Charged Defects and Phonon Hall Effects in Ionic Crystals

TL;DR

This paper investigates how charged defects in ionic crystals can cause phonon skew-scattering under magnetic fields, leading to observable thermal Hall effects consistent with recent experimental data.

Contribution

It introduces a mechanism involving charged defect scattering and Lorentz forces as a source of phonon Hall effects in ionic crystals, explaining recent experimental observations.

Findings

Charged defects induce skew-scattering of phonons under magnetic fields.

Lorentz forces on charged defects produce significant thermal Hall conductivity.

The proposed mechanism aligns with experimental ratios of thermal Hall to longitudinal conductivities.

Abstract

It has been known for decades that a magnetic field can deflect phonons as they flow in response to a thermal gradient, producing a thermal Hall effect. Several recent experiments have revealed ratios of the phonon Hall conductivity to the phonon longitudinal conductivity in oxide dielectrics that are larger than $10^{-3}$ when phonon mean-free-paths exceed phonon wavelengths. At the same time $\kappa_{H}/\kappa_{L}$ is not strongly temperature dependent. We argue that these two properties together imply a mechanism related to phonon scattering from defects that break time-reversal symmetry, and we show that Lorentz forces acting on charged defects produce substantial skew-scattering amplitudes, and related thermal Hall effects that are consistent with recent observations.