Violation of the orbital depairing limit in a nonunitary state: High-field phase in the heavy fermion superconductor ${\mathrm{UTe}}_{2}$

Violation of the orbital depairing limit in a nonunitary state: High-field phase in the heavy fermion superconductor ${UTe}_{2}$

Kazushige Machida

Phys. Rev. B 107, 224512 – Published 21 June 2023

Abstract

A theoretical study is reported on the origin of an extremely high upper critical field $\sim 70$ T observed in ${UTe}_{2}$ with the transition temperature $T_{c} = 1.6$ –2 K, which exceeds the conventional orbital depairing limit set by the Fermi velocity and $T_{c}$ for a superconductor (SC) in the clean limit. We investigate possible violation of the orbital limit in terms of a spin-triplet nonunitary state, which is effectively coupled to the underlying magnetization induced by an external field. This produces the reduced internal field by canceling it via magnetization. We formulate a theory within the Ginzburg-Landau framework to describe this orbital limit violation and analyze experimental data on the upper critical fields for various field orientations in ${UTe}_{2}$ . We show that the orbital limit violation for a spin-triplet SC, as well as the Pauli-Clogston limit violation for a spin-singlet SC, constitutes a complete and useful framework for examining the high field physics of superconductors in the clean limit.