Thermal Hall effect driven by phonon-magnon hybridization in a honeycomb antiferromagnet

The underlying mechanism of the thermal Hall effect (THE) generated by phonons in a variety of insulators is yet to be identified. Here, we report on a sizeable thermal Hall conductivity in NiPS_3, a van de Waals stack of honeycomb layers with a zigzag antiferromagnetic order below T_N = 155 K. The longitudinal (κ_aa) and the transverse (κ_ab) thermal conductivities peak at the same temperature and the thermal Hall angle (κ_ab/κ_aa/B) respects a previously identified bound. The amplitude of κ_ab is extremely sensitive to the amplitude of magnetization along the b-axis, in contrast to the phonon mean free path, which is not at all. We show that the magnon and acoustic phonon bands cross each other along the b^∗ orientation in the momentum space. The exponential temperature dependence of κ_ab above its peak reveals an energy scale on the order of magnitude of the gap expected to be opened by magnon-phonon hybridization. This points to an intrinsic scenario for THE with possible relevance to other magnetic insulators.