Nonunitary Triplet Pairing on the Border of Magnetism and Upper Critical Field in UTe₂

The discovery of superconductivity in the uranium compound UTe₂ in 2019 and the identification of it likely being a triplet superconductor was one of the most tantalizing recent developments in the field of unconventional superconductivity. Despite many experimental probes, the precise nature of the superconducting order parameter is still under debate. In this talk, I will show, using a combination of phenomenological free-energy analysis and the weak-coupling BCS theory, coupled with the ab initio band structure calculations, that a time-reversal breaking superconducting phase, so called nonunitary state, is stabilized in UTe₂ [1]. In the past, such nonunitary states have been proposed for ferromagnetic superconductors such as UGe₂, whereas in the present proposal, the nonunitary chiral state is predicted to be stable even in the non-magnetic phase of UTe₂. This result has a number of consequences - from chiral edge modes, recently observed in STM, to non-vanishing magneto-optical Kerr effect. Furthermore, we predict the nodal points of the gap to be topologically stable, with associated surface Majorana states and their predicted contribution to thermal Hall effect [1].
Recent studies of superconductivity under the applied hydrostatic pressure and magnetic field have revealed a complex phase diagram with what appears to be two superconducting phases whose upper critical fields are suppressed by pressure [2]. I will present a semi-phenomenological theory [2] describing the behaviour of the upper critical field on pressure and the interplay between superconductivity and the meta-magnetic phase transition observed in UTe₂.

[1] A. H. Nevidomskyy, arXiv:2001.02699 (2020).
[2] W.-C. Lin, et al. npj Quant. Materials 5, 68 (2020).