Observation of half-integer thermal Hall conductance in a Kitaev quantum magnet

The systems composed of interacting 1/2 spins on a honeycomb lattice with bond-directional exchange interactions are of vital interest, as they host a ground state of Kitaev quantum spin liquid (QSL), where spins are fractionalized into emergent Majorana fermions. Here we report on the measurements of thermal Hall effect in a candidate Kitaev magnet α-RuCl₃. Although α-RuCl₃ exhibits antiferromagnetic (AFM) order in zero field, the application of in-plane magnetic field melts the AFM order, leading to the Kitaev QSL. We discover that in the Kitaev QSL state the 2D thermal Hall conductance reaches a quantum plateau as a function of applied magnetic field, which is exactly half of the quantization value reported for the integer quantum Hall system. The observed half-integer thermal Hall conductance in a bulk material is a direct signature of topologically protected chiral edge current of Majorana fermion. The thermal conductivity vanishes rapidly upon entering the forced ferromagnetic state, suggesting the topological quantum phase transition between the states with and without chiral Majorana edge states.