Condensed-Matter Physics & Materials Science Seminar

Understanding the relationship between magnetic order and unconventional superconductivity in strongly correlated electron systems poses a problem for condensed matter physics. There now are several examples of superconductivity appearing near a pressure-induced, zero-temperature magnetic-nonmagnetic boundary in heavy-electron compounds, raising the possibility that unconventional superconductivity may be mediated by long-lived, long-ranged fluctuations of the magnetic order parameter. In these prior cases, rather high pressures and low superconducting transition temperatures and critical fields have made it difficult to explore the relationship among magnetic, superconducting and associated non-Fermi-liquid normal states. Barriers to progress are largely overcome in the ‘115’ family of heavy-electron metals CeMIn5 (M=Co, Rh, Ir) whose readily accessible magnetic or superconducting groundstates are tuned easily by pressure, magnetic field and hole doping. Recent studies of these systems strongly suggest that unconventional (d-wave) superconductivity in CeCoIn5 and CeRhIn5 coexists microscopically with long-range magnetic order and that both broken symmetries involve the same electronic degrees of freedom, with superconductivity in both materials emerging from a distinctly non-Fermi-liquid normal state associated with proximity to an unconventional quantum-phase transition. On the other hand, magnetic order and superconductivity are separated in CeIrIn5 by a non-Fermi-liquid phase, suggesting that the symmetry of its superconducting order parameter may be different from that of the Co- and Rh115’s and that pairing is influenced by some broken symmetry yet to be identified. Though the detailed interplay of magnetism and unconventional superconductivity remains unsolved, progress made from studying these CeMIn5 materials is defining research directions that hold promise for providing a solution.