Thermal Hall effect of Kitaev quantum spin liquid candidate materials*

α-RuCl₃ represents one of the most intriguing quantum materials of the past years. Numerous evidence for a significant Kitaev interaction between the j=1/2 moments which are located at the vertices of a honeycomb lattice has nourished hope to find fingerprints of the celebrated Kitaev model in this compound. Indeed, reports of a half-quantized thermal Hall effect in α-RuCl₃ seem to match perfectly with the predicted thermal transport due to an emergent Majorana edge mode of the Kitaev quantum spin liquid (QSL). On the other hand, we have demonstrated that the longitudinal thermal transport of α-RuCl₃ is due to phonons and is determined by a strong phonon-spin scattering. Interestingly, its temperature dependence is very similar to that of the thermal Hall conductivity which suggests the possibility of an alternative phononic scenario for the thermal Hall effect. Recently, Na₂Co₂TeO₆ emerged as a new and interesting candidate materials for a Kitaev QSL. Our data for the longitudinal thermal conductivity reveal a striking similarity to that of α-RuCl₃, as well as a large thermal Hall effect. Thus, with these two materials at hand, one can hope to obtain fresh information as to whether the thermal Hall effect in the Kitaev QSL compounds is due to genuine Majorana heat transport or due to to phonons.