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 NiPS₃, a van de Waals stack of honeycomb layers with a zigzag antiferromagnetic order below TN = 155 K. The longitudinal (κₐₐ) and the transverse (κab) thermal conductivities peak at the same temperature and the thermal Hall angle (κab/κₐₐ/B) respects a previously identified bound. The amplitude of κab 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 κab 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.