Enhanced Thermal Hall Effect in Nearly Ferroelectric Insulators

In the context of recent experimental observations of an unexpectedly large thermal Hall conductivity, κH, in insulating La₂CuO₄ (LCO) and SrTiO₃ (STO), we theoretically explore conditions under which acoustic phonons can give rise to such a large κH. Both the intrinsic and extrinsic contributions to κH are large in proportion to the dielectric constant, ε, 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 κH in certain insulating perovskites.