Research: Spin gap compounds

		Spin-dimer compounds provide fertile ground for studying novel ordered states in the context of a magnetic material. Based on strongly coupled pairs of spins (dimers), which by virtue of the crystal structure only weakly interact with each other, antiferromagnetic intradimer exchange results in a ground state that is a product of singlets and there is a gap to excited triplet states (the spin gap). Large magnetic fields Zeeman split the triplet states and hence can close the spin gap, resulting in a state characterized by long-range magnetic order. Details of both the physical lattice and the resulting spin Hamiltonian determine the nature of this ordered state. A field-tuned quantum critical point (QCP) separates the quantum paramagnetic phase from the ordered state at the critical field. Depending on the balance of potential and kinetic energy of the triplets, the ground state can be variously approximated as a Bose-Einstein condensate (canted XY antiferromagnet), a triplet crystal (inhomogeneous Ising order) or even a supersolid. Significantly, quantum fluctuations play a prominent role in these systems, dynamically tuning various physical quantities. Our research seeks to understand the factors determining the various magnetically ordered phases observed in spin dimer compounds, and most recently, to explore the effects of disorder in these materials. The figure shows the phase diagram of the S=1 spin dimer compound Ba₃Mn₂O₈, which exhibits successive triplet and quintuplet condensation.
Publications
A few key publications:
Asymmetric Quintuplet Condensation in the Frustrated S=1 Spin Dimer Compound Ba₃Mn₂O₈ Ordered magnetic phases of the frustrated spin-dimer compound Ba₃Mn₂O₈ Dimensional reduction at a quantum critical point
Recent publications:
Field-induced spin density wave and spiral phases in a layered antiferromagnet Pressure Dependent Diffraction and Spectroscopy of a Dimerized Antiferromagnet Persistence of magnons in a site-diluted dimerized frustrated antiferromagnet Heat capacity of the site-diluted spin dimer system Ba₃(Mn_1-xV_x)₂O₈ Nonuniversal magnetization at the BEC critical field: Application to the spin dimer compound Ba₃Mn₂O₈ Anisotropic phase diagram of the frustrated spin dimer compound Ba₃Mn₂O₈ Click for additional publications Asymmetric Quintuplet Condensation in the Frustrated S=1 Spin Dimer Compound Ba₃Mn₂O₈ Ordered magnetic phases of the frustrated spin-dimer compound Ba₃Mn₂O₈ Singlet-Triplet Dispersion Reveals Additional Frustration in the Triangular-Lattice Dimer Compound Ba₃Mn₂O₈ Dispersive magnetic excitations in the S = 1 antiferromagnet Ba₃Mn₂O₈ Nuclear magnetic resonance evidence for a strong modulation of the Bose-Einstein condensate in BaCuSi2O6 Geometric Frustration and Dimensional Reduction at a Quantum Critical Point Multiple Magnon Modes and Consequences for the Bose-Einstein Condensed Phase in BaCuSi2O6 Bose-Einstein condensation in BaCuSi2O6 Role of anisotropy in the spin dimer compound BaCuSi2O6 Dimensional reduction at a quantum critical point Low temperature structural phase transition and incommensurate lattice modulation in the spin gap compound BaCuSi2O6 Characteristic BEC scaling close to QCP in BaCuSi2O6 High field behavior of the spin gap compound Sr2Cu(BO3)2
Theses
Eric Samulon: Magnetic phases of the frustrated spin dimer compound Ba₃Mn₂O₈ Suchitra Sebastian: Bose-Einstein condensation in spin dimer compounds
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Fisher Research Group
Geballe Laboratory for Advanced Materials
Dept. of Applied Physics
Stanford University
CA 94305-4045

Last Updated: Dec 30th 2017

Research: Quantum magnetism of spin dimer compounds