Robustness of the thermal Hall effect close to half-quantization in α-RuCl 3

A key feature of quantum spin liquids is the predicted formation of fractionalized excitations. They are expected to produce changes in the physical response, providing a way to observe the quantum spin liquid state 1 . In the honeycomb magnet α-RuCl 3 , a quantum spin liquid has been proposed to explain the behaviour observed on applying an in-plane magnetic field H || . Previous work reported that the thermal Hall conductivity took on a half-integer quantized value and suggested this as a signature of a fractionalized Majorana edge mode predicted to exist in Kitaev quantum spin liquids 2 . However, the temperature and magnetic-field range of the half-quantized signal 2 – 4 and its association with Majorana edge modes are still under debate 5 , 6 . Here we present a comprehensive study of the thermal Hall conductivity in α-RuCl 3 showing that approximately half-integer quantization exists in an extended region of the phase diagram, particularly across a plateau-like parameter regime for H || exceeding 10 T and temperature below 6.5 K. At lower fields, the thermal Hall conductivity exhibits correlations with complex anomalies in the longitudinal thermal conductivity and magnetization, and is suppressed by cooling to low temperatures. Our results can be explained by the existence of a topological state in magnetic fields above 10 T.