Survey of Superconducting Materials from Heavy Fermion Compounds to Iron-Pnictides and Beyond
Abstract: Many classes of novel superconducting materials have been discovered in the long history of this subject. In this talk, we survey the superconducting and normal-state properties of three classes of correlated electron materials that exhibit unconventional superconductivity that emerges from magnetically ordered phases: heavy fermion f-electron materials, layered copper oxides (cuprates), and iron pnictides and chalcogenides. The layered cuprates and the Fe-based pnictides and chalcogenides have crystal structures that consist of conducting layers (CuO2 layers for the cuprates and FePn or FeCh layers for the Fe pnictides and chalcogenides) separated by “blocking layers” that act as charge reservoirs and control the charge carrier concentration within the conducting layers. We also discuss a new class of superconducting materials that have recently been discovered that are based on the semiconducting parent compound LnOBiS2, where Ln is a lanthanide. These compounds have crystal structures that are similar to those of the Fe pnictide and chalcogenide superconductors and consist of conducting BiS2 layers separated by blocking layers. Subsequent studies demonstrated that superconductivity is induced in general by electron doping in the blocking layers, by means of the substitution of F for O or tetravalent Ti, Zr, Hf, or Th for Ln. The semiconducting behavior can be suppressed by application of pressure, leading to a transition to another superconducting phase with a higher Tc.