Geometry dependence of the thermal Hall effect in chiral spin liquids

Recent thermal-transport experiments on the Kitaev magnet α-RuCl₃ highlight the challenge in identifying chiral quantum spin liquids through their quantized thermal Hall effect. Here, we theoretically propose and experimentally demonstrate that the origin of the thermal Hall effect can be determined by varying the underlying sample geometry through, for example, introducing constrictions. By studying standard phenomenological heat-transport equations based on minimal assumptions, we show that, whereas a conventional thermal Hall effect due to phonons or magnons is completely geometry independent, a thermal Hall effect originating from a chiral fermion edge mode is significantly enhanced by geometric constrictions at low temperatures. Motivated by this result, we measure the thermal Hall effect in both constricted and unconstricted samples of α-RuCl₃; the comparison of these two measurements reveals a pronounced geometry dependence of the thermal Hall effect and thus provides compelling evidence of a chiral fermion edge mode connected to a field-induced chiral spin liquid.