Pair-Kondo effect: a mechanism for time-reversal broken superconductivity in UTe$_2$

TL;DR

This paper proposes a novel mechanism called pair-Kondo effect that explains the emergence of time-reversal broken superconductivity in UTe$_2$ without requiring degenerate order parameters, aligning with experimental observations.

Contribution

It introduces the pair-Kondo coupling mechanism, driven by magnetic fluctuations, as a way to produce time-reversal symmetry breaking in superconductors near magnetic transitions.

Findings

Pair-Kondo coupling can induce time-reversal symmetry breaking.

The transition can be weakly first order and occur over an extended phase region.

Experimental signatures include specific thermodynamic and ultrasound responses.

Abstract

An important open puzzle in the superconductivity of UTe$_2$ is the emergence of time-reversal broken superconductivity from a non-magnetic normal state. Breaking time-reversal symmetry in a single second-order superconducting transition requires the existence of two degenerate superconducting order parameters, which is not natural for orthorhombic UTe$_2$. Moreover, experiments under pressure (Braithwaite et. al., Comm. Phys. \bf{2}, 147 (2019), arXiv:1909.06074 [cond-mat.str-el]) suggest that superconductivity sets in at a single transition temperature in a finite parameter window, in contrast to the splitting between the symmetry breaking temperatures expected for accidental degenerate orders. Motivated by these observations, we propose a mechanism for the emergence of time-reversal breaking superconductivity without accidental or symmetry-enforced order parameter degeneracies in systems close to a magnetic phase transition. We demonstrate using Landau theory that a cubic coupling between incipient magnetic order and magnetic moments of Cooper pairs (pair-Kondo coupling) can drive time-reversal symmetry breaking superconductivity that onsets in a single, weakly first order transition over an extended region of the phase diagram. We discuss the experimental signatures of such transition in thermodynamic and resonant ultrasound measurements. A microscopic origin of pair-Kondo coupling is identified as screening of magnetic moments by chiral Cooper pairs, built out of two non-degenerate order parameters - an extension of Kondo screening to unconventional pairs.