Thermodynamic origin of the phonon Hall effect in a honeycomb antiferromagnet

TL;DR

This paper investigates the phonon-mediated thermal Hall effect in NiPS3, revealing a thermodynamic, intrinsic origin linked to magnon-phonon interactions in a honeycomb antiferromagnet.

Contribution

It identifies the thermodynamic origin of the phonon Hall effect in NiPS3 and highlights the role of magnon-phonon band crossing in an antiferromagnetic honeycomb structure.

Findings

Significant thermal Hall conductivity observed in NiPS3.

Thermal Hall angle respects a known bound at peak.

The effect is sensitive to magnetization, not phonon mean free path.

Abstract

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 der Waals stack of honeycomb layers with a zigzag antiferromagnetic order below $T_N$ = 155 K. The longitudinal ($\kappa_{aa}$) and the transverse ($\kappa_{ab}$) thermal conductivities peak at the same temperature and the thermal Hall angle, at this peak, respects a previously identified bound. The amplitude of $\kappa_{ab}$ 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 $b^\ast$ orientation in the momentum space. The relevance of a thermodynamic property, combined with the irrelevance of the mean free path, points to an intrinsic origin.