Violation of the orbital depairing limit in a nonunitary state: High-field phase in the heavy fermion superconductor UTe2

Abstract

A theoretical study is reported on the origin of an extremely high upper critical field $\ensuremath{\sim}70$ T observed in ${\mathrm{UTe}}_{2}$ with the transition temperature ${T}_{\mathrm{c}}=1.6$--2 K, which exceeds the conventional orbital depairing limit set by the Fermi velocity and ${T}_{\mathrm{c}}$ 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 ${\mathrm{UTe}}_{2}$. 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.