Paramagnetic effects of j-electron superconductivity and application to UTe2

TL;DR

This paper explains the unusual superconducting properties of UTe2, a potential spin-triplet superconductor, by analyzing its multi-orbital f-electron bands and spin-orbit coupling effects, accounting for its magnetic anisotropy and high critical fields.

Contribution

It provides a theoretical understanding of the paramagnetic effects and magnetic anisotropy in UTe2 based on multi-orbital band structure and spin-orbit interactions, explaining experimental observations.

Findings

Suppression of Pauli depairing effect for hard-axis magnetic fields.

Tiny decrease of Knight shift below Tc for hard-axis fields.

Strong suppression of spin susceptibility decrease below Tc.

Abstract

The putative spin-triplet superconductor UTe2 has an Ising-like magnetic anisotropy, indicating the Cooper pair spins parallel to the easy-axis. However, the upper critical field for the hard-axis direction is much larger than that for the easy-axis, implying the strong suppression of the Pauli depairing effect even for the hard-axis direction. This observation is consistent with the recent NMR measurements which show the tiny decrease of the Knight shift below the transition temperature Tc for hard-axis magnetic fields. We clarify that these behaviors are naturally understood as a result of multi-orbital f -electron bands with total angular momentum j=5/2. Taking into account spin-orbit couplings for an orthorhombic structure, we demonstrate that for hard-axis magnetic fields, the decrease of the spin susceptibility below Tc and the Pauli depairing effect are strongly suppressed.