Unconventional superconductivity

The recently discovered high-temperature superconductors have a number of very unusual properties. Nor only is the superconducting transition temperature T-c much higher than ever achieved before, but evidence is now growing that they may also be unconventional superconductors. An unconventional superconductor is one which has fundamentally different symmetries from those of other superconductors. Previously unconventional superconductivity was known to occur only in the heavy-fermion systems, such as the compounds UPt3 and UBe13. This review examines the basic concepts of symmetry applied to superconductors. I show how conventional (s-wave) and unconventional (p-wave and d-wave) superconductors arise naturally from the spontaneous symmetry breaking which lends to superconductivity. I then discuss the key experiments which can test for unconventional superconductivity in the high T-c and heavy-fermion systems. These experiments include measurements of the energy gap, phase-sensitive Josephson junction experiments, and observations of multiple superconducting phases.