Effects of Impurities with Singlet-Triplet Configuration on Multiband Superconductors

TL;DR

This paper explores how impurities with singlet-triplet states influence multiband superconductors, revealing that inelastic scattering involving quadrupole or octupole moments can enhance the transition temperature and affect critical field anisotropy.

Contribution

It introduces a model where impurity multipole states affect superconductivity, explaining Tc enhancement and field-dependent anisotropy in Pr-doped LaOs4Sb12.

Findings

Inelastic interband scattering increases Tc.

Magnetic fields modify pairing interactions.

Field orientation affects upper critical field anisotropy.

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 $T_{\rm c}$ is increased by Pr substitution for La in LaOs$_4$Sb$_{12}$. The most important contribution to the $T_{\rm c}$ 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 $H_{{\rm c}2}$ in the vicinity of $T_{\rm c}$, 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$_{12}$.