Violation of the orbital depairing limit in a non-unitary state --on the high field phase in the heavy Fermion superconductor UTe$_2$--

TL;DR

This paper presents a theoretical explanation for the exceptionally high upper critical field in UTe$_2$, suggesting a violation of the orbital depairing limit due to a nonunitary spin-triplet state coupled with magnetization.

Contribution

It introduces a Ginzburg-Landau based theory explaining the orbital limit violation in UTe$_2$ through a nonunitary spin-triplet state and analyzes experimental data accordingly.

Findings

High upper critical field (~70T) exceeds conventional limits.

Nonunitary spin-triplet state can reduce internal magnetic field.

Framework combines orbital and Pauli limits for high-field superconductivity.

Abstract

A theoretical study is reported on the origin of extremely high upper critical field $\sim$70T observed in UTe$_2$ with the transition temperature T$_{\rm c}$=1.6K-2K, far exceeding the conventional orbital depairing limit set by the Fermi velocity and T$_{\rm c}$ 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 external field. This in turn produces the reduced internal field by cancelling it via magnetization. We formulate a theory within Ginzburg-Landau framework to describe this orbital limit violation and analyze experimental data on the upper critical fields for various field orientations in UTe$_2$. It is pointed out that the orbital limit violation for a spin-triplet SC together with the Pauli-Clogston limit violation for a spin-singlet SC constitutes a complete and useful framework to examine the high field physics in superconductors in the clean limit.