A mechanism for time-reversal broken superconductivity in UTe2

An important open puzzle in the superconductivity of UTe2 is the emergence of time-reversal breaking superconductivity from a non-magnetic normal state. Breaking time-reversal symmetry in a homogenous sample with a single superconducting transition requires the existence of two degenerate superconducting order parameters, which is not natural for the orthorhombic UTe2.

To address this puzzle, we study the thermodynamic phase diagrams of superconductors with two non-degenerate order parameters close to a magnetic instability using Landau theory. We demonstrate that finite coupling between magnetic and superconducting orders can drive a weakly first order transition from a metal to a time-reversal breaking superconductor. A microscopic origin of this coupling is identified as screening of magnetic moments by chiral Cooper pairs, built out of the two non-degenerate pairs - an extension of Kondo screening to unconventional pairs. We discuss the experimental signatures of such a weakly first order transition and the resulting phase diagram.