Planar thermal Hall effect from phonons in a Kitaev candidate material.

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