Lu Chen
A surprising “planar” thermal Hall effect, whereby the field is parallel to the heat current, has recently been observed in a Kitaev candidate material α-RuCl3, and this has been attributed to exotic excitations such as Majorana fermions [1] or chiral magnons [2]. Here we report the first observation of a planar κ_xy generated by phonons in another Kitaev candidate material Na2Co2TeO6 [3] and cuprates [4]. Based on the striking similarity between κ_xy and the phonon-dominated thermal conductivity κ_xx, we attribute the effect to phonons. We observe a large difference in κ_xy between different configurations of heat current and magnetic field, which reveals that the direction of heat current matters in determining the planar κ_xy. Our findings call for a careful re-examination of the planar κ_xy observed in α-RuCl3 and the mechanisms responsible for the phonon thermal Hall effect in insulators in general.
[1] Yokoi, T. et al. Half-integer quantized anomalous thermal Hall effect in the Kitaev material candidate α-RuCl3. Science 373, 568-572 (2021).
[2] Czajka, P. et al. Planar thermal Hall effect of topological bosons in the Kitaev magnet α-RuCl3. Nat. Mater. 22, 36–41 (2023).
[3] Chen, L. et al. Planar thermal Hall effect from phonons in a Kitaev candidate material, Nat. Commun. 15, 3513 (2024).
[4] Chen, L. et al. Planar thermal Hall effect from phonons in cuprates, Phys. Rev. X 14, 041011 (2024).