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 κH to the phonon longitudinal conductivity κL in oxide dielectrics that are larger than 10^-3 when phonon mean-free paths exceed phonon wavelengths. At the same time κH/κL 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 can produce substantial skew-scattering amplitudes and related thermal Hall effects.