Two mechanisms for enhanced thermal Hall effect: ferroelectric fluctuations & resonant scattering

In the context of recent experimental observations of an unusually large thermal Hall conductivity in insulating materials, in this talk we explore theoretical conditions under which acoustic phonons can give rise to the large effect. We will discuss two mechanisms that can be important. In the first part, we will show that the coupling of phonons to the magnetic field can be greatly enhanced in certain nearly ferroelectric insulators. We further provide a phenomenological model of extrinsic phonon thermal Hall effects that explains the SrTiO₃ experiment. In the second part, we will discuss the extrinsic mechanism of resonant skew scattering, which can be a fundamental ingredient for the enhanced extrinsic thermal Hall effect in more general materials.