Pair Kondo effect: A mechanism for time-reversal symmetry breaking superconductivity in UTe2

Abstract

An important open puzzle in the superconductivity of UTe2 is the emergence of broken time reversal superconductivity from a nonmagnetic 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 UTe2. Moreover, experiments under pressure [D. Braithwaite et al., Commun. Phys. 2, 147 (2019)] 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 broken time reversal 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 proximate 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 nondegenerate order parameters, an extension of Kondo screening to unconventional pairs.

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