Thermodynamic origin of the phonon Hall effect in a honeycomb antiferromagnet.

The underlying mechanism of the thermal Hall effect generated by phonons in a variety of insulators is yet to be identified. In this work, we report on a sizable thermal Hall conductivity in nickel phosphorus trisulfide (NiPS3), a van der Waals stack of honeycomb layers with a zigzag antiferromagnetic order. The longitudinal (κaa) and the transverse (κab) thermal conductivities peak at the same temperature, and the thermal Hall angle, at this peak, respects a previously identified bound. The amplitude of κab is extremely sensitive to the amplitude of magnetization along the b axis but is not correlated with the phonon mean free path, which points to an intrinsic origin.