Enhanced thermal Hall effect in nearly ferroelectric insulators*

In the context of recent experimental observations of an unexpectedly large thermal Hall conductivity in insulating La_2-xSr_xCuO₄ and SrTiO₃, we theoretically explore conditions under which acoustic phonons can give rise to such a large thermal Hall effect. Both the intrinsic and extrinsic contributions to the thermal Hall conductivity are large in proportion to the dielectric constant and the flexoelectric coupling. 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 SrTiO₃. We predict a larger intrinsic contribution to thermal Hall effect in certain insulating perovskites.