Theory of Thermal Conductivity in UTe2

The superconducting state of the heavy-fermion metal UTe2 has attracted considerable interest because of evidence for spin-triplet Cooper pairing, including remarkable resilient behavior in high magnetic fields and potential non-trivial topology. Progress on the nature of superconductivity in this fascinating system would be greatly assisted by proper identification of the superconducting order parameter, particularly its nodal structure. Thermal conductivity is an excellent tool to probe nodal structure since it can be extended to millikelvin temperatures, where the electronic quasiparticle contribution tends to dominate. I will review thermal conductivity experiments on UTe2, highlighting anisotropy, temperature and field dependence[1]. Particular focus will be on what variation over several generations of improved quality sample tells us qualitatively. The theory of triplet state thermal conductivity[2] including disorder- and spin-fluctuation limited electronic thermal conductivity, as well as the phonon contribution, will then be presented, including the effect weak magnetic fields via the quasiparticle Doppler shifts they induce. The predictions will be compared in detail to applied to recent data of Hayes et al. In addition, I will discuss the possible existence of time reversal symmetry nonunitary triplet states, and their signatures in transport and NMR. Finally, I review new measurements of Hayes et al. on field angle-dependent thermal conductivity that have the potential to pin down the nodal structure definitively.