Aspects of nodal superconductivity

Abstract

Until recently, the precise gap function Δ( k ) of many nodal superconductors has been unknown. The most notable exception are the high-temperature cuprate superconductors for which angular resolved photoemission spectroscopy and Josephson interferometry have succeeded to clearly identify d x 2 − y 2 -wave symmetry. Unfortunately, it has turned out to be difficult to apply these methods to some of the other nodal superconductors. Recently it has been recognized that the quasiparticle spectrum of nodal superconductors in the vortex state is very sensitive to the symmetry of the gap function. In particular, Izawa et al. have succeeded in identifying an f-wave order parameter in Sr 2 RuO 4 , d x 2 − y 2 -wave order in CeCoIn 5 and κ -(ET) 2 Cu(NCS) 2 , and s + g-wave superconductivity in YNi 2 B 2 C and PrOs 4 Sb 12 . In this paper we review some of the surprises that were encountered in this exploration.