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 κxy that is topological in origin. Here we report observation of a large κxy in the Kagome magnet Cu(1-3, bdc) which orders magnetically at 1.8 K. The observed κxy 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.
@article{arxiv.1502.05688,
title = {The thermal Hall effect of spin excitations in a Kagome magnet},
author = {Max Hirschberger and Robin Chisnell and Young S. Lee and N. P. Ong},
journal= {arXiv preprint arXiv:1502.05688},
year = {2015}
}