Majorana-fermion origin of the planar thermal Hall effect in the Kitaev magnet $\alpha$-RuCl$_3$

TL;DR

This study provides conclusive evidence that the planar thermal Hall effect in $ extalpha$-RuCl$_3$ originates from Majorana fermions, through low-temperature specific heat and thermal Hall measurements revealing a topological transition.

Contribution

It demonstrates that the thermal Hall effect in $ extalpha$-RuCl$_3$ is due to Majorana fermions, confirming their role in the topological properties of the material.

Findings

Observation of a bulk gap closure at specific angles.

Sign reversal of the thermal Hall effect with angle rotation.

Evidence of a topological transition induced by in-plane field rotation.

Abstract

The field-induced quantum disordered state of layered honeycomb magnet $\alpha$-RuCl$_3$ is a prime candidate for Kitaev spin liquids hosting Majorana fermions and non-Abelian anyons. Recent observations of anomalous planar thermal Hall effect demonstrate a topological edge mode, but whether it originates from Majorana fermions or bosonic magnons remains controversial. Here we distinguish these origins from low-temperature measurements of high-resolution specific heat and thermal Hall conductivity with rotating in-plane fields. In the honeycomb bond direction, a distinct closure of the low-energy bulk gap is observed concomitantly with the sign reversal of the Hall effect. General discussions of topological bands show that this is the hallmark of an angle-rotation-induced topological transition of fermions, providing conclusive evidence for the Majorana-fermion origin of the thermal Hall effect in $\alpha$-RuCl$_3$.