Enhanced Thermal Hall Effect in Nearly Ferroelectric Insulators

In the context of recent experimental observations of an unexpectedly large thermal Hall conductivity, kappa(H), in insulating La2CuO4 (LCO) and SrTiO3 (STO), we theoretically explore conditions under which acoustic phonons can give rise to such a large kappa(H). Both the intrinsic and extrinsic contributions to kappa(H) are large in proportion to the dielectric constant, epsilon, and the "flexoelectric" coupling, F. While the intrinsic contribution is still orders of magnitude smaller than the observed effect, an extrinsic contribution proportional to the phonon mean-free path appears likely to account for the observations, at least in STO. We predict a larger intrinsic kappa(H) in certain insulating perovskites.