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 (NiPS 3 ), a van der Waals stack of honeycomb layers with a zigzag antiferromagnetic order. The longitudinal (κ aa ) and the transverse (κ ab ) thermal conductivities peak at the same temperature, and the thermal Hall angle, at this peak, respects a previously identified bound. The amplitude of κ ab 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.