Effects of Impurities with Singlet–Triplet Configuration on Multiband Superconductors

Abstract

Roles of multipole degrees of freedom in multiband superconductors are investigated in a case of impurities whose low-lying states consist of singlet ground and triplet excited states, which is related to the experimental fact that the transition temperature T c is increased by Pr substitution for La in LaOs 4 Sb 12 . The most important contribution to the T c increase comes from the inelastic interband scattering of electrons coupled to quadrupole or octupole moments of impurities. It is found that a magnetic field modifies an effective pairing interaction and the scattering anisotropy appears in the field-orientation dependence of the upper critical field H c2 in the vicinity of T c , although a uniaxial anisotropic field is required for experimental detection. This would be proof that the Pr internal degrees of freedom are relevant to the stability of superconductivity in (La 1- x Pr x )Os 4 Sb 12 .