Thermal Hall Effect of Spin Excitations in a Kagome Magnet

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. However, in the kagome lattice, theory predicts that the Berry curvature leads to a thermal Hall conductivity κxy. 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. The close correlation between κxy and κxx firmly precludes a phonon origin for the thermal Hall effect.