Spontaneous thermal Hall conductance in superconductors with broken time-reversal symmetry

Abstract

The off-diagonal components of thermal conductance tensor, thermal Hall conductivities (THCs), have extensively been studied in recent condensed matter experiments to investigate fractionalized quantum spin liquids, and quantum Hall systems. Under zero magnetic field, THCs spontaneously become non-zero for time-reversal symmetry (TRS) broken systems, and can have contributions from topologically protected edge states. Here we focus on an additional bulk effect, the impurity mechanism in TRS broken superconductors. Inspired by $Sr_2 Ru O_4$, the low temperature THC was calculated [Sup. Sci. and Tech. 29, 085006 (2016)] for the chiral p-wave superconductors induced by point impurities. Compared to topological part of THC, this contribution can be orders of magnitude larger as it scales with the density of states at the Fermi level. Motivated by TRS broken superconductors, URu$_2$Si$_2$ and SrPtAs and Sr$_2$RuO$_4$ as recently also been suggested as d-wave possibly, we calculate the THCs to $i.$ finite temperatures $ii.$ d-wave pairing states, $iii.$ finite size impurities. For this study, the non-equilibrium quasi-classical Keldysh Green's function formalism is utilized. The THCs are calculated by the systematic expansion of the quasiclassical transport equation in the center of mass gradients, self-consistently. $\kappa_{ij}$ are obtained analytically at low temperatures ($T \to 0$) and numerically at finite temperatures. We find that the impurity mechanism is dominant in $\kappa_{yx}$ at finite temperatures when compared to the topological part except at very low temperatures.There are two experimental signatures of IM on $\kappa_{yx}$: A non-monotonic temperature dependence and a sign change as a function of temperature depending on the scattering process.