Phonon Thermal Hall Effect in Strontium Titanate

It has been known for more than a decade that phonons can produce an off-diagonal thermal conductivity in the presence of a magnetic field. Recent studies of thermal Hall conductivity, kappa(xy), in a variety of contexts, however, have assumed a negligibly small phonon contribution. We present a study of kappa(xy) in quantum paraelectric SrTiO3, which is a nonmagnetic insulator and find that its peak value exceeds what has been reported in any other insulator, including those in which the signal has been qualified as giant. Remarkably, kappa(xy) (T) and kappa(T) peak at the same temperature and the former decreases faster than the latter at both sides of the peak. Interestingly, in the case of La2CuO4 and alpha-RuCl3, kappa(xy) (T) and kappa(T) peak also at the same temperature. We also studied KTaO3 and found a small signal, indicating that a sizable kappa(xy)(T) is not a generic feature of quantum paraelectrics. Combined to other observations, this points to a crucial role played by antiferrodistortive domains in generating kappa(xy) of this solid.