Phase Diagrams of Noncentrosymmetric Superconductors

Abstract

Noncentrosymmetric superconductors with various types of pairing interactions are systematically examined with particular focus on phenomena that originate from the differences between Fermi surfaces split by a strong spin–orbit coupling. In particular, when the spin–orbit coupling increases and one of the split Fermi surfaces disappears, the phase diagram and the structure of the gap function change drastically. For example, we examine the conditions for the transition from full-gap states to line–node states (FLT), which may explain the differences in the experimental results between the noncentrosymmetric superconductors Li2Pd3B and Li2Pt3B discovered recently. The dominant pairing interactions and gap functions can be predicted to some extent by comparing the theoretical and experimental results for these compounds. For example, if the FLT occurs by replacing Pd with Pt, it is most likely that the superconductivity is mainly induced by charge–charge interactions, and if this is the case, the superconductivities in Li2Pd3B and Li2Pt3B are an s-wave nearly spin-triplet state and a d-wave state that has both spin-singlet and triplet components of comparable weights, respectively. Comparing the theoretical phase diagrams in simple models, it is found that the FLT occurs in a wider realistic parameter region for charge–charge interactions, i.e., where short-range Coulomb repulsion is strong and p- and d-wave interactions are attractive, while it occurs in narrower rather unrealistic parameter regions for interactions of magnetic origin. It is also found that d-wave spin-triplet pairing may occur, when pairing interactions are of magnetic origin and anisotropic in spin space.