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, κxy, in a variety of contexts, however, have assumed a negligibly small phonon contribution. We present a study of κxy 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, κxy(T) and κ(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 α-RuCl₃, κxy(T) and κ(T) peak also at the same temperature. We also studied KTaO₃ and found a small signal, indicating that a sizable κxy(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 κxy of this solid.