Enhanced thermal Hall effect in nearly ferroelectric insulators

TL;DR

This paper investigates the large thermal Hall effect in insulators like La2CuO4 and SrTiO3, suggesting extrinsic phonon contributions related to dielectric properties and flexoelectric coupling as key factors.

Contribution

It identifies conditions under which phonons can produce large thermal Hall conductivity, highlighting the role of extrinsic effects and predicting larger effects in some perovskites.

Findings

Extrinsic phonon contribution proportional to phonon mean free path can explain observations in STO.

Intrinsic contribution to $$ is much smaller than experimental values.

Larger intrinsic $$ predicted in certain insulating perovskites.

Abstract

In the context of recent experimental observations of an unexpectedly large thermal Hall conductivity, $\kappa_H$, in insulating $\mathrm{La_2CuO_4}$ (LCO) and $\mathrm{SrTiO_3}$ (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.