The thermal Hall effect of spin excitations in a Kagome magnet

TL;DR

This paper reports the observation of a significant thermal Hall effect in a Kagome magnet, demonstrating topological heat transport due to Berry curvature in spin excitations, with sign-reversal phenomena linked to magnon band topology.

Contribution

It provides experimental evidence of a topological thermal Hall effect in a Kagome magnet, highlighting the role of Berry curvature in heat transport of spin excitations.

Findings

Large thermal Hall conductivity observed in Cu(1-3, bdc)

Sign-reversal of $_{xy}$ with temperature and magnetic field

Sign alternation of Chern flux between magnon bands

Abstract

At low temperatures, the thermal conductivity of spin excitations in a magnetic insulator can exceed that of phonons. However, because they are charge neutral, the spin waves are not expected to display a thermal Hall effect in a magnetic field. Recently, this semiclassical notion has been upended in quantum magnets in which the spin texture has a finite chirality. In the Kagome lattice, the chiral term generates a Berry curvature. This results in a thermal Hall conductivity $\kappa_{xy}$ that is topological in origin. Here we report observation of a large $\kappa_{xy}$ in the Kagome magnet Cu(1-3, bdc) which orders magnetically at 1.8 K. The observed $\kappa_{xy}$ undergoes a remarkable sign-reversal with changes in temperature or magnetic field, associated with sign alternation of the Chern flux between magnon bands. We show that thermal Hall experiments probe incisively the effect of Berry curvature on heat transport.