Discovery of Universal Phonon Thermal Hall Effect in Crystals

Thermal Hall effect (THE) in insulators is a remarkable phenomenon that arises from the motion of chargeless quasiparticles under a magnetic field. While magnons or various exotic excitations were considered as the origin of THE in some magnetic materials, there is more and more evidence suggesting that phonons play a significant role. However, the mechanism behind phonon THE is still unknown. Here we report the observation of THE, including planar THE, in a broad range of nonmagnetic insulators and semiconductors: SrTiO_{3}, SiO_{2} (quartz), MgO, MgAl_{2}O_{4}, Si, and Ge. While the presence of antiferrodistortive domains in SrTiO_{3} and chiral structure in SiO_{2} may complicate the interpretation of THE, the striking observations of THE in trivial insulators MgO and MgAl_{2}O_{4}, as well as in high-purity intrinsic semiconductors Si and Ge, demonstrate that phonon THE is a universal property of crystals. Without other effects on phonons such as from magnons, this universal phonon THE is characterized by a scaling behavior of |κ_{xy}|∼κ_{xx}^{2}. Our results experimentally discover a fundamental physics of phonons in the magnetic field, which should come from the direct coupling between atom vibrations and the field. Starting from this universal phonon THE in crystals, previous interpretations of THE in magnetic or nonmagnetic materials need to be reconsidered.