Charged defects and phonon Hall effects in ionic crystals

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 κ_H to the phonon longitudinal conductivity κ_L in oxide dielectrics that are larger than 10⁻³ when phonon mean-free paths exceed phonon wavelengths. At the same time, κ_H/κ_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.