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With Tc values up to 55 K, the Fe-based superconductors have attracted considerable recent attention. The key structural element appears to be a square planar arrangement of Fe ions, tetrahedrally coordinated by a pnictogen or chalcogen, as exemplified by BaFe2As2 shown above. Parent compounds are antiferromagnetic, with a coupled orthorhombic distortion, but suppression of the long range magnetic order leads to superconducting behavior (as illustrated for the case of Co-doping above). Our research seeks to address the nature of the magnetic, "nematic", and superconducting states in these families of compounds, and to explore the range of occurrence of superconductivity. Experiments emphasize magnetotransport and quantum oscillations to determine Fermi surface topology and quasiparticle properties, and the use of symmetry-breaking anisotropic strain as a probe of the nematic behavior. In particular, we have developed a new experimental tool, elastoresistivity, as a probe of the nematic susceptibility, revealing that the structural phase transition is driven by electronic nematic order.
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