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 $${{\alpha }}$$ α -RuCl3, the thermal Hall conductivity $${{{\kappa }}}_{{{xy}}}$$ κ x y has been attributed to Majorana fermions, chiral magnons, or phonons. Theoretically, the former two types of heat carriers can generate a “planar” $${{{\kappa }}}_{{{xy}}}$$ κ x y , whereby the magnetic field is parallel to the heat current, but it is unknown whether phonons also could. Here we show that a planar $${{{\kappa }}}_{{{xy}}}$$ κ x y is present in another Kitaev candidate material, Na2Co2TeO6. Based on the striking similarity between $${{{\kappa }}}_{{{xy}}}$$ κ x y and the phonon-dominated thermal conductivity $${{{\kappa }}}_{{{xx}}}$$ κ x x , we attribute the effect to phonons. We observe a large difference in $${{{\kappa }}}_{{{xy}}}$$ κ x y between different configurations of heat current and magnetic field, which reveals that the direction of heat current matters in determining the planar $${{{\kappa }}}_{{{xy}}}$$ κ x y . Our observation calls for a re-evaluation of the planar $${{{\kappa }}}_{{{xy}}}$$ κ x y observed in $$\,{{\alpha }}$$ α -RuCl3.