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

热霍尔效应已成为自旋液体中奇异激发的潜在探针。在基塔耶夫磁体\({{\alpha }}\) -RuCl ₃中，霍尔热导率\({{{\kappa }}}_{{{xy}}}\)归因于马约拉纳费米子、手性磁子，或声子。理论上，前两种热载体可以产生“平面” \({{{\kappa }}}_{{{xy}}}\)，从而使磁场与热流平行，但实际上不知道声子是否也可以。在这里，我们表明平面\({{{\kappa }}}_{{{xy}}}\)存在于另一种 Kitaev 候选材料 Na ₂ Co ₂ TeO ₆中。基于\({{{\kappa }}}_{{{xy}}}\)和声子主导的热导率\({{{\kappa }}}_{{{xx}}之间惊人的相似性}\)，我们将效应归因于声子。我们观察到不同热流和磁场配置之间的\({{{\kappa }}}_{{{xy}}}\)存在很大差异，这表明热流方向对于确定平面\ ({{{\kappa }}}_{{{xy}}}\)。我们的观察要求重新评估在\(\,{{\alpha }}\) -RuCl _{3中观察到的平面}\({{{\kappa }}}_{{{xy}}}\)。