state governed by the Hund’s interaction, the control and mechanisms of electronic nematicity, the impact of magnetic fluctuations and quantum criticality, and the importance of topology in correlated states. Over the fourteen years since their discovery, iron-based superconductors have proven to be a testing ground for the development of novel experimental tools and theoretical approaches, both of which have extensively influenced the wider field of quantum materials.

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298. Anomalous thermal transport and strong violation of Wiedemann-Franz law in the critical regime of a charge density wave transition
Erik D. Kountz, Jiecheng Zhang, Joshua A. W. Straquadine, Anisha G. Singh, Maja D. Bachmann, Ian R. Fisher, Steven A. Kivelson, and Aharon Kapitulnik,
Phys. Rev. B 104, L241109 (2021).

ErTe₃ is a model system used to explore thermal transport in a layered charge density wave (CDW) material. We present thermal diffusivity, resistivity, and specific-heat data: There is a sharp decrease in thermal conductivity parallel and perpendicular to the primary CDW at the CDW transition temperature. Yet, the resistivity changes more gradually. Using the Wiedemann-Franz law well above and below Tc a consistent description of the thermal transport applies with essentially independent electron and phonon contributions. In the critical regime, no such description is possible; the observed behavior corresponds to a strongly coupled electron-phonon critical 'soup.'

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297. Expanding the momentum field of view in angle-resolved photoemission systems with hemispherical analyzers
Nicolas Gauthier, Jonathan A. Sobota, Heike Pfau, Alexandre Gauthier, Hadas Soifer, Maja D. Bachmann, Ian R. Fisher, Zhi-Xun Shen and Patrick S. Kirchmann,
Review of Scientific Instruments 92, 123907 (2021).

In photoelectron spectroscopy, the measured electron momentum range is intrinsically related to the excitation photon energy. Low photon energies <10 eV are commonly encountered in laser-based photoemission and lead to a momentum range that is smaller than the Brillouin zones of most materials. This can become a limiting factor when studying condensed matter with laser-based photoemission. An additional restriction is introduced by widely used hemispherical analyzers that record only electrons photoemitted in a solid angle set by the aperture size at the analyzer entrance. Here, we present an upgrade to increase the effective solid angle that is measured with a hemispherical analyzer. We achieve this by accelerating the photoelectrons toward the analyzer with an electric field that is generated by a bias voltage on the sample. Our experimental geometry is comparable to a parallel plate capacitor, and therefore, we approximate the electric field to be uniform along the photoelectron trajectory. With this assumption, we developed an analytic, parameter-free model that relates the measured angles to the electron momenta in the solid and verify its validity by comparing with experimental results on the charge density wave material TbTe₃. By providing a larger field of view in momentum space, our approach using a bias potential considerably expands the flexibility of laser-based photoemission setups.

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296. Anisotropic nematic fluctuations above the ferroquadrupolar transition in TmVO₄
Z. Wang, I. Vinograd, Z. Mei, P. Menegasso, D. Garcia, P. Massat, I. R. Fisher, and N. J. Curro,
Phys. Rev. B 118, 205137 (2021).

Ferroquadrupole order of local atomic orbitals provides a specific realization of electronic nematic order. TmVO₄ is an insulator and undergoes ferroquadrupolar order associated with the local Tm 4f orbitals at T_Q = 2.15 K. The material is a model system to study nematic order and the roles played by nematic fluctuations. Here we present ⁵¹V nuclear magnetic resonance data as a function of field orientation in a single crystal. Although the spectra are well understood in terms of direct dipolar hyperfine couplings, the spin-lattice relaxation rate exhibits strong anisotropy that cannot be understood in terms of magnetic fluctuations. We find that the spinlattice relaxation rate scales with the shear elastic constant associated with the ferroquadrupole phase transition, suggesting that quadrupole (nematic) fluctuations dominate the spin-lattice relaxation for in-plane fields.

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295. Role of Equilibrium Fluctuations in Light-Induced Order
Alfred Zong, Pavel E. Dolgirev, Anshul Kogar, Yifan Su, Xiaozhe Shen, Joshua A. W. Straquadine, Xirui Wang, Duan Luo, Michael E. Kozina, Alexander H. Reid, Renkai Li, Jie Yang, Stephen P. Weathersby, Suji Park, Edbert J. Sie, Pablo Jarillo-Herrero, Ian R. Fisher, Xijie Wang, Eugene Demler, and Nuh Gedik,
Phys. Rev. Lett 127, 227401 (2021).

Engineering novel states of matter with light is at the forefront of materials research. An intensely studied direction is to realize broken-symmetry phases that are 'hidden' under equilibrium conditions but can be unleashed by an ultrashort laser pulse. Despite a plethora of experimental discoveries, the nature of these orders and how they transiently appear remain unclear. To this end, we investigate a nonequilibrium charge density wave (CDW) in rare-earth tritellurides, which is suppressed in equilibrium but emerges after photoexcitation. Using a pump-pump-probe protocol implemented in ultrafast electron diffraction, we demonstrate that the light-induced CDW consists solely of order parameter fluctuations, which bear striking similarities to critical fluctuations in equilibrium despite differences in the length scale. By calculating the dynamics of CDW fluctuations in a nonperturbative model, we further show that the strength of the light-induced order is governed by the amplitude of equilibrium fluctuations. These findings highlight photoinduced fluctuations as an important ingredient for the emergence of transient orders out of equilibrium. Our results further suggest that materials with strong fluctuations in equilibrium are promising platforms to host hidden orders after laser excitation.

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294. Elastocaloric signature of nematic fluctuations
Matthias S. Ikeda, Thanapat Worasaran, Elliott W. Rosenberg, Johanna C. Palmstrom, Steven A. Kivelson, Ian R. Fisher,
PNAS 118 (37) e2105911118 (2021).

The elastocaloric effect (ECE) relates changes in entropy to changes in strain experienced by a material. As such, ECE measurements can provide valuable information about the entropy landscape proximate to strain-tuned phase transitions. For ordered states that break only point symmetries, bilinear coupling of the order parameter with strain implies that the ECE can also provide a window on fluctuations above the critical temperature and hence, in principle, can also provide a thermodynamic measure of the associated susceptibility. To demonstrate this, we use the ECE to sensitively reveal the presence of nematic fluctuations in the archetypal Fe-based superconductor Ba(Fe_1-xCo_x)₂As₂. By performing these measurements simultaneously with elastoresistivity in a multimodal fashion, we are able to make a direct and unambiguous comparison of these closely related thermodynamic and transport properties, both of which are sensitive to nematic fluctuations. As a result, we have uncovered an unanticipated doping dependence of the nemato-elastic coupling and of the magnitude of the scattering of low-energy quasi-particles by nematic fluctuations - while the former weakens, the latter increases dramatically with increasing doping.

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293. Anderson localization of electron states in a quasicrystal
Shuvam Sarkar, Marian Krajcí, Pampa Sadhukhan, Vipin Kumar Singh, Andrei Gloskovskii, Prabhat Mandal, Vincent Fournée, Marie-Cecile de Weerd, Julian Ledieu, Ian R. Fisher, and Sudipta Roy Barman,
Phys. Rev. B 103, L241106 (2021).

The influence of disorder on the critical electron states in a quasiperiodic lattice is a subject of intense research. In this work, we report the occurrence of Anderson localization in an icosahedral (i) polygrain quasicrystal Al-Pd-Re due to site disorder using hard x-ray photoemission spectroscopy, resistivity, and density functional theory (DFT). Photoemission spectroscopy shows that the density of states is enhanced at the Fermi level in polygrain i-Al-Pd-Re compared to single-grain i-Al-Pd-Re. In contrast, the conductivity of the former is an order of magnitude reduced compared to the latter, indicating that these electron states are localized. DFT shows that these states originate primarily from Re 5d-Pd 4d hybridization and are enhanced in polygrain i-Al-Pd-Re due to compositional difference, but are broadened because of disorder that brings about Anderson localization. This is established by the Mott variable range hopping behavior of conductivity, and the estimated localization length is 23 Angstroms.

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292. Nematic quantum criticality in an Fe-based superconductor revealed by strain-tuning
Thanapat Worasaran, Matthias S. Ikeda, Johanna C. Palmstrom, Joshua A. W. Straquadine, Steven A. Kivelson, Ian R. Fisher,
Science 372, Issue 6545, pp. 973-977 (2021) .

Quantum criticality may be essential to understanding a wide range of exotic electronic behavior; however, conclusive evidence of quantum critical fluctuations has been elusive in many materials of current interest. An expected characteristic feature of quantum criticality is power-law behavior of thermodynamic quantities as a function of a nonthermal tuning parameter close to the quantum critical point (QCP). Here, we observed power-law behavior of the critical temperature of the coupled nematic/structural phase transition as a function of uniaxial stress in a representative family of iron-based superconductors, providing direct evidence of quantum critical nematic fluctuations in this material. These quantum critical fluctuations are not confined within a narrow regime around the QCP but rather extend over a wide range of temperatures and compositions.

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291. Nonequilibrium charge-density-wave order beyond the thermal limit
J. Maklar, Y. W. Windsor, C. W. Nicholson, M. Puppin, P. Walmsley, V. Esposito, M. Porer, J. Rittmann, D. Leuenberger, M. Kubli, M. Savoini, E. Abreu, S. L. Johnson, P. Beaud, G. Ingold, U. Staub, I. R. Fisher, R. Ernstorfer, M. Wolf & L. Rettig,
Nature Communications 12, Article number: 2499 (2021).

The interaction of many-body systems with intense light pulses may lead to novel emergent phenomena far from equilibrium. Recent discoveries, such as the optical enhancement of the critical temperature in certain superconductors and the photo-stabilization of hidden phases, have turned this field into an important research frontier. Here, we demonstrate nonthermal charge-density-wave (CDW) order at electronic temperatures far greater than the thermodynamic transition temperature. Using time- and angle-resolved photoemission spectroscopy and time-resolved X-ray diffraction, we investigate the electronic and structural order parameters of an ultrafast photoinduced CDW-to-metal transition. Tracking the dynamical CDW recovery as a function of electronic temperature reveals a behaviour markedly different from equilibrium, which we attribute to the suppression of lattice fluctuations in the transient nonthermal phonon distribution. A complete description of the system’s coherent and incoherent order-parameter dynamics is given by a time-dependent Ginzburg-Landau framework, providing access to the transient potential energy surfaces.

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290. Anisotropic quasiparticle coherence in nematic BaFe₂As₂ studied with strain-dependent ARPES
H. Pfau, S. D. Chen, M. Hashimoto, N. Gauthier, C. R. Rotundu, J. C. Palmstrom, I. R. Fisher, S.-K. Mo, Z.-X. Shen, and D. Lu,
Phys. Rev. B 103, 165136 (2021).

The hallmark of nematic order in iron-based superconductors is a resistivity anisotropy but it is unclear to which extent quasiparticle dispersions, lifetimes, and coherence contribute. While the lifted degeneracy of the Fe d_xz and d_yz dispersions has been studied extensively, only little is known about the two other factors. Here, we combine in situ strain tuning with ARPES and study the nematic response of the spectral weight in BaFe₂As₂. The symmetry analysis of the ARPES spectra demonstrates that the d_xz band gains quasiparticle spectral weight compared to the d_yz band for negative antisymmetric strain, suggesting the same response inside the nematic phase. Our results are compatible with a different coherence of the d_xz and d_yz orbital within a Hund’s metal picture. We also discuss the influence of orbital mixing.

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289. Signatures of two-dimensional superconductivity emerging within a three-dimensional host superconductor
Carolina Parra, Francis C. Niestemski, Alex W. Contryman, Paula Giraldo-Gallo, Theodore H. Geballe, Ian R. Fisher, and Hari C. Manoharan,
PNAS 118 e2017810118 (2021).

Spatial disorder has been shown to drive two-dimensional (2D) superconductors to an insulating phase through a superconductor–insulator transition (SIT). Numerical calculations predict that with increasing disorder, emergent electronic granularity is expected in these materials - a phenomenon where superconducting (SC) domains on the scale of the material's coherence length are embedded in an insulating matrix and coherently coupled by Josephson tunneling. Here, we present spatially resolved scanning tunneling spectroscopy (STS) measurements of the three-dimensional (3D) superconductor BaPb_1-xBi_xO₃ (BPBO), which surprisingly demonstrate three key signatures of emergent electronic granularity, having only been previously conjectured and observed in 2D thin-film systems. These signatures include the observation of emergent SC domains on the scale of the coherence length, finite energy gap over all space, and strong enhancement of spatial anticorrelation between pairing amplitude and gap magnitude as the SIT is approached. These observations are suggestive of 2D SC behavior embedded within a conventional 3D s-wave host, an intriguing but still unexplained interdimensional phenomenon, which has been hinted at by previous experiments in which critical scaling exponents in the vicinity of a putative 3D quantum phase transition are consistent only with dimensionality d = 2.

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288. Proposal for methods to measure the octupole susceptibility in certain cubic Pr compounds
M. E. Sorensen and I. R. Fisher,
Phys. Rev. B 103, 155106 (2021).

Direct means of measuring the susceptibility toward an octupole order parameter are proposed via a sixth rank tensor property. Equivalent derivatives of more conventionally measured tensor properties, including elastic stiffness, magnetic susceptibility, and elastoresistivity, are written in full, as constrained by the symmetry of the experimentally motivated O_h point group. For simplicity, we consider the specific case of Pr³⁺ ions in a cubic point symmetry with a Γ₃ crystal-field ground state, but the ideas are somewhat general. The experimental feasibility of measuring these various derivatives of tensor quantities is discussed.

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287. Ultrafast formation of domain walls of a charge density wave in SmTe₃
M. Trigo, P. Giraldo-Gallo, J. N. Clark, M. E. Kozina, T. Henighan, M. P. Jiang, M. Chollet, I. R. Fisher, J. M. Glownia, T. Katayama, P. S. Kirchmann, D. Leuenberger, H. Liu, D. A. Reis, Z. X. Shen, and D. Zhu,
Phys. Rev. B 103, 054109 (2021).

We study ultrafast x-ray diffraction on the charge density wave (CDW) of SmTe₃ using an x-ray free-electron laser. The high momentum and time resolution afforded by the x-ray laser enabled capturing fine wave-vector and time-dependent features of the CDW that originate from fast (in time) and sharp (in real space) variations of the CDW lattice distortion, which we attribute to an inversion of the order parameter. These domain inversions occur near the surface and are caused by the short penetration depth of the near-infrared pump with the wavelength centered at 800 nm, resulting in CDW domain walls perpendicular to the sample surface. These domain walls break the CDW long-range order on the scale of the x-ray probe depth, controlled experimentally by the x-ray incidence angle and suppress the diffraction intensity of the CDW for times much longer than the ~1 ps recovery of the electronic gap observed in time and angle-resolved photoemission spectroscopy. We model the spatial and temporal dependences of the order parameter using a simple Ginzburg-Landau model with all the parameters obtained from the published literature. We find reasonable agreement between the calculated and the measured diffraction across the momentum, time, fluence, and incidence angle dependence without adjusting any parameters. We reconstruct the spatial and temporal dependences of the lattice order parameter and find that at long times, depending on the pump fluence, multiple domain walls remain at distances of a few nanometers from the surface.

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286. Robust superconductivity intertwined with charge density wave and disorder in Pd-intercalated ErTe₃
Alan Fang, Anisha G. Singh, Joshua A. W. Straquadine, Ian R. Fisher, Steven A. Kivelson, and Aharon Kapitulnik,
Phys. Rev. Research 2, 043221 (2020).

Pd-intercalated ErTe₃ is studied as a model system to explore the effect of “intertwined” superconducting and charge density wave (CDW) orders. Despite the common wisdom that superconductivity emerges only when CDW is suppressed, we present data from STM and ac susceptibility measurements that show no direct competition between CDW order and superconductivity. Both coexist over most of the intercalation range, with uniform superconductivity over length scales that exceed the superconducting coherence length. This is despite persisting short-range CDW order and increased scattering from the Pd intercalation. While superconductivity is insensitive to local defects in either of the bidirectional CDWs, vestiges of the Fermi-level distortions are observed in the properties of the superconducting state.

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285. Frequency-dependent sensitivity of AC elastocaloric effect measurements explored through analytical and numerical models
J. A. W. Straquadine, M.S. Ikeda and I. R. Fisher,
Review of Scientific Instruments 91, 083905 (2020).

We present a comprehensive study of the frequency-dependent sensitivity for measurements of the AC elastocaloric effect by applying both exactly soluble models and numerical methods to the oscillating heat flow problem. These models reproduce the finer details of the thermal transfer functions observed in experiments, considering here representative data for single-crystal Ba(Fe_1-xCo_x)₂As₂. Based on our results, we propose a set of practical guidelines for experimentalists using this technique. This work establishes a baseline against which the frequency response of the AC elastocaloric technique can be compared and provides intuitive explanations of the detailed structure observed in experiments.

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284. Magnetic breakdown and charge density wave formation: A quantum oscillation study of the rare-earth tritellurides
P. Walmsley, S. Aeschlimann, J. A. W. Straquadine, P. Giraldo-Gallo, S. C. Riggs, M. K. Chan, R. D. McDonald, and I. R. Fisher,
Phys. Rev. B 102, 045150 (2020), Editors’ Suggestion.

The rare-earth tritellurides (RTe₃, where R = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Y) form a charge density wave state consisting of a single unidirectional charge density wave for lighter R, with a second unidirectional charge density wave, perpendicular and in addition to the first, also present at low temperatures for heavier R. We present a quantum oscillation study in magnetic fields up to 65 T that compares the single charge density wave state with the double charge density wave state both above and below the magnetic breakdown field of the second charge density wave. In the double charge density wave state it is observed that there remain several small, light pockets, with the largest occupying around 0.5 percent of the Brillouin zone. By applying magnetic fields above the independently determined magnetic breakown field, the quantum oscillation frequencies of the single charge density wave state are recovered, as expected in a magnetic breakdown scenario. Measurements of the electronic effective mass do not show any divergence or significant increase on the pockets of Fermi surface observed here as the putative quantum phase transition between the single and the double charge density wave states is approached.

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283. Interplay of charge density wave states and strain at the surface of CeTe₂
Bishnu Sharma, Manoj Singh, Burhan Ahmed, Boning Yu, Philip Walmsley, Ian R. Fisher and Michael C. Boyer,
Phys. Rev. B 101, 245423 (2020).

We use scanning tunneling microscopy (STM) to study charge density wave (CDW) states in the rare-earth ditelluride, CeTe₂. Our STM measurements surprisingly detect a unidirectional CDW with q ~ 0.28 a^*, which differs from previous experimental and first-principles studies of the rare-earth ditellurides, and which is very close to what is found in experimental measurements of the related rare-earth tritellurides. Furthermore, in the vicinity of an extended subsurface defect, we find spatially-separated as well as spatially-coexisting unidirectional CDWs at the surface of CeTe₂. We quantify the nanoscale strain and its variations induced by this defect, and establish a correlation between local lattice strain and the locally-established CDW states; this suggests that lattice strain plays an important role in determining the specific characteristics of the established CDW state. Our measurements probe the fundamental properties of a weakly-bound two-dimensional Te sheet, which experimental and theoretical work has previously established as the fundamental component driving much of the essential physics in both the rare-earth di- and tritelluride compounds.

Link to full article.

282. High resolution time- and angle-resolved photoemission spectroscopy with 11 eV laser pulses
Changmin Lee, Timm Rohwer, Edbert J. Sie, Alfred Zong, Edoardo Baldini, Joshua Straquadine, Philip Walmsley, Dillon Gardner, Young S. Lee, Ian R. Fisher and Nuh Gedik,
Reviews of Scientific Instruments 91, 043102 (2020).

Performing time- and angle-resolved photoemission (tr-ARPES) spectroscopy at high momenta necessitates extreme ultraviolet laser pulses, which are typically produced via high harmonic generation (HHG). Despite recent advances, HHG-based setups still require large pulse energies (from hundreds of μJ to mJ) and their energy resolution is limited to tens of meV. Here, we present a novel 11 eV tr-ARPES setup that generates a flux of 5 × 10¹⁰ photons/s and achieves an unprecedented energy resolution of 16 meV. It can be operated at high repetition rates (up to 250 kHz) while using input pulse energies down to 3 μJ. We demonstrate these unique capabilities by simultaneously capturing the energy and momentum resolved dynamics in two well-separated momentum space regions of a charge density wave material ErTe₃. This novel setup offers the opportunity to study the non-equilibrium band structure of solids with exceptional energy and time resolutions at high repetition rates.

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281. Nematic transitions in iron pnictide superconductors imaged with a quantum gas,
Fan Yang, Stephen F. Taylor, Stephen D. Edkins, Johanna C. Palmstrom, Ian R. Fisher and Benjamin L. Lev
Nat. Physics 16, 514 (2020).

The Scanning Quantum Cryogenic Atom Microscope (SQCRAMscope) uses an atomic Bose–Einstein condensate to measure magnetic fields emanating from solid-state samples. The quantum sensor does so with unprecedented d.c. sensitivity at micrometre resolution, from room to cryogenic temperatures. An additional advantage of the SQCRAMscope is the preservation of optical access to the sample so that magnetometry imaging of, for example, electron transport may be performed in concert with other imaging techniques. Here, we apply this multimodal imaging capability to the study of nematicity in iron pnictide high-temperature superconductors, where the relationship between electronic and structural symmetry breaking resulting in a nematic phase is under debate. We combine the SQCRAMscope with an in situ microscope that measures optical birefringence near the surface. This enables simultaneous and spatially resolved detection of both bulk and near-surface manifestations of nematicity via transport and structural deformation channels, respectively. By performing local measurements of emergent resistivity anisotropy in iron pnictides, we observe sharp, nearly concurrent transport and structural transitions. More broadly, these measurements demonstrate the SQCRAMscope’s ability to reveal important insights into the physics of complex quantum materials.

Link to full article.

280. Low work function in the 122-family of iron-based superconductors
H. Pfau, H. Soifer, J. A. Sobota, A. Gauthier, C. R. Rotundu, J. C. Palmstrom, I. R. Fisher, G.-Y. Chen, H.-H. Wen, Z.-X. Shen, and P. S. Kirchmann,
Phys. Rev. Materials 4, 034801 (2020).

We determine the work functions of the iron arsenic compounds AFe₂As₂ (A = Ca, Ba, Cs) using photoemission spectroscopy to be 2.7 eV for CaFe₂As₂, 1.8 eV for BaFe₂As₂, and 1.3 eV for CsFe₂As₂. The work functions of these 122 iron-based superconductors track those of the elementary metal A but are substantially smaller. The most likely explanation of this observation is that the cleaving surface exposes only half an A-layer. The low work function and good photoemission cross section of BaFe₂As₂ and CsFe₂As₂ enable photoemission even from a common white LED light.

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279. Light-induced charge density wave in LaTe₃,
Anshul Kogar, Alfred Zong, Pavel E Dolgirev, Xiaozhe Shen, Joshua Straquadine, Ya-Qing Bie, Xirui Wang, Timm Rohwer, I-Cheng Tung, Yafang Yang, Renkai Li, Jie Yang, Stephen Weathersby, Suji Park, Michael E Kozina, Edbert J Sie, Haidan Wen, Pablo Jarillo-Herrero, Ian R Fisher, Xijie Wang and Nuh Gedik
Nat. Physics 16, 159 (2020).

When electrons in a solid are excited by light, they can alter the free energy landscape and access phases of matter that are out of reach in thermal equilibrium. This accessibility becomes important in the presence of phase competition, when one state of matter is preferred over another by only a small energy scale that, in principle, is surmountable by the excitation. Here, we study a layered compound, LaTe₃, where a small lattice anisotropy in the a–c plane results in a unidirectional charge density wave (CDW) along the c axis. Using ultrafast electron diffraction, we find that, after photoexcitation, the CDW along the c axis is weakened and a different competing CDW along the a axis subsequently emerges. The timescales characterizing the relaxation of this new CDW and the reestablishment of the original CDW are nearly identical, which points towards a strong competition between the two orders. The new density wave represents a transient non-equilibrium phase of matter with no equilibrium counterpart, and this study thus provides a framework for discovering similar states of matter that are ‘trapped’ under equilibrium conditions.

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278. Charge Kondo Effect induced by valence skipping dopants in Pb_1-xTl_xTe and Pb_1-xNa_xTe probed by ¹²⁵Te-NMR
Ryusei Horikawa, Mitsuharu Yashima, Takashi Matsumura, Shota Maki, Hidekazu Mukuda, Kazumasa Miyake, Hironaru Murakami, Philip Walmsley, Paula Giraldo-Gallo, Theodore H. Geballe, and Ian R. Fisher,
JPS Conf. Proc. 30, 011126 (2020).

We have performed schematic ¹²⁵Te-NMR measurements in Pb_1-xTl_xTe (x = 0, 0.35, 1.0%) and Pb_1-xNa_xTe (x = 0.46, 1.45%). Superconductivity occurs above x ~ 0.3% in Pb_1-xTl_xTe and super- conducting temperature T_c reaches about 1 K at x = 1.0%. In Pb_0.99Tl_0:01Te, the ¹²⁵Te nuclear spin relaxation rate (1/T₁T) for Te sites near Tl dopants is unexpectedly enhanced in the normal state below a characteristic temperature of ~ 10 K, below which the resistivity experiences an upturn. In contrast, no enhancement of 1/T₁T is observed at low temperatures for Te sites both near and far from non-valence-skipping Na dopants in Pb_1-xNa_xTe with x = 1.45%. These results suggest the existence of valence fluctuations associated with the charge Kondo effect arising from Tl dopants in the superconducting sample Pb_1-xTl_xTe with x = 1.0%.

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277. Disorder-induced suppression of charge density wave order: STM study of Pd-intercalated ErTe₃
Alan Fang, Joshua A. W. Straquadine, Ian R. Fisher, Steven A. Kivelson, and Aharon Kapitulnik,
Phys. Rev. B 100, 235446 (2019).

Pd-intercalated ErTe₃ is studied as a model system to explore the effect of increasing disorder on an incommensurate two-component charge density wave (CDW). The ordering vectors of the CDW components lie along the two in-plane principal axes of the nearly tetragonal crystal structure. Using scanning tunneling microscopy (STM), we show that introducing Pd intercalants (i.e., disorder) induces CDW dislocations, which appear to be associated with each CDW component separately. Increasing Pd concentration has a stronger effect on the secondary CDW order, manifested in a higher density of dislocations, and thus increases the anisotropy (nematic character) of the CDW. Suggestive evidence of Bragg glass phases at weak disorder is also discussed.

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276. Anisotropic Superconducting Gap in Optimally Doped Iron–Based Material,
A. Pal, M. Chinotti, J.-H. Chu, H.-H. Kuo, I. R. Fisher, L. Degiorgi
Journal of Superconductivity and Novel Magnetism 33, 2313–2318 (2020).

We offer a comprehensive optical investigation of the optimally hole-doped Ba_0.6K_0.4Fe₂As₂ over a broad spectral range, as a function of temperature and of tunable applied stress, which acts as an external symmetry breaking field. We show that there is a large electronic nematicity at optimal doping which extends right under the superconducting dome and implies an anisotropy of the superconducting energy gaps.

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275. Possible scale invariant linear magnetoresistance in pyrochlore iridates Bi₂Ir₂O₇
Jiun-Haw Chu, Jian Liu, Han Zhang, Kyle Noordhoek, Scott C Riggs, Maxwell Shapiro, Claudy Ryan Serro, Di Yi, M Mellisa, S.J. Suresha, Carlos Frontera, E. Arenholz, Ashvin Vishwanath, Xavi Marti, I.R. Fisher and R. Ramesh,
New Journal of Physics 21, 113041 (2019).

We report the observation of a linear magnetoresistance in single crystals and epitaxial thin films of the pyrochlore iridate Bi₂Ir₂O₇. The linear magnetoresistance is positive and isotropic at low temperatures, without any sign of saturation up to 35 T. As temperature increases, the linear field dependence gradually evolves to a quadratic field dependence. The temperature and field dependence of magnetoresistance of Bi₂Ir₂O₇ bears strikingly resemblance to the scale invariant magnetoresistance observed in the strange metal phase in high Tc cuprates. However, the residual resistivity of Bi₂Ir₂O₇ is more than two orders of magnitude higher than the curpates. Our results suggest that the correlation between linear magnetoresistance and quantum fluctuations may exist beyond high temperature superconductors.

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274. Growth of nematic susceptibility in the field-induced normal state of an iron-based superconductor revealed by elastoresistivity measurements in a 65 T pulsed magnet
J. A. W. Straquadine, J. C. Palmstrom, P. Walmsley, A. T. Hristov, F. Weickert, F. F. Balakirev, M. Jaime, R. McDonald, and I. R. Fisher,
Phys. Rev. B 100, 125147 (2019), Editors' Suggestion.

In iron-based superconductors, both nematic and magnetic fluctuations are expected to enhance superconductivity and may originate from a quantum critical point hidden beneath the superconducting dome. The behavior of the nonsuperconducting state can be an important piece of the puzzle, motivating, in this paper, the use of high magnetic fields to suppress superconductivity and measure the nematic susceptibility of the normal state at low temperatures. We describe experimental advances which make it possible to measure a resistive gauge factor (which is a proxy for the nematic susceptibility) in the field-induced normal state in a 65 T pulsed magnet, and report measurements of the gauge factor of a micromachined single crystal of Ba(Fe_0.926Co_0.074)₂As₂ at temperatures down to 1.2 K. The nematic susceptibility increases monotonically in the field-induced normal state as the temperature decreases, consistent with the presence of a quantum critical point nearby in composition.

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273. Dynamical Slowing-Down in an Ultrafast Photoinduced Phase Transition
Alfred Zong, Pavel E. Dolgirev, Anshul Kogar, Emre Ergeçen, Mehmet B. Yilmaz, Ya-Qing Bie, Timm Rohwer, I-Cheng Tung, Joshua Straquadine, Xirui Wang, Yafang Yang, Xiaozhe Shen, Renkai Li, Jie Yang, Suji Park, Matthias C. Hoffmann, Benjamin K. Ofori-Okai, Michael E. Kozina, Haidan Wen, Xijie Wang, Ian R. Fisher, Pablo Jarillo-Herrero, and Nuh Gedik,
Phys. Rev. Lett. 123, 097601 (2019).

Complex systems, which consist of a large number of interacting constituents, often exhibit universal behavior near a phase transition. A slowdown of certain dynamical observables is one such recurring feature found in a vast array of contexts. This phenomenon, known as critical slowing-down, is well studied mostly in thermodynamic phase transitions. However, it is less understood in highly nonequilibrium settings, where the time it takes to traverse the phase boundary becomes comparable to the timescale of dynamical fluctuations. Using transient optical spectroscopy and femtosecond electron diffraction, we studied a photoinduced transition of a model charge-density-wave (CDW) compound LaTe₃. We observed that it takes the longest time to suppress the order parameter at the threshold photoexcitation density, where the CDW transiently vanishes. This finding can be captured by generalizing the time-dependent Landau theory to a system far from equilibrium. The experimental observation and theoretical understanding of dynamical slowing-down may offer insight into other general principles behind nonequilibrium phase transitions in many-body systems.

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272. Momentum dependence of the nematic order parameter in iron-based superconductors
H. Pfau, S. D. Chen, M. Yi, M. Hashimoto, C. R. Rotundu, J. C. Palmstrom, T. Chen, P. -C. Dai, J. Straquadine, A. Hristov, R. J. Birgeneau, I. R. Fisher, D. Lu, and Z. -X. Shen,
Phys. Rev. Lett. 123, 066402 (2019).

The momentum dependence of the nematic order parameter is an important ingredient in the microscopic description of iron-based high-temperature superconductors. While recent reports on FeSe indicate that the nematic order parameter changes sign between electron and hole bands, detailed knowledge is still missing for other compounds. Combining angle-resolved photoemission spectroscopy with uniaxial strain tuning, we measure the nematic band splitting in both FeSe and BaFe₂As₂ without interference from either twinning or magnetic order. We find that the nematic order parameter exhibits the same momentum dependence in both compounds with a sign change between the Brillouin center and the corner. This suggests that the same microscopic mechanism drives the nematic order in spite of the very different phase diagrams.

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271. AC Elastocaloric effect as a probe for thermodynamic signatures of continuous phase transitions
Matthias S. Ikeda, Joshua A.W. Straquadine, Alexander T. Hristov, Thanapat Worasaran, Johanna C. Palmstrom, Matthew Sorensen, Philip Walmsley, Ian R. Fisher,
Review of Scientific Instruments 90, 083902 (2019).

Studying the response of materials to strain can elucidate subtle properties of the electronic structure in strongly correlated materials. Here, we focus on the elastocaloric coefficients, forming a second rank tensor quantity describing the relation between entropy and strain. In contrast to the better-known elastoresistivity, the elastocaloric effect is a thermodynamic quantity. Experimentally, elastocaloric effect measurements are demanding since the thermodynamic conditions during the measurement have to be well controlled. In this work, we present a technique to measure the elastocaloric effect under quasiadiabatic conditions. The technique is based on oscillating strain, which allows for increasing the frequency of the elastocaloric effect above the thermal relaxation rate of the sample. We apply the technique to Co-doped iron pnictide superconductors and show that the thermodynamic signatures of second order phase transitions in the elastocaloric effect closely follow those observed in calorimetry experiments. In contrast to heat capacity, elastocaloric effect measurements allow for the electronic signatures to be measured against a small phononic background even at high temperatures and in addition give information on the symmetry of the involved order parameters. This establishes the technique as a powerful complimentary tool for extracting the entropy landscape as a function of strain proximate to a continuous phase transition.

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270. Phase transition preceding magnetic long-range order in the double perovskite Ba₂NaOsO₆
Kristin Willa, Roland Willa, Ulrich Welp, Ian R. Fisher, Andreas Rydh, Wai-Kwong Kwok, and Zahir Islam,
Phys. Rev. B 100, 041108 (R) (2019).

Recent theoretical studies [G. Chen et al., Phys. Rev. B 82, 174440 (2010); H. Ishizuka et al., Phys. Rev. B 90, 184422 (2014)] for the magnetic Mott insulator Ba₂NaOsO₆ have proposed a low-temperature order parameter that breaks lattice rotational symmetry without breaking time reversal symmetry, leading to a nematic phase just above the magnetic ordering temperature. We present high-resolution calorimetric and magnetization data of the same Ba₂NaOsO₆ single crystal and show evidence for a weakly field-dependent phase transition occurring at a temperature of Ts ˜ 9.5 K, above the magnetic ordering temperature of Tc ˜ 7.5 K. This transition appears as a broadened step in the low-field temperature dependence of the specific heat. The evolution of the phase boundary with applied magnetic field suggests that this phase coincides with the phase of broken local point symmetry seen in NMR experiments at high fields [L. Lu et al., Nat. Commun. 8, 14407 (2017)]. Furthermore, the magnetic field dependence of the specific heat provides clear indications for magnetic correlations persisting at temperatures between Tc and Ts where long-range magnetic order is absent, giving support for the existence of the proposed nematic phase.

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269. Imaging anisotropic vortex dynamics in FeSe
Irene P. Zhang, Johanna C. Palmstrom, Hilary Noad, Logan Bishop-Van Horn, Yusuke Iguchi, Zheng Cui, Eli Mueller, John R. Kirtley, Ian R. Fisher, and Kathryn A. Moler,
Phys. Rev. B 100, 024514 (2019).

Strong vortex pinning in FeSe could be useful for technological applications and could provide clues about the coexistence of superconductivity and nematicity. To characterize the pinning of individual, isolated vortices, we simultaneously apply a local magnetic field and image the vortex motion with scanning superconducting quantum interference devices susceptibility. We find that the pinning is highly anisotropic: the vortices move easily along directions that are parallel to the orientations of twin domain walls and pin strongly in a perpendicular direction. These results are consistent with a scenario in which the anisotropy arises from vortex pinning on twin domain walls and quantify the dynamics of individual vortex pinning in FeSe.

Link to full article.

268. Suppression of charge density wave order by disorder in Pd-intercalated ErTe₃
J.A.W. Straquadine, F. Weber, S. Rosenkranz, A.H. Said, and I.R. Fisher,
Phys. Rev. B 99, 235138 (2019).

Disorder is generically anticipated to suppress long range charge density wave (CDW) order. We report transport, thermodynamic, and scattering experiments on Pd_xErTe₃, a model CDW system with disorder induced by intercalation. The pristine parent compound (x = 0) shows two separate, mutually perpendicular, incommensurate unidirectional CDW phases setting in at 270 K and 165 K. In this work we present measurements on a finely-spaced series of single crystal samples, in which we track the suppression of signatures corresponding to these two parent transitions as the Pd concentration increases. At the largest values of x, we observe complete suppression of long range CDW order in favor of superconductivity. We also report evidence from electron and x-ray diffraction which suggests a tendency toward short-range ordering along both wave vectors which persists even well above the crossover temperature and comment on the origin and consequences of this effect. Based on this work, Pd_xErTe₃ appears to provide a promising model system for the study of the interrelation of charge order and superconductivity in the presence of quenched disorder, for pseudotetragonal materials.

Link to full article.

267. Lattice dynamics, crystal-field excitations, and quadrupolar fluctuations of YbRu2Ge2
Mai Ye, E. W. Rosenberg, I. R. Fisher, and G. Blumberg,
Phys. Rev. B 99, 235104 (2019).

We employ polarization-resolved Raman scattering spectroscopy to study ferroquadrupolar (FQ) fluctuations and crystal-field (CF) excitations in the YbRu₂Ge₂ heavy-fermion metal with FQ transition at T_Q = 10K. We demonstrate that the electronic static Raman susceptibilities in quadrupolar symmetry channels exhibit nearly Curie law behavior and that the electron-lattice coupling is essential for the FQ transition at T_Q. We establish the CF level scheme of the Yb³⁺ ground state ²F_7/2 multiplet. We study the lattice dynamics and demonstrate coupling between CF transitions and phonon modes.

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266. Coherent order parameter dynamics in SmTe₃
M. Trigo, P. Giraldo-Gallo, M. E. Kozina, T. Henighan, M. P. Jiang, H. Liu, J. N. Clark, M. Chollet, J. M. Glownia, D. Zhu, T. Katayama, D. Leuenberger, P. S. Kirchmann, I. R. Fisher, Z. X. Shen, and D. A. Reis,
Phys. Rev. B 99, 104111 (2019).

We present ultrafast optical pump-probe and ultrafast x-ray diffraction measurements of the charge density wave dynamics in SmTe₃ at 300 K. We performed ultrafast x-ray diffraction measurements at the Linac Coherent Light Source to directly probe the dynamics of the finite-wave-vector order parameter. The dynamics reveal coherent oscillations at ~1.6 THz that become overdamped with increasing fluence. We identify this oscillation with the lattice component of the amplitude mode. Furthermore, our data allow for a clear identification of the amplitude mode frequency in the optical pump-probe data. In both measurements, the system reaches the symmetric phase at high fluence, where the order parameter vanishes and the response (reflectivity and x-ray intensity) is quadratic in the order parameter. This is observed in the x-ray diffraction as a small overdamped modulation near zero intensity. Similar overdamped features are observed in the optical reflectivity at high fluence. A time-dependent Ginzburg-Landau model captures qualitatively the essential features of the experimental observations.

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265. Optical evidence of an enhanced electronic effective mass in the anomalous Pb_1-xTl_xTe superconductor
A. Pal, M. Chinotti, L. Degiorgi, P. Walmsley, and I. R. Fisher,
Phys. Rev. Materials 3, 054801 (2019).

The narrow band-gap semiconductor PbTe exhibits a number of striking properties upon thallium (Tl)-doping, including the onset of superconductivity at temperatures Tc which are substantially higher than for materials with equivalent charge carrier concentration. Here we provide a thorough optical investigation of Pb_1-xTl_xTe over a very broad spectral range and contrast its normal-state, complete excitation spectrum with the optical response of the nonsuperconducting analog Na-doped PbTe.We capture the relevant energy scales shaping their electronic structure and uncover the formation of an impurity band upon doping with Tl, which evolves into a resonant state for large doping. This implies a large density of states and an enhancement of the optical effective mass m*/m_e of the itinerant charge carriers, which is stronger for Tl- than for Na-doping. Since the enhancement of m*/m_e particularly occurs upon crossing a critical concentration x_c in Tl-doped PbTe for which Tc is not zero, we advance its relevance for the onset of superconductivity.

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264. Divergence of the quadrupole-strain susceptibility of the electronic nematic system YbRu₂Ge₂
Elliott Rosenberg, Jiun-Haw Chu, Jacob Ruff, Alexander Hristov, Ian Fisher,
PNAS 116 7232-7237 (2019).

Ferroquadrupole order associated with local 4f atomic orbitals of rare-earth ions is a realization of electronic nematic order. However, there are relatively few examples of intermetallic materials which exhibit continuous ferroquadrupole phase transitions, motivating the search for additional materials that fall into this category. Furthermore, it is not clear a priori whether experimental approaches based on transport measurements which have been successfully used to probe the nematic susceptibility in materials such as the Fe-based superconductors will be as effective in the case of 4f intermetallic materials, for which the important electronic degrees of freedom are local rather than itinerant and are consequently less strongly coupled to the charge-carrying quasiparticles near the Fermi energy. In the present work, we demonstrate that the intermetallic compound YbRu₂Ge₂ exhibits a tetragonal-to-orthorhombic phase transition consistent with ferroquadrupole order of the Yb ions and go on to show that elastoresistivity measurements can indeed provide a clear window on the diverging nematic susceptibility in this system. This material provides an arena in which to study the causes and consequences of electronic nematicity.

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263. Evidence for topological defects in a photoinduced phase transition
A. Zong, A. Kogar, Y. Bie, T. Rohwer, C. Lee, E. Baldini, E. Ergecen, M. B. Yilmaz, B. Freelon, E. J. Sie, H. Zhou, J. Straquadine, P. Walmsley, P. E. Dolgirev, A. V. Rozhkov, I. R. Fisher, P. Jarillo-Herrero, V. B. Fine, N. Gedik,
Nature Physics 15, 27-31 (2019).

Upon excitation with an intense laser pulse, a symmetry-broken ground state can undergo a non-equilibrium phase transition through pathways different from those in thermal equilibrium. The mechanism underlying these photoinduced phase transitions has long been researched in the study of condensed matter systems, but many details in this ultrafast, non-adiabatic regime still remain to be clarified. To this end, we investigate the light-induced melting of a unidirectional charge density wave (CDW) in LaTe₃. Using a suite of time-resolved probes, we independently track the amplitude and phase dynamics of the CDW. We find that a fast (approximately 1 picosecond) recovery of the CDW amplitude is followed by a slower re-establishment of phase coherence. This longer timescale is dictated by the presence of topological defects: long-range order is inhibited and is only restored when the defects annihilate. Our results provide a framework for understanding other photoinduced phase transitions by identifying the generation of defects as a governing mechanism.

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262. Multi-band mass enhancement towards critical doping in a pnictide superconductor
Camilla Margaret Moir, Scott Chandler Riggs, Jose Augusto Galvis, Xiujun Lian, Paula Giraldo-Gallo, Jiun-Haw Chu, Philip Walmsley, Ian Randal Fisher, Arkady Shekhter & Gregory Scott Boebinger,
npj Quantum Materials volume 4, Article number: 8 (2019).

Near critical doping, high-temperature superconductors exhibit multiple anomalies associated with enhanced electronic correlations and quantum criticality. Quasiparticle mass enhancement approaching optimal doping has been reported in quantum oscillation measurements in both cuprate and pnictide superconductors. Although the data are suggestive of enhanced interactions, the microscopic theory of quantum oscillation measurements near a quantum critical point is not yet firmly established. It is therefore desirable to have a direct thermodynamic measurement of quasiparticle mass. Here we report high magnetic field measurements of heat capacity in the doped pnictide superconductor BaFe₂(As_1-xP_x)₂. We observe saturation of the specific heat at high magnetic field in a broad doping range above optimal doping which enables a direct determination of the electronic density of states recovered when superconductivity is suppressed. Our measurements find a strong total mass enhancement in the Fermi pockets that superconduct. This mass enhancement extrapolates to a mass divergence at a critical doping of x = 0.28.

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261. Elastoresistive and elastocaloric anomalies at magnetic and electronic-nematic critical points
Alexander T. Hristov, Matthias S. Ikeda, Johanna C. Palmstrom, Philip Walmsley, and Ian R. Fisher,
Phys. Rev. B 99, 100101(R) (2019).

Using Ba(Fe_0.975Co_0.025)₂As₂ as an exemplar material exhibiting second-order electronic-nematic and antiferromagnetic transitions, we present measurements that reveal anomalies in the elastoresistance (dρ_ij/dε_kl) and elastocaloric effect (dT/dε_kl) at both phase transitions for induced strains ε_kl that do not share the symmetry of either order parameter. Both effects are understood to arise from the effect of strain on the transition temperatures; in the region close to the phase transitions this leads to (1) similarity between the strain and temperature derivatives of the resistivity and (2) similarity between the elastocaloric effect and the singular part of the specific heat. These mechanisms for elastoresistance and elastocaloric effect should be anticipated for any material in which mechanical deformation changes the transition temperature. Furthermore, these measurements provide evidence that the Fisher-Langer relation ρ^(c) ∝ U^(c) between the scattering from critical degrees of freedom and their energy density, respectively, holds near each of the transitions in the material studied under varying strain as it does for varying temperature.

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260. Electronic structure of the quadrupolar ordered heavy-fermion compound YbRu₂Ge₂ measured by angle-resolved photoemission
H. Pfau, E. W. Rosenberg, P. Massat, B. Moritz, M. Hashimoto, D. Lu, I. R. Fisher, and Z.-X. Shen,
Phys. Rev. B 99, 075159 (2019).

We studied the electronic structure of the heavy-fermion compound Yb(Ru_1-xRh_x)₂Ge₂ with x = 0 and nominally x = 0.125 using ARPES and LDA calculations. We find a valence band structure of Yb corresponding to a noninteger valence close to 3+. The three observed crystal electric field levels with a splitting of 32 and 75 meV confirm the suggested configuration with a quasiquartet ground state. The experimentally determined band structure of the conduction electrons with predominantly Ru 4d character is well reproduced by our calculations. YbRu₂Ge₂ undergoes a nonmagnetic phase transition into a ferroquadrupolar ordered state below 10.2 K and then to an antiferromagnetically ordered state below 6.5 K. A small hole Fermi surface shows nesting features in our calculated band structure and its size determined by ARPES is close to themagnetic ordering wave vector found in neutron scattering. The transitions are suppressed when YbRu₂Ge₂ is doped with 12.5% Rh. The electron doping leads to a shift of the band structure and successive Lifshitz transitions.

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259. Optical anisotropy in optimally doped iron-based superconductor
Anirban Pal, Manuel Chinotti, Jiun-Haw Chu, Hseuh-Hui Kuo, Ian Randal Fisher & Leonardo Degiorgi,
npj Quantum Materials volume 4, Article number: 3 (2019).

The divergent nematic susceptibility, obeying a simple Curie-Weiss power law over a large temperature interval, is empirically found to be a ubiquitous signature in several iron-based materials across their doping-temperature phase diagram. The composition at which the associated Weiss temperature extrapolates to zero is found to be close to optimal doping, boosting the debate to what extent nematic fluctuations contribute to the pairing-mechanism and generally affect the electronic structure of iron-based superconductors. Here, we offer a comprehensive optical investigation of the optimally hole-doped Ba_0.6K_0.4Fe₂As₂ over a broad spectral range, as a function of temperature and of tunable applied stress, which acts as an external symmetry breaking field. We show that the stress-induced optical anisotropy in the infrared spectral range is reversible upon sweeping the applied stress and occurs only below the superconducting transition temperature. These findings demonstrate that there is a large electronic nematicity at optimal doping which extends right under the superconducting dome.

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258. Detailed band structure of twinned and detwinned BaFe₂As₂ studied with angle-resolved photoemission spectroscopy
H. Pfau, C. R. Rotundu, J. C. Palmstrom, S. D. Chen, M. Hashimoto, D. Lu, A. F. Kemper, I. R. Fisher, and Z.-X. Shen,
Phys. Rev. B 99, 035118 (2019).

We study the band structure of twinned and detwinned BaFe₂As₂ using angle-resolved photoemission spectroscopy. The combination of measurements in the ordered and normal states along four high-symmetry momentum directions Γ/Z - X/Y enables us to identify the complex reconstructed band structure in the ordered state in great detail. We clearly observe the nematic splitting of the d_xz and d_yz orbitals as well as folding due to magnetic order with a wave vector of (π,π,π). We are able to assign all observed bands. In particular we suggest an assignment of the electron bands different from previous reports. The high-quality spectra allow us to achieve a comprehensive understanding of the band structure of BaFe₂As₂.

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257. Sharp increase in the density of states in PbTe upon approaching a saddle point in the band structure
P. Walmsley, D. M. Abrams, J. Straquadine, M. K. Chan, R. D. McDonald, P. Giraldo-Gallo, and I. R. Fisher,
Phys. Rev. B 99, 035105 (2019).

PbTe is a leading mid-range thermoelectric material with a zT that has been enhanced by, amongst other methods, band engineering. Here we present an experimental study of the Hall effect, quantum oscillations, specific heat, and electron microprobe analysis that explores the evolution of the electronic structure of PbTe heavily doped with the 'ideal' acceptor Na up to the solubility limit. We identify two phenomenological changes that onset as the electronic structure deviates from a Kane-type dispersion at around 180 meV; a qualitative change in the field dependence of the Hall effect indicative of an increase in the high-field limit and a change in the Fermiology, and a sharp increase in the density of states as a function of energy. Following consideration of three possible origins for the observed phenomenology we conclude that the most likely source is nonellipsoidicity of the L pocket upon approach to a saddle point in the band structure, which is evidenced directly by our quantum oscillation measurements. Comparison to density functional theory calculations imply that this evolution of the electronic structure may be a key contributor to the large thermopower in PbTe.

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256. Symmetric and antisymmetric strain as continuous tuning parameters for electronic nematic order
M. S. Ikeda, T. Worasaran, J. C. Palmstrom, J. A. W. Straquadine, P. Walmsley, and I. R. Fisher,
Phys. Rev. B 98, 245133 (2018).

We report the separate response of the critical temperature of the nematic phase transition T_S to symmetric and antisymmetric strains for the prototypical underdoped iron pnictide Ba(Fe_0.975Co_0.025)₂As₂. This decomposition is achieved by comparing the response of T_S to in-plane uniaxial stress and hydrostatic pressure. In addition to quantifying the two distinct linear responses to symmetric strains, we find a quadratic variation of T_S as a response to antisymmetric strains ε_B1g = (ε_xx-ε_yy)/2, exceeding the nonlinear response to symmetric strains by at least two orders of magnitude. These observations establish orthogonal antisymmetric strain as a powerful tuning parameter for nematic order.

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255. Direct spectroscopic evidence for mixed-valence Tl in the low carrier-density superconductor Pb_1-xTl_xTe
P. Walmsley, C. Liu, A. D. Palczewski, P. Giraldo-Gallo, C. G. Olson, I. R. Fisher, and A. Kaminski,
Phys. Rev. B 98, 184506 (2018).

Upon doping with Tl the narrow band-gap semiconductor PbTe exhibits anomalously high-temperature superconductivity despite a very low carrier-density as well as signatures of the Kondo effect despite an absence of magnetic moments. These phenomena have been explained by invoking 2e fluctuations of the valence of the Tl dopants, but a direct measurement of the mixed valency implied by such a mechanism has not been reported to date. In this work we present the unambiguous observation of multiple valences of Tl in Tl-doped PbTe via photoemission spectroscopy measurements. It is shown via a quantitative analysis that the suppression of the carrier density in compositions exhibiting superconductivity and Kondo-like behavior can be accounted for by mixed valency, thus arguing against a self-compensation scenario proposed elsewhere for this material and strengthening the case for valence fluctuation models. In addition to the identification of Tl⁺ and Tl³⁺ a possible third intermediate local charge-density is suggested by full fits to the data, the origins of which are also discussed but remain unclear.

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254. Evidence of Incoherent Carriers Associated with Resonant Impurity Levels and Their Influence on Superconductivity in the Anomalous Superconductor Pb_1-xTl_xTe
P. Giraldo-Gallo, P. Walmsley, B. Sangiorgio, S.?C. Riggs, R.?D. McDonald, L. Buchauer, B. Fauque, Chang Liu, N.?A. Spaldin, A. Kaminski, K. Behnia, and I.?R. Fisher,
Phys. Rev. Lett. 121, 207001 (2018).

We present a combined experimental and theoretical study of the evolution of the Fermi surface of the anomalous superconductor Pb_1-xTl_xTe as a function of thallium concentration, drawing on a combination of magnetotransport measurements (Shubnikov–de Haas oscillations and the Hall coefficient), angle resolved photoemission spectroscopy, and density functional theory calculations of the electronic structure. Our results indicate that for Tl concentrations beyond a critical value, the Fermi energy coincides with resonant impurity states in Pb_1-xTl_xTe, and we rule out the presence of an additional valence band maximum at the Fermi energy. A comparison to nonsuperconducting Pb_1-xNa_xTe implies that the presence of these impurity states at the Fermi energy provides the enhanced pairing interaction and thus also the anomalously high temperature superconductivity in this material.

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253. Measurement of elastoresistivity at finite frequency by amplitude demodulation
Alexander T. Hristov, Johanna C. Palmstrom, Joshua A. W. Straquadine, Tyler A. Merz, Harold Y. Hwang, and Ian R. Fisher,
Rev. Sci. Instrum. 89, 103901 (2018).

Elastoresistivity, the relation between resistivity and strain, can elucidate the subtle properties of the electronic structure of a material and is an increasingly important tool for the study of strongly correlated materials. To date, elastoresistivity measurements have predominantly been performed with quasi-static (DC) strain. In this work, we demonstrate a method using AC strain in elastoresistivity measurements. A sample experiencing AC strain has a time-dependent resistivity, which modulates the voltage produced by an AC current; this effect produces time-dependent variations in resistivity that are directly proportional to the elastoresistivity, and which can be measured more quickly, with less strain on the sample, and with less stringent requirements for temperature stability than the previous DC technique. Example measurements between 10 Hz and 3 kHz are performed on a material with a large, well-characterized and temperature dependent elastoresistivity: the representative iron-based superconductor Ba(Fe_0.975Co_0.025)₂As₂. These measurements yield a frequency independent elastoresistivity and reproduce results from previous DC elastoresistivity methods to within experimental accuracy. We emphasize that the dynamic (AC) elastoresistivity is a distinct material-specific property that has not previously been considered.

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252. Competing soft phonon modes at the charge-density-wave transitions in DyTe₃
M. Maschek, D. A. Zocco, S. Rosenkranz, R. Heid, A. H. Said, A. Alatas, P. Walmsley, I. R. Fisher, and F. Weber,
Phys. Rev. B 98, 094304 (2018).

The family of rare-earth (R) tritellurides RTe₃ features charge-density-wave (CDW) order related to strongly momentum-dependent electron-phonon coupling. Similar to other CDW compounds, superconductivity is observed when the CDW order is suppressed via hydrostatic pressure [J. J. Hamlin et al., Phys. Rev. Lett. 102, 177002 (2009)]. What sets the heavier members of the RTe₃ series apart is the observation of a second CDW transition at lower temperatures having an in-plane ordering wave vector q_CDW,2 || [100] of almost the same magnitude but orthogonal to the ordering wave vector q_CDW,1 || [001] observed at higher temperatures [N. Ru et al., Phys. Rev. B 77, 035114 (2008)]. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of DyTe₃ In particular, we show that there are several phonon modes along both in-plane directions, which respond to the onset of the CDW transition at T_CDW,1 = 308 K. Surprisingly, these soft modes close to q_CDW,2 = (0.68, 0, 0) show strong softening near T_CDW,1 but do not exhibit any response to the lower-temperature transition at T_CDW,2 = 68 K. Our results indicate that the low-temperature CDW order is not just the 90^o rotated analogue of the one appearing at high temperatures.

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251. Interplay of lattice, electronic, and spin degrees of freedom in detwinned BaFe₂As₂: A Raman scattering study
A. Baum, Ying Li, M. Tomic, N. Lazarevic, D. Jost, F. Loffler, B. Muschler, T. Bohm, J.-H. Chu, I. R. Fisher, R. Valenti, I. I. Mazin, and R. Hackl,
Phys. Rev. B 98, 075113 (2018).

We report results of Raman scattering experiments on twin-free BaFe₂As₂ with the main focus placed on understanding the influence of electronic and spin degrees of freedom on the lattice dynamics. In particular, we scrutinize the E_g modes and the As A_1g mode. Each of the two E_g phonons in the tetragonal phase is observed to split into a B_2g and a B_3g mode upon entering the orthorhombic stripe-magnetic phase. The splitting amounts to approximately 10 cm^-1 and less than 5 cm^-1 for the low- and the high-energy E_g mode, respectively. The detailed study of the fully symmetric As mode using parallel incident and outgoing photon polarizations along either the antiferromagnetic or the ferromagnetic Fe-Fe direction reveals an anisotropic variation of the spectral weight with the energy of the exciting laser indicating a polarization-dependent resonance effect. Along with the experiments we present results from density functional theory calculations of the phonon eigenvectors, the dielectric function, and the Raman tensor elements. The comparison of theory and experiment indicates that (i) orbital-selective electronic correlations are crucial to understand the lattice dynamics and (ii) all phonon anomalies originate predominantly from the magnetic ordering and the corresponding reconstruction of the electronic bands at all energies.

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250. Nature of lattice distortions in the cubic double perovskite Ba₂NaOsO₆
W. Liu, R. Cong, A. P. Reyes, I. R. Fisher and V. F. Mitrovic,
Phys. Rev. B 97, 224103 (2018).

We present detailed calculations of the electric field gradient (EFG) using a point charge approximation in Ba₂NaOsO₆, a Mott insulator with strong spin-orbit interaction. Recent ²³Na nuclear magnetic resonance (NMR) measurements found that the onset of local point symmetry breaking, likely caused by the formation of quadrupolar order [Chen, Pereira, and Balents, Phys. Rev. B 82, 174440 (2010)], precedes the formation of long range magnetic order in this compound [Lu et al., Nat. Commun. 8, 14407 (2017); Liu et al., Physica B 536, 863 (2018)]. An extension of the static ²³Na NMR measurements as a function of the orientation of a 15 T applied magnetic field at 8 K in the magnetically ordered phase is reported. Broken local cubic symmetry induces a nonspherical electronic charge distribution around the Na site and thus finite EFG, affecting the NMR spectral shape. We combine the spectral analysis as a function of the orientation of the magnetic field with calculations of the EFG to determine the exact microscopic nature of the lattice distortions present in low temperature phases of this material. We establish that orthorhombic distortions, constrained along the cubic axes of the perovskite reference unit cell, of oxygen octahedra surrounding Na nuclei are present in the magnetic phase. Other common types of distortions often observed in oxide structures are considered as well.

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249. Phase diagram of Ba₂NaOsO₆, a Mott insulator with strong spin orbit interactions
W. Liu, R. Cong, E. Garcia, A.P. Reyes, H.O. Lee, I.R. Fisher, V.F. Mitrovic,
Physica B: Condensed Matter 536, 863-866 (2018).

We report ²³Na nuclear magnetic resonance (NMR) measurements of the Mott insulator with strong spin-orbit interaction Ba₂NaOsO₆ as a function of temperature in different magnetic fields ranging from 7 T to 29 T. The measurements, intended to concurrently probe spin and orbital/lattice degrees of freedom, are an extension of our work at lower fields reported in Lu et al. (2017) [1]. We have identified clear quantitative NMR signatures that display the appearance of a canted ferromagnetic phase, which is preceded by local point symmetry breaking. We have compiled the field temperature phase diagram extending up to 29 T. We find that the broken local point symmetry phase extends over a wider temperature range as magnetic field increases.

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248. Anomalous ¹²⁵Te Nuclear Spin Relaxation Coincident with Charge Kondo Behavior in Superconducting Pb_1-xTl_xTe
Hidekazu Mukuda, Takashi Matsumura, Shota Maki, Mitsuharu Yashima, Yoshio Kitaoka, Kazumasa Miyake, Hironaru Murakami, Paula Giraldo-Gallo, Theodore H. Geballe, and Ian R. Fisher
J. Phys. Soc. Jpn 87, 023706 (2018).

We report the results of a ¹²⁵Te NMR study of single crystalline Pb_1-xTl_xTe (x = 0, 0.35, 1.0%) as a window on the novel electronic states associated with the thallium impurities in PbTe. The Knight shift is enhanced as x increases, corresponding to an increase in the average density of states (DOS) coupled to a strong spatial variation in the local DOS surrounding each Tl dopant. Remarkably, for the superconducting composition (x = 1.0%), the ¹²⁵Te nuclear spin relaxation rate (1/T₁T) for Te ions that are close to the Tl dopants is unexpectedly enhanced in the normal state below a characteristic temperature of ~10 K, below which the resistivity experiences an upturn. Such a simultaneous upturn in both the resistivity and (1/T₁T) was not suppressed in the high magnetic field. We suggest that these observations are consistently accounted for by dynamical charge fluctuations in the absence of paramagnetism, which is anticipated by the charge Kondo scenario associated with the Tl dopants. In contrast, such anomalies were not detected in the nonsuperconducting samples (x = 0 and 0.35%), suggesting a connection between dynamical valence fluctuations and the occurrence of superconductivity in Pb_1-xTl_xTe.

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247. Transverse fields to tune an Ising-nematic quantum phase transition,
Akash V. Maharaj, Elliott W. Rosenberg, Alexander T. Hristov, Erez Berg, Rafael M. Fernandes, Ian R. Fisher, and Steven A. Kivelson
PNAS 114, 13430-13434 (2017).

The paradigmatic example of a continuous quantum phase transition is the transverse field Ising ferromagnet. In contrast to classical critical systems, whose properties depend only on symmetry and the dimension of space, the nature of a quantum phase transition also depends on the dynamics. In the transverse field Ising model, the order parameter is not conserved, and increasing the transverse field enhances quantum fluctuations until they become strong enough to restore the symmetry of the ground state. Ising pseudospins can represent the order parameter of any system with a twofold degenerate broken-symmetry phase, including electronic nematic order associated with spontaneous point-group symmetry breaking. Here, we show for the representative example of orbital-nematic ordering of a non-Kramers doublet that an orthogonal strain or a perpendicular magnetic field plays the role of the transverse field, thereby providing a practical route for tuning appropriate materials to a quantum critical point. While the transverse fields are conjugate to seemingly unrelated order parameters, their nontrivial commutation relations with the nematic order parameter, which can be represented by a Berry-phase term in an effective field theory, intrinsically intertwine the different order parameters.

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246. Critical divergence of the symmetric (A_1g) nonlinear elastoresistance near the nematic transition in an iron-based superconductor,
J. C. Palmstrom, A. T. Hristov, S. A. Kivelson, J.-H. Chu, and I. R. Fisher
Phys. Rev. B 96, 205133 (2017).

We report the observation of a nonlinear elastoresistivity response for the prototypical underdoped iron pnictide Ba(Fe_0.975Co_0.025)₂As₂. Our measurements reveal a large quadratic term in the isotropic (A_1g) electronic response that was produced by a purely shear (B_2g) strain. The divergence of this quantity upon cooling towards the structural phase transition reflects the temperature dependence of the nematic susceptibility. This observation shows that nematic fluctuations play a significant role in determining even the isotropic properties of this family of compounds.

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245. Tuning Perpendicular Magnetic Anisotropy by Oxygen Octahedral Rotations in (La_1-xSr_xMnO₃)/(SrIrO₃) Superlattices,
Di Yi, Charles L. Flint, Purnima P. Balakrishnan, Krishnamurthy Mahalingam, Brittany Urwin, Arturas Vailionis, Alpha T. N’Diaye, Padraic Shafer, Elke Arenholz, Yongseong Choi, Kevin H. Stone, Jiun-Haw Chu, Brandon M. Howe, Jian Liu, Ian R. Fisher, and Yuri Suzuki
Phys. Rev. Lett 119, 077201 (2017).

Perpendicular magnetic anisotropy (PMA) plays a critical role in the development of spintronics, thereby demanding new strategies to control PMA. Here we demonstrate a conceptually new type of interface induced PMA that is controlled by oxygen octahedral rotation. In superlattices comprised of La_1-xSr_xMnO₃ and SrIrO₃, we find that all superlattices exhibit ferromagnetism despite the fact that La_1-xSr_xMnO₃ is antiferromagnetic for x>0.5. PMA as high as 4×10⁶erg/cm³ is observed by increasing x and attributed to a decrease of oxygen octahedral rotation at interfaces. We also demonstrate that oxygen octahedral deformation cannot explain the trend in PMA. These results reveal a new degree of freedom to control PMA, enabling discovery of emergent magnetic textures and topological phenomena.

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244. Anomalous relaxation kinetics and charge-density wave correlations in underdoped BaPb_1-xBi_xO₃,
D. Nicoletti, E. Casandruc, D. Fua, P. Giraldo-Gallo, I. R. Fisher & A. Cavalleri
PNAS 114, 9020-9025 (2017).

Superconductivity often emerges in proximity of other symmetrybreaking ground states, such as antiferromagnetism or charge density- wave (CDW) order. However, the subtle interrelation of these phases remains poorly understood, and in some cases even the existence of short-range correlations for superconducting compositions is uncertain. In such circumstances, ultrafast experiments can provide new insights by tracking the relaxation kinetics following excitation at frequencies related to the broken-symmetry state. Here, we investigate the transient terahertz conductivity of BaPb_1-xBi_xO₃––a material for which superconductivity is “adjacent” to a competing CDW phase––after optical excitation tuned to the CDW absorption band. In insulating BaBiO₃ we observed an increase in conductivity and a subsequent relaxation, which are consistent with quasiparticles injection across a rigid semiconducting gap. In the doped compound BaBi_0.72Pb_0.28O₃ (superconducting below T_C = 7 K), a similar response was also found immediately above T_c. This observation evidences the presence of a robust gap up to T ~ 40 K, which is presumably associated with short-range CDW correlations. A qualitatively different behavior was observed in the same material for T > 40 K. Here, the photoconductivity was dominated by an enhancement in carrier mobility at constant density, suggestive of melting of the CDW correlations rather than excitation across an optical gap. The relaxation displayed a temperature-dependent, Arrhenius-like kinetics, suggestive of the crossing of a free-energy barrier between two phases. These results support the existence of short-range CDW correlations above T_C in underdoped BaPb_1-xBi_xO₃, and provide information on the dynamical interplay between superconductivity and charge order.

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243. Determination of the resistivity anisotropy of orthorhombic materials via transverse resistivity measurements,
P. Walmsley & I. R. Fisher
Review of Scientific Instruments 88, 043901 (2017).

Measurements of the resistivity anisotropy can provide crucial information about the electronic structure and scattering processes in anisotropic and low-dimensional materials, but quantitative measurements by conventional means often suffer very significant systematic errors. Here we describe a novel approach to measuring the resistivity anisotropy of orthorhombic materials, using a single crystal and a single measurement that is derived from a π/4 rotation of the measurement frame relative to the crystallographic axes. In this new basis, the transverse resistivity gives a direct measurement of the resistivity anisotropy, which combined with the longitudinal resistivity also gives the in-plane elements of the conventional resistivity tensor via a 5-point contact geometry. This is demonstrated through application to the charge-density wave compound ErTe₃, and it is concluded that this method presents a significant improvement on existing techniques, particularly when measuring small anisotropies.

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242. Magnetism and local symmetry breaking in a Mott insulator with strong spin orbit interactions,
L. Lu, M. Song, W. Liu, A.P. Reyes, P. Kuhns, H.O. Lee, I.R. Fisher & V.F. Mitrovic
Nature Communications 8, 14407 (2017).

Study of the combined effects of strong electronic correlations with spin-orbit coupling (SOC) represents a central issue in quantum materials research. Predicting emergent properties represents a huge theoretical problem since the presence of SOC implies that the spin is not a good quantum number. Existing theories propose the emergence of a multitude of exotic quantum phases, distinguishable by either local point symmetry breaking or local spin expectation values, even in materials with simple cubic crystal structure such as Ba₂NaO_sO₆. Experimental tests of these theories by local probes are highly sought for. Our local measurements designed to concurrently probe spin and orbital/lattice degrees of freedom of Ba₂NaO_sO₆ provide such tests. Here we show that a canted ferromagnetic phase which is preceded by local point symmetry breaking is stabilized at low temperatures, as predicted by quantum theories involving multipolar spin interactions.

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241. Fermi surface evolution of Na-doped PbTe studied through density functional theory calculations and Shubnikov–de Haas measurements,
P. Giraldo-Gallo, B. Sangiorgio, P. Walmsley, H. J. Silverstein, M. Fechner, S. C. Riggs, T. H. Geballe, N. A. Spaldin & I. R. Fisher
Phys. Rev. B 94, 195141 (2016).

We present a combined experimental and theoretical study of the evolution of the low-temperature Fermi surface of lead telluride (PbTe) when holes are introduced through sodium substitution on the lead site. Our Shubnikov-de Haas measurements for samples with carrier concentrations up to 9.4 × 10¹⁹ cm^-3 (0.62 Na at. %) show the qualitative features of the Fermi surface evolution (topology and effective mass) predicted by our density functional (DFT) calculations within the generalized gradient approximation (GGA): we obtain perfect ellipsoidal L pockets at low and intermediate carrier concentrations, evolution away from ideal ellipsoidicity for the highest doping studied, and cyclotron effective masses increasing monotonically with doping level, implying deviations from perfect parabolicity throughout the whole band. Our measurements show, however, that standard DFT calculations underestimate the energy difference between the L point and Σ-line valence band maxima, since our data are consistent with a single-band Fermi surface over the entire doping range studied, whereas the calculations predict an occupation of the Σ pockets at higher doping. Our results for low and intermediate compositions are consistent with a nonparabolic Kane-model dispersion, in which the L pockets are ellipsoids of fixed anisotropy throughout the band, but the effective masses depend strongly on Fermi energy.

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240. Multiple charge density wave states at the surface of TbTe₃,
L. Fu, A. M. Kraft, B. Sharma, M. Singh, P. Walmsley, I. R. Fisher & M. C. Boyer
Phys. Rev. B 94, 205101 (2016).

We studied TbTe₃ using scanning tunneling microscopy (STM) in the temperature range of 298–355 K. Our measurements detect a unidirectional charge density wave (CDW) state in the surface Te layer with a wave vector consistent with that of the bulk q_CDW = 0.30 ± 0.01 c*. However, unlike previous STM measurements, and differing from measurements probing the bulk, we detect two perpendicular orientations for the unidirectional CDW with no directional preference for the in-plane crystal axes (a or c axis) and no noticeable difference in wave vector magnitude. In addition, we find regions in which the bidirectional CDW states coexist. We propose that observation of two unidirectional CDW states indicates a decoupling of the surface Te layer from the rare-earth block layer below, and that strain variations in the Te surface layer drive the local CDW direction to the specific unidirectional or, in rare occurrences, bidirectional CDW orders observed. This indicates that similar driving mechanisms for CDW formation in the bulk, where anisotropic lattice strain energy is important, are at play at the surface. Furthermore, the wave vectors for the bidirectional order we observe differ from those theoretically predicted for checkerboard order competing with stripe order in a Fermi-surface nesting scenario, suggesting that factors beyond Fermi-surface nesting drive CDW order in TbTe₃. Finally, our temperature-dependent measurements provide evidence for localized CDW formation above the bulk transition temperature T_CDW.

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239. Charge density wave modulation and gap measurements in CeTe₃,
U. Ralevic, N. Lazarevic, A. Baum, H.-M. Eiter, R. Hackl, P. Giraldo-Gallo, I. R. Fisher, C. Petrovic, R. Gajic & Z. V. Popovic
Phys. Rev. B 94, 165132 (2016).

We present a study of charge density wave (CDW) ordering in CeTe₃ at room temperature using a scanning tunneling microscope and Raman spectroscopy. Two characteristic CDW ordering wave vectors obtained from the Fourier analysis are assessed to be |c* - q| = 4.19 nm^-1 and |q| = 10.26 nm^-1 where |c*| = 2π/c is the reciprocal lattice vector. The scanning tunneling spectroscopy measurements, along with inelastic light (Raman) scattering measurements, show a CDW gap Δ_max of approximately 0.37 eV. In addition to the CDW modulation, we observe an organization of the Te sheet atoms in an array of alternating V- and N-shaped groups along the CDW modulation, as predicted in the literature.

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238. Scaling of the Stress and Temperature Dependence of the Optical Anisotropy in Ba(Fe_1-xCo_x)₂As₂,
C. Mirri, A. Dusza1, S. Bastelberger, J. -H. Chu, H. -H. Kuo, I. R. Fisher, L. Degiorgi
Journal of Superconductivity and Novel Magnetism 29, 3053-3057 (2016).

We revisit our recent investigations of the optical properties in the underdoped regime of the title compounds with respect to their anisotropic behavior as a function of both temperature and uniaxial stress across the ferroelastic tetragonal-to-orthorhombic transition. By exploiting a dedicated pressure device, we can tune and control uniaxial stress in situ thus changing the degree of detwinning of the samples in the orthorhombic SDW state as well as pressure-inducing an orthorhombicity in the paramagnetic tetragonal phase. We discover a hysteretic behavior of the optical anisotropy; its stress versus temperature dependence across the structural transition bears testimony to the analogy with the magnetic-field versus temperature dependence of the magnetization in a ferromagnet when crossing the Curie temperature. In this context, we find furthermore an intriguing scaling of the stress and temperature dependence of the optical anisotropy in Ba(Fe_1-xCo_x)₂As₂2.

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237. High Current Density and Low Thermal Conductivity of Atomically Thin Semimetallic WTe₂,
M. J. Mleczko, R. L. Xu, K. Okabe, H.-H. Kuo, I. R. Fisher, H.-S. P. Wong, Y. Nishi & E. Pop
ACS Nano 10, 7507-7514 (2016).

Two-dimensional (2D) semimetals beyond graphene have been relatively unexplored in the atomically thin limit. Here, we introduce a facile growth mechanism for semimetallic WTe₂ crystals and then fabricate few-layer test structures while carefully avoiding degradation from exposure to air. Low-field electrical measurements of 80 nm to 2 μm long devices allow us to separate intrinsic and contact resistance, revealing metallic response in the thinnest encapsulated and stable WTe₂ devices studied to date (3-20 layers thick). High-field electrical measurements and electrothermal modeling demonstrate that ultrathin WTe₂ can carry remarkably high current density (approaching 50 MA/cm², higher than most common interconnect metals) despite a very low thermal conductivity (of the order ~3 Wm^-1K^-1). These results suggest several pathways for air-stable technological viability of this layered semimetal.

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236. Superconductivity and fluctuations in Ba_1-pK_pFe₂As₂ and Ba(Fe_1-nCo_n)₂As₂,
T. Bohm, R. Hosseinian Ahangharnejhad, D. Jost, A. Baum, B. Muschler, F. Kretzschmar, P. Adelmann, T. Wolf, H.-H. Wen, J.-H. Chu, I. R. Fisher, and R. Hackl
Physica Status Solidi 254, 1600308 (2016).

We study the interplay of fluctuations and superconductivity in BaFe₂As₂ (Ba-122) compounds with Ba and Fe substituted by K (p doping) and Co (n doping), respectively. To this end, we measured electronic Raman spectra as a function of polarization and temperature. We observe gap excitations and fluctuations for all doping levels studied. The response from fluctuations is much stronger for Co substitution and, according to the selection rules and the temperature dependence, originates from the exchange of two critical spin fluctuations with characteristic wave vectors (±π, 0) and (0,±π). At 22% K doping (p = 0.22), we find the same selection rules and spectral shape for the fluctuations but the intensity is smaller by a factor of 5. Since there exists no nematic region above the orthorhombic spin-density wave (SDW) phase, the identification of the fluctuations via the temperature dependence is not possible. The gap excitations in the superconducting state indicate strongly anisotropic near-nodal gaps for Co substitution which make the observation of collective modes difficult. The variation with doping of the spectral weights of the A_1g and B_1g gap features does not support the influence of fluctuations on Cooper pairing. Therefore, the observation of Bardasis–Schrieffer modes inside the nearly clean gaps on the K-doped side remains the only experimental evidence for the relevance of fluctuations for pairing.

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235. Measurement of the B_1g and B_2g components of the elastoresistivity tensor for tetragonal materials via transverse resistivity configurations,
M. C. Shapiro, A. T. Hristov, J. C. Palmstrom, J. H. Chu and I. R. Fisher
Review of Scientific Instruments 87, 063902 (2016).

The elastoresistivity tensor m_ij,kl relates changes in resistivity to strains experienced by a material. As a fourth-rank tensor, it contains considerably more information about the material than the simpler (second-rank) resistivity tensor; in particular, for a tetragonal material, the B_1g and B_2g components of the elastoresistivity tensor (m_xx,xx - m_xx,yy and 2m_xy,xy, respectively) can be related to its nematic susceptibility. Previous experimental probes of this quantity have focused exclusively on differential longitudinal elastoresistance measurements, which determine the induced resistivity anisotropy arising from anisotropic in-plane strain based on the difference of two longitudinal resistivity measurements. Here we describe a complementary technique based on transverse elastoresistance measurements. This new approach is advantageous because it directly determines the strain-induced resistivity anisotropy from a single transverse measurement. To demonstrate the efficacy of this new experimental protocol, we present transverse elastoresistance measurements of the 2m_xy,xy elastoresistivity coefficient of BaFe₂As₂, a representative iron-pnictide that has previously been characterized via differential longitudinal elastoresistance measurements.

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234. Ubiquitous signatures of nematic quantum criticality in optimally doped Fe-based superconductors,
H. H. Kuo, J. H. Chu, J. C. Palmstrom,S. A. Kivelson, I. R. Fisher
Science 352,6288 (2016).

A key actor in the conventional theory of superconductivity is the induced interaction between electrons mediated by the exchange of virtual collective fluctuations (phonons in the case of conventional s-wave superconductors). Other collective modes that can play the same role, especially spin fluctuations, have been widely discussed in the context of high-temperature and heavy Fermion superconductors. The strength of such collective fluctuations is measured by the associated susceptibility. Here we use differential elastoresistance measurements from five optimally doped iron-based superconductors to show that divergent nematic susceptibility appears to be a generic feature in the optimal doping regime of these materials. This observation motivates consideration of the effects of nematic fluctuations on the superconducting pairing interaction in this family of compounds and possibly beyond.

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233. NMR Evidence for Inhomogeneous Nematic Fluctuations in BaFe₂(As_1-xP_x)₂,
A. P. Dioguardi, T. Kissikov, C. H. Lin, K. R. Shirer, M. M. Lawson, H.-J. Grafe, J.-H. Chu, I. R. Fisher, R. M. Fernandes & N. J. Curro
Phys. Rev. Lett. 116,107202 (2016).

We present evidence for nuclear spin-lattice relaxation driven by glassy nematic fluctuations in isovalent P-doped BaFe₂As₂ single crystals. Both the ⁷⁵As and ³¹P sites exhibit a stretched-exponential relaxation similar to the electron-doped systems. By comparing the hyperfine fields and the relaxation rates at these sites we find that the As relaxation cannot be explained solely in terms of magnetic spin fluctuations. We demonstrate that nematic fluctuations couple to the As nuclear quadrupolar moment and can explain the excess relaxation. These results suggest that glassy nematic dynamics are a common phenomenon in the ironbased superconductors.

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232. Electrodynamic response in the electronic nematic phase of BaFe₂As₂,
C. Mirri, A. Dusza, S. Bastelberger, M. Chinotti, J.-H. Chu, H.-H. Kuo, I. R. Fisher & L. Degiorgi
Phys. Rev. B 93, 085114 (2016).

We perform, as a function of uniaxial stress, a temperature-dependent optical-reflectivity investigation of the parent Fe-arsenide compound BaFe₂As₂ over a broad spectral range, from the far infrared up to the ultraviolet, across the coincident structural tetragonal-to-orthorhombic and spin-density-wave (SDW) phase transitions at T_s,N = 135 K. Our results provide knowledge to the complete electrodynamic response of the title compound over a wide energy range as a function of both tunable variables. For temperatures below T_s,N, varying the uniaxial stress in situ affects the twin domain population and yields hysteretic behavior of the optical properties as the stress is first increased and then decreased, whereas for temperatures above T_s,N the stress-induced optical anisotropy is reversible, as anticipated. In particular, by analyzing the low-frequency infrared response, we obtain detailed insight to the effects determining the intrinsic anisotropy of the (metallic) charge dynamics in the orthorhombic state, and similarly the induced one due to applied uniaxial stress at higher temperatures in the tetragonal phase. The low-frequency optical conductivity thus allows establishing a link to the dc transport properties and reveals that they are determined almost exclusively by changes in the Drude weight, therefore by the anisotropy in the Fermi surface parameters. Finally, we show that the spectral weight distribution in the SDW state occurs for energies below approximately 1 eV, and therefore points towards a correlation mechanism due to Hund’s coupling rather than on-site Coulomb interactions.

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231. Critical spin fluctuations and the origin of nematic order in Ba(Fe_1-xCo_x)₂As₂,
F. Kretzschmar, T. Bohm, U. Karahasanovic, B. Muschler, A. Baum, D. Jost, J. Schmalian, S. Caprara, M. Grilli, C. Di Castro, J. G. Analytis, J.-H. Chu, I. R. Fisher & R. Hackl
Nat. Physics 12, 560 (2016).

Nematic fluctuations and order play a prominent role in material classes such as the cuprates, some ruthenates or the iron-based compounds and may be interrelated with superconductivity. In iron-based compounds signatures of nematicity have been observed in a variety of experiments. However, the fundamental question as to the relevance of the related spin, charge or orbital fluctuations remains open. Here, we use inelastic light (Raman) scattering and study Ba(Fe_1-xCo_x)₂As₂ (0 < x < 0.085) for getting direct access to nematicity and the underlying critical fluctuations with finite characteristic wavelengths. We show that the response from fluctuations appears only in B_1g (x² - y²) symmetry (1 Fe unit cell). The scattering amplitude increases towards the structural transition at T_s but vanishes only below the magnetic ordering transition at T_SDW, suggesting a magnetic origin of the fluctuations. The theoretical analysis explains the selection rules and the temperature dependence of the fluctuation response. These results make magnetism the favourite candidate for driving the series of transitions.

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230. Persistent order due to transiently enhanced nesting in an electronically excited charge density wave,
L. Rettig, R. Cortés, J.-H. Chu, I. R. Fisher, F. Schmitt, R. G. Moore, Z.-X. Shen, P. S. Kirchmann, M. Wolf & U. Bovensiepen
Nat. Commun. 7:10459 doi: 10.1038/ncomms10459 (2016).

Non-equilibrium conditions may lead to novel properties of materials with broken symmetry ground states not accessible in equilibrium as vividly demonstrated by non-linearly driven mid-infrared active phonon excitation. Potential energy surfaces of electronically excited states also allow to direct nuclear motion, but relaxation of the excess energy typically excites fluctuations leading to a reduced or even vanishing order parameter as characterized by an electronic energy gap. Here, using femtosecond time- and angle-resolved photoemission spectroscopy, we demonstrate a tendency towards transient stabilization of a charge density wave after near-infrared excitation, counteracting the suppression of order in the non-equilibrium state. Analysis of the dynamic electronic structure reveals a remaining energy gap in a highly excited transient state. Our observation can be explained by a competition between fluctuations in the electronically excited state, which tend to reduce order, and transiently enhanced Fermi surface nesting stabilizing the order.

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229. Ultrafast resonant soft x-ray diffraction dynamics of the charge density wave in TbTe₃,
R. G. Moore, W. S. Lee, P. S. Kirchman, Y. D. Chuang, A. F. Kemper, M. Trigo, L. Patthey, D. H. Lu, O. Krupin, M. Yi, D. A. Reis, D. Doering, P. Denes, W. F. Schlotter, J. J. Turner, G. Hays, P. Hering, T. Benson, J.-H. Chu, T. P. Devereaux, I. R. Fisher, Z. Hussain, and Z.-X. Shen,
Phys. Rev. B 93, 024304 (2016).

Understanding the emergence of collective behavior in correlated electron systems remains at the forefront of modern condensed matter physics. Disentangling the degrees of freedom responsible for collective behavior can lead to insights into the microscopic origins of emergent properties and phase transitions. Utilizing an optical pump, resonant soft x-ray diffraction probe we are able to track, in real time, the dynamics of the charge density wave (CDW) in TbTe₃, a model system that violates traditional views of a Fermi surface nested CDW. We observe coherent oscillations corresponding to the CDW amplitude mode at 2.4 THz and a coherent optical phonon mode at ~1.7 THz. We show how such observations reveal the anisotropic energy optimization between in-plane Te charge density modulations and the three-dimensional lattice coupling.

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228. Dichroism in the Parent Ferropnictide BaFe₂As₂ Across the Nematic Phase Transition,
M. Chinotti, C. Mirri, A. Dusza, S. Bastelberger, L. Degiorgi, J.-H. Chu, H.-H. Kuo, & I. R. Fisher,
J. Supercond. Nov. Magn. 29, 667-672 (2016).

We investigate the anisotropy of the optical response (i.e., dichroism) in the representative ‘parent’ ferropnictide BaFe₂As₂. The dichroism is achieved through measurements of the optical reflectivity in a broad spectral range, across the tetragonal-to-orthorhombic and spin density- wave (SDW) phase transitions, from which we extract the real part of the optical conductivity. The collection of data on the specimen with variable degree of detwinning is made possible via our novel pressure device which allows tuning in situ uniaxial stress. The dichroism bears testimony to an important electronic polarization, reflecting the impact of the nematic phase and its fluctuations on the electronic structure.

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227. Symmetry constraints on the elastoresistivity tensor,
M. C. Shapiro, Patrik Hlobil, A. T. Hristov, Akash V. Maharaj, and I. R. Fisher,
Phys. Rev. B 92, 235147 (2015).

The elastoresistivity tensor m_ij,kl characterizes changes in a material’s resistivity due to strain. As a fourth-rank tensor, elastoresistivity can be a uniquely useful probe of the symmetries and character of the electronic state of a solid. We present a symmetry analysis of m_ij,kl (both in the presence and absence of a magnetic field) based on the crystalline point group, focusing for pedagogic purposes on the D_4h point group (of relevance to several materials of current interest). We also discuss the relation between m_ij,kl and various thermodynamic susceptibilities, particularly where they are sensitive to critical fluctuations proximate to a critical point at which a point-group symmetry is spontaneously broken.

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226. Stripe-like nanoscale structural phase separation in superconducting BaPb_1-xBi_xO₃,
P. Giraldo-Gallo, Y. Zhang, C. Parra, H.C. Manoharan, M.R. Beasley, T.H. Geballe, M.J. Kramer and I.R. Fisher
Nat. Commun. 6:8231 doi: 10.1038/ncomms9231 (2015).

The phase diagram of BaPb_1-xBi_xO₃ exhibits a superconducting dome in the proximity of a charge density wave phase. For the superconducting compositions, the material coexists as two structural polymorphs. Here we show, via high-resolution transmission electron microscopy, that the structural dimorphism is accommodated in the form of partially disordered nanoscale stripes. Identification of the morphology of the nanoscale structural phase separation enables determination of the associated length scales, which we compare with the Ginzburg–Landau coherence length. We find that the maximum T_c occurs when the superconducting coherence length matches the width of the partially disordered stripes, implying a connection between the structural phase separation and the shape of the superconducting dome.

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225. Origin of the resistive anisotropy in the electronic nematic phase of BaFe₂As₂ revealed by optical spectroscopy,
C. Mirri, A. Dusza, S. Bastelberger, M. Chinotti, J.-H. Chu, H.-H. Kuo, I.R. Fisher, L. Degiorgi,
Phys. Rev. Lett. 115, 107001 (2015).

We perform, as a function of uniaxial stress, an optical-reflectivity investigation of the representative "parent" ferropnictide BaFe₂As₂ in a broad spectral range, across the tetragonal-to-orthorhombic phase transition and the onset of the long-range antiferromagnetic (AFM) order. The infrared response reveals that the dc transport anisotropy in the orthorhombic AFM state is determined by the interplay between the Drude spectral weight and the scattering rate, but that the dominant effect is clearly associated with the metallic spectral weight. In the paramagnetic tetragonal phase, though, the dc resistivity anisotropy of strained samples is almost exclusively due to stress-induced changes in the Drude weight rather than in the scattering rate, definitively establishing the anisotropy of the Fermi surface parameters as the primary effect driving the dc transport properties in the electronic nematic state.

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224. Transfer of spectral weight across the gap of Sr₂IrO₄ induced by La doping,
Veronique Brouet, Joseph Mansart, Luca Perfetti, Christian Piovera, Ivana Vobornik, Patrick Le Fèvre, François Bertran, Scott C. Riggs, M. C. Shapiro, Paula Giraldo-Gallo, and Ian R. Fisher,
Phys. Rev. B 92, 081117(R) (2015).

We study with angle-resolved photoelectron spectroscopy the evolution of the electronic structure of Sr₂IrO₄, when holes or electrons are introduced, through Rh or La substitutions. At low dopings, the added carriers occupy the first available states, at the bottom or top of the gap, revealing an asymmetric gap of 0.7 eV, in good agreement with scanning tunnel microscope measurements. At further doping, we observe a reduction of the gap and a transfer of spectral weight across the gap, although the quasiparticle weight remains very small. We discuss the origin of the in-gap spectral weight as a local distribution of gap values.

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223. Observation of universal strong orbital-dependent correlation effects in iron chalcogenides,
M. Yi, Z. K. Liu, Y. Zhang R. Yu, J. X. Zhu, J. J. Lee, R. G. Moore, F. T. Schmitt, W. Li, S. C. Riggs, J. H. Chu, B. Lv, J. Hu, M. Hashimoto, S. K. Mo, Z. Hussain, Z. Q. Mao, C. W. Chu, I. R. Fisher, Q. Si, Z. X. Shen, D. H. Lu,
Nat. Commun. 6:7777 doi: 10.1038/ncomms8777 (2015).

Establishing the appropriate theoretical framework for unconventional superconductivity in the iron-based materials requires correct understanding of both the electron correlation strength and the role of Fermi surfaces. This fundamental issue becomes especially relevant with the discovery of the iron chalcogenide superconductors. Here, we use angle-resolved photoemission spectroscopy to measure three representative iron chalcogenides, FeTe_0.56Se_0.44, monolayer FeSe grown on SrTiO₃ and K_0.76Fe_1.72Se₂. We show that these superconductors are all strongly correlated, with an orbital-selective strong renormalization in the d_xy bands despite having drastically different Fermi surface topologies. Furthermore, raising temperature brings all three compounds from a metallic state to a phase where the dxy orbital loses all spectral weight while other orbitals remain itinerant. These observations establish that iron chalcogenides display universal orbital-selective strong correlations that are insensitive to the Fermi surface topology, and are close to an orbital-selective Mott phase, hence placing strong constraints for theoretical understanding of iron-based superconductors.

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222. Field-induced spin density wave and spiral phases in a layered antiferromagnet,
M. B. Stone, M. D. Lumsden, V. O. Garlea, B. Grenier, E. Ressouche, E. C. Samulon, and I. R. Fisher,
Phys. Rev. B 92, 020415(R) (2015).

We determine the low-field ordered magnetic phases of the S = 1 dimerized antiferromagnet Ba₃Mn₂O₈ using single-crystal neutron diffraction. We find that for magnetic fields between μ₀H = 8.80 T and 10.56 T applied along the [1¯10] direction the system exhibits spin density wave order with incommensurate wave vectors of type (η,η,ε). For μ₀H >10.56 T, the magnetic order changes to a spiral phase with incommensurate wave vectors only along the [hh0] direction. For both field-induced ordered phases, the magnetic moments are lying in the plane perpendicular to the field direction. The nature of these two transitions is fundamentally different: the low-field transition is a second-order transition to a spin density wave ground state, while the one at higher field, toward the spiral phase, is of first order.

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221. Elastoconductivity as a probe of broken mirror symmetries,
Patrik Hlobil, Akash V. Maharaj, Pavan Hosur, M.C. Shapiro, I.R. Fisher, S. Raghu,
Phys. Rev. B 92, 035148 (2015).

We propose the possible detection of broken mirror symmetries in correlated two-dimensional materials by elastotransport measurements. Using linear response theory we calculate the "shear conductivity" Γ_xx,xy, defined as the linear change of the longitudinal conductivity σ_xx due to a shear strain ε_xy . This quantity can only be nonvanishing when in-plane mirror symmetries are broken and we discuss how candidate states in the cuprate pseudogap regime (e.g., various loop current or charge orders) may exhibit a finite shear conductivity. We also provide a realistic experimental protocol for detecting such a response.

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220. Wave-vector-dependent electron-phonon coupling and the charge-density-wave transition in TbTe₃,
M. Maschek, S. Rosenkranz, R. Heid, A. H. Said, P. Giraldo-Gallo, I.R. Fisher and F. Weber,
Phys. Rev. B 91, 235146 (2015).

We present a high-energy-resolution inelastic x-ray scattering investigation of the soft phonon mode in the charge-density-wave (CDW) system TbTe₃. We analyze our data based on lattice dynamical calculations using density-functional-perturbation theory and find clear evidence that strongly momentum-dependent electronphonon coupling defines the periodicity of the CDW superstructure: Our experiment reveals strong phonon softening and increased phonon linewidths over a large part in reciprocal space adjacent to the CDW ordering vector q_CDW = (0,0,0.3). Further, q_CDW is clearly offset from the wave vector of (weak) Fermi surface nesting q_FS = (0,0,0.25), and our detailed analysis indicates that electron-phonon coupling is responsible for this shift. Hence, we can add TbTe₃, which was previously considered as a canonical CDW compound following the Peierls scenario, to the list of distinct charge-density-wave materials characterized by momentum-dependent electron-phonon coupling.

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219. Bulk electronic structure of Zn-Mg-Y and Zn-Mg-Dy icosahedral quasicrystals,
J. Nayak, M. Maniraj, A. Gloskovskii, M. Krajci, S. Sebastian, I. R. Fisher, K. Horn, and S. R. Barman,
Phys. Rev. B 91, 235116 (2015).

We use bulk sensitive hard x-ray photoelectron spectroscopy to provide unambiguous evidence for a pseudogap in the density of states around the Fermi level in icosahedral Zn-Mg-Y and Zn-Mg-Dy quasicrystals, in agreement with our density functional theory calculations. The pseudogap in these Zn-based quasicrystals is less pronounced compared to the Al-based ones such as Al-Pd-Mn and Al-Cu-Fe [J. Nayak et al., Phys. Rev. Lett. 109, 216403 (2012)]. This observation is in agreement with transport studies that indicate a larger charge carrier concentration in the Zn-based quasicrystals. Compared to Zn-Mg-Dy, the pseudogap is somewhat deeper in Zn-Mg-Y. The larger width of the Mg 1s and Zn 2p core-level spectra in Zn-Mg-Y is explained by different configurations of the local atomic surrounding compared to Zn-Mg-Dy.

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218. Classification of Collective Modes in a Charge Density Wave by Momentum-Dependent Modulation of the Electronic Band Structure,
D. Leuenberger, J. A. Sobota, S.-L. Yang, A. F. Kemper, P. Giraldo-Gallo, R. G. Moore, I. R. Fisher, P. S. Kirchmann, T. P. Devereaux, Z.-X. Shen,
Phys. Rev. B 91, 201106(R) (2015).

We present time- and angle-resolved photoemission spectroscopy (trARPES) measurements on the charge density wave system CeTe₃. Optical excitation transiently populates the unoccupied band structure and reveals a gap size of 2Δ = 0.59 eV. The occupied Te-5p band dispersion is coherently modified by three modes at Ω1 = 2.2, Ω2 = 2.7, and Ω3 = 3 THz. All three modes lead to small rigid energy shifts whereas Δ is only affected by Ω1 and Ω2. Their spatial polarization is analyzed by fits of a transient model dispersion and density-functional theory frozen phonon calculations.We conclude that modes Ω1 and Ω2 result from in-plane ionic lattice motions, which modulate the charge order and that Ω3 originates from a generic out-of-plane A_1g phonon. We thereby demonstrate how the rich information from trARPES allows identification of collective modes and their spatial polarization, which explains the mode-dependent coupling to charge order.

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217. Pressure dependence of the charge-density-wave and superconducting states in GdTe₃, TbTe₃, and DyTe₃,
D. A. Zocco, J. J. Hamlin, K. Grube, J.-H. Chu, H.-H. Kuo, I. R. Fisher, and M. B. Maple,
Phys. Rev. B 91, 205114 (2015).

We present electrical resistivity and ac-susceptibility measurements of GdTe₃, TbTe₃, and DyTe₃ performed under pressure. An upper charge-density-wave (CDW) is suppressed at a rate of dT_CDW,1/dP ~ -85 K/GPa. For TbTe₃ and DyTe₃, a second CDW below T_CDW,2 increases with pressure until it reaches the T_CDW,1(P) line. For GdTe₃, the lower CDW emerges as pressure is increased above ~1 GPa. As these two CDW states are suppressed with pressure, superconductivity (SC) appears in the three compounds at lower temperatures. Ac-susceptibility experiments performed on TbTe₃ provide compelling evidence for bulk SC in the low-pressure region of the phase diagram. We provide measurements of superconducting critical fields and discuss the origin of a high-pressure superconducting phase occurring above 5 GPa.

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216. Evidence for a nematic component to the hidden-order parameter in URu₂Si₂ from differential elastoresistance measurements
Scott C. Riggs, M.C. Shapiro, Akash V Maharaj, S. Raghu, E.D. Bauer, R.E. Baumbach, P. Giraldo-Gallo, Mark Wartenbe & I.R. Fisher,
Nature Communications 6, 6425 (2015).

For materials that harbour a continuous phase transition, the susceptibility of the material to various fields can be used to understand the nature of the fluctuating order and hence the nature of the ordered state. Here we use anisotropic biaxial strain to probe the nematic susceptibility of URu₂Si₂, a heavy fermion material for which the nature of the low temperature 'hidden order' state has defied comprehensive understanding for over 30 years. Our measurements reveal that the fluctuating order has a nematic component, confirming reports of twofold anisotropy in the broken symmetry state and strongly constraining theoretical models of the hidden-order phase.

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215. Monotonic Doping-Dependence in the Anisotropy of the Drude Weight and Scattering Rate of Detwinned Ba(Fe_1-xCo_x)₂As₂ Established from the Optical Conductivity
A. Dusza, A. Lucarelli, J.-H. Chu, I. R. Fisher and L. Degiorgi,
Journal of Superconductivity and Novel Magnetism 28, 1261–1266 (2015).

We investigate the anisotropic metallic response in the optical conductivity of fully detwinned Ba(Fe_1-xCo_x)₂As₂ in their underdoped regime. We estimate the Drude weight and the scattering rates of the itinerant charge carriers across the structural (at T_s) and magnetic (at T_N) phase transition. Our findings support a monotonic doping dependence of the anisotropy of both parameters determining the transport properties, opposite to the non-monotonic anisotropy of the dc resistivity between the crystallographic axes. The capability of optical methods to address a broad energy interval extending far away from the Fermi level allows emphasizing the prominent role of the electronic structure rather than the impurity scattering when establishing a direct connection with the long-range ferro-orbital and antiferromagnetic orders.

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214. Disorder Driven Metal-Insulator Transition in BaPb_1-xBi_xO₃ and Inference of Disorder-Free Critical Temperature
Katherine Luna, Paula Giraldo-Gallo, Theodore Geballe, Ian Fisher, and Malcolm Beasley,
Phys. Rev. Lett. 113, 177004 (2014).

We performed point-contact spectroscopy tunneling measurements on single crystal
BaPb_1-xBi_xO₃ for 0≤x≤0.28 at temperatures T=2-40K and find a suppression in the density of states at low bias voltages that is characteristic of disordered metals. Both the correlation gap and the zero-temperature conductivity are zero at a critical concentration x_c = 0.30. Not only does this suggests that a disorder driven metal-insulator transition occurs before the onset of the charge disproportionated charge density wave insulator, but we also explore whether a scaling theory is applicable. In addition, we estimate the disorder-free critical temperature and compare these results to Ba_1-xK_xBiO₃.

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213. Nematic-driven anisotropic electronic properties of underdoped detwinned Ba(Fe_1-xCo_x)₂As₂ revealed by optical spectroscopy
C. Mirri, A. Dusza, S. Bastelberger, J.-H. Chu, H.-H. Kuo, I. R. Fisher, and L. Degiorgi,
Phys. Rev. B 90, 155125 (2014).

We collect optical reflectivity data as a function of temperature across the structural tetragonal-to-orthorhombic phase transition at T_s on Ba(Fe_1-xCo_x)₂As₂ for x = 0%, 2.5%, and 4.5%, with uniaxial and in situ tunable applied pressure in order to detwin the sample and to exert on it an external symmetry-breaking field. At T < T_s , we discover a remarkable hysteretic optical anisotropy as a function of the applied pressure at energies far away from the Fermi level. Such an anisotropy turns into a reversible linear pressure dependence at T > T_s . Moreover, the optical anisotropy gets progressively depleted with increasing Co content in the underdoped regime, consistent with the doping dependence of the orthorhombicity but contrary to the nonmonotonic behavior observed for the dc anisotropy.

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212. Distinguishing bulk and surface electron-phonon coupling in the topological insulator Bi₂Se₃ using time-resolved photoemission spectroscopy
J.A. Sobota, S.-L. Yang, D. Leuenberger, A.F. Kemper, J.G. Analytis, I.R. Fisher, P.S. Kirchmann, T.P. Devereaux, and Z.-X. Shen,
Phys. Rev. Lett. 113, 157401 (2014).

We report time- and angle-resolved photoemission spectroscopy measurements on the topological insulator Bi₂Se₃. We observe oscillatory modulations of the electronic structure of both the bulk and surface states at a frequency of 2.23 THz due to coherent excitation of an A_1g phonon mode. A distinct, additional frequency of 2.05 THz is observed in the surface state only. The lower phonon frequency at the surface is attributed to the termination of the crystal and thus reduction of interlayer van der Waals forces, which serve as restorative forces for out-of-plane lattice distortions. Density functional theory calculations quantitatively reproduce the magnitude of the surface phonon softening. These results represent the first band-resolved evidence of the A_1g phonon mode coupling to the surface state in a topological insulator.

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211. Ultrafast electron dynamics in the topological insulator Bi₂Se₃ studied by time-resolved photoemission spectroscopy
J.A. Sobota, S.-L. Yang, D. Leuenberger, A.F. Kemper, J.G. Analytis, I.R. Fisher, P.S. Kirchmann, T.P. Devereaux, Z.-X. Shen,
Journal of Electron Spectroscopy and Related Phenomena 195, 249-257 (2014).

We characterize the topological insulator Bi₂Se₃ using time- and angle-resolved photoemission spectroscopy. By employing two-photon photoemission, a complete picture of the unoccupied electronic structure from the Fermi level up to the vacuum level is obtained. We demonstrate that the unoccupied states host a second Dirac surface state which can be resonantly excited by 1.5 eV photons. We then study the ultrafast relaxation processes following optical excitation. We find that they culminate in a persistent non-equilibrium population of the first Dirac surface state, which is maintained by a meta-stable population of the bulk conduction band. Finally, we perform a temperature-dependent study of the electron–phonon scattering processes in the conduction band, and find the unexpected result that their rates decrease with increasing sample temperature. We develop a model of phonon emission and absorption from a population of electrons, and show that this counter-intuitive trend is the natural consequence of fundamental electron–phonon scattering processes. This analysis serves as an important reminder that the decay rates extracted by time-resolved photoemission are not in general equal to single electron scattering rates, but include contributions from filling and emptying processes from a continuum of states.

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210. Effect of Disorder on the Resistivity Anisotropy Near the Electronic Nematic Phase Transition in Pure and Electron-Doped BaFe₂As₂
Hsueh-Hui Kuo and Ian R. Fisher,
Phys. Rev. Lett. 112, 227001 (2014).

We show that the strain-induced resistivity anisotropy in the tetragonal state of the representative underdoped Fe arsenides BaFe₂As₂, Ba(Fe_1-xCo_x)₂As₂ and Ba(Fe_1-xNi_x)₂As₂ is independent of disorder over a wide range of defect and impurity concentrations. This result demonstrates that the anisotropy in the in-plane resistivity in the paramagnetic orthorhombic state of this material is not due to elastic scattering from anisotropic defects. Conversely, our result can be most easily understood if the resistivity anisotropy arises primarily from an intrinsic anisotropy in the electronic structure.

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209. Bandgap closure and reopening in CsAuI₃ at high pressure
Shibing Wang, Alexander F. Kemper, Maria Baldini, M. C. Shapiro, Scott C. Riggs, Zhao Zhao, Zhenxian Liu, Thomas P. Devereaux, Ted H. Geballe, Ian R. Fisher, and Wendy L. Mao,
Phys. Rev. B 89, 245109 (2014).

Results of high-pressure infrared (IR) and Raman spectroscopy measurements are presented for the mixed valence compound CsAuI₃, where Au adopts Au^I and Au^III valency. Raman spectroscopy shows softening with pressure of the vibration modes in the Au^III-I₄ square planar units in the tetragonal phase, indicating a similar pressure-induced lattice distortion as found for the closely related compounds CsAuCl₃ and CsAuBr₃. Multiple features in the higher pressure spectra confirm that the high-pressure phase has a lower symmetry than the ambient pressure tetragonal structure, consistent with an orthorhombic structure discovered recently by x-ray diffraction measurements. From IR spectroscopy, we observed rapid bandgap closure at a rate of 0.2 eV/GPa in the tetragonal phase of CsAuI₃, close to the tetragonal-orthorhombic phase transition. The IR reflectivity shows a Drude-like behavior implying metallic conductivity. However, as the compound fully transforms to the orthorhombic phase, the bandgap reopens and the Drude behavior in the reflectivity disappears.

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208. Dynamic competition between spin-density wave order and superconductivity in underdoped Ba_1-xK_xFe₂As₂
M. Yi, Y. Zhang, Z.-K. Liu, X. Ding, J.-H. Chu, A.F. Kemper, N. Plonka, B. Moritz, M. Hashimoto, S.-K. Mo, Z. Hussain, T.P. Devereaux, I.R. Fisher, H.H. Wen, Z.-X. Shen & D.H. Lu ,
Nature Communications 5, 3711 (2014).

An intriguing aspect of unconventional superconductivity is that it always appears in the vicinity of other competing phases, whose suppression brings the full emergence of superconductivity. In the iron pnictides, these competing phases are marked by a tetragonalto- orthorhombic structural transition and a collinear spin-density wave (SDW) transition. There has been macroscopic evidence for competition between these phases and superconductivity as the magnitude of both the orthorhombicity and magnetic moment are suppressed in the superconducting state. Here, using angle-resolved photoemission spectroscopy on detwinned underdoped Ba_1-xK_xFe₂As₂, we observe a coexistence of both the SDW gap and superconducting gap in the same electronic structure. Furthermore, our data reveal that following the onset of superconductivity, the SDW gap decreases in magnitude and shifts in a direction consistent with a reduction of the orbital anisotropy. This observation provides direct spectroscopic evidence for the dynamic competition between superconductivity and both SDW and electronic nematic orders in these materials.

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207. Spectrally resolved femtosecond reflectivity relaxation dynamics in undoped spin-density wave 122-structure iron-based pnictides
A. Pogrebna, N.Vujicic, T. Mertelj, T. Borzda, G. Cao, Z. A. Xu, J.-H. Chu, I. R. Fisher, and D. Mihailovic,
Phys. Rev. B 89, 165131 (2014).

We systematically investigate temperature- and spectrally dependent optical reflectivity dynamics in AAs₂Fe₂, (A = Ba, Sr, and Eu), iron-based superconductors parent spin-density wave (SDW) compounds. Two different relaxation processes are identified. The behavior of the slower process, which is strongly sensitive to the magnetostructural transition, is analyzed in the framework of the relaxation-bottleneck model involving magnons. The results are compared to recent time-resolved angular photoemission results (TR-ARPES) and possible alternative assignment of the slower relaxation to the magnetostructural order parameter relaxation is discussed.

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206. Hysteretic behavior in the optical response of the underdoped Fe-arsenide
Ba(Fe_1-xCo_x)₂As₂ in the electronic nematic phase
C. Mirri, A. Dusza, S. Bastelberger, J.-H. Chu, H.-H. Kuo, I. R. Fisher, and L. Degiorgi,
Phys. Rev. B 89, 060501(R) (2014) .

We use a newtechnique that allows in situ variation of uniaxial stress to probe the polarization dependence of the optical reflectivity of the representative underdoped iron-arsenide Ba(Fe_1-xCo_x)₂As₂ through the tetragonal-toorthorhombic structural transition and with respect to their electronic nematic phase. These measurements reveal a hysteretic behavior of the anisotropic optical response to uniaxial stress in the orthorhombic state associated with twin boundary motion, whereas the pressure-induced anisotropy of the optical response is reversible in the tetragonal state.

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205. Transport near a quantum critical point in BaFe₂(As_1-xP_x)₂
James G. Analytis, H-H. Kuo, Ross D. McDonald, MarkWartenbe, P. M. C. Rourke, N. E. Hussey and I. R. Fisher,
Nature Physics 10, 194 - 197 (2014).

The physics of quantum critical phase transitions connects to some of the most difficult problems in condensed matter physics, including metal–insulator transitions, frustrated magnetism and high-temperature superconductivity. Near a quantum critical point, a new kind of metal emerges, the thermodynamic and transport properties of which do not fit into the unified phenomenology for conventional metals—the Landau Fermi-liquid theory—characterized by a low-temperature limiting T-linear specific heat and a T² resistivity. Studying the evolution of the temperature dependence of these observables as a function of a control parameter leads to the identification of both the presence and the nature of the quantum phase transition in candidate systems. In this study we measure the transport properties of BaFe₂(As_1-xP_x)₂ below the critical temperature T_c by suppressing superconductivity with high magnetic fields. At sufficiently low temperatures, the resistivity of all compositions (x >0.31) crosses over from a linear to a quadratic temperature dependence, consistent with a low-temperature Fermi-liquid ground state. As compositions with optimal T_c are approached from the overdoped side, this crossover becomes steeper, consistent with models of quantum criticality where the effective Fermi temperature T_F goes to zero.

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204. Direct Optical Coupling to an Unoccupied Dirac Surface State in the Topological Insulator Bi₂Se₃
J. A. Sobota, S.-L. Yang, A. F. Kemper, J. J. Lee, F. T. Schmitt, W. Li, R. G. Moore, J. G. Analytis, I. R. Fisher, P. S. Kirchmann, T. P. Devereaux, and Z.-X. Shen,
Phys. Rev. Lett. 111, 136802 (2013).

We characterize the occupied and unoccupied electronic structure of the topological insulator Bi₂Se₃ by one-photon and two-photon angle-resolved photoemission spectroscopy and slab band structure calculations. We reveal a second, unoccupied Dirac surface state with similar electronic structure and physical origin to the well-known topological surface state. This state is energetically located 1.5 eV above the conduction band, which permits it to be directly excited by the output of a Ti:sapphire laser. This discovery demonstrates the feasibility of direct ultrafast optical coupling to a topologically protected, spin-textured surface state.

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203. Measurement of the elastoresistivity coefficients of the underdoped iron arsenide Ba(Fe_0.975Co_0.025)₂As₂
Hsueh-Hui Kuo, Maxwell C. Shapiro, Scott C. Riggs, and Ian R. Fisher,
Phys. Rev. B 88, 085113 (2013).

A new method is presented for measuring terms in the elastoresistivity tensor m_ij of single crystal samples with tetragonal symmetry. The technique is applied to a representative underdoped Fe-arsenide, Ba(Fe_0.975Co_0.025)₂As₂, revealing an anomalously large and anisotropic elastoresistance in comparison to simple metals. The m₆₆ coefficient follows a Curie-Weiss temperature dependence, providing direct evidence that the tetragonal-to-orthorhombic structural phase transition that occurs at T_s = 97.5 K in this material is not the result of a true-proper ferroelastic transition. Rather, the material suffers a pseudoproper transition for which the lattice strain is not the primary order parameter.

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202. Direct Measurement of Current-Phase Relations in Superconductor/Topological Insulator/Superconductor Junctions
Ilya Sochnikov, Andrew J. Bestwick, James R. Williams, Thomas M. Lippman, Ian R. Fisher, David Goldhaber-Gordon, John R. Kirtley, and Kathryn A. Moler,
Nano Letters 13, 3086–3092 (2013).

Proximity to a superconductor is predicted to induce exotic quantum phases in topological insulators. Here, scanning superconducting quantum interference device (SQUID) microscopy reveals that aluminum superconducting rings with topologically insulating Bi₂Se₃ junctions exhibit a conventional, nearly sinusoidal 2π-periodic current-phase relations. Pearl vortices occur in longer junctions, indicating suppressed superconductivity in aluminum, probably due to a proximity effect. Our observations establish scanning SQUID as a general tool for characterizing proximity effects and for measuring current-phase relations in new materials systems.

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201. Infrared study of the electronic structure of the metallic pyrochlore iridate Bi₂Ir₂O₇
Y. S. Lee, S. J. Moon, Scott C. Riggs, M. C. Shapiro, I. R. Fisher, Bradford W. Fulfer, Julia Y. Chan, A. F. Kemper, and D. N. Basov,
Phys. Rev. B 87, 195143 (2013).

We investigated the electronic properties of a single crystal of metallic pyrochlore iridate Bi₂Ir₂O₇ by means of infrared spectroscopy. Our optical conductivity data show the splitting of t_2g bands into J_eff ones due to strong spin-orbit coupling. We observed a sizable midinfrared absorption near 0.2 eV which can be attributed to the optical transition within the J_eff,1/2 bands. More interestingly, we found an abrupt suppression of optical conductivity in the very far-infrared region. Our results suggest that the electronic structure of Bi₂Ir₂O₇ is governed by the strong spin-orbit coupling and correlation effects, which are a prerequisite for theoretically proposed nontrivial topological phases in pyrochlore iridates.

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200. Charge transfer and multiple density waves in the rare earth tellurides
A. Banerjee, Yejun Feng, D. M. Silevitch, Jiyang Wang, J. C. Lang, H.-H. Kuo, I. R. Fisher, and T. F. Rosenbaum
Phys. Rev. B 87, 155131 (2013).

We use high-resolution synchrotron x-ray diffraction to uncover a second, low-temperature, charge density wave (CDW) in TbTe₃. Its T_c2 = 41.0 +/- 0.4 K is the lowest discovered so far in the rare earth telluride series. The CDW wave vectors of the high temperature and low temperature states differ significantly and evolve in opposite directions with temperature, indicating that the two nested Fermi surfaces are separated and the CDWs coexist independently. Both the in-plane and out-of-plane correlation lengths are robust, implying that the density waves on different Te layers are well coupled through the TbTe layers. Finally, we rule out any low-temperature CDW in GdTe₃ for temperatures above 8 K, an energy scale sufficiently low to make pressure tuning of incipient CDW order a realistic possibility.

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199. Incoherent Topological Defect Recombination Dynamics in TbTe₃
T. Mertelj, P. Kusar, V.V. Kabanov, P. Giraldo-Gallo, I. R. Fisher, and D. Mihailovic,
Phys. Rev. Lett. 110, 156401 (2013).

We study the incoherent recombination of topological defects created during a rapid quench of a charge-density-wave system through the electronic ordering transition. Using a specially devised three-pulse femtosecond optical spectroscopy technique we follow the evolution of the order parameter over a wide range of time scales. By careful consideration of thermal processes we can clearly identify intrinsic topological defect annihilation processes on a time scale 30 ps and find a possible signature of extrinsic defect-dominated relaxation dynamics occurring on longer time scales.

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198. Distribution of Optical Spectral Weight in Detwinned Ba(Fe_1-xCo_x)₂As₂,
A. Dusza, A. Lucarelli, J.-H. Chu, I.R. Fisher and L. Degiorgi
Journal of Superconductivity and Novel Magnetism 26, 2603–2606 (2013).

We analyze our recent optical investigation on detwinned Ba(Fe_1-xCo_x)₂As₂ materials in the underdoped regime from the perspective of the spectral weight (SW) distribution. We identify its evolution for both in-plane crystallographic axes as a function of temperature across the structural tetragonal-orthorhombic phase transition. We can thus disentangle the anisotropy of SW occurring in the orthorhombic magnetic phase, from where we identify the relevant energy scales arising from interactions with spin fluctuations.

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197. Inhomogeneous Superconductivity in BaPb_1-xBi_xO₃,
P. Giraldo-Gallo, Hanoh Lee, M.R. Beasley, T.H. Geballe and I.R. Fisher,
J. Supercond. Nov. Magn. 26, 2675-2678 (2013).

Transport and magnetization measurements of single crystas of BaPb_1-xBi_xO₃, for Bi compositions spanning the superconducting phase diagram, show signatures of granular and inhomogeneous superconductivity, possibly associated with the nanoscale structural phase separation found for compositions around optimal doping. The material exhibits a field-tuned superconductor-to-insulator transition for Bi compositions 0.24 < x < 0.28. For optimally doped samples (x ~ 0.25), the magnetoresistance curves show scaling, suggesting some kind of critical behavior.

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196. Dynamics of Photoexcited Carriers in Ba(Fe_1-xCo_x)₂As₂ Single Crystals with Spin-Density-Wave Ordering,
L. Stojchevska, T. Mertelj, Juin-Haw Chu, Ian R. Fisher, D. Mihailovic,
J. Supercond. Nov. Magn. 26, 2593–2596 (2013).

We apply the femtosecond optical pump-probe spectroscopy to study the relaxation dynamics in photoexcited Co-doped Ba(Fe_1-xCo_x)₂As₂ single crystals in the underdoped spin-density wave (SDW) state region of the x - T phase diagram. Underdoped SDW samples with Co- 0 % and Co-2.5 % show a bottleneck in the relaxation as a consequence of the partial charge gap opening in the orthorhombic SDW phase, similar to previous results in other SDW iron-pnictides. Moreover, the charge gap magnitude decreases with increasing doping. The sample with Co- 5.1 % displays both a SDW ordering and superconductivity at low T .We were able to observe a 2-fold anisotropy in our samples, existing up to ~200 K, without any applied uniaxial stress. We associate the 2-fold symmetry breaking in nominally tetragonal phase with nematic ordering of the Fe d orbitals.

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195. Pressure-induced symmetry breaking in tetragonal CsAuI₃,
Shibing Wang, Shigeto Hirai, Max C. Shapiro, Scott C. Riggs, Ted H. Geballe, Wendy L. Mao, and Ian R. Fisher,
Phys. Rev. B. 87, 054104 (2013).

Results of in situ high-pressure x-ray powder diffraction on the mixed-valence compound Cs₂Au^IAu^IIII₆ (CsAuI₃) are reported for pressures up to 21 GPa in a diamond-anvil cell under hydrostatic conditions. We find a reversible pressure-induced tetragonal-to-orthorhombic structural transition at 5.5–6 GPa and reversible amorphization at 12–14 GPa. Two alternative structures are proposed for the high-pressure orthorhombic phase and are discussed in the context of a possible Au valence transition.

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194. Observation of Temperature-Induced Crossover to an Orbital-Selective Mott Phase in A_xFe_2-ySe₂ (A = K, Rb) Superconductors
M. Yi, D. H. Lu, R. Yu, S. C. Riggs, J.-H. Chu, B. Lv, Z. K. Liu, M. Lu, Y.-T. Cui, M. Hashimoto, S.-K. Mo, Z. Hussain, C. W. Chu, I. R. Fisher, Q. Si, and Z.-X. Shen,
Phys. Rev. Lett. 110, 067003 (2013).

Using angle-resolved photoemission spectroscopy, we observe the low-temperature state of the A_xFe_2-ySe₂ (A = K, Rb) superconductors to exhibit an orbital-dependent renormalization of the bands near the Fermi level—the d_xy bands heavily renormalized compared to the d_xz/d_yz bands. Upon raising the temperature to above 150 K, the system evolves into a state in which the dxy bands have depleted spectral weight while the d_xz=d_yz bands remain metallic. Combined with theoretical calculations, our observations can be consistently understood as a temperature-induced crossover from a metallic state at low temperatures to an orbital-selective Mott phase at high temperatures. Moreover, the fact that the superconducting state of AxFe2-ySe2 is near the boundary of such an orbital-selective Mott phase constrains the system to have sufficiently strong on-site Coulomb interactions and Hund’s coupling, highlighting the nontrivial role of electron correlation in this family of iron-based superconductors.

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193. Giant atomic displacement at a magnetic phase transition in metastable Mn₃O₄,
S. Hirai, A. M. dos Santos, M. C. Shapiro, J. J. Molaison, N. Pradhan, M. Guthrie, C. A. Tulk, I. R. Fisher, and W. L. Mao,
Phys. Rev. B 87, 014417 (2013).

We present x-ray, neutron scattering, and heat capacity data that reveal a coupled first-order magnetic and structural phase transition of the metastable mixed-valence postspinel compound Mn₃O₄ at 210 K. Powder neutron diffraction measurements reveal a magnetic structure in which Mn³⁺ spins align antiferromagnetically along the edge-sharing a axis, with a magnetic propagation vector k = [1/2,0,0]. In contrast, the Mn²⁺ spins, which are geometrically frustrated, do not order until a much lower temperature. Although the Mn²⁺ spins do not directly participate in the magnetic phase transition at 210 K, structural refinements reveal a large atomic shift at this phase transition, corresponding to a physical motion of approximately 0.25 A, even though the crystal symmetry remains unchanged. This “giant” response is due to the coupled effect of built-in strain in the metastable postspinel structure with the orbital realignment of the Mn³⁺ ion.

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192. Alternative route to charge density wave formation in multiband systems
Hans-Martin Eiter, Michela Lavagnini, Rudi Hackl, Elizabeth A. Nowadnick, Alexander F. Kemper, Thomas P. Devereaux, Jiun-Haw Chu, James G. Analytis, Ian R. Fisher, and Leonardo Degiorgi,
PNAS 110, 64-69 (2013).

Charge and spin density waves, periodic modulations of the electron, and magnetization densities, respectively, are among the most abundant and nontrivial low-temperature ordered phases in condensed matter. The ordering direction is widely believed to result from the Fermi surface topology. However, several recent studies indicate that this common view needs to be supplemented. Here, we show how an enhanced electron–lattice interaction can contribute to or even determine the selection of the ordering vector in the model charge densitywave system ErTe₃. Our joint experimental and theoretical study allows us to establish a relation between the selection rules of the electronic light scattering spectra and the enhanced electron–phonon coupling in the vicinity of band degeneracy points. This alternative proposal for charge density wave formation may be of general relevance for driving phase transitions into other brokensymmetry ground states, particularly in multiband systems, such as the iron-based superconductors.

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191. Magneto-elastically coupled structural, magnetic and superconducting order parameters in BaFe₂(As_1-xP_x)₂
H-.H. Kuo, J. G. Analytis, J.-H. Chu, R.M. Fernandes, J. Schmalian and I. R. Fisher,
Phys. Rev. B 86, 134507 (2012).

We measure the transport properties of mechanically strained single crystals of BaFe₂(As_1-xP_x)₂ over a wide range of x. The Neel transition is extremely sensitive to stress and this sensitivity increases as optimal doping is approached (doping with the highest superconducting Tc), even though the magnetic transition itself is strongly suppressed. Furthermore, we observe significant changes in the superconducting transition temperature with applied strain, which mirror changes in the composition x. These experiments are a direct illustration of the intimate coupling between different degrees of freedom in iron-based superconductors, revealing the importance of magnetoelastic coupling to the magnetic and superconducting transition temperatures.

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190. Unconventional Josephson Effect in Hybrid Superconductor-Topological Insulator Devices
J. R. Williams, A. J. Bestwick, P. Gallagher, Seung Sae Hong, Y. Cui, Andrew S. Bleich, J. G. Analytis, I. R. Fisher, and D. Goldhaber-Gordon
Phys. Rev. Lett. 109, 056803 (2012).

We report on transport properties of Josephson junctions in hybrid superconducting-topological insulator devices, which show two striking departures from the common Josephson junction behavior: a characteristic energy that scales inversely with the width of the junction, and a low characteristic magnetic field for suppressing supercurrent. To explain these effects, we propose a phenomenological model which expands on the existing theory for topological insulator Josephson junctions.

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189. Divergent nematic susceptibility in an iron arsenide superconductor
Jiun-Haw Chu, Hsueh-Hui Kuo, James G. Analytis and Ian R. Fisher
Science, 337, 710-712 (2012).

Within the Landau paradigm of continuous phase transitions, ordered states of matter are characterized by a broken symmetry. Although the broken symmetry is usually evident, determining the driving force behind the phase transition can be complicated by coupling between distinct order parameters. We show how measurement of the divergent nematic susceptibility of the iron pnictide superconductor Ba(Fe_1-xCo_x)₂As₂ distinguishes an electronic nematic phase transition from a simple ferroelastic distortion. These measurements also indicate an electronic nematic quantum phase transition near the composition with optimal superconducting transition temperature.

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188. Doping dependence of femtosecond quasiparticle relaxation dynamics in Ba(Fe,Co)₂As₂ single crystals: Evidence for normal-state nematic fluctuations
L. Stojchevska, T. Mertelj, Jiun-Haw Chu, Ian R. Fisher and D. Mihailovic
Phys. Rev. B 86, 024519 (2012).

We systematically investigate the photoexcited (PE) quasiparticle (QP) relaxation and low-energy electronic structure in electron doped Ba(Fe_1-xCo_x)₂As₂ single crystals as a function of Co doping. The evolution of the photoinduced reflectivity transients with x proceeds with no abrupt changes. In the orthorhombic spin-density-wave (SDW) state, a bottleneck associated with a partial charge-gap opening is detected, similar to previous results in different SDW iron pnictides. The relative charge gap magnitude 2Δ(0)/k_BT_s decreases with increasing x. In the superconducting (SC) state, an additional relaxational component appears due to a partial (or complete) destruction of the SC state proceeding on a sub-0.5-picosecond timescale. From the SC component saturation behavior the optical SC-state destruction energy, U_p/k_B = 0.3 K/Fe, is determined near the optimal doping. The subsequent relatively slow recovery of the SC state indicates clean SC gaps. The T dependence of the transient reflectivity amplitude in the normal state is consistent with the presence of a pseudogap in the QP density of states. The polarization anisotropy of the transients suggests that the pseudogap-like behavior might be associated with a broken fourfold rotational symmetry resulting from nematic electronic fluctuations persisting up to T ~ 200 K at any x. The second moment of the Eliashberg function, obtained from the relaxation rate in the metallic state at higher temperatures, indicates a moderate electron phonon coupling, λ ~ 0.3, that decreases with increasing doping.

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187. Single crystal growth by self-flux method of the mixed valence gold halides
Cs₂[Au^IX₂][Au^IIIX₄] (X=Br,I)
Scott C. Riggs, M.C. Shapiro, F. Corredor, T.H. Geballe, I.R. Fisher, Gregory T. McCandless and Julia Y. Chan
Journal of Crystal Growth, 355, 13-16 (2012).

High quality single crystals of Cs₂Au₂X₆ (X=Br,I) were grown using a ternary self-flux method. Structural refinements based on single crystal X-ray diffraction measurements show that both materials have a distorted perovskite structure belonging to the I4/mmm space group with full site occupancy. Transport measurements reveal a large bandgap of 550+/-100 meV for Cs₂Au₂I₆ and 520+/-80 meV for Cs₂Au₂Br₆. Initial attempts at chemical substitution are described.

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186. Susceptibility anisotropy in an iron arsenide superconductor revealed by x-ray diffraction in pulsed magnetic fields
J. P. C. Ruff, J.-H. Chu, H.-H. Kuo, R. K. Das, H. Nojiri, I. R. Fisher and Z. Islam
Phys. Rev. Lett. 109, 027004 (2012).

In addition to unconventional high-Tc superconductivity, the iron arsenides exhibit strong magnetoelastic coupling and a notable electronic anisotropy within the a-b plane. We relate these properties by studying underdoped Ba(Fe_1-xCo_x)₂As₂ by x-ray diffraction in pulsed magnetic fields up to 27.5 T. We exploit magnetic detwinning effects to demonstrate anisotropy in the in-plane susceptibility, which develops at the structural phase transition despite the absence of magnetic order. The degree of detwinning increases smoothly with decreasing temperature, and a single-domain condition is realized over a range of field and temperature. At low temperatures we observe an activated behavior, with a large hysteretic remnant effect. Detwinning was not observed within the superconducting phase for accessible magnetic fields.

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185. Structure and Magnetic Properties of the Pyrochlore Iridate Y₂Ir₂O₇
M. C. Shapiro, S. C. Riggs, M. B. Stone, C. R. de la Cruz, S. Chi, A. A. Podlesnyak & I. R. Fisher
Phys. Rev. B. 85, 214434 (2012).

Neutron powder diffraction and inelastic measurements were performed examining the 5d pyrochlore Y₂Ir₂O₇. Temperature-dependent measurements were performed between 3.4 K and 290 K, spanning the magnetic transition at 155 K. No sign of any structural or disorder-induced phase transition was observed over the entire temperature range. In addition, no sign of magnetic long-range order was observed to within the sensitivity of the instrumentation. These measurements do not rule out long-range magnetic order, but the neutron-powder-diffraction structural refinements do put an upper bound for the ordered iridium moment of ~0.2 μ_B/Ir (for a magnetic structure with wave vector Q ≠ 0) or ~0.5 μ_B/Ir (for Q = 0).

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184. Principles of crystal growth of intermetallic and oxide compounds from molten solutions
I. R. Fisher, M. Shapiro and J. G. Analytis
Phil. Mag. 92, 2401–2435 (2012).

We present a tutorial on the principles of crystal growth of intermetallic and oxide compounds from molten solutions, with an emphasis on the fundamental principles governing the underlying phase equilibria and phase diagrams of multicomponent systems.

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183. STM Imaging of Impurity Resonances on Bi₂Se₃
Zhanybek Alpichshev, Rudro R. Biswas, Alexander V. Balatsky, J. G. Analytis, J.-H. Chu, I. R. Fisher, and A. Kapitulnik
Phys. Rev. Lett. 108, 206402 (2012).

In this Letter we present detailed study of the density of states near defects in Bi₂Se₃. In particular, we present data on the commonly found triangular defects in this system. While we do not find any measurable quasiparticle scattering interference effects, we do find localized resonances, which can be well fitted by theory [R. R. Biswas and A.V. Balatsky, Phys. Rev. B 81, 233405(R) (2010)] once the potential is taken to be extended to properly account for the observed defects. The data together with the fits confirm that while the local density of states around the Dirac point of the electronic spectrum at the surface is significantly disrupted near the impurity by the creation of low-energy resonance state, the Dirac point is not locally destroyed. We discuss our results in terms of the expected protected surface state of topological insulators.

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182. Field-tuned superconductor-insulator transition in BaPb_1-xBi_xO₃
P. Giraldo-Gallo, Hanoh Lee, Y. Zhang, M. J. Kramer, M. R. Beasley, T. H. Geballe, and I. R. Fisher
Phys. Rev. B 85, 174503 (2012).

BaPb_1-xBi_xO₃ is found to exhibit a field-tuned superconductor to insulator transition for Bi compositions 0.24 < x < 0.29. The magnetoresistance of optimally doped samples manifests a temperature-independent crossing point and scaling of the form ρ(T,H) = ρ_cF(|H - H_c|T^-1/zν), where H_c is the field determined by the temperature-independent crossing point, and zν = 0.69+/-0.03. High-resolution transmission electronmicroscopy measurements reveal a complex intergrown nanostructure comprising tetragonal and orthorhombic polymorphs. Data are analyzed in terms of both a classical effectivemedium theory and a field-tuned quantum phase transition, neither of which provides a completely satisfactory explanation for this remarkable phenomenology.

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181. Resonant enhancement of charge density wave diffraction in the rare-earth tritellurides
W. S. Lee, A. P. Sorini, M. Yi, Y. D. Chuang, B. Moritz, W. L. Yang, J.-H. Chu, H. H. Kuo, A. G. Cruz Gonzalez, I. R. Fisher, Z. Hussain, T. P. Devereaux, and Z. X. Shen
Phys. Rev. B 85, 155142 (2012).

We performed resonant soft x-ray diffraction on known charge density wave (CDW) compounds, rare-earth tritellurides. Near the M₅ (3d-4f ) absorption edge of rare-earth ions, an intense diffraction peak is detected at a wave vector identical to that of the CDW state hosted on Te₂ planes, indicating a CDW-induced modulation on the rare-earth ions. Surprisingly, the temperature dependence of the diffraction peak intensity demonstrates an exponential increase at low temperatures, vastly different than that of the CDW order parameter. Assuming 4f multiplet splitting due to the CDW states, we present a model to calculate x-ray-absorption spectrum and resonant profile of the diffraction peak, agreeing well with experimental observations. Our results demonstrate a situation where the temperature dependence of resonant x-ray-diffraction peak intensity is not directly related to the intrinsic behavior of the order parameter associated with the electronic order, but is dominated by the thermal occupancy of the valence states.

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180. Pressure dependence of the BaFe₂As₂ Fermi surface within the spin density wave state
D. Graf, R. Stillwell, T. P. Murphy, J.-H. Park, E. C. Palm, P. Schlottmann, R. D. McDonald, J. G. Analytis, I. R. Fisher, and S. W. Tozer
Phys. Rev. B 85, 134503 (2012).

Measuring surface conductivity we have observed the evolution of Shubnikov de Haas oscillations under quasihydrostatic pressure for the pnictide parent compound BaFe₂As₂. Prior results in the reconstructed state have observed small pockets which emerge from zone folding as a result of structural changes with cooling. For pressures below 20 kbar, both Fermi surface orbits grow in size. The effective masses increase with pressure suggesting enhanced correlation in the system, and a series of magnetic breakdown orbits are observed confirming that band structure calculations setting them in close proximity are correct.

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179. Ultrafast Optical Excitation of a Persistent Surface-State Population in the Topological Insulator Bi₂Se₃
J. A. Sobota, S. Yang, J. G. Analytis, Y. L. Chen, I. R. Fisher, P. S. Kirchmann, and Z.-X. Shen
Phys. Rev. Lett. 108, 117403 (2012).

Using femtosecond time- and angle-resolved photoemission spectroscopy, we investigated the nonequilibrium dynamics of the topological insulator Bi₂Se₃.We studied p-type Bi₂Se₃, in which the metallic Dirac surface state and bulk conduction bands are unoccupied. Optical excitation leads to a metastable population at the bulk conduction band edge, which feeds a nonequilibrium population of the surface state persisting for >10 ps. This unusually long-lived population of a metallic Dirac surface state with spin texture may present a channel in which to drive transient spin-polarized currents.

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178. Anisotropic in-plane optical conductivity in detwinned Ba(Fe_1-xCo_x)₂As₂
A. Dusza, A. Lucarelli, A. Sanna, S. Massidda, J.-H. Chu, I. R. Fisher and L. Degiorgi
New Journal of Physics 14, 023020 (2012).

We study the anisotropic in-plane optical conductivity of detwinned Ba(Fe_1-xCo_x)₂As₂ single crystals for x = 0, 2.5 and 4.5% in a broad energy range (3 meV–5 eV) across their structural and magnetic transitions. For temperatures below the Neel transition, the topology of the reconstructed Fermi surface, combined with the distinct behavior of the scattering rates, determines the anisotropy of the low-frequency optical response. For the itinerant charge carriers, we are able to disentangle the evolution of the Drude weights and scattering rates and to observe their enhancement along the orthorhombic antiferromagnetic a-axis with respect to the ferromagnetic b-axis. For temperatures above the structural phase transition, uniaxial stress induces a finite in-plane anisotropy. The anisotropy of the optical conductivity, leading to significant dichroism, extends to high frequencies in the mid- and near-infrared regions. The temperature dependence of the dichroism at all dopings scales with the anisotropy ratio of dc conductivity, suggesting the electronic nature of the structural transition. Our findings bear testimony to a large nematic susceptibility that couples very effectively to the uniaxial lattice strain. In order to clarify the subtle interplay of magnetism and Fermi surface topology we compare our results with theoretical calculations obtained from density functional theory within the full-potential linear augmented plane-wave method.

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177. Ultrafast pump-probe measurements of short small-polaron lifetimes in the mixed-valence perovskite Cs₂Au₂I₆ under high pressures
M. Trigo, J. Chen, M. P. Jiang, W. L. Mao, S. C. Riggs, M. C. Shapiro, I. R. Fisher, and D. A. Reis
Phys. Rev. B 85, 081102(R) (2012).

We study the ultrafast phonon response of mixed-valence perovskite Cs₂Au₂I₆ using pump-probe spectroscopy under high pressure in a diamond-anvil cell. We observed a remarkable softening and broadening of the Au-I stretching phonon mode with both applied pressure and photoexcitation. Using a double-pump scheme we measured a lifetime of the charge-transfer excitation into single-valence Au²⁺ of less than 4 ps, which is an indication of the local character of the Au²⁺ excitation. Furthermore, the strong similarity between the pressure and fluence dependence of the phonon softening shows that the intervalence charge transfer plays an important role in the structural transition.

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176. Low-energy quasiparticles probed by heat transport in the iron-based superconductor LaFePO
Mike Sutherland, J. Dunn, W. H. Toews, Eoin O’Farrell, James Analytis, Ian Fisher, and R. W. Hill
Phys. Rev. B 85, 014517 (2012).

We have measured the thermal conductivity of the iron pnictide superconductor LaFePO down to temperatures as low as T = 60 mK and in magnetic fields up to 5 T. The data show a large residual contribution that is linear in temperature, consistent with the presence of low-energy electronic quasiparticles. We interpret the magnitude of the linear term, as well as the field and temperature dependence of thermal transport in several pairing scenarios. The presence of an unusual supralinear temperature dependence of the electronic thermal conductivity in zero magnetic field, and a high scattering rate with minimal Tc suppression argues for a sign-changing nodal s+- state.

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175. Incommensurate magnetic order in TbTe₃
F Pfuner, S N Gvasaliya, O Zaharko, L Keller, J Mesot, V Pomjakushin, J-H Chu, I R Fisher and L Degiorgi,
J. Phys.: Condens. Matter 24, 036001 (2012).

We report a neutron diffraction study of the magnetic phase transitions in the charge-density wave (CDW) TbTe₃ compound. We discover that in the paramagnetic phase there are strong 2D-like magnetic correlations, consistent with the pronounced anisotropy of the chemical structure. A long-range incommensurate magnetic order emerges in TbTe₃ at T_mag1 = 5.78 K as a result of continuous phase transitions. We observe that near the temperature T_mag1 the magnetic Bragg peaks appear around the position (0, 0, 0.24) (or its rational multiples), that is fairly close to the propagation vector (0, 0, 0.29) associated with the CDW phase transition in TbTe₃. This suggests that correlations leading to the long-range magnetic order in TbTe₃ are linked to the modulations that occur in the CDW state.

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174. Pressure Dependent Diffraction and Spectroscopy of a Dimerized Antiferromagnet
M. B. Stone, C. A. Tulk, A. dos Santos, J. J. Molaison, S. Chang, J. B. Leao, E. C. Samulon, M. C. Shapiro, and I. R. Fisher,
J. Phys. Soc. Jpn. 80, SB005 (2011).

We present pressure dependent neutron diffraction and inelastic neutron scattering measurements of the dimerized antiferromagnet Ba₃Mn₂O₈. The room temperature diffraction measurements reveal a linear decrease in lattice constant as a function of applied pressure. No structural transitions are observed. The low-temperature neutron spectroscopy measurements indicate a small change in magnetic scattering intensity in the vicinity of the spin gap for pressures up to P = 0.6 Gpa.

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173. Widespread spin polarization effects in photoemission from topological insulator
C. Jozwiak, Y L. Chen, A V. Fedorov, J G. Analytis, C R. Rotundu, A K. Schmid, J D. Denlinger, Y.-D. Chuang, D.-H. Lee, I R. Fisher, R J. Birgeneau, Z.-X. Shen, Z. Hussain, and A. Lanzara,
Phys. Rev. B 84, 165113 (2011).

High-resolution spin- and angle-resolved photoemission spectroscopy (spin-ARPES) was performed on the three-dimensional topological insulator Bi₂Se₃ using a recently developed high-efficiency spectrometer. The topological surface state’s helical spin structure is observed, in agreement with theoretical prediction. Spin textures of both chiralities, at energies above and below the Dirac point, are observed, and the spin structure is found to persist at room temperature. The measurements reveal additional unexpected spin polarization effects, which also originate from the spin-orbit interaction, but are well differentiated from topological physics by contrasting momentum and photon energy and polarization dependencies. These observations demonstrate significant deviations of photoelectron and quasiparticle spin polarizations. Our findings illustrate the inherent complexity of spin-resolved ARPES and demonstrate key considerations for interpreting experimental results.

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172. Polarized neutron diffraction study of the field-induced magnetization in the normal and superconducting states of Ba(Fe_1-xCo_x)₂As₂ (x=0.65),
C. Lester, Jiun-Haw Chu, J. G. Analytis, A. Stunault, I. R. Fisher, and S. M. Hayden,
Phys. Rev. B 84, 134514 (2011).

We use polarized neutron diffraction to study the induced magnetization density of near optimally doped Ba(Fe_0.935Co_0.065)₂As₂ (T_C = 24 K) as a function of magnetic field (1 < μ₀H < 9 T) and temperature (2 < T < 300 K). The T dependence of the induced moment in the superconducting state is consistent with the Yosida function, characteristic of spin-singlet pairing. The induced moment is proportional to applied field for μ₀H < 9 T ~ μ₀H_c2/6. In addition to the Yosida spin-susceptibility, our results reveal a large zero-field contribution M(H --> 0,T --> 0)/H ~ 2/3χ_normal which does not scale with the field or number of vortices and is most likely due to the van Vleck susceptibility. Magnetic structure factors derived from the polarization dependence of 15 Bragg reflections were used to make a maximum entropy reconstruction of the induced magnetization distribution in real space. The magnetization is confined to the Fe atoms, and the measured density distribution is in good agreement with LAPW band structure calculations which suggest that the relevant bands near the Fermi energy are of the d_xz/yz and d_xy type.

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171. Ambipolar field effect in the ternary topological insulator (Bi_xSb_1-x)₂Te₃ by composition tuning
Desheng Kong, Yulin Chen, Judy J. Cha, Qianfan Zhang, James G. Analytis, Keji Lai, Zhongkai Liu, Seung Sae Hong, Kristie J. Koski, Sung-Kwan Mo, Zahid Hussain, Ian R. Fisher, Zhi-Xun Shen & Yi Cui,
Nature Nanotechnology 6, 705 (2011).

Topological insulators exhibit a bulk energy gap and spin-polarized surface states that lead to unique electronic properties with potential applications in spintronics and quantum information processing. However, transport measurements have typically been dominated by residual bulk charge carriers originating from crystal defects or environmental doping and these mask the contribution of surface carriers to charge transport in these materials. Controlling bulk carriers in current topological insulator materials, such as the binary sesquichalcogenides Bi₂Te₃, Sb₂Te₃ and Bi₂Se₃, has been explored extensively by means of material doping and electrical gating, but limited progress has been made to achieve nanostructures with low bulk conductivity for electronic device applications. Here we demonstrate that the ternary sesquichalcogenide (Bi_xSb_1-x)₂Te₃ is a tunable topological insulator system. By tuning the ratio of bismuth to antimony, we are able to reduce the bulk carrier density by over two orders of magnitude, while maintaining the topological insulator properties. As a result, we observe a clear ambipolar gating effect in (Bi_xSb_1-x)₂Te₃ nanoplate field-effect transistor devices, similar to that observed in graphene field-effect transistor devices. The manipulation of carrier type and density in topological insulator nanostructures demonstrated here paves the way for the implementation of topological insulators in nanoelectronics and spintronics.

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170. Persistence of magnons in a site-diluted dimerized frustrated antiferromagnet
M. B. Stone, A. Podlesnyak, G. Ehlers, A. Huq, E. C. Samulon, M. C. Shapiro and I. R. Fisher,
J. Phys.: Condens. Matter 23 416003 (2011).

We present inelastic neutron scattering and thermodynamic measurements characterizing the magnetic excitations in a disordered spin-liquid antiferromagnet with non-magnetic substitution. The parent compound Ba₃Mn₂O₈ is a dimerized, quasi-two-dimensional geometrically frustrated quantum disordered antiferromagnet. We substitute this compound with non-magnetic V⁵⁺ for the S = 1 Mn⁵⁺ ions, Ba₃(Mn_1-xV_x)₂O₈, and find that the singlet–triplet excitations which dominate the spectrum of the parent compound persist for the full range of substitution examined, up to x =0.3. We also observe additional low-energy magnetic fluctuations which are enhanced at the greatest substitution values.

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169. In-plane electronic anisotropy of underdoped ‘122’ Fe-arsenides revealed by measurements of detwinned single crystals
I. R. Fisher, L. Degiorgi and Z. X. Shen,
Rep. Prog. Phys 74, 124506 (2011)

The parent phases of the Fe-arsenide superconductors harbor an antiferromagnetic ground state. Significantly, the Neel transition is either preceded or accompanied by a structural transition that breaks the four-fold symmetry of the high-temperature lattice. Borrowing language from the field of soft condensed matter physics, this broken discrete rotational symmetry is widely referred to as an Ising nematic phase transition. Understanding the origin of this effect is a key component of a complete theoretical description of the occurrence of superconductivity in this family of compounds, motivating both theoretical and experimental investigation of the nematic transition and the associated in-plane anisotropy. Here we review recent experimental progress in determining the intrinsic in-plane electronic anisotropy as revealed by resistivity, reflectivity and angle-resolved photoemission spectroscopy measurements of detwinned single crystals of underdoped Fe-arsenide superconductors in the ‘122’ family of compounds.

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168. Revealing the dual nature of magnetism in iron pnictides and iron chalcogenides using x-ray emission spectroscopy
H. Gretarsson, A. Lupascu, Jungho Kim, D. Casa, T. Gog, W. Wu, S. R. Julian, Z. J. Xu, J. S. Wen, G. D. Gu, R. H. Yuan, Z. G. Chen, N.-L. Wang, S. Khim, K. H. Kim, M. Ishikado, I. Jarrige, S. Shamoto, J.-H. Chu, I. R. Fisher and Young-June Kim,
Phys. Rev. B 84, 100509(R) (2011)

We report a Fe Kβ x-ray emission spectroscopy study of local magnetic moments in various iron-based superconductors in their paramagnetic phases. Local magnetic moments are found in all samples studied: PrFeAsO, Ba(Fe,Co)₂As₂, LiFeAs, Fe_1+x(Te,Se), and A₂Fe₄Se₅ (where A=K, Rb, and Cs). The moment size is independent of temperature or carrier concentration but varies significantly across different families. Specifically, all iron pnictide samples have local moments of about 1μ_B/Fe, while FeTe and K₂Fe₄Se₅ families have much larger local moments of 2μ_B/Fe and 3.3μ_B/Fe, respectively. Our results illustrate the importance of multiorbital physics in describing magnetism of these compounds

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167. Possible origin of the nonmonotonic doping dependence of the in-plane resistivity anisotropy of Ba(Fe_1-xT_x)₂As₂ (T=Co, Ni and Cu)
H.-H. Kuo, J.-H. Chu, S. C. Riggs, L. Yu, P. L. McMahon, K. De Greve, Y. Yamamoto, J. G. Analytis and I. R. Fisher,
Phys. Rev. B 84, 054540 (2011)

The in-plane resistivity anisotropy has been measured for detwinned single crystals of Ba(Fe_1-xNi_x)₂As₂ and Ba(Fe_1-xCu_x)₂As₂. The data reveal a nonmonotonic doping dependence, similar to previous observations for Ba(Fe_1-xCo_x)₂As₂. Magnetotransport measurements of the parent compound reveal a nonlinear Hall coefficient and a large linear term in the transverse magnetoresistance. Both effects are rapidly suppressed with chemical substitution over a similar compositional range as the onset of the large in-plane resistivity anisotropy. This suggests that the relatively small in-plane anisotropy of the parent compound in the spin-density wave state is due to the presence of an isotropic, high mobility pocket of the reconstructed Fermi surface. Progressive suppression of the contribution to the conductivity arising from this isotropic pocket with chemical substitution eventually reveals the underlying in-plane anisotropy associated with the remaining Fermi surface pockets.

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166. Heat capacity of the site-diluted spin dimer system Ba₃(Mn_1-xV_x)₂O₈
E. C. Samulon, M. C. Shapiro, and I. R. Fisher,
Phys. Rev. B 84, 054417 (2011)

Heat-capacity and susceptibility measurements have been performed on the diluted spin dimer compound Ba₃(Mn_1-xV_x)₂O₈. The parent compound Ba₃Mn₂O₈ is a spin dimer system based on pairs of antiferromagnetically coupled S = 1, 3d² Mn⁵⁺ ions such that the zero-field ground state is a product of singlets. Substitution of nonmagnetic S = 0, 3d⁰ V⁵⁺ ions leads to an interacting network of unpaired Mn moments, the low-temperature properties of which are explored in the limit of small concentrations 0 < x < 0.05. The zero-field heat capacity of this diluted system reveals a progressive removal of magnetic entropy over an extended range of temperatures, with no evidence for a phase transition. The concentration dependence does not conform to expectations for a spin-glass state. Rather, the data suggest a low-temperature random singlet phase, reflecting the hierarchy of exchange energies found in this system.

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165. Nonuniversal magnetization at the BEC critical field: Application to the spin dimer compound Ba₃Mn₂O₈
S. Suh, K. A. Al-Hassanieh, E. C. Samulon, I. R. Fisher, S. E. Brown, and C. D. Batista,
Phys. Rev. B 84, 054413 (2011)

Ba₃Mn₂O₈ is a hexagonally coordinated Mn⁵⁺ S = 1 spin dimer system with small uniaxial single-ion anisotropy. ^135,137Ba NMR spectroscopy is used to measure the longitudinal (M_l) magnetization in the vicinity of the critical field at H_c1 for the onset of magnetic order for H || c and H perp c. M_l||(T,H_c1), M_lperp(T,H_c1) are reproduced by solving a low-energy model for a dilute gas of interacting bosons.

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164. STM imaging of a bound state along a step on the surface of the topological insulator Bi₂Te₃
Zhanybek Alpichshev, J. G. Analytis, J.-H. Chu, I. R. Fisher, and A. Kapitulnik,
Phys. Rev. B 84, 041104 (2011)

We present a detailed study of the local density of states (LDOS) associated with the surface-state band near a step edge of the strong topological insulator Bi₂Te₃ and reveal a one-dimensional bound state that runs parallel to the step edge and is bound to it at some characteristic distance. This bound state is clearly observed in the bulk gap region, while it becomes entangled with the oscillations of the warped surface band at high energy, and with the valence-band states near the Dirac point. We obtain excellent fits to theoretical predictions that properly incorporate the three-dimensional nature of the problem to the surface state. Fitting the data at different energies, we can recalculate the LDOS originating from the Dirac band without the contribution of the bulk bands or incoherent tunneling effects.

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163. Ultrafast electron dynamics in the charge density wave material TbTe₃
F. Schmitt, P. S. Kirchmann, U. Bovensiepen, R. G. Moore, J.-H. Chu, D. H. Lu, L. Rettig, M. Wolf, I. R. Fisher, and Z. X. Shen,
New Journal of Physics 13, 063022 (2011)

Gaining insights into the mechanisms of how order and broken symmetry emerges from many-particle interactions is a major challenge in solid state physics. Most experimental techniques—such as angle-resolved photoemission spectroscopy (ARPES)—probe the single-particle excitation spectrum and extract information about the ordering mechanism and collective effects, often indirectly through theory. Time-resolved ARPES (tr-ARPES) makes collective dynamics of a system after optical excitation directly visible through their influence on the quasi-particle band structure. Using this technique, we present a systematic study of TbTe₃, a metal that exhibits a charge-density wave (CDW) transition. We discuss time-resolved data taken at different positions in the Brillouin zone (BZ) and at different temperatures. The transient change in the band structure due to the excitation is qualitatively different between the region gapped by the CDW order vector and an ungapped but otherwise equivalent region. Also, we discovered two distinct collective modes at roughly 3.5 and 2.5 THz, the latter of which only occurs in the CDW band near the gapped region, demonstrating the strength of tr-ARPES in discerning the origin of the modes from the way in which they couple to the quasi-particle bands. In addition, a systematic pump fluence dependence in the gapped region documents the crossover from a weakly perturbed to a strongly perturbed regime, which can be related to a crossover from a regime where mainly the amplitude mode gets excited to a regime where the CDW gap closes at least partially.

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162. Rapid Surface Oxidation as a Source of Surface Degradation Factor for Bi₂Se₃
Desheng Kong, Judy J. Cha, Keji Lai, Hailin Peng, James G. Analytis, Stefan Meister, Yulin Chen, Hai-Jun Zhang, Ian R. Fisher, Zhi-Xun Shen, Yi Cui
ACS Nano 5 (6), 4698-4703 (2011)

Bismuth selenide (Bi₂Se₃) is a topological insulator with metallic surface states (SS) residing in a large bulk bandgap. In experiments, synthesized Bi₂Se₃ is often heavily n-type doped due to selenium vacancies. Furthermore, it is discovered from experiments on bulk single crystals that Bi₂Se₃ gets additional n-type doping after exposure to the atmosphere, thereby reducing the relative contribution of SS in total conductivity. In this article, transport measurements on Bi₂Se₃ nanoribbons provide additional evidence of such environmental doping process. Systematic surface composition analyses by X-ray photoelectron spectroscopy reveal fast formation and continuous growth of native oxide on Bi₂Se₃ under ambient conditions. In addition to n-type doping at the surface, such surface oxidation is likely the material origin of the degradation of topological SS. Appropriate surface passivation or encapsulation may be required to probe topological SS of Bi₂Se₃ by transport measurements.

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161. Symmetry breaking orbital anisotropy observed for detwinned Ba(Fe_1-xCo_x)₂As₂ above the spin density wave transition
M. Yi, D. H. Lu, J.-H. Chu, J. G. Analytis, A. P. Sorini, A. F. Kemper, B. Moritz, S.-K. Mo, R. G. Moore, M. Hashimoto, W. S. Lee, Z. Hussain, T. P. Devereaux, I. R. Fisher, Z.-X. Shen
PNAS 108 (17) 6878-6883 (2011).

Nematicity, defined as broken rotational symmetry, has recently been observed in competing phases proximate to the superconducting phase in the cuprate high-temperature superconductors. Similarly, the new iron-based high-temperature superconductors exhibit a tetragonal-to-orthorhombic structural transition (i.e., a broken C4 symmetry) that either precedes or is coincident with a collinear spin density wave (SDW) transition in undoped parent compounds, and superconductivity arises when both transitions are suppressed via doping. Evidence for strong in-plane anisotropy in the SDW state in this family of compounds has been reported by neutron scattering, scanning tunneling microscopy, and transport measurements. Here, we present an angle-resolved photoemission spectroscopy study of detwinned single crystals of a representative family of electron-doped iron-arsenide superconductors, Ba(Fe_1-xCo_x)₂As₂ in the underdoped region. The crystals were detwinned via application of in-plane uniaxial stress, enabling measurements of single domain electronic structure in the orthorhombic state. At low temperatures, our results clearly demonstrate an in-plane electronic anisotropy characterized by a large energy splitting of two orthogonal bands with dominant d_xz and d_yz character, which is consistent with anisotropy observed by other probes. For compositions x > 0, for which the structural transition (T_S) precedes the magnetic transition (T_SDW), an anisotropic splitting is observed to develop above T_SDW, indicating that it is specifically associated with T_S. For unstressed crystals, the band splitting is observed close to T_S, whereas for stressed crystals,the splitting is observed to considerably higher temperatures, revealing the presence of a surprisingly large in-plane nematic susceptibility in the electronic structure.

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160. Behavior of vortices near twin boundaries in underdoped Ba(Fe_1-xCo_x)₂As₂
B. Kalisky, J. R. Kirtley, J. G. Analytis, J.-H. Chu, I. R. Fisher, and K. A. Moler
Phys. Rev. B. 83, 064511 (2011).

We use scanning superconducting quantum interference device (SQUID) microscopy to investigate the behavior of vortices in the presence of twin boundaries in the pnictide superconductor Ba(Fe_1-xCo_x)₂As₂. We show that the vortices avoid pinning on twin boundaries. Individual vortices move in a preferential way when manipulated with the SQUID: They tend to not cross a twin boundary, but rather to move parallel to it. This behavior can be explained by the observation of enhanced superfluid density on twin boundaries in Ba(Fe_1-xCo_x)₂As₂. The observed repulsion from twin boundaries may be a mechanism for enhanced critical currents observed in twinned samples in pnictides and other superconductors.

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159. Local Measurement of the Superfluid Density in the Pnictide Superconductor Ba(Fe_1-xCo_x)₂As₂ across the Superconducting Dome
Lan Luan, Thomas M. Lippman, Clifford W. Hicks, Julie A. Bert, Ophir M. Auslaender, Jiun-Haw Chu, James G. Analytis, Ian R. Fisher, and Kathryn A. Moler,
Phys. Rev. Lett. 106, 067001 (2011).

We measure the penetration depth λ_ab(T) in Ba(Fe_1-xCo_x)₂As₂ using local techniques that do not average over the sample. The superfluid density ρ_s(T)=1/λ_ab(T)2 has three main features. First, ρ_s(T=0) falls sharply on the underdoped side of the dome. Second, λ_ab is flat at low T at optimal doping, indicating fully gapped superconductivity, but varies more strongly in underdoped and overdoped samples, consistent with either a power law or a small second gap. Third, ρ_s(T) varies steeply near Tc for optimal and underdoping. These observations are consistent with an interplay between magnetic and superconducting phases.

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158. Anisotropic charge dynamics in detwinned Ba(Fe_1-xCo_x)₂As₂
A. Dusza, A. Lucarelli, F. Pfuner, J.-H. Chu, I. R. Fisher and L. Degiorgi,
Euro. Phys. Letters 93, 37002 (2011).

We investigate the optical conductivity with light polarized along the in-plane orthorhombic a and b axes of Ba(Fe_1-xCo_x)₂As₂ for x=0 and 2.5% under uniaxial pressure across their structural and magnetic transitions. The charge dynamics at low frequencies and temperatures on these detwinned, single-domain samples reveals an enhancement of both the scattering rate and Drude weight of the charge carriers along the antiferromagnetic a-axis with respect to the ferromagnetic b-axis. Our findings also allow us to estimate the dichroism, which extends to high frequencies and temperatures. These results reveal a nematic susceptibility as well as the electronic nature of the structural transition.

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157. Anharmonic order-parameter oscillations and lattice coupling in strongly driven 1T-TaS₂ and TbTe₃ charge-density-wave compounds: A multiple-pulse femtosecond laser spectroscopy study
P. Kusar, T. Mertelj, V. V. Kabanov, J.-H. Chu, I. R. Fisher, H. Berger, L. Forró, and D. Mihailovic
Phys. Rev. B. 83, 035104 (2011).

The anharmonic response of charge-density wave (CDW) order to strong laser-pulse perturbations in 1 T -TaS₂ and TbTe₃ is investigated by means of multiple-pump-pulse time-resolved femtosecond optical spectroscopy.We observe remarkable anharmonic effects hitherto undetected in systems exhibiting collective charge ordering. The efficiency for additional excitation of the amplitude mode by a laser pulse becomes periodically modulated after the mode is strongly excited into a coherently oscillating state. A similar effect is observed also for some other phonons, where the cross-modulation at the amplitude-mode frequency indicates anharmonic interaction of those phonons with the amplitude mode. By analyzing the observed phenomena in the framework of time-dependent Ginzburg-Landau theory we attribute the effects to the anharmonicity of the mode potentials inherent in the broken symmetry state of the CDW systems.

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156. Single Dirac Cone Topological Surface State and Unusual Thermoelectric Property of Compounds from a New Topological Insulator Family
Y. L. Chen, Z. K. Liu, J. G. Analytis, J.-H. Chu, H. J. Zhang, B. H. Yan, S.-K. Mo, R. G. Moore, D. H. Lu, I. R. Fisher, S. C. Zhang, Z. Hussain, and Z.-X. Shen
Phys. Rev. Lett. 105, 266401 (2010).

Angle resolved photoemission spectroscopy study on TlBiTe₂ and TlBiSe₂ from a thallium-based ternary chalcogenides family revealed a single surface Dirac cone at the center of the Brillouin zone for both compounds. For TlBiSe₂, the large bulk gap (~200 meV) makes it a topological insulator with better mechanical properties than the previous binary 3D topological insualtor family. For TlBiTe₂, the observed negative bulk gap indicates it as a semimetal, instead of a narrow-gap semiconductor as conventionally believed; this semimetality naturally explains its mysteriously small thermoelectric figure of merit comparing to other compounds in the family. Finally, the unique band structures of TlBiTe₂ also suggest it as a candidate for topological superconductors.

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155. Two-dimensional surface state in the quantum limit of a topological insulator
J. G. Analytis, R. D. McDonald, S. C. Riggs, J.-H. Chu, G. S. Boebinger, and I. R. Fisher
Nature Physics 6, 960 (2010).

The topological insulator is a unique state of matter that possesses a metallic surface state of massless particles known as Dirac fermions, which have coupled spin and momentum quantum numbers. Owing to the preservation of time-reversal symmetry, this coupling protects the wavefunctions against disorder. The experimental realization of this state of matter in Bi₂Se₃ and Bi₂Te₃ has sparked considerable interest owing both to their potential use in spintronic devices and in the investigation of the fundamental nature of topologically nontrivial quantum matter. However, the conductivity of these compounds tends to be dominated by the bulk of the material because of chemical imperfection, making the transport properties of the surface nearly impossible to measure. We have systematically reduced the number of bulk carriers in Bi₂Se₃ to the point where a magnetic field can collapse them to their lowest Landau level. Beyond this field, known as the three-dimensional (3D) 'quantum limit', the signature of the 2D surface state can be seen. At still higher fields, we reach the 2D quantum limit of the surface Dirac fermions. In this limit we observe an altered phase of the oscillations, which is related to the peculiar nature of the Landau quantization of topological insulators at high field. Furthermore, we observe quantum oscillations corresponding to fractions of the Landau integers, suggesting that correlation effects can be observed in this new state of quantum matter.

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154. Enhanced Fermi surface nesting in superconducting BaFe₂(As_1-xP_x)₂ revealed by the de Haas-van Alphen effect
J.G. Analytis, J.-H. Chu, R.D. McDonald, S. C. Riggs and I.R. Fisher
Phys. Rev. Lett. 105, 207004 (2010).

ABSTRACT
The three-dimensional Fermi-surface morphology of superconducting BaFe₂(As_0.37P_0.63)₂ with T_c=9 K is determined using the de Haas–van Alphen effect. The inner electron pocket has a similar area and k_z interplane warping to the observed hole pocket, revealing that the Fermi surfaces are geometrically well nested in the (π,π) direction. These results are in stark contrast to the fermiology of the nonsuperconducting phosphides (x=1), and therefore suggest an important role for nesting in pnictide superconductivity.
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153. Pinpointing gap minima in Ba(Fe_0.94Co_0.06)₂As₂ via band-structure calculations and electronic Raman scattering
I. I. Mazin, T. P. Devereaux, J. G. Analytis, Jiun-Haw Chu, I. R. Fisher, B. Muschler, and R. Hackl,
Phys. Rev. B 82, 180502 (R) (2010).

ABSTRACT
A detailed knowledge of the gap structure for the Fe-pnictide superconductors is still rather rudimentary with several conflicting reports of either nodes, deep gap minima, or fully isotropic gaps on the Fermi-surface sheets, both in the k_x-k_y plane and along the c axis. In this Rapid Communication, we present considerations for electronic Raman scattering which can help clarify the gap structure and topology using different light-scattering geometries. Using density-functional calculations for the Raman vertices, it is shown that the location of the gap minima may occur on loops stretching over a portion of the c axis in Ba(Fe_0.94Co_0.06)₂As₂.
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152. Coherent dynamics of macroscopic electronic order through a symmetry breaking transition
R. Yusupov, T. Mertelj, V. V. Kabanov, S. Brazovskii, P. Kusar, J.-H. Chu, I. R. Fisher and D. Mihailovic
Nature Physics 6, 681 (2010).

The study of the temporal evolution of systems undergoing symmetry breaking phase transitions — whether it is in condensed-matter physics, cosmology or finance — is difficult because they are hard to repeat, or they occur very rapidly. Here we report a high-time-resolution study of the evolution of both bosonic and fermionic excitations through an electronic charge-ordering symmetry breaking phase transition. Periodically quenching our system with femtosecond optical pulses, we subsequently detect hitherto-unrecorded coherent aperiodic undulations of the order parameter, critical slowing down of the collective mode and evolution of the particle–hole gap as the system evolves through the transition. Modelling on the basis of Ginzburg–Landau theory is used to reproduce the observations without free parameters. Of particular interest is the observation of spectrotemporal distortions arising from spontaneous annihilation of topological defects, analogous to those discussed by the Kibble–Zurek cosmological model.

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151. In-plane resistivity anisotropy in an underdoped iron pnictide superconductor
Jiun-Haw Chu, James G. Analytis, Kristiaan De Greve, Peter L. McMahon, Zahirul Islam, Yoshihisa Yamamoto, and Ian R. Fisher,
Science 329, 824 (2010).

ABSTRACT
High-temperature superconductivity often emerges in the proximity of a symmetry-breaking ground state. For superconducting iron arsenides, in addition to the antiferromagnetic ground state, a small structural distortion breaks the crystal’s C4 rotational symmetry in the underdoped part of the phase diagram. We reveal that the representative iron arsenide Ba(Fe_1-xCo_x)₂As₂ develops a large electronic anisotropy at this transition via measurements of the in-plane resistivity of detwinned single crystals, with the resistivity along the shorter b axis ρ_b being greater than ρ_a. The anisotropy reaches a maximum value of ~2 for compositions in the neighborhood of the beginning of the superconducting dome. For temperatures well above the structural transition, uniaxial stress induces a resistivity anisotropy, indicating a substantial nematic susceptibility.

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150. Massive Dirac Fermion on the Surface of a Magnetically Doped Topological Insulator
Y. L. Chen, J.-H. Chu, J. G. Analytis, Z. K. Liu, K. Igarashi, H.-H. Kuo, X. L. Qi, S. K. Mo, R. G. Moore, D. H. Lu, M. Hashimoto, T. Sasagawa, S. C. Zhang, I. R. Fisher, Z. Hussain, and Z. X. Shen,
Science 329, 659 (2010).

ABSTRACT
In addition to a bulk energy gap, topological insulators accommodate a conducting, linearly dispersed Dirac surface state. This state is predicted to become massive if time reversal symmetry is broken, and to become insulating if the Fermi energy is positioned inside both the surface and bulk gaps. We introduced magnetic dopants into the three-dimensional topological insulator dibismuth triselenide (Bi2Se3) to break the time reversal symmetry and further position the Fermi energy inside the gaps by simultaneous magnetic and charge doping. The resulting insulating massive Dirac fermion state, which we observed by angle-resolved photoemission, paves the way for studying a range of topological phenomena relevant to both condensed matter and particle physics.

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149. Charge dynamics of Co-doped BaFe₂As₂
A. Lucarelli, A. Dusza, F. Pfuner, P. Lerch, J. G. Analytis, J.-H. Chu, I. R. Fisher and L. Degiorgi ,
New J. Phys. 12, 073036 (2010).

ABSTRACT
We report on a thorough optical investigation over a broad spectral range and as a function of temperature of the charge dynamics in Ba(Fe_1-xCo_x)₂As₂ compounds for Co-doping ranging between 0 and 18%. For the parent compound as well as for x = 0.025, we observe the opening of a pseudogap, due to the spin-density-wave phase transition and inducing a reshuffling of spectral weight from low to high frequencies. For compounds with 0.051 < x < 0.11, we detect the superconducting gap, while at x = 0.18 the material stays metallic at all temperatures. We describe the effective metallic contribution to the optical conductivity with two Drude terms, representing the combination of a coherent and an incoherent component, and extract the respective scattering rates. We establish that the dc transport properties in the normal phase are dominated by the coherent Drude term for 0 < x < 0.051 and by the incoherent one for 0.061 < x < 0.18, respectively. Finally, through spectral weight arguments, we give clear-cut evidence of moderate electronic correlations for 0 < x < 0.061, which then cross over to values appropriate for a regime of weak interacting and nearly-free electron metals for x > 0.11.

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148. In-plane electronic anisotropy in underdoped Ba(Fe_1-xCo_x)₂As₂ revealed by partial detwinning in a magnetic field
J.-H. Chu, J. Analytis, D. Press, K. De Greve, T. D. Ladd, Y. Yamamoto, I. R. Fisher,
Phys. Rev. B. 81, 214502, (2010).

ABSTRACT
We present results of angle-dependent magnetoresistance measurements and direct optical images of underdoped Ba(Fe_1-xCo_x)₂As₂ which reveal partial detwinning by action of a 14 T magnetic field. The relative change in the twin domain population for the given field is modest, of order 5-15%. The associated fieldinduced hysteretic changes in the resistivity are up to 5% for intermediate Co concentrations, implying a large in-plane resistivity anisotropy in the broken-symmetry state. Based on the generic anisotropic susceptibility of collinear antiferromagnets, we infer a smaller resistivity along the antiferromagnetic ordering direction. The observation of field-induced motion of twin boundaries indicates a substantial magnetoelastic coupling in this material.

Link to full article.

147. Stripes of increased diamagnetic susceptibility in underdoped superconducting Ba(Fe_1-xCo_x)₂As₂ single crystals: Evidence for an enhanced superfluid density at twin boundaries
B. Kalisky, J. R. Kirtley, J. Analytis, J.-H. Chu, A. Vailionis, I. R. Fisher, and K. A. Moler,
Phys. Rev. B. 81, 184513, (2010). (Editor's Suggestion)

ABSTRACT
Superconducting quantum interference device microscopy shows stripes of increased diamagnetic susceptibility in the superconducting state of twinned, orthorhombic, underdoped crystals of Ba(Fe_1-xCo_x)₂As₂, but not in tetragonal overdoped crystals. These stripes are consistent with enhanced superfluid density on twin boundaries.

Link to full article.

146. Temperature dependence of the excitation spectrum in the charge-density-wave ErTe₃ and HoTe₃ systems
F. Pfuner, P. Lerch, J.-H. Chu, H.-H. Kuo, I. R. Fisher, and L. Degiorgi,
Phys. Rev. B. 81, 195110, (2010).

ABSTRACT
We provide optical reflectivity data collected over a broad spectral range and as a function of temperature on the ErTe₃ and HoTe₃ materials, which undergo two consecutive charge-density-wave (CDW) phase transitions at T_CDW1=265 and 288 K and at T_CDW2=157 and 110 K, respectively. We observe the temperature dependence of both the Drude component, due to the itinerant charge carriers, and the single-particle peak, ascribed to the charge-density-wave gap excitation. The CDW gap progressively opens while the metallic component gets narrow with decreasing temperature. An important fraction of the whole Fermi surface seems to be affected by the CDW phase transitions. It turns out that the temperature and the previously investigated pressure dependence of the most relevant CDW parameters share several common features and behaviors. Particularly, the order parameter of the CDW state is in general agreement with the predictions of the BCS theory.

Link to full article.

145. Bulk Fermi surface coexistence with Dirac surface state in Bi₂Se₃: A comparison of photoemission and Shubnikov–de Haas measurements
J. G. Analytis, J.-H. Chu, Y. Chen, F. Corredor, R. D. McDonald, Z. X. Shen and I. R. Fisher,
Phys. Rev. B. 81, 205407, (2010).

ABSTRACT
Shubnikov-de Haas (SdH) oscillations and angle-resolved photoemission spectroscopy (ARPES) are used to probe the Fermi surface of single crystals of Bi₂Se₃. We find that SdH and ARPES probes quantitatively agree on measurements of the effective mass and bulk band dispersion. In high carrier density samples, the two probes also agree in the exact position of the Fermi level E_F, but for lower carrier density samples discrepancies emerge in the position of E_F. In particular, SdH reveals a bulk three-dimensional Fermi surface for samples with carrier densities as low as 10¹⁷ cm^-3. We suggest a simple mechanism to explain these differences and discuss consequences for existing and future transport studies of topological insulators.

Link to full article.

144. Correlation of anomalous normal state properties with superconductivity
in Pb_1-x-yTl_xIn_yTe
A. S. Erickson, N. P. Breznay, E. A. Nowadnick, T. H. Geballe and I. R. Fisher
Phys. Rev. B. 81, 134521, (2010).

ABSTRACT
Recent evidence for a charge-Kondo effect in superconducting samples of Pb_1-xTl_xTe has brought renewed attention to the possibility of negative U superconductivity in this material, associated with valence fluctuations on the Tl impurity sites. Here, we use indium as an electron donor to counterdope Pb_0.99Tl_0.01Te and study the effect of changing the chemical potential on the Kondo-type physics and the superconductivity. We find a clear correlation between these two effects, providing further evidence that both are induced by the same source, as anticipated in the charge-Kondo model.

Link to full article.

143. Anisotropic phase diagram of the frustrated spin dimer compound Ba₃Mn₂O₈
E. C. Samulon, K. A. Al-Hassanieh, Y.-J. Jo, M. C. Shapiro, L. Balicas, C. D. Batista, and I. R. Fisher
Phys. Rev. B. 81, 104421, (2010).

ABSTRACT
Heat-capacity and magnetic torque measurements are used to probe the anisotropic temperature-field phase diagram of the frustrated spin dimer compound Ba₃Mn₂O₈ in the field range from 0 to 18 T. For fields oriented along the c axis, a single magnetically ordered phase is found in this field range whereas for fields oriented along the a axis, two distinct phases are observed. The present measurements reveal a surprising nonmonotonic evolution of the phase diagram as the magnetic field is rotated in the [001]-[100] plane. The angle dependence of the critical field (H_c1) that marks the closing of the spin gap can be quantitatively accounted for using a minimal spin Hamiltonian comprising superexchange between nearest and next-nearest Mn ions, the Zeeman energy, and single-ion anisotropy. This Hamiltonian also predicts a nonmonotonic evolution of the transition between the two ordered states as the field is rotated in the a-c plane. However, the observed effect is found to be significantly larger in magnitude, implying that either this minimal spin Hamiltonian is incomplete or that the magnetically ordered states have a slightly different structure than previously proposed.

Link to full article.

142. Local measurement of the penetration depth in the pnictide superconductor Ba(Fe_0.95Co_0.05)₂As₂
Lan Luan, Ophir M. Auslaender, Thomas M. Lippman, Clifford W. Hicks, Beena Kalisky, Jiun-Haw Chu, James G. Analytis, I. R. Fisher, John R. Kirtley, Kathryn A. Moler ,
Phys. Rev. B. 81, 100501(R) , (2010). (Editor's Suggestion)

ABSTRACT
We use magnetic force microscopy (MFM) to measure the local penetration depth λ in Ba(Fe_0.95Co_0.05)₂As₂ single crystals and use scanning superconducting quantum interference device susceptometry to measure its temperature variation down to 0.4 K. We observe that superfluid density ρ_s over the full temperature range is well described by a clean two-band fully gapped model. We demonstrate that MFM can measure the important and hard-to-determine absolute value of λ, as well as obtain its temperature dependence and spatial homogeneity. We find ρ_s to be uniform on the submicron scale despite the highly disordered vortex pinning.

Link to full article.

141. Raman scattering evidence for a cascade evolution of the charge-density-wave collective amplitude mode ,
M. Lavagnini, H.-M. Eiter, L. Tassini, B. Muschler, R. Hackl, R. Monnier, J.-H. Chu, I. R. Fisher, and L. Degiorgi,
Phys. Rev. B. 81, 081101, (2010).

ABSTRACT
The two-dimensional rare-earth tritellurides undergo a unidirectional charge-density-wave (CDW) transition at high temperature and, for the heaviest members of the series, a bidirectional one at low temperature. Raman scattering experiments as a function of temperature on DyTe₃ and on LaTe₃ at 6 GPa provide a clear-cut evidence for the emergence of the respective collective CDW amplitude excitations. In the unidirectional CDW phase, we discover that the amplitude mode develops as a succession of two mean-field BCS-like transitions with different critical temperatures, which we associate with the presence of two adjacent Te planes in the structure.

Link to full article.

140. Bulk electronic structure of optimally doped Ba(Fe_1-xCo_x)₂As₂,
C. Utfeld, J. Laverock, T. D. Haynes, S. B. Dugdale, J. A. Duffy, M. W. Butchers, J. W. Taylor, S. R. Giblin, J. G. Analytis, J.-H. Chu, I. R. Fisher, M. Itou, and Y. Sakurai,
Phys. Rev. B. 81, 064509 (2010).

ABSTRACT
We report high-resolution, bulk Compton scattering measurements unveiling the Fermi surface of an optimally doped iron-arsenide superconductor, Ba(Fe_0.93Co_0.07)₂As₂. Our measurements are in agreement with first-principles calculations of the electronic structure, revealing both the X-centered electron pockets and the Γ-centered hole pockets. Moreover, our data are consistent with the strong three dimensionality of one of these sheets that has been predicted by electronic structure calculations at the local-density-approximation-minimum As position. Complementary calculations of the noninteracting susceptibility, suggest that the broad peak that develops due to interband Fermi-surface nesting, and which has motivated several theories of superconductivity in this class of material, survives the measured three dimensionality of the Fermi surface in this family.

Link to full article.

139. Microwave surface impedance measurements of Tl_xPb_1-xTe : A proposed negative- U induced superconductor
P. J. Baker, R. J. Ormeno, C. E. Gough, Y. Matsushita, and I. R. Fisher ,
Phys. Rev. B. 81, 064506 , (2010). (Editor's suggestion)

ABSTRACT
The microwave surface impedance of the proposed negative-U induced superconductor Tl(1.4%)PbTe has been measured for a number of single crystals at several frequencies from 4.5 to 15.2 GHz in both the zero-field and mixed superconducting states. For crystals with the best-quality surfaces, R_s and X_s at low temperatures showed no significant power-law T dependence. From the low-temperature values of X_s, we extracted λ(0)=1.9±0.2 μm, in good agreement with rigid-band model predictions. A coherence peak was observed in σ₁ below T_c consistent with BCS predictions. However, the observed temperature dependence of the superfluid fraction [λ(0)/λ(T)]² fitted BCS predictions best if a somewhat larger value of λ(0)=2.7 μm was assumed, which may be associated with remanent surface defects in even our best sample. In addition to investigating the field dependence of the microwave properties in the mixed state, a B_c3 transition region was observed. The absence of a significant power-law temperature dependence at low temperatures in our best sample and the observation of a B_c3 transition are strongly suggestive of conventional s-wave superconductivity.

Link to full article.

138. Dispersive Spin Fluctuations in the near optimally-doped superconductor Ba(Fe_1-xCo_x)₂As₂ (x=0.065),
C. Lester, Jiun-Haw Chu, J. G. Analytis, T. G. Perring, I. R. Fisher, S.M. Hayden ,
Phys. Rev. B. 81, 064505, (2010). (Editor's Suggestion)

ABSTRACT
Inelastic neutron scattering is used to probe the collective spin excitations of the near optimally doped superconductor Ba(Fe_1-xCo_x)₂As₂ (x=0.065). Previous measurements on the antiferromagnetically ordered parents of this material show a strongly anisotropic spin-wave velocity. Here we measure the magnetic excitations up to 80 meV and show that a similar anisotropy persists for superconducting compositions. The dispersive mode measured here connects directly with the spin resonance previously observed in this compound. When placed on an absolute scale, our measurements show that the local- or wave-vector-integrated susceptibility is larger in magnitude than that of the ordered parents over the energy range probed.

Link to full article.

137. Fermi surface evolution across multiple CDW transitions in ErTe₃,
R. G. Moore, V. Brouet, R. He, D. H. Lu, N. Ru, J.-H. Chu, I. R. Fisher, and Z.-X. Shen,
Phys. Rev. B. 81, 073102, (2010).

ABSTRACT
The Fermi surface (FS) of ErTe₃ is investigated using angle-resolved photoemission spectroscopy. Low-temperature measurements reveal two incommensurate charge-density wave (CDW) gaps created by perpendicular FS nesting vectors. A large Δ₁ = 175 meV gap arising from a CDW with c*-q_CDW1 ~ 0.70(0)c* is in good agreement with the expected value. A second, smaller Δ₂ = 50 meV gap is due to a second CDW with a*-q_CDW2 ~ 0.68(5)a*. The temperature dependence of the FS, the two gaps, and possible interaction between the CDWs are examined.

Link to full article.

136. STM Imaging of Electronic Waves on the Surface of Bi₂Te₃: Topologically Protected Surface States and Hexagonal Warping Effects,
Zhanybek Alpichshev, J. G. Analytis, J.-H. Chu, I. R. Fisher, Y. L. Chen, Z. X. Shen, A. Fang, and A. Kapitulnik,
Phys. Rev. Lett. 104, 016401, (2010).

ABSTRACT
Scanning tunneling spectroscopy studies on high-quality Bi₂Te₃ crystals exhibit perfect correspondence to angle-resolved photoemission spectroscopy data, hence enabling identification of different regimes measured in the local density of states (LDOS). Oscillations of LDOS near a step are analyzed. Within the main part of the surface band oscillations are strongly damped, supporting the hypothesis of topological protection. At higher energies, as the surface band becomes concave, oscillations appear, dispersing with a wave vector that may result from a hexagonal warping term.

Link to full article.

135. Observation of two separate charge density wave transitions in Gd₂Te₅ via transmission electron microscopy and high-resolution X-ray diffraction,
K.Y. Shin, N. Ru, I.R. Fisher, C.L. Condron, M.F. Toney, Y.Q. Wu, M.J. Kramer ,
Journal of Alloys and Compounds, 489, (2010) 332-335.

ABSTRACT
Gd₂Te₅ is a layered material consisting of alternating single and double square planar Te sheets. At room temperature the material hosts a complex lattice modulation characterized by multiple in-plane wavevectors. Diffraction measurements performed via transmission electron microscopy and highresolution X-ray scattering reveal two distinct transitions at T_c1 = 410(3) and T_c2 = 532(3) K, associated with an on-axis incommensurate lattice modulation and an off-axis commensurate lattice modulation respectively. Our results show that the two lattice modulations are separate in origin but that there is some coupling between them.

Link to full article.

134. Evidence for a charge Kondo effect in Pb_1-xTl_xTe from measurements of thermoelectric power
M. Matusiak, E. M. Tunnicliffe, J. R. Cooper, Y. Matsushita, and I. R. Fisher,
Phys. Rev. B, 80, 220403 (2009) (R).

ABSTRACT
We report measurements of the thermoelectric power (TEP) for a series of Pb_1-xTl_xTe crystals with x=0.0 to 1.3%. Although the TEP is very large for x=0.0, using a single-band analysis based on older work for dilute magnetic alloys we do find evidence for a Kondo contribution of 11–18 μV/K. This analysis suggests that T_K is ~50-70 K, a factor 10 higher than previously thought.

Link to full article.

133. Band- and momentum-dependent electron dynamics in superconducting Ba(Fe_{1-x}Co_x)2As2 as seen via electronic Raman scattering,
B. Muschler, W. Prestel,, R. Hackl, T. P. Devereaux, J. G. Analytis, Jiun-Haw Chu and I. R. Fisher,
Phys. Rev. B. 80, 180510(R) (2009). (Editor's suggestion)

ABSTRACT
We present details of carrier properties in high quality Ba(Fe_{1-x}Co_x)2As2 single crystals obtained from electronic Raman scattering. The experiments indicate a strong band and momentum anisotropy of the electron dynamics above and below the superconducting transition, highlighting the importance of complex banddependent interactions. The presence of low-energy spectral weight deep in the superconducting state suggests a gap with accidental nodes, which may be lifted by doping and/or impurity scattering. When combined with other measurements, our observation of band- and momentum-dependent carrier dynamics indicate that theiron arsenides may have several competing superconducting ground states.

Link to full article.

132. Unconventional electronic reconstruction in undoped (Ba,Sr)Fe2As2 across the spin density wave transition,
M. Yi, D. H. Lu, J. G. Analytis, J.-H. Chu, S.-K. Mo, R.-H. He, M. Hashimoto, R. G. Moore, I. I. Mazin, D. J. Singh, Z. Hussain, I. R. Fisher, and Z.-X.Shen,
Phys. Rev. B. 80, 174510 (2009). (Editor's suggestion)

ABSTRACT
Through a systematic high-resolution angle-resolved photoemission study of the iron pnictide compounds (Ba,Sr)Fe2As2, we show that the electronic structures of these compounds are significantly reconstructed across the spin density wave transition, which cannot be described by a simple folding scenario of conventional density wave ordering. Moreover, we find that LDA calculations with an incorporated suppressed magnetic moment of 0.5 Bohr magnetons can match well the details in the reconstructed electronic structure, suggesting that the nature of magnetism in the pnictides is more itinerant than local, while the origin of suppressed magnetic moment remains an important issue for future investigations

Link to full article.

131. Evidence for a Nodal Energy Gap in the Iron-Pnictide Superconductor LaFePO from Penetration Depth Measurements by Scanning SQUID Susceptometry,
Clifford W. Hicks, Thomas M. Lippman, Martin E. Huber, James G. Analytis, Jiun-Haw Chu, Ann S. Erickson, Ian R. Fisher, and Kathryn A. Moler,
Phys. Rev. Lett. 103, 127003 (2009).

ABSTRACT
We measure changes in the penetration depth lambda of the Tc ~ 6 K superconductor LaFePO. In the process, scanning SQUID susceptometry is demonstrated as a technique for accurately measuring local temperature-dependent changes in lambda, ideal for studying early or difficult-to-grow materials. Lambda is found to vary linearly with temperatures from 0.36 to 2 K, with a slope of 143 +/- 15 Angstroms/K, suggesting line nodes in the superconducting order parameter. The linear dependence up to ~Tc/3, similar to the cuprate superconductors, indicates well-developed nodes.

Link to full article.

130. Experimental Realization of a Three-Dimensional Topological Insulator, Bi 2Te3,
Y. L. Chen, J. G. Analytis, J.-H. Chu, Z. K. Liu, S.-K. Mo, X. L. Qi, H. J. Zhang, D. H. Lu, X. Dai, Z. Fang, S. C. Zhang, I. R. Fisher, Z. Hussain, Z.-X. ,
Science 325, 178-181 (2009).

ABSTRACT
Three-dimensional topological insulators are a new state of quantum matter with a bulk gap and odd number of relativistic Dirac fermions on the surface. By investigating the surface state of Bi2Te3 with angle-resolved photoemission spectroscopy, we demonstrate that the surface state consists of a single nondegenerate Dirac cone. Furthermore, with appropriate hole doping, the Fermi level can be tuned to intersect only the surface states, indicating a full energy gap for the bulk states. Our results establish that Bi2Te3 is a simple model system for the three-dimensional topological insulator with a single Dirac cone on the surface. The large bulk gap of Bi2Te3 also points to promising potential for high-temperature spintronics applications.

Link to full article.

129. Quantum oscillations in the parent pnictide BaFe2As2: Itinerant electrons in the reconstructed state
James G. Analytis, Ross D. McDonald, Jiun-Haw Chu, Scott C. Riggs, Alimamy F. Bangura, Chris Kucharczyk, Michelle Johannes, and I. R. Fisher,
Phys. Rev. B 80, 064507 (2009).

ABSTRACT
We report quantum-oscillation measurements that enable the direct observation of the Fermi surface of the low-temperature ground state of BaFe2As2. From these measurements we characterize the low-energy excitations, revealing that the Fermi surface is reconstructed in the antiferromagnetic state, but leaving itinerant electrons in its wake. The present measurements are consistent with a conventional band folding picture of the antiferromagnetic ground state, placing important limits on the topology and size of the Fermi surface.

Link to full article.

128. Fermi Surface of SrFe2P2 Determined by the de Haas-van Alphen Effect
J. G. Analytis, C. M. J. Andrew, A. I. Coldea, A. McCollam, J.-H. Chu, R. D. McDonald,I. R. Fisher, and A. Carrington,
Phys. Rev. Lett. 103, 076401, (2009).

ABSTRACT
We report measurements of the Fermi surface (FS) of the ternary iron-phosphide SrFe2P2 using the de Haas-van Alphen effect. The calculated FS of this compound is very similar to SrFe2As2, the parent compound of the high temperature superconductors. Our data show that the Fermi surface is composed of two electron and two hole sheets in agreement with band-structure calculations. Several of the sheets show strong c-axis warping emphasizing the importance of three dimensionality in the nonmagnetic state of the ternary pnictides. We find that the electron and hole pockets have a different topology, implying that this material does not satisfy a (pi,pi) nesting condition.

Link to full article.

127. Evidence for weak electronic correlations in iron pnictides
W. L. Yang, A. P. Sorini, C-C. Chen, B. Moritz, W.-S. Lee, F. Vernay, P. Olalde-Velasco, J. D. Denlinger, B. Delley, J.-H. Chu, J. G. Analytis, I. R. Fisher, Z. A. Ren, J. Yang, W. Lu, Z. X. Zhao, J. van den Brink, Z. Hussain, Z.-X. Shen, and T. P. Devereaux,
Phys. Rev. B 80, 014508, (2009). (Editor's suggestion.)

ABSTRACT
Using x-ray absorption (XAS) and resonant inelastic x-ray scattering (RIXS), charge dynamics at and near the Fe L edges is investigated in Fe-pnictide materials and contrasted to that measured in other Fe compounds. It is shown that the XAS and RIXS spectra for 122 and 1111 Fe pnictides are each qualitatively similar to Fe metal. Cluster diagonalization, multiplet, and density-functional calculations show that Coulomb correlations are much smaller than in the cuprates, highlighting the role of Fe metallicity and strong covalency in these materials. The best agreement with experiment is obtained using Hubbard parameters U~2 eV and J~0.8 eV.

Link to full article.

126. Electronic structure of the BaFe2As2 family of iron-pnictide superconductors
M. Yi, D. H. Lu, J. G. Analytis, J.-H. Chu, S.-K. Mo, R.-H. He, R. G. Moore, X. J. Zhou, G. F. Chen, J. L. Luo, N. L. Wang, Z. Hussain, D. J. Singh, I. R. Fisher, and Z.-X. Shen,
Phys. Rev. B 80, 024515, (July 24,2009).

ABSTRACT
We use high-resolution angle-resolved photoemission to study the electronic structure of the BaFe2As2 pnictides. We observe two electron bands and two hole bands near the X point,(,) of the Brillouin zone, in the paramagnetic state for electron-doped Ba(Co0.06Fe0.94)2As2, undoped BaFe2As2, and hole-doped Ba0.6K0.4Fe2As2. Among these bands, only the electron bands cross the Fermi level, forming two electron pockets around X while the hole bands approach but never reach the Fermi level. We show that the band structure of the BaFe2As2 family matches reasonably well with the prediction of local-density approximation calculations after a momentum-dependent shift and renormalization. Our finding resolves a number of inconsistencies regarding the electronic structure of pnictides.

Link to full article.

125. Asymmetric Quintuplet Condensation in the Frustrated S = 1 Spin Dimer Compound Ba3Mn2O8
E. C. Samulon, Y. Kohama, R. D. McDonald, M. C. Shapiro, K. A. Al-Hassanieh,C. D. Batista, M. Jaime, and I. R. Fisher,
Phys. Rev. Lett. 103, 047702 (2009).

ABSTRACT
Ba3Mn2O8 is a spin-dimer compound based on pairs of S=1, 3d2, Mn5+ ions arranged on a triangular lattice. Antiferromagnetic intradimer exchange leads to a singlet ground state in zero field, with excited triplet and quintuplet states at higher energy. High field thermodynamic measurements are used to establish the phase diagram, revealing a substantial asymmetry of the quintuplet condensate. This striking effect, all but absent for the triplet condensate, is due to a fundamental asymmetry in quantum fluctuations of the paramagnetic phases near the various critical fields.

Link to full article.

124. Optical properties of the charge density wave rare earth tritelluride compounds: a view on PrTe3,
F. Pfuner, L. DeGiorgi, J.-H. Chu, N. Ru, K. Y. Shin and I. R. Fisher,
Physica B. 404, 533-536 (2009).

ABSTRACT
We report on our recent optical measurements of the Pr tri-telluride charge-density-wave system. Our data, collected over an extremely broad spectral range, allow us to observe both the Drude component and the single-particle peak, ascribed to the contributions due to the free charge carriers and to the charge-density-wave gap excitation, respectively. Our findings perfectly fit within the scenario based on a diminishing impact of the charge-density-wave condensate on the electronic properties upon compressing the lattice, as evinced from our previous investigations on these systems both with chemical as well as with applied pressure.

Link to full article.

123. Topological Change of the Fermi Surface in Ternary Iron Pnictides with Reduced c/a Ratio: A de Haas-van Alphen Study of CaFe2P2 ,
Amalia I. Coldea, C. M. J. Andrew, J. G. Analytis, R. D. McDonald, A. F. Bangura, J.-H. Chu, I. R. Fisher, and A. Carrington,
Phys. Rev. Lett. 103, 026404 (2009).

ABSTRACT
We report a de Haas-van Alphen effect study of the Fermi surface of CaFe2P2 using low-temperature torque magnetometry up to 45 T. This system is a close structural analog of the collapsed tetragonal nonmagnetic phase of CaFe2As2.We find the Fermi surface of CaFe2P2 to differ from other related ternary phosphides in that its topology is highly dispersive in the c axis, being three dimensional in character and with identical mass enhancement on both electron and hole pockets. This suggests that when the bonding between pnictogen layers becomes important nesting conditions are not fulfilled.

Link to full article.

122. Quantum oscillation studies of the Fermi surface of LaFePO,
A. Carrington, A. I. Coldea, J. D. Fletcher, N. E. Hussey, C. M. J. Andrew, A. F. Bangura, J. G. Analytis, J.-H. Chu, A. S. Erickson , I. R. Fisher and R. D. McDonald,
Physica C. 469, 459-468 (2009).

ABSTRACT
We review recent experimental measurements of the Fermi surface of the iron-pnictide superconductor LaFePO using quantum oscillation techniques. These studies show that the Fermi surface topology is close to that predicted by first principles density functional theory calculations, consisting of quasi-twodimensional electron-like and hole-like sheets. The total volume of the two hole sheets is almost equal to that of the two electron sheets, and the hole and electron Fermi surface sheets are close to a nesting condition. No evidence for the predicted three-dimensional pocket arising from the Fe dz2 band is found. Measurements of the effective mass suggest a renormalisation of around two, close to the value for the overall band renormalisation found in recent angle resolved photoemission measurements.

Link to full article.

121. ARPES studies of the electronic structure of LaOFe(P,As),
D. H. Lu, M. Yi, S.-K. Mo, J. G. Analytis, J.-H. Chu, A. S. Erickson, D. J. Singh, Z. Hussain, T. H. Geballe, I. R. Fisher, and Z. X Shen,
Physica C. 469, 452-458 (2009).

ABSTRACT
We report a comparison study of LaOFeP and LaOFeAs, two parent compounds of recently discovered iron-pnictide superconductors, using angle-resolved photoemission spectroscopy. Both systems exhibit some common features that are very different from well-studied cuprates. In addition, important differences have also been observed between these two ferrooxypnictides. For LaOFeP, quantitative agreement can be found between our photoemission data and the LDA band structure calculations, suggesting that a weak coupling approach based on an itinerant ground state may be more appropriate for understanding this new superconducting compound. In contrast, the agreement between LDA calculations and experiments in LaOFeAs is relatively poor, as highlighted by the unexpected Fermi surface topology around (pi,pi). Further investigations are required for a comprehensive understanding of the electronic structure of LaOFeAs and related compounds.

Link to full article.

120. Pressure induced superconducting phase in the charge density wave compound TbTe3,
J. J. Hamlin, D. A. Zocco, T. A. Sayles, M. B. Maple, J. -H. Chu and I. R. Fisher,
Phys. Rev. Lett. 102, 177002 (2009). (Editor's Suggestion.)

ABSTRACT
A series of high-pressure electrical resistivity measurements on single crystals of TbTe3 reveal a complex phase diagram involving the interplay of superconducting, antiferromagnetic and charge-densitywave order. The onset of superconductivity reaches a maximum of almost 4 K (onset) near 12.4 GPa.

Link to full article.

119. Evidence for a Nodal-Line Superconducting State in LaFePO,
J. D. Fletcher, A. Serafin, L. Malone, J. G. Analytis, J.-H. Chu, A. S. Erickson, I. R. Fisher, and A. Carrington,
Phys. Rev. Lett. 102, 147001 (2009).

ABSTRACT
In several iron-arsenide superconductors there is strong evidence for a fully gapped superconducting state consistent with either a conventional s-wave symmetry or an unusual x state where the gap changes sign between the electron and hole Fermi-surface sheets. Here we report measurements of the penetration depth in very clean samples of the related iron-phosphide superconductor, LaFePO, at temperatures down to _100 mK. We find that Lamda varies approximately linearly with T strongly suggesting the presence of gap nodes in this compound. Taken together with other data, this suggests the gap function is not universal for all pnictide superconductors.

Link to full article.

118. Charge dynamics of the spin density wave state in BaFe2As2,
F. Pfuner, J. G. Analytis , J.-H. Chu, I. R. Fisher, and L. Degiorgi,
Eur. Phys. J. B 67, 513-517 (2009).

ABSTRACT
We report on a thorough optical investigation of BaFe2As2 over a broad spectral range and as a function of temperature, focusing our attention on its spin-density-wave (SDW) phase transition at TSDW = 135 K. While BaFe2As2 remains metallic at all temperatures, we observe a depletion in the far infrared energy interval of the optical conductivity below TSDW, ascribed to the formation of a pseudogaplike feature in the excitation spectrum. This is accompanied by the narrowing of the Drude term consistent with the dc transport results and suggestive of suppression of scattering channels in the SDW state. About 20% of the spectral weight in the far infrared energy interval is affected by the SDW phase transition.

Link to full article.

117. Pressure-induced quenching of the charge-density-wave state in rare-earth tritellurides observed by x-ray diffraction,
A. Sacchetti, C. L. Condron, S. N. Gvasaliya, F. Pfuner, M. Lavagnini, M. Baldini, M. F. Toney, M. Merlini, M. Hanfland, J. Mesot, J.-H. Chu, I. R. Fisher, P. Postorino, and L. Degiorgi,
Phys. Rev. B. 79, 201101(R) (2009).

ABSTRACT
We report an x-ray diffraction study on the charge-density-wave LaTe3 and CeTe3 compounds as a function of pressure. We extract the lattice constants and the CDW modulation wave vector. We observe that the intensity of the CDW satellite peaks tend to zero with increasing pressure, thus providing direct evidence for a pressure-induced quenching of the CDW phase. Our findings further support the equivalence between chemical and applied pressures in RTe3, put forward by our previous optical investigations, but reveal some subtle differences. We offer a possible explanation for these differences.

Link to full article.

116. Neutron scattering study of the interplay between structure and magnetism in Ba(Fe_{1-x}Co_{x})2As2 ,
C. Lester, Jiun-Haw Chu, J. G. Analytis, S. C. Capelli, A. S. Erickson, C. L. Condron, M. F. Toney, I. R. Fisher and S. M. Hayden,
Phys. Rev. B. 79, 144523 (2009).

ABSTRACT
Single-crystal neutron diffraction is used to investigate the magnetic and structural phase diagrams of the electron-doped superconductor Ba(Fe_{1-x}Co_{x})2As2. Heat-capacity and resistivity measurements have demonstrated that Co doping this system splits the combined antiferromagnetic and structural transition present in BaFe2As2 into two distinct transitions. For x=0.025, we find that the upper transition is between the hightemperature tetragonal and low-temperature orthorhombic structures with T_{to}=99+/-0.5K and the antiferromagnetic transition occurs at T_{AF}=93+/-O.5K We find that doping rapidly suppresses the antiferromagnetism, with antiferromagnetic order disappearing at x0.055. However, there is a region of coexistence of antiferromagnetism and signatures of superconductivity obtained from thermodynamic and transport properties. For all the compositions studied, we find two anomalies in the temperature dependence of the structural Bragg peaks from both neutron scattering and x-ray diffraction at the same temperatures where anomalies in the heat capacity and resistivity have been previously identified. Thus for x=0.025, where we have shown that the lower anomaly occurs at TAF, we infer that there is strong coupling between the antiferromagnetism and the crystal lattice which may persist to larger x.

Link to full article.
Erratum.

115. High pressure transport and thermodynamic properties of CeTe3,
D. A. Zocco, J. J. Hamlin, T. A. Sayles, M. B. Maple, J. -H. Chu and I. R. Fisher ,
Phys. Rev. B. 79, 134428 (2009).

ABSTRACT
We have performed high-pressure, electrical resistivity, and specific heat measurements on CeTe3 single crystals. Two magnetic phases with nonparallel magnetic easy axes were detected in electrical resistivity and specific heat at low temperatures. We also observed the emergence of an additional phase at high pressures and low temperatures and a possible structural phase transition detected at room temperature and at 45 kbar, which can possibly be related with the lowering of the charge-density wave transition temperature known for this compound.

Link to full article.

114. Fine Structure in the Electronic Density of States near the Fermi Energy of Al-Ni-Co Decagonal Quasicrystal from Ultrafast Time-Resolved Optical Reflectivity,
T. Mertelj, A. Olak, J. Dolinek, I. R. Fisher, V. V. Kabanov and D. Mihailovic,
Phys. Rev. Lett. 102, 086405 (2009).

ABSTRACT
We measured the temperature and fluence dependence of the time-resolved photoinduced optical reflectivity in a decagonal Al71.9Ni11.1Co17.0 quasicrystal. We find no evidence for the relaxation of a hot thermalized electron gas as observed in metals. Instead, a quick diffusion of the hot nonthermal carriers 40 nm into the bulk is detected, enhanced by the presence of a broad pseudogap. From the relaxation dynamics we find evidence for the suppression of the electronic density of states (DOS) at the Fermi energy with respect to the electronic DOS at 13 meV away from the Fermi energy which is consistent with recent theoretical calculations.

Link to full article.

113. Pressure dependence of the single particle excitation in the charge-density-wave CeTe3 system,
M. Lavagnini, A. Sacchetti, C. Marini, M. Valentini, R. Sopracase, A. Perucchi, P. Postorino, S. Lupi, J.-H. Chu, I. R. Fisher, and L. Degiorgi,
Phys. Rev. B, 79, 075117 (2009).

ABSTRACT
We present data on the pressure dependence at 300 K of the optical reflectivity of CeTe3, which undergoes a charge-density-wave phase transition well above room temperature. The collected data cover an unprecedented broad spectral range from the infrared up to the ultraviolet, which allows a robust determination of the gap as well as of the fraction of the Fermi surface affected by the formation of the CDW condensate. Upon compressing the lattice there is a progressive closing of the gap, inducing a transfer of spectral weight from the gap feature into the Drude component. At frequencies above the CDW gap we also identify a power-law behavior, consistent with findings along the RTe3 series (i.e., chemical pressure) and suggestive of a Tomonaga-Luttinger liquid scenario at high-energy scales. This set of data is placed in the context of our previous investigations of this class of materials and allows us to revisit important concepts for the physics of CDW state in layeredlike two-dimensional systems.

Link to full article.

112. Enhanced superconducting pairing interaction in indium-doped tin telluride,
A. S. Erickson, J. -H. Chu, M. F. Toney, T. H. Geballe and I. R. Fisher,
Phys. Rev. B, 79, 024520 (2009). (Editor's Suggestion.)

ABSTRACT
The ferroelectric degenerate semiconductor Sn_{1-delta}Te exhibits superconductivity with critical temperatures, Tc, of up to 0.3 K for hole densities of order 10^21 cm^3. When doped on the tin site with greater than xc 1.7(3)% indium atoms, however, superconductivity is observed up to 2 K, though the carrier density does not change significantly. We present specific-heat data showing that a stronger pairing interaction is present for x>xc than for x <xc. By examining the effect of In dopant atoms on both Tc and the temperature of the ferroelectric structural phase transition, T_SPT, we show that phonon modes related to this transition are not responsible for this Tc enhancement, and discuss a plausible candidate based on negative U pairing associated with mixed valency on the indium impurity sites.

Link to full article.

111. Determination of the phase diagram of the electron doped superconductor Ba(Fe_{1-x}Co_{x})2As2,
J.-H. Chu, J. G. Analytis, C. Kucharczyk & I. R.Fisher,
Phys. Rev. B, 79, 014506 (2009).

ABSTRACT
Systematic measurements of the resistivity, heat capacity, susceptibility, and Hall coefficient are presented for single-crystal samples of the electron-doped superconductor Ba(Fe_{1-x}Co_{x})2As2. These data delineate an x-T phase diagram in which the single magnetic/structural phase transition that is observed for undoped BaFe2As2 at 134 K appears to split into two distinct phase transitions, both of which are rapidly suppressed with increasing Co concentration. Superconductivity emerges for Co concentrations above x=0.025 and appears to coexist with the broken-symmetry state for an appreciable range of doping up to x=0.06. The optimal superconducting transition temperature appears to coincide with the Co concentration at which the magnetic/ structural phase transitions are totally suppressed, at least within the resolution provided by the finite-step size between crystals prepared with different doping levels. Superconductivity is observed for a further range of Co concentrations before being completely suppressed for x=0.18 and above. The form of this x-T phase diagram is suggestive of an association between superconductivity and a quantum critical point arising from suppression of the magnetic and/or structural phase transitions.

Link to full article.

110. Single-particle and collective mode couplings associated with 1- and 2-directional electronic ordering in metallic RTe_3 (R = Ho, Dy, Tb)
R. V. Yusupov, T. Mertelj, J.-H. Chu, I. R. Fisher and D. Mihailovic,
Phys. Rev. Lett. 101, 246402 (2008).

ABSTRACT
The coupling of phonons with collective modes and single-particle gap excitations associated with one- (1d) and two-directional (2d) electronically driven charge-density wave (CDW) ordering in metallic RTe3 is investigated as a function of rare-earth ion chemical pressure (R = Tb; Dy; Ho) using femtosecond pump-probe spectroscopy. From the T dependence of the CDW gap delta_CDW and the amplitude mode, we find that while the transition to a 1d-CDWordered state at Tc1 initially proceeds in an exemplary meanfield- like fashion, below Tc1, delta_CDW is depressed and departs from the mean-field behavior. The effect is apparently triggered by resonant mode mixing of the amplitude mode with a totally symmetric phonon at 1.75 THz. At low temperatures, when the state evolves into a 2d-CDWordered state at Tc2 in the DyTe3 and HoTe3, additional much weaker mode mixing is evident but no soft mode is observed.

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109. Fermi Surface of Superconducting LaFePO Determined from Quantum Oscillations
A.I. Coldea, J.D. Fletcher, A. Carrington, J.G. Analytis, A.F. Bangura, J.-H. Chu, A.S. Erickson, I.R. Fisher, N.E. Hussey, R.D. McDonald,
Phys. Rev. Lett. 101, 216402 (2008).

ABSTRACT
We report extensive measurements of quantum oscillations in the normal state of the Fe-based superconductor LaFePO, (Tc~6 K) using low temperature torque magnetometry and transport in high static magnetic fields (45 T). We find that the Fermi surface is in broad agreement with the band-structure calculations with the quasiparticle mass enhanced by a factor ~2. The quasi-two-dimensional Fermi surface consists of nearly nested electron and hole pockets, suggesting proximity to a spin or charge density wave instability.

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108. Evidence for coupling between charge density waves and phonons in two-dimensional rare earth tellurides
M. Lavagnini, M. Baldini, A. Sacchetti, D. Di Castro, B. Delley, R. Monnier, J.-H. Chu, N. Ru, I.R. Fisher, P. Postorino and L. Degiorgi,
Phys. Rev. B 78, 201101(R) (2008).

ABSTRACT
We report on a Raman-scattering investigation of the charge density wave (CDW) quasi-two-dimensional rare-earth tritellurides RTe3 (R=La, Ce, Pr, Nd, Sm, Gd, and Dy) at ambient pressure, and of LaTe3 and CeTe3 under externally applied pressure. The observed phonon peaks can be ascribed to the Raman-active modes for both the undistorted and the distorted lattices in the CDW state by means of a first-principles calculation. The latter also predicts the Kohn anomaly in the phonon dispersion, driving the CDW transition. The integrated intensity of the two most prominent modes scales as a characteristic power of the CDW-gap amplitude upon compressing the lattice, which provides clear evidence for the tight coupling between the CDW condensate and the vibrational modes.

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107. Transient Electronic Structure and Melting of a Charge Density Wave in TbTe3
F. Schmitt, P. S. Kirchmann, U. Bovensiepen, R. G. Moore, L. Rettig, M. Krenz, J.-H. Chu, N. Ru, L. Perfetti, D. H. Lu, M. Wolf, I. R. Fisher and Z.-X. Shen,
Science 321, 1649-1652 (2008).

ABSTRACT
Obtaining insight into microscopic cooperative effects is a fascinating topic in condensed matter research because, through self-coordination and collectivity, they can lead to instabilities with macroscopic impacts like phase transitions. We used femtosecond time- and angle-resolved photoelectron spectroscopy (trARPES) to optically pump and probe TbTe3, an excellent model system with which to study these effects. We drove a transient charge density wave melting, excited collective vibrations in TbTe3, and observed them through their time-, frequency-, and momentum-dependent influence on the electronic structure. We were able to identify the role of the observed collective vibration in the transition and to document the transition in real time. The information that we demonstrate as being accessible with trARPES will greatly enhance the understanding of all materials exhibiting collective phenomena.

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106. Electronic structure of the iron-based superconductor LaOFeP
D. H. Lu, M. Yi, S.-K. Mo, A. S. Erickson, J. Analytis, J.-H. Chu, D. J. Singh, Z. Hussain, T. H. Geballe, I. R. Fisher & Z.-X. Shen,
Nature 455, 81-84 (2008).

ABSTRACT
The recent discovery of superconductivity in the iron oxypnictide family of compounds has generated intense interest. The layered crystal structure with transition-metal ions in planar square-lattice form and the discovery of spin-density-wave order near 130 K seem to hint at a strong similarity with the copper oxide superconductors. An important current issue is the nature of the ground state of the parent compounds. Two distinct classes of theories, distinguished by the underlying band structure, have been put forward: a local-moment antiferromagnetic ground state in the strong-coupling approach, and an itinerant ground state in the weak-coupling approach. The first approach stresses onsite correlations, proximity to a Mott-insulating state and, thus, the resemblance to the high-transition-temperature copper oxides, whereas the second approach emphasizes the itinerant-electron physics and the interplay between the competing ferromagnetic and antiferromagnetic fluctuations. The debate over the two approaches is partly due to the lack of conclusive experimental information on the electronic structures. Here we report angle-resolved photoemission spectroscopy (ARPES) of LaOFeP (superconducting transition temperature, Tc=5.9 K), the first-reported iron-based superconductor. Our results favour the itinerant ground state, albeit with band renormalization. In addition, our data reveal important differences between these and copper-based superconductors.

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105. De Haas-van Alphen oscillations in the charge-density wave compound lanthanum tritelluride (LaTe3)
N. Ru, R. A. Borzi, A. Rost , A. P. Mackenzie , J. Laverock, S.B. Dugdale, and I. R. Fisher,
Phys. Rev. B 78, 045123 (2008).

ABSTRACT
de Haas–van Alphen oscillations were measured in lanthanum tritelluride (LaTe3) to probe the partially gapped Fermi surface (FS) resulting from charge density wave (CDW) formation. Three distinct frequencies were observed, one of which can be correlated with a FS sheet that is unaltered by CDW formation. The other two frequencies arise from FS sheets that have been reconstructed in the CDW state. All three pockets are quasi-two-dimensional, consistent with expectations from band-structure calculations. Analysis of the field dependence of the oscillations is used to provide an estimate of the geometry of the reconstructed FS sections.

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104. Magnetic properties of the charge density wave compounds RTe3, R=Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er & Tm
N. Ru, J.-H. Chu and I. R. Fisher,
Phys. Rev. B 78, 012410 (2008).

ABSTRACT
The antiferromagnetic transition is investigated in the rare-earth (R) tritelluride RTe3 family of charge-density wave (CDW) compounds via specific heat, magnetization, and resistivity measurements. Observation of the opening of a superzone gap in the resistivity of DyTe3 indicates that additional nesting of the reconstructed Fermi surface in the CDW state plays an important role in determining the magnetic structure.

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103. Ordered magnetic phases of the frustrated spin-dimer compound Ba3Mn2O8
E. C. Samulon, Y.-J. Jo, P. Sengupta, C. D. Batista, M. Jaime, L. Balicas and I. R. Fisher,
Phys. Rev. B 77, 214441 (2008).

ABSTRACT
Ba3Mn2O8 is a spin-dimer compound based on pairs of S=1, 3d2, Mn5+ ions arranged on a triangular lattice. Antiferromagnetic intradimer exchange leads to a singlet ground state in zero field. Here we present the first results of thermodynamic measurements for single crystals probing the high-field ordered states of this material associated with closing the spin gap to the excited triplet states. Specific heat, magnetocaloric effect, and torque magnetometry measurements were performed in magnetic fields up to 32 T and temperatures down to 20 mK. For fields above Hc1~8.7 T, these measurements reveal a single magnetic phase for H parallel c but two distinct phases (approximately symmetric about the center of the phase diagram) for H perpendicular to c. Analysis of the simplest possible spin Hamiltonian describing this system yields candidates for these ordered states corresponding to a simple spiral structure for H parallel to c and to two distinct modulated phases for H perpendicular to c. Both single-ion anisotropy and geometric frustration play crucial roles in defining the phase diagram.

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102. Singlet-Triplet Dispersion Reveals Additional Frustration in the Triangular-Lattice Dimer Compound Ba3Mn2O8
M. B. Stone, M. D. Lumsden, S. Chang, E. C. Samulon, C. D. Batista, and I. R. Fisher,
Phys. Rev. Lett 100, 237201 (2008).

ABSTRACT
We present single crystal inelastic neutron scattering measurements of the S=1 dimerized quasi-two-dimensional antiferromagnet Ba3Mn2O8. The singlet-triplet dispersion reveals nearest-neighbor and next-nearest-neighbor ferromagnetic interactions between adjacent bilayers that compete against each other. Although the interbilayer exchange is comparable to the intrabilayer exchange, this additional frustration reduces the effective coupling along the c axis and leads to a quasi-two-dimensional behavior. In addition, the obtained exchange values are able to reproduce the four critical fields in the phase diagram.

Link to full article.
Erratum.

101. Optical properties of the charge-density-wave polychalcogenide compounds R2Te5 (R=Nd, Sm and Gd)
F. Pfuner, L. Degiorgi, K.Y. Shin and I. R. Fisher,
Eur. Phys. J. B 63, 11–16 (2008).

ABSTRACT
We investigate the rare-earth polychalcogenide R2Te5 (R=Nd, Sm and Gd) charge-density wave (CDW) compounds by optical reflectivity measurements. We obtain the optical conductivity through Kramers-Kronig transformation of the reflectivity spectra. From the real part of the optical conductivity we then extract the excitation energy of the CDW gap and estimate the fraction of the Fermi surface which is gapped by the formation of the CDW condensate. In analogy to previous findings on the related RTen (n = 2 and 3) families, we establish the progressive closing of the CDW gap and the moderate enhancement of the metallic component upon chemically compressing the lattice.

Link to full article.

100. Angle-resolved photoemission study of the evolution of band structure and charge density wave properties in RTe3 ( R=Y , La, Ce, Sm, Gd, Tb, and Dy)
V. Brouet, W. L. Yang, X. J. Zhou, Z. Hussain, R. G. Moore, R. He, D. H. Lu, Z. X. Shen, J. Laverock, S. B. Dugdale, N. Ru, and I. R. Fisher,
Phys. Rev. B 77, 235104 (2008).

ABSTRACT
We present a detailed angle-resolved photoemission spectroscopy (ARPES) investigation of the RTe3 family, which sets this system as an ideal "textbook" example for the formation of a nesting driven charge density wave (CDW). This family indeed exhibits the full range of phenomena that can be associated to CDW instabilities, from the opening of large gaps on the best nested parts of Fermi surface (up to 0.4 eV), to the existence of residual metallic pockets. ARPES is the best suited technique to characterize these features, thanks to its unique ability to resolve the electronic structure in k space. An additional advantage of RTe3 is that the band structure can be very accurately described by a simple two dimensional tight-binding (TB) model, which allows one to understand and easily reproduce many characteristics of the CDW. In this paper, we first establish the main features of the electronic structure by comparing our ARPES measurements with the linear muffin-tin orbital band calculations. We use this to define the validity and limits of the TB model. We then present a complete description of the CDW properties and of their strong evolution as a function of R. Using simple models, we are able to reproduce perfectly the evolution of gaps in k space, the evolution of the CDW wave vector with R, and the shape of the residual metallic pockets. Finally, we give an estimation of the CDW interaction parameters and find that the change in the electronic density of states n(EF), due to lattice expansion when different R ions are inserted, has the correct order of magnitude to explain the evolution of the CDW properties.

Link to full article.

99. Dispersive magnetic excitations in the S = 1 antiferromagnet Ba3Mn2O8
M. B. Stone, M. D. Lumsden, Y. Qiu, E. C. Samulon, C. D. Batista and I. R. Fisher,
Phys. Rev. B 77, 134406 (2008).

ABSTRACT
We present powder inelastic neutron scattering measurements of the S=1 dimerized antiferromagnet Ba3Mn2O8. The T = 1.4 K magnetic spectrum exhibits a spin gap of 1.0 meV and a dispersive spectrum with a bandwidth of approximately 1.5 meV. A comparison to coupled dimer models accurately describes the dispersion and scattering intensity and determines the exchange constants in Ba3Mn2O8. The wave vector dependent scattering intensity confirms the proposed S=1 dimer bond. Temperature dependent measurements of the magnetic excitations indicate the presence of both singlet-triplet and thermally activated triplet-quintet excitations.

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98. Pressure dependence of the optical properties of the charge-density-wave compound LaTe2
M. Lavagnini, A. Sacchetti, L. Degiorgi, E. Arcangeletti, L. Baldassarre, P. Postorino, S. Lupi, A. Perucchi, K. Y. Shin and I. R. Fisher,
Phys. Rev. B 77, 165132 (2008).

ABSTRACT
We report the pressure dependence of the optical response of LaTe2, which is deep in the charge-density-wave (CDW) ground state even at 300 K. The reflectivity spectrum is collected in the midinfrared spectral range at room temperature and at pressures between 0 and 7 GPa. We extract the energy scale due to the single-particle excitation across the CDW gap and the Drude weight. We establish that the gap decreases upon compressing the lattice while the Drude weight increases. This signals a reduction in the quality of nesting upon applying pressure, therefore inducing a lesser impact of the CDW condensate on the electronic properties of LaTe2. The consequent suppression of the CDW gap leads to a release of additional charge carriers, manifested by the shift of weight from the gap feature into the metallic component of the optical response. On the contrary, the power-law behavior, seen in the optical conductivity at energies above the gap excitation and indicating a weakly interacting limit within the Tomonaga-Luttinger liquid scenario, seems to be only moderately dependent on pressure.

Link to full article.

97. Charge density wave formation in R2Te5 (R = Nd, Sm, Gd)
K. Y. Shin, J. Laverock, Y. Q. Wu, C. L. Condron, M.F. Toney, S. B. Dugdale, M. J. Kramer and I. R. Fisher,
Phys. Rev. B 77, 165101 (2008). (Editor's Suggestion.)

ABSTRACT
The rare earth (R) tellurides R2Te5 have a crystal structure intermediate between that of RTe2 and RTe3, consisting of alternating single and double Te planes sandwiched between RTe block layers. We have successfully grown single crystals of Nd2Te5, Sm2Te5, and Gd2Te5 from a self-flux and we describe here evidence for charge density wave formation in these materials. The superlattice patterns for all three compounds are relatively complex, consisting at room temperature of at least two independent wave vectors. Consideration of the electronic structure indicates that, to a large extent, these wave vectors are separately associated with sheets of the Fermi surface which are principally derived from the single and double Te layers.

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96. Effect of chemical pressure on the charge density wave transition in rare-earth tritellurides RTe3
N. Ru, C. L. Condron, G. Y. Margulis, K. Y. Shin, J. Laverock, S. B. Dugdale, M. F. Toney, and I. R. Fisher,
Phys. Rev. B 77, 035114 (2008).

ABSTRACT
The charge density wave transition is investigated in the bilayer family of rare-earth tritelluride RTe3 compounds (R=Sm, Gd, Tb, Dy, Ho, Er, and Tm) via high-resolution x-ray diffraction and electrical resistivity. The transition temperature increases monotonically with increasing lattice parameter from 244(3) K for TmTe3 to 416(3) K for SmTe3. The heaviest members of the series, R=Dy, Ho, Er, and Tm, are observed to have a second transition at a lower temperature, which marks the onset of an additional charge density wave with wave vector almost equal in magnitude to the first, but oriented in the perpendicular direction.

Link to full article.
Erratum.

95. Reply to Comment on "Extrinsic origin of the insulating behavior of polygrain icosahedral Al-Pd-Re quasicrystals"
J. Dolinsek, P. J. McGuiness, M. Klanjsek, I. Smiljanic, A. Smontara, E. S. Zijlstra, S. K. Bose, I. R. Fisher, M. J. Kramer and P. C. Canfield,
Phys. Rev. B 76, 216202 (2007).

ABSTRACT
We clarify issues raised in the preceding Comment regarding the viewpoint that the highly porous and oxidized arc-melted polygrain i-Al-Pd-Re samples could reveal intrinsic electrical transport properties of this icosahedral family. Flux-grown single-grain i-Al-Pd-Re samples of superior structural quality do not show insulatinglike behavior, and their transport properties are on common ground with all other Al-based icosahedral families.

Link to full article.

94. Nuclear magnetic resonance evidence for a strong modulation of the Bose-Einstein condensate in BaCuSi2O6
S. Krämer R. Stern, M. Horvatic, C. Berthier, T. Kimura, and I. R. Fisher
Phys. Rev. B 76, 100406(R) (2007).

ABSTRACT
We present a 63,65Cu and 29Si NMR study of the quasi-2D coupled spin 1/2 dimer compound BaCuSi2O6 in the magnetic field range 13–26 T and at temperatures as low as 50 mK. NMR data in the gapped phase reveal that below 90 K different intradimer exchange couplings and different gaps (B/A=1.16) exist in every second plane along the c axis, in addition to a planar incommensurate (IC) modulation. 29Si spectra in the field induced magnetic ordered phase reveal that close to the quantum critical point at Hc1=23.35 T the average boson density of the Bose-Einstein condensate is strongly modulated along the c axis with a density ratio for every second plane A/B5. An IC modulation of the local density is also present in each plane.

Link to full article.

93. STM Studies of TbTe3: Evidence for a Fully Incommensurate Charge Density Wave
A. Fang, N. Ru, I. R. Fisher and A. Kapitulnik
Phys. Rev. Lett 99, 046401 (2007).

ABSTRACT
We observe unidirectional charge density wave (CDW) ordering on the quasi-2D material TbTe3 with a scanning tunneling microscope at ~6 K. Our analysis indicates that the CDW is fully incommensurate, with wave vector q_CDW ~ 0.71×2pi/c. By imaging at various tip-sample voltages, we highlight effects of the subsurface layer and its effect on the CDW. We also observe an additional (possibly surface) dimerization and 0.68×2pi/a ordering perpendicular to the CDW.

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92. Ferromagnetism in the Mott Insulator Ba2NaOsO6
A. S. Erickson, S. Misra, G. J. Miller, R. R. Gupta, Z. Schlesinger, W. A. Harrison, J. M. Kim, and I. R. Fisher
Phys. Rev. Lett 99, 016404 (2007).

ABSTRACT
Results are presented of single crystal structural, thermodynamic, and reflectivity measurements of the double-perovskite Ba2NaOsO6. These characterize the material as a 5d1 ferromagnetic Mott insulator with an ordered moment of ~0.2µ_B per formula unit and T_C=6.8(3) K. The magnetic entropy associated with this phase transition is close to Rln2, indicating that the quartet ground state anticipated from consideration of the crystal structure is split, consistent with a scenario in which the ferromagnetism is associated with orbital ordering.

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91. Geometric frustration and dimensional reduction at a quantum critical point
C. D. Batista, J. Schmalian, N. Kawashima, P. Sengupta, S. E. Sebastian, N. Harrison, M. Jaime and I. R. Fisher
Phys. Rev. Lett 98, 257201 (2007).

ABSTRACT
We show that the spatial dimensionality of the quantum critical point associated with Bose-Einstein condensation at T=0 is reduced when the underlying lattice comprises layers coupled by a frustrating interaction. Our theoretical predictions for the critical behavior correspond very well with recent measurements in BaCuSi2O6 [ S. E. Sebastian et al., Nature (London) 441, 617 (2006)].

Link to full article.

90. Optical properties of the Ce and La ditelluride charge density wave compounds
M. Lavagnini, A. Sacchetti, L. DeGiorgi, K. Y. Shin and I. R. Fisher
Phys. Rev. B 75, 205133 (2007).

ABSTRACT
The La and Ce ditellurides LaTe2 and CeTe2 are deep in the charge-density-wave (CDW) ground state even at 300 K. We have collected their electrodynamic response over a broad spectral range from the far infrared up to the ultraviolet. We establish the energy scale of the single particle excitation across the CDW gap. Moreover, we find that the CDW collective state gaps a large portion of the Fermi surface. Based on the observation of a power-law behavior in the optical conductivity, similar to that found for the related rare-earth tritellurides but over a reduced frequency range, we suggest that interactions and umklapp processes may play a role in the onset of the CDW broken-symmetry ground state.

Link to full article.

89. Pressure Dependence of the Charge-Density-Wave Gap in Rare-Earth Tritellurides
A. Sacchetti, E. Arcangeletti, A. Perucchi, L. Baldassarre, P. Postorino, S. Lupi, N. Ru, I. R. Fisher and L. Degiorgi
Phys. Rev. Lett. 98, 026401 (2007).

ABSTRACT
We investigate the pressure dependence of the optical properties of CeTe3, which exhibits an incommensurate charge-density-wave (CDW) state already at 300 K. Our data are collected in the midinfrared spectral range at room temperature and at pressures between 0 and 9 GPa. The energy for the single particle excitation across the CDW gap decreases upon increasing the applied pressure, similarly to the chemical pressure by rare-earth substitution. The broadening of the bands upon lattice compression removes the perfect nesting condition of the Fermi surface and therefore diminishes the impact of the CDW transition on the electronic properties of RTe3.

Link to full article.

88. Multiple Magnon Modes and Consequences for the Bose-Einstein Condensed Phase in BaCuSi2O6
Ch. Rüegg, D. F. McMorrow, B. Normand, H. M. Ronnow, S. E. Sebastian, I. R. Fisher, C. D. Batista, S. Gvasaliya, Ch. Niedermayer, J. Stahn
Phys. Rev. Lett. 98, 017202 (2007).

ABSTRACT
The compound BaCuSi2O6 is a quantum magnet with antiferromagnetic dimers of S=1/2 moments on a quasi-2D square lattice. We have investigated its spin dynamics by inelastic neutron scattering experiments on single crystals with an energy resolution considerably higher than in an earlier study. We observe multiple magnon modes, indicating clearly the presence of magnetically inequivalent dimer sites. The more complex spin Hamiltonian revealed in our study leads to a distinct form of magnon Bose-Einstein condensate phase with a spatially modulated condensate amplitude.

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87. Bose-Einstein condensation in BaCuSi2O6
N. Harrison, S. E. Sebastian, C. D. Batista, M. Jaime, L. Balicas, P. A. Sharma, N. Kawashima and I. R. Fisher
J. Phys.: Conf. Ser. 51, 9-14 (2006).

ABSTRACT
BaCuSi2O6 is a model spin dimer system in which a BEC QCP is realised. Universal BEC power law scaling is experimentally observed, with 3d critical behaviour above 0.5 K, but a crossover to 2d BEC critical scaling down to 30 mK. Here we briefly review and expand on the results presented in the recent Nature paper.

Link to full article.

86. Role of anisotropy in the spin-dimer compound BaCuSi2O6
S. E. Sebastian, P. Tanedo, P. A. Goddard, S.-C. Lee, A. Wilson, S. Kim, S. Cox, R. D. McDonald, S. Hill, N. Harrison, C. D. Batista and I. R. Fisher
Phys. Rev. B. 74, 180401(R) (2006).

ABSTRACT
We present results of magnetization and electron paramagnetic resonance experiments on the spin dimer system BaCuSi2O6. Evidence indicates that the origin of anisotropic terms in the spin Hamiltonian lies in magnetic dipolar interactions. Axial symmetry breaking is on a very small energy scale of < 11 mK, confirming Bose Einstein condensation critical scaling over an extended temperature range in the vicinity of the quantum critical point.

Link to full article.

85. Type II superconducting parameters of Tl-doped PbTe determined from heat capacity and electronic transport measurements
Y. Matsushita, P. Wianicki, A. T. Sommer, T. H. Geballe and I. R. Fisher
Phys. Rev. B. 74, 134512 (2006).

ABSTRACT
Tl-doped PbTe (Pb_{1-x}Tl_{x}Te) is an anomalous superconductor with a remarkably high maximum T_c value given its relatively low carrier concentration. Here, we present results of systematic measurements of superconducting parameters for this material, for Tl concentrations up to x=1.4%. We find that it is a Type II, weak-coupled BCS superconductor in the dirty limit and discuss implications for the applicability of the charge Kondo model recently proposed to account for superconductivity in this system.

Link to full article.

84. Extrinsic origin of the insulating behavior of polygrain icosahedral Al-Pd-Re quasicrystals
J. Dolinsek, P. J. McGuiness, M. Klanjsek, I. Smiljanic, A. Smontara, E. S. Zijlstra, S. K. Bose, I. R. Fisher, M. J. Kramer and P. C. Canfield
Phys. Rev. B. 74, 134201 (2006).

ABSTRACT
Polygrain icosahedral i-Al-Pd-Re quasicrystals are known to exhibit dramatically different electronic transport properties to other Al-based quasicrystals. By performing comparative experimental and theoretical studies of the electronic transport and electronic structure of polygrain and monocrystalline i-Al-Pd-Re samples, we show that the extraordinarily high electrical resistivity and the metal-to-insulator transition in the polygrain material are not intrinsic properties of the quasiperiodic lattice, but are of extrinsic origin due to the high porosity and the oxygen-rich weakly insulating regions in the material. We also compare theoretical electronic structures and experimental electrical resistivities of monocrystalline i-Al-Pd-Re and i-Al-Pd-Mn quasicrystals and show that there are no significant differences between these two isomorphous compounds, suggesting that i-Al-Pd-Re is on common ground with other Al-based quasicrystals. We present a structural model of i-Al-Pd-Re.

Link to full article.

83. Chemical pressure and hidden one-dimensional behavior in rare earth tri-telluride charge density wave compounds
A. Sacchetti, L. Degiorgi, T. Giamarchi, N. Ru and I. R. Fisher,
Phys. Rev. B. 74, 125115 (2006).

ABSTRACT
We report on the first optical measurements of the rare-earth tri-telluride charge-density wave systems. Our data, collected over an extremely broad spectral range, allow us to observe both the Drude component and the single-particle peak, ascribed to the contributions due to the free charge carriers and to the charge-density wave gap excitation, respectively. The data analysis displays a diminishing impact of the charge-density wave condensate on the electronic properties with decreasing lattice constant across the rare-earth series. We propose a possible mechanism describing this behavior and we suggest the presence of a one-dimensional character in these two-dimensional compounds. We also envisage that interactions and umklapp processes might play a relevant role in the formation of the charge-density wave state in these compounds.

Link to full article.

82. Comment on "Bose-Einstein condensation of magnons in Cs2CuCl4"
S. E. Sebastian, V. S. Zapf, N. Harrison, C. D. Batista, P. A. Sharma, M. Jaime, I. R. Fisher and A. Lacerda
Phys. Rev. Lett. 96, 189703 (2006).

Link to full article.

81. Use of periodic approximants in a dynamical LEED study of the quasicrystalline tenfold surface of decagonal Al-Ni-Co
K. Pussi, N. Ferralis, M. Mihalkovic, M. Widom, S. Curtarolo, M. Gierer, C. J. Jenks, P. C. Canfield, I. R. Fisher and R. D. Diehl
Phys. Rev. B. 73, 184203 (2006).

ABSTRACT
The determination of quasicrystal (QC) surface structures is a challenge to current surface structure techniques. Low-energy electron diffraction (LEED) is the primary technique for the determination of periodic surface structures, but application of dynamical LEED to quasicrystals requires the use of many approximations. In this study, two different approaches were used to apply dynamical LEED to the structure of the tenfold surface of decagonal Al73Ni10Co17. One method (method 1) involves the use of a quasicrystalline model along with approximations that average over the composition and local geometries. The other method (method 2) uses periodic models that approximate the actual local QC structure (approximants) in more exact, atomistic calculations. Although the results using the two methods were consistent, the results of the approximant analysis (method 2) suggested a different way to apply the approximations in method 1, resulting in a better fit between experimental and calculated beams. Thus, periodic approximant structure models can provide a simpler and more efficient method for the determination of local geometries in QC surfaces, and may also facilitate analyses using quasicrystal models.

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80. Dimensional reduction at a quantum critical point
S. E. Sebastian, N. Harrison, C. D. Batista, L. Balicas, M. Jaime, P. A. Sharma, N. Kawashima and I. R. Fisher
Nature vol. 441, number 7093, pp 617-620 (June 1st 2006).

ABSTRACT
Competition between electronic ground states near a quantum critical point (QCP) — the location of a zero-temperature phase transition driven solely by quantum-mechanical fluctuations — is expected to lead to unconventional behaviour in low-dimensional systems. New electronic phases of matter have been predicted to occur in the vicinity of a QCP by two-dimensional theories, and explanations based on these ideas have been proposed for signifi- cant unsolved problems in condensed-matter physics, such as non-Fermi-liquid behaviour and high-temperature superconductivity. But the real materials to which these ideas have been applied are usually rendered three-dimensional by a finite electronic coupling between their component layers; a two-dimensional QCP has not been experimentally observed in any bulk threedimensional system, and mechanisms for dimensional reduction have remained the subject of theoretical conjecture. Here we show evidence that the Bose–Einstein condensate of spin triplets in the three-dimensional Mott insulator BaCuSi2O6 provides an experimentally verifiable example of dimensional reduction at a QCP. The interplay of correlations on a geometrically frustrated lattice causes the individual two-dimensional layers of spin-1/2 Cu2+ pairs (spin dimers) to become decoupled at the QCP, giving rise to a two-dimensional QCP characterized by linear power law scaling distinctly different from that of its three-dimensional counterpart. Thus the very notion of dimensionality can be said to acquire an ‘emergent’ nature: although the individual particles move on a three-dimensional lattice, their collective behaviour occurs in lower-dimensional space.

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79. Low temperature structural phase transition and incommensurate lattice modulation in the spin gap compound BaCuSi2O6
E. C. Samulon, Z. Islam, S. E. Sebastian, P. B. Brooks, M. K. McCourt Jr., J. Ilavsky and I. R. Fisher
Phys. Rev. B. 73, 100407(R) (2006).

ABSTRACT
Results of high resolution x-ray diffraction experiments are presented for single crystals of the spin gap compound BaCuSi2O6 in the temperature range from 16 to 300 K. The data show clear evidence of a transition from the room temperature tetragonal phase into an incommensurately modulated orthorhombic structure below ~100 K. This lattice modulation is characterized by a resolution-limited reduced wavevector q=(0,0.129+/-0.001,0) (reciprocal lattice units) referred to the orthorhombic lattice, and its 2nd and 3rd harmonics. The phase transition is first order and exhibits considerable hysteresis. This observation implies that the spin Hamiltonian representing the system is more complex than originally thought.

Link to full article.

78. Scanning Hall probe imaging of ErNi2B2C
H. Bluhm, S. E. Sebastian, J. W. Guikema, I. R. Fisher and K. A. Moler
Phys. Rev. B. 73, 014514 (2006).

ABSTRACT
We report scanning Hall probe imaging of ErNi2B2C in the superconducting, antiferromagnetic and weakly ferromagnetic regimes in magnetic fields up to 20 Oe, well below Hc1, with two results. First, imaging isolated vortices shows that they spontaneously rearrange on cooling through the antiferromagnetic transition temperature TN = 6 K to pin on twin boundaries, forming a striped pattern. Second, a weak, random magnetic signal appears in the ferromagnetic phase below TWFM = 2.3 K, and no spontaneous vortex lattice is present down to 1.9 K. We conclude that ferromagnetism coexists with superconductivity either by forming small ferromagnetic domains or with oscillatory variation of the magnetization on sub-penetration depth length scales.

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77. Thermodynamic and transport properties of YTe3, LaTe3 and CeTe3
N. Ru and I. R. Fisher
Phys. Rev. B. 73, 033101 (2006).

ABSTRACT
Measurements of heat capacity, susceptibility and electrical resistivity are presented for single crystals of the charge density wave compounds YTe3, LaTe3 and CeTe3. The materials are metallic to low temperatures, but have a small density of states due to the charge density wave gapping large portions of the Fermi surface. CeTe3 is found to be a weak Kondo lattice, with an antiferromagnetic groundstate and T_N = 2.8 K. The electrical resistivity of all three compounds is highly anisotropic, confirming the weak dispersion perpendicular to Te planes predicted by band structure calculations.

Link to full article.

76. Characteristic BEC scaling close to Quantum Critical Point in BaCuSi2O6
S. E. Sebastian, P. A. Sharma, M. Jaime, N. Harrison, V. Correa, L. Balicas, N. Kawashima, C. D. Batista and I. R. Fisher
Phys. Rev. B. 72, 100404(R) (2005).

ABSTRACT
We report an experimental determination of the phase boundary approaching the quantum critical point separating a quantum paramagnetic state and the proposed spin Bose-Einstein condensate of triplons in the spin dimer compound BaCuSi2O6. The ordering temperature is related to the proximity to a quantum critical point at the lower critical magnetic field Hc1 = 23.52 +/- 0.03 T by a power law parameterized by critical exponent nu. We obtain an experimental estimate of nu = 0.63 +/- 0.03 down to a temperature of 0.61 K, which is in good agreement with the mean field prediction of nu = 2/3 for the 3D Bose-Einstein condensation universality class.

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75. Evidence for charge Kondo effect in superconducting Tl-doped PbTe
I. R. Fisher, Y. Matsushita, H. Bluhm and T. H. Geballe
Proc. SPIE Vol. 5932, 59321Y (Aug. 30, 2005).

ABSTRACT
We report results of low temperature thermodynamic and transport measurements of Pb(1-x)Tl(x)Te single crystals for Tl concentrations up to the solubility limit of approximately 1.5 %. The material superconducts for x > 0.3 %, with a maximum Tc of 1.5 K for the highest Tl concentrations. All superconducting samples exhibit an anomalous resistivity upturn at low temperatures, whereas non-superconducting samples (x < 0.3%) do not. The temperature and field dependence of this resistivity upturn are consistent with a charge Kondo effect involving degenerate Tl valence states differing by two electrons, with a characteristic Kondo temperature TK ~ 6 K. The observation of such an effect supports an electronic pairing mechanism for superconductivity in this material and may account for the anomalously high Tc values.

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74. Electronic structure and charge density wave formation in LaTe1.95 and CeTe2.00
K. Y. Shin, V. Brouet, N. Ru, Z. X. Shen and I. R. Fisher
Phys. Rev. B. 72, 085132 (2005).

ABSTRACT
Results are presented of complementary measurements that probe the electronic structure and charge density wave (CDW) modulation in the quasi-2D compounds LaTe1.95 and CeTe2. Transmission electron micrographs show that the modulation wave vectors associated with the CDW are different for the two materials, and in both cases are incommensurate with the underlying lattice. These wavevectors are shown to correspond to nesting features of a simplified model of the Fermi surface. Angle Resolved PhotoEmission Spectroscopy is used to reveal the electronic structure and Fermi surface topology in the CDW state. The data indicate a large CDW gap that varies in magnitude around the Fermi surface rather differently for the two compounds. Differences in the volume of the original Fermi surface are related to the doping effect of Te vacancies. Heat capacity measurements at low temperatures indicate a very small electronic density of states, consistent with the electrical resistivity, which appears to be semiconducting or semimetallic.

Link to full article.

73. Magnetic structure of GdCo2Ge2
W. Good, J. Kim, A.I. Goldman, D. Wermeille, P. C. Canfield, Z. Islam, J. C. Lang, G. Srajer and I. R. Fisher,
Phys. Rev. B. 71, 224427 (2005).

ABSTRACT
Resonant and nonresonant magnetic x-ray scattering studies of GdCo2Ge2 were performed to determine its magnetic structure at low temperature. This compound orders in an incommensurate antiferromagnetic (AF) structure characterized by a propogation wave vector t=(0,0,tz). The value of tz is temperature dependent and approaches 0.930 reciprocal lattice units well below TN = 33.25 K. A peak corresponding to 3tz was also observed, indicating either a squaring up of the magnetic structure or the presence of a noncollinear amplitude modulated structure below TN. Fitting the angular dependence of the magnetic scattering integrated intensities to the relevant resonant and nonresonant scattering cross sections revealed that the moment direction lies primarily in the tetragonal basal plane. Scattering measurements at the Co K-edge failed to detect any resonant signal, consistent with the absence of a magnetic moment on the Co sites.

Link to full article.

72. High-field behavior of the spin-gap compound Sr2Cu(BO3)2
S.E. Sebastian, D. Yin, P. Tanedo, N. Harrison, G. Jorge, M. Jaime, Y. Moyzerov, G. Miller and I. R. Fisher,
Phys. Rev. B. 71, 212405 (2005).

ABSTRACT
We report magnetization and heat capacity measurements of single crystal samples of the spin gap compound Sr2Cu(BO3)2. Low-field data show that the material has a singlet ground state comprising dimers with intradimer coupling J = 100 K. High field data reveal the role of weak interdimer coupling. For fields that are large compared to the spin gap, triplet excitations are observed for significantly smaller fields than predicted for isolated dimers, indicating that weak inter-dimer coupling leads to triplet delocalization. High field magnetization behavior at low temperatures suggests additional cooperative effects.

Link to full article.

71. Evidence for charge Kondo effect in superconducting Tl-doped PbTe
Y. Matsushita, H. Bluhm, T. H. Geballe and I. R. Fisher,
Phys. Rev. Lett., 94, 157002 (2005).

ABSTRACT
We report results of low-temperature thermodynamic and transport measurements of Pb_{1-x}Tl_{x}Te single crystals for Tl concentrations up to the solubility limit of approximately x = 1.5%. For all doped samples, we observe a low-temperature resistivity upturn that scales in magnitude with the Tl concentration. The temperature and field dependence of this upturn are consistent with a charge Kondo effect involving degenerate Tl valence states differing by two electrons, with a characteristic Kondo temperature T_K ~ 6 K. The observation of such an effect supports an electronic pairing mechanism for superconductivity in this material and may account for the anomalously high Tc values.

Link to full article.

70. Fermi surface nesting and charge-density wave formation in rare-earth tritellurides
J. Laverock, S. B. Dugdale, Zs. Major, M. A. Alam, N. Ru, I. R. Fisher, G. Santi, and E. Bruno,
Phys. Rev. B. 71, 085114 (2005).

ABSTRACT
The Fermi surface of rare earth tritellurides (RTe3) is investigated in terms of the nesting-driven charge- density wave formation using positron annihilation and first-principles linear muffin-tin orbital calculations. Fermi surface nesting is revealed as a strong candidate for driving charge density wave formation in these compounds. The nesting vector obtained from positron annihilation experiments on GdTe3 is determined to be q = (0.28+/-0.02,0,0)a* (a* = 2pi/a), in excellent agreement with previous experimental and theoretical studies.

Link to full article.

69. Magneto-optic spectrum and electronic structure of single-crystal MnBi
D. P. Brammeier, J.-M. Park, C. G. Olsen, I. R. Fisher and D. W. Lynch,
J. Magnetism and Magnetic Materials, 283, 95-102 (2004).
ABSTRACT
Single crystals of MnBi were used for magneto-optic Kerr-angle spectroscopy and photoelectron spectroscopy. The Kerr-angle peak at 3.35 eV was observed for our nearly oxygen-free bulk samples, but the role of the oxide overlayer, demonstrated by Auger spectroscopy, as a possible origin of this peak cannot be fully assessed. The energy bands from photoelectron spectroscopy were in general agreement with calculated band structures, but were not extensive enough, due to sample cleavage properties, to distinguish reliably between band-structure calculations that disagree.

68. Fermi surface reconstruction in the CDW state of CeTe3 observed by photoemission
V. Brouet, W. L. Yang, X. J. Zhou, Z. Hussain, N. Ru, K. Y. Shin, I. R. Fisher and Z. X. Shen,
Phys. Rev. Lett, 93, 126405 (2004).

ABSTRACT
CeTe3 is a layered compound where an incommensurate charge density wave (CDW) opens a large gap (~400 meV) in optimally nested regions of the Fermi surface (FS), whereas other sections with poorer nesting remain ungapped. Through angle-resolved photoemission, we identify bands back-folded according to the CDW periodicity. They define FS pockets formed by the intersection of the original FS and its CDW replica. Such pockets illustrate very directly the role of nesting in the CDW formation but they could not be detected so far in a CDW system. We address the reasons for the weak intensity of the folded bands, by comparing different foldings coexisting in CeTe3.

Link to full article.

67. An STM and SXPS study of the interaction of C60 with the ten-fold surface of the Al72Ni11Co17 quasicrystal
E.J. Cox, J. Ledieu, V.R. Dhanak, S.D. Barrett, C.J. Jenks, I. R. Fisher, and R. McGrath,
Surface Science, 566-568, 1200-1205 (2004).

66. Structure of the ten-fold d-AlNiCo quasicrystal surface
N. Ferralis, K. Pussi, E.J. Cox, M. Gierer, J. Ledieu, I. R. Fisher, C. J. Jenks, M. Lindroos, R. McGrath, and R.D. Diehl,
Phys. Rev. B, 69, 153404 (2004).

65. Magnetic properties of single grain R-Mg-Cd primitive icosahedral quasicrystals (R = Y, Gd, Tb, Dy)
S. E. Sebastian, T. Huie, I. R. Fisher, K. W. Dennis and M. J. Kramer
Phil. Mag. B 84, 1029-1037 (2004).

ABSTRACT
The growth and characterization of single grains of the primitive icosahedral quasicrystal R-Mg-Cd (R = Y, Gd, Tb or Dy) are described. From the thermodynamic properties, it is confirmed that the system is a spin glass exhibiting the characteristic spin-freezing transition of such systems. There is no evidence for two distinct freezing transitions previously observed for polygrain samples. The bulk magnetic properties and the effect of crystalline electric fields on the spin-freezing temperature are found to be very similar to those of the face-centred icosahedral quasicrystal R-Mg-Zn.

64. Low energy electron diffraction study of Xe adsorption on the ten-fold decagonal Al-Ni-Co quasicrystal surface
N. Ferralis, R.D. Diehl, K. Pussi, M. Lindroos, I. Fisher, and C.J. Jenks,
Phys. Rev. B. 69, 075410 (2004).

63. Phonon dispersion curve of Mg-Zn-Y quasicrystals
R. A. Brand, J. Voss, F. Hippert, M. Krisch, R. Sterzel, W. Assmus and I. R. Fisher,
Journal of noncrystalline solids, 334&335, 207-209 (2004).

62. Degree of structural perfection of icosahedral quasicrystalline grainsinvestigated by synchrotron x-ray diffractometry and imaging techniques
J. Gastaldi, S. Agliozzo, A. Letoublon, J. Wang, L. Mancini, H. Klein, J. Hartwig, J. Baruchel, I. R. Fisher, T. Sato, A. P. Tsai and A. M. de Boissieu,
Phil. Mag. 83, 1-29 (2003).

61. Surface structures of approximant phases in the Al-Pd-Mn system
V. Fournee, A. R. Ross, T. A. Lograsso, J. W. Anderegg, C. Dong, K. Kramer, I.R. Fisher, P. C. Canfield, and P. A. Thiel,
Phys. Rev. B. 66, 165423 (2002).

60. Quasicrystal surfaces: potential as templates for molecular adsorption
R. McGrath, J. Ledieu, E. J. Cox, S. Haq, R. D. Diehl, C. J. Jenks, I. R. Fisher, A. R. Ross and T. A. Lograsso,
Journal of Alloys and Compounds 342, 432 (2002).

59. Transport properties of icosahedral quasicrystal Al72Pd19.5Mn8.5
A. Bilusic, Z. Budrovic, A. Smontara, J. Dolinsek, P. C. Canfield, and I. R. Fisher,
Journal of Alloys and Compounds 342, 413-415 (2002).

58. The local atomic structure of R-Mg-Zn (R=Y, Gd, Dy and Tb)
M. J. Kramer, S. T. Hong, P. C. Canfield, I. R. Fisher, J. D. Corbett, Y. Zhu, and A. I. Goldman,
Journal of Alloys and Compounds 342, 82-86 (2002).

57. The electrical conductivity of single-grain Al-Pd-Re quasicrystals
I. R. Fisher, X. P. Xie, I. Tudosa, C. W. Gao, C. Song, P. C. Canfield, A. Kracher, K. Dennis, D. Abanoz and M.J. Kramer
Phil. Mag. B 82, 1089 (2002).

ABSTRACT
Systematic electrical transport data are presented for single quasicrystals in the Al-Pd-Re family across the accessible width of formation. The temperature dependence of the electrical conductivity can be accounted for in terms of weak localization and electron-electron interactions for all compositions studied, in contrast to previous studies of cast and annealed polygrain material. These results indicate that the conductivity mechanism in the Al-Pd-Re system is not dramatically different from that of other quasicrystal families.

56. Surface preparation and characterization of the icosahedral Al-Ga-Pd-Mn quasicrystal
M. Heinzig, C. J. Jenks, M. Van Hove, I. R. Fisher, P. C. Canfield and P. A. Thiel,
J. Alloys and Compounds 338, 248-252 (2002).

55. The ten-fold surface of the decagonal Al72Ni11Co17 quasicrystal studied by LEED, SPA-LEED, AES and STM
E. J. Cox, J. Ledieu, R. McGrath, R. D. Diehl, C. J. Jenks, and I. R. Fisher,
Materials Research Society Symposium Proceedings (2001), 643, K11.3.1-K11.3.6 (2002).

54. The thermal stability of a single-grain Mg-Zn-Y icosahedral quasicrystal
Z. P. Luo, Y. L. Tang, D. J. Miller, M. J. Kramer, I. R. Fisher, and P. C. Canfield,
Materials Research Society Symposium Proceedings (2001), 643, K9.4.1-K9.4.6 (2002).

53. The influence of growth rate on porosity in icosahedral Al-Pd-Mn quasicrystals,
A. R. Ross, I. R. Fisher, P. C. Canfield and T. A. Lograsso,
Materials Research Society Symposium Proceedings (2001), 643, K1.5.1-K1.5.5. (2002).

52. Icosahedral quasicrystal Al-Pd-Mn and its xsi' approximant: linear expansivity, specific heat, magnetic susceptibility, electrical resistivity and elastic constants
C. Swenson, I. R. Fisher, N. E. Anderson Jr., P. C. Canfield and A. Migliori,
Phys. Rev. B. 65, 184206 (2002).

51. Microwave conductivity and penetration depth in the heavy fermion superconductor CeCoIn5
R. J. Ormeno, A. Sibley, C. E. Gough, S. Sebastian and I. R. Fisher,
Phys. Rev. Lett. 88, 047005 (2002).

50. Infrared and optical properties of pure and cobalt doped LuNi2B2C
M. Windt, J. J. McGuire, T. Room, A. Pronin, T. Timusk, I. R. Fisher, and P. C. Canfield,
Phys. Rev. B. 65, 064506/1-064506/9 (2002).

49. High-temperature solution growth of single crystals and quasicrystals
P. C. Canfield and I. R. Fisher,
J. Crystal Growth, 225, 155-161 (2001).

48. Observation of a spin reorientation in TbCu2Ge2 from resonant and non-resonant x-ray scattering
C. Song, D. Johnson, D. Wermeille, A. I. Goldman, S. L. Bud'ko, I. R. Fisher, and P. C. Canfield,
Phys. Rev. B 64, 224414 (2001).

47. Unusual spin glass phase in icosahedral Tb-Mg-Zn quasicrystals
J. Dolinsek, Z. Jaglicic, M. A. Chernikov, I. R. Fisher, and P. C. Canfield,
Phys. Rev. B 64, 224209 (2001).

46. Reentrant behavior in the temperature dependence of metamagnetic transitions in single crystal Nd6Fe13-xAl1+x
R. W. McCallum, I. R. Fisher, N. E. Anderson, P. C. Canfield, M. J. Kramer and K. W. Dennis
IEEE Transactions on Magnetics 37(4, Pt. 1), 2147 (2001).

45. R9Mg34Zn57 icosahedral quasicrystals: The tuning of a model spin glass
P.C. Canfield and I.R. Fisher,
J. Alloys and Compounds 317-318, 443-447 (2001).

44. Structure and physical properties of the new Pseudo-binary intermetallic compound Ti11(Sb,Sn)8
H. Kim, M.M. Olmstead, J.Y. Chan, P.C. Canfield, I.R. Fisher, R.W. Henning, A.J. Schultz and S.M. Kauzlarich,
J. Solid State Chemistry 157, 225-232 (2001).

43. Investigation of the effect of Ga doping on the thermoelectric properties of the A1PdMn quasicrystalline system.
D.W. Winkler, A.L. Pope, T.M. Tritt, I.R. Fisher, T.A. Wiener and P.C. Canfield,
Proc. 18th Int. Conf. Thermoelectr. 398-401 (1999).

42. Plasticity of icosahedral Zn-Mg-Dy single quasicrystals
M. Heggen, P. Schall, M. Feuerbacher, H. Klein, I. R. Fisher, P.C. Canfield and K. Urban,
Materials Science and Engineering A 294-296, 781-785 (2000).

41. Heat capacities of icosahedral and hexagonal phases of Zn-Mg-Y system
A. Inaba, H. Takakura, A. P. Tsai, I. R. Fisher, and P. C. Canfield,
Materials Science & Engineering, A 294-296 723-726 (2000).

40. Electrical resistivity, thermopower, and thermal conductivity of single-grained (Y, Tb, Ho, Er)-Mg-Zn icosahedral quasicrystals.
K. Gianno, A.V. Sologubenko, M.A. Chernikov, H.R. Ott, I. R. Fisher and P.C. Canfield,
Materials Science and Engineering A 294-296, 715-718 (2000).

39. Diffusion of 57Co in decagonal Al-Ni-Co quasicrystals
C. Khoukaz, R. Galler, H. Mehrer, P.C. Canfield, I.R. Fisher and M. Feuerbacher
Materials Science and Engineering A 294-296, 697-701 (2000).

38. Phason strain and structural perfection in the R-Mg-Zn icosahedral phases
A. Letoublon, I.R. Fisher, T.J. Sato, M. de Boissieu, M. Boudard, S. Agliozzo, L. Mancini, J. Gastaldi, P.C. Canfield, A.I Goldman and A.P. Tsai,
Materials Science and Engineering A 294-296, 127-130 (2000).

37. Formation and morphological development of porosity in icosahedral Al-Pd-Mn alloys.
A. R. Ross, T.A. Wiener, I.R. Fisher, P.C. Canfield and T.A. Lograsso,
Materials Science and Engineering A 294-296, 53-56 (2000).

36. Growth of large single-grain quasicrystals from high-temperature metallic solutions
I.R. Fisher, M.J. Kramer, Z. Islam, T.A. Wiener, A. Kracher, A.R. Ross, T.A. Lograsso, A.I. Goldman and P.C. Canfield,
Materials Science and Engineering A 294-296, 10-16 (2000).

35. Design of a metallic Ising spin glass in the Y1-xTbxNi2Ge2 system
T.A. Wiener, I. R. Fisher, S.L. Bud'ko, A. Kracher and P.C. Canfield,
Phys. Rev. B. 62, 15056-15066 (2000).

34. Nonlocal effects in magnetization of high-kappa superconductors
V.G. Kogan, S.L. Bud'ko, I. R. Fisher and P.C. Canfield,
Phys. Rev. B. 62, 9077-9082 (2000).

33. Observation of a metamagnetic phase transition in an itinerant 4f system via the magneto-optic Kerr effect: Ce(Fe1-xCox)2
R.J. Lange, I.R. Fisher, P.C. Canfield, V.P. Antropov, S.J. Lee, B.N. Harmon and D.W. Lynch,
Phys. Rev. B. 62, 7084-7092 (2000).

32. Yb14ZnSb11: charge balance in Zintl compounds as a route to intermediate Yb valence
I.R. Fisher, S.L. Bud'ko, C. Song, P.C. Canfield, T.C. Ozawa and S.M. Kauzlarich,
Phys. Rev. Lett. 85, 1120-1123 (2000).

31. Low-temperature thermal conductivity of a single-grain Y-Mg-Zn icosahedral quasicrystal
K. Giannò, A.V. Sologubenko, M.A. Chernikov, H.R. Ott, I.R. Fisher, and P.C. Canfield,
Phys. Rev. B 62, 292-300 (2000).

30. Low-temperature transport, thermal and optical properties of single-grain quasicrystals of icosahedral phases in the Y-Mg-Zn and Tb-Mg-Zn alloy systems
M.A. Chernikov, S. Paschen, E. Felder, P. Vorburger, B. Ruzicka, L. Degiorgi, H.R. Ott, I.R. Fisher, and P.C. Canfield,
Phys. Rev. B 62, 262-272 (2000).

29. Non-locality and the flux line lattice square to hexagonal symmetry transition in the borocarbide superconductors
M.R. Eskildsen, I.R. Fisher, P.L. Gammel, D.J. Bishop, N.H. Andersen, K. Mortensen and P.C. Canfield,
Physica C 332, 320-326 (2000).

28. The magnetic characteristics of the Tb(Ni1-xCox)2Ge2 system
T.A. Wiener, I.R. Fisher and P.C. Canfield,
J. Alloys and Compounds 303-304, 289-292 (2000).

27. Magnetic properties of icosahedral R-Mg-Zn quasicrystals
I.R. Fisher, Z. Islam J. Zarestky, C. Stassis, M.J. Kramer, A.I. Goldman and P.C. Canfield,
J. Alloys and Compounds 303-304, 223-227 (2000).

26. Recent advances in the study of quasicrystals
I.R. Fisher, M.J. Kramer and A.I. Goldman,
Micron 31, 469-473 (2000).

25. Interwoven magnetic and flux line structures in single crystal (Tm,Er)Ni2B2C
P.L. Gammel, D. Lopez, D.J. Bishop, M.R. Eskildsen, N.H. Andersen, K. Mortensen, I.R. Fisher, K.O. Cheon and P.C. Canfield
Journal of Applied Physics 87, 5544-5548 (2000).

24. A comparison of structural stabilities of the three high-symmetry surfaces of Al-Pd-Mn bulk quasicrystals using LEED
Z. Shen, W. Raberg, M. Heinzig, C.J. Jenks, V. Fournée, M.A. Van Hove, T.A. Lograsso, D. Delaney, T. Cai, P.C. Canfield, I.R. Fisher, A.I. Goldman, M.J. Kramer, and P.A. Thiel
Surface Science 450, 1-11 (2000).

23. Optical properties and electronic structure of single crystals of LuAl2 and YbAl2
S.J. Lee, S.Y. Hong, I.R. Fisher, P.C. Canfield, B.N. Harmon and D.W. Lynch
Phys. Rev. B 61, 10076-10083 (2000).

22. Plastic deformation of icosahedral Zn-Mg-Dy single quasicrystals
M. Heggen, M. Feuerbacher, P. Schall, H. Klein, I.R. Fisher, P.C. Canfield and K. Urban
Phil. Mag. Letters, 80, 129-136 (2000).

21. Nesting properties and anisotropy of the Fermi surface of LuNi2B2C
S.B. Dugdale, M.A. Alam, I. Wilkinson, R.J. Hughes, I.R. Fisher, P.C. Canfield, T. Jarlborg and G. Santi,
Phys. Rev. Lett. 83, 4824-4827 (1999).

20. On the growth of icosahedral Al-Pd-Mn quasicrystals from the ternary melt
I.R. Fisher, M.J. Kramer, T.A. Wiener, Z. Islam, A.R. Ross, T.A. Lograsso, A. Kracher, A.I. Goldman and P.C. Canfield,
Phil. Mag. B 79, 1673-1684 (1999).

19. Critical doping in overdoped high-Tc superconductors: a quantum critical point?
J.L. Tallon, J.W. Loram, G.V.M. Williams, J.R. Cooper, I.R. Fisher, J.D. Johnson, M.P. Staines and C. Bernhard,
Physica Status Solidi B 215, 531-540 (1999).

18. Anisotropy and metamagnetism in the RNi2Ge2 (R = Y, La-Nd, Sm-Lu) series
S.L. Bud'ko, Z. Islam, T.A. Wiener, I.R. Fisher, P.C. Canfield and A.H. Lacerda
J. Magn. Magn. Mat. 205, 53 - 78 (1999).

17. Systematic study of anisotropic transport and magnetic properties of RAgSb2 (R = Y, La-Nd, Sm, Gd-Tm)
K.D Myers, S.L. Bud'ko, I.R. Fisher, Z. Islam, H. Kleinke, P.C. Canfield and A.H. Lacerda,
J. Magn. Magn. Mat. 205, 27 - 52 (1999).

16. Magnetic and transport properties of single-crystal R2Cu2In (R = Gd-Tm, Lu)
I.R. Fisher, Z. Islam and P.C. Canfield,
J. Magn. Magn. Mat. 202, 1-10 (1999).

15. Thermodynamic and Transport Properties of Single-Crystal Yb14MnSb11
I.R. Fisher, T. A. Wiener, S. L. Bud'ko P. C. Canfield, J. Y. Chan and S. M. Kauzlarich,
Phys. Rev. B 59, 13829-13834 (1999).

14. Systematic studies of the square-hexagonal flux line lattice transition in Lu(Ni1-xCox)2B2C: the role of non-locality
P.L. Gammel, D.J. Bishop, M.R. Eskildsen, K. Mortensen, N.H. Andersen, I.R. Fisher, K.O. Cheon, P.C. Canfield and V.G. Kogan,
Phys. Rev. Lett. 82, 4082-4085 (1999).

13. On the growth of decagonal Al-Ni-Co quasicrystals from the ternary melt
I.R. Fisher, M.J. Kramer, Z. Islam, A.R. Ross, A. Kracher, T. Wiener, M.J. Sailer, A.I. Goldman and P.C. Canfield,
Phil. Mag. B 79, 425-434 (1999).

12. Boron isotope effect in single-crystal YNi2B2C and LuNi2B2C superconductors
K.O. Cheon, I.R. Fisher and P.C. Canfield,
Physica C 312, 35-39 (1999).

11. Magnetic and transport properties of single-grain R-Mg-Zn icosahedral quasicrystals [R = Y, (Y1-xGdx), (Y1-xTbx), Tb, Dy, Ho and Er]
I.R. Fisher, K.O. Cheon, A.F. Panchula, P.C. Canfield, M. Chernikov, H.R. Ott, and K. Dennis,
Phys. Rev. B 59, 308-321 (1999).

10. Resistivity and magnetic susceptibility of single-crystal Lu(Ni1-xCox)2B2C
K.O. Cheon, I.R. Fisher, V.G. Kogan, P.C. Canfield, P. Miranovic and P.L. Gammel,
Phys. Rev. B 58, 6463-6467 (1998).

9. Reinvestigation of long-range magnetic ordering in icosahedral Tb-Mg-Zn
Z. Islam, I.R. Fisher, J. Zarestky, P.C. Canfield, C. Stassis and A.I. Goldman
Phys. Rev. B 57, R11047-R11050 (1998).

8. Growth of large-grain R-Mg-Zn quasicrystals from the ternary melt (R = Y, Er, Ho, Dy and Tb)
I.R. Fisher, Z. Islam, A.F. Panchula, K.O. Cheon, M.J. Kramer, P.C. Canfield and A.I. Goldman,
Phil. Mag. B 77, 1601-1615 (1998).

7. Anisotropic resistivity and normal-state magnetoresistance of RNi2B2C (R = Y, Lu, Er, Ho)
I.R. Fisher, J.R. Cooper and P.C. Canfield,
Phys. Rev. B 56, 10820-10823 (1997).

6. Angular dependence of the c-axis normal-state magnetoresistance in single-crystal Tl2Ba2CuO6
N.E. Hussey, J.R. Cooper, J.M. Wheatley, I.R. Fisher, A. Carrington, A.P. Mackenzie, C.T. Lin and O. Milat,
Phys. Rev. Lett. 76, 122-125 (1996).

5. Angular dependence of the c-axis normal-state magnetoresistance in single-crystal Tl2Ba2CuO6
N.E. Hussey, J.R. Cooper, I.R. Fisher, A.P. Mackenzie and J.M. Wheatley,
Proc. 10th Anniv HTS Workshop Phys., Mater. Appl., (Ed. B. Batlogg, World Scientific, Singapore) p330-331 (1996).

4. Normal-state properties of RNi2B2C (R = Y, Ho, La) and anisotropic resistivity of single-crystal RNi2B2C (R = Lu, Er, Ho)
I.R. Fisher, J.R. Cooper, K.R. Locherer, R.J. Cava and P.C. Canfield
J. Low T. Phys. 105, Nos. 5/6, 1623-1628 (1996).

3. Effect of sample density on the resistivity and Hall coefficient of polycrystalline La1.83Sr0.17CuO4
I.R. Fisher and J.R. Cooper,
Physica C 272, 125-130 (1996).

2. Normal-state transport and magnetic properties of RNi2B2C (R = Y, Ho, La)
I.R. Fisher, J.R. Cooper and R.J. Cava,
Phys. Rev. B 52, 15086-15089 (1995).

1. Hall effect and thermoelectric power of Y0.9Ca0.1Ba2Cu3O7-delta
I.R. Fisher, P.S.I.P.N. de Silva, J.W. Loram, J.L. Tallon, A. Carrington and J.R. Cooper,
Physica C 235-240, 1497-1498 (1994).

Fisher Research Group
Geballe Laboratory for Advanced Materials
Dept. of Applied Physics
Stanford University
CA 94305-4045

Last Updated: May 27th 2023