Disorder Induced Anomalous Thermal Hall Effect in Chiral Phases of Superfluid 3 He

Abstract

NMR experiments on liquid $^3$He infused into uniaxially anisotropic silica aerogels show the stabilisation of two equal-spin-pairing chiral phases on cooling from the normal phase. The alignment of the chiral axis relative to the anisotropy axis for these phases is predicted to depend upon temperature. A chiral A-like phase is also stabilized when $^3$He is confined to a slab of thickness $D\sim \xi$, the superfluid coherence length. For both types of confinement, scattering of quasiparticles by the random potential - aerogel or surface - is pair breaking and generates a sub-gap density of quasiparticle states. The random field also conspires with the chiral order parameter to generate skew scattering of quasiparticles in the plane normal to the chiral axis. This scattering mechanism leads to anomalous thermal Hall transport for nonequilibrium quasiparticles driven by a thermal gradient. We report theoretical results for the longitudinal as well as anomalous thermal Hall conductivity for theoretical models for chiral phases of $^3$He in both anisotropic aerogel and slabs. The anomalous thermal Hall effect provides an important tool to identify signatures of broken time-reversal and mirror symmetries and topology in chiral superconductors/superfluids. We compare our theoretical predictions with existing experimental measurements of thermal transport in chiral phases of $^3$He.