Planar thermal Hall effect from phonons in a Kitaev candidate material

Abstract

The thermal Hall effect has emerged as a potential probe of exotic excitations in spin liquids. In the Kitaev magnet α-RuCl3, the thermal Hall conductivity κxy has been attributed to Majorana fermions, chiral magnons, or phonons. Theoretically, the former two types of heat carriers can generate a “planar” κxy, whereby the magnetic field is parallel to the heat current, but it is unknown whether phonons also could. Here we show that a planar κxy is present in another Kitaev candidate material, Na2Co2TeO6. Based on the striking similarity between κxy and the phonon-dominated thermal conductivity κxx, we attribute the effect to phonons. We observe a large difference in κxy between different configurations of heat current and magnetic field, which reveals that the direction of heat current matters in determining the planar κxy. Our observation calls for a re-evaluation of the planar κxy observed inα-RuCl3.