Unusual thermal Hall effect in a Kitaev spin liquid candidate $\alpha$-RuCl$_3$

TL;DR

This study reports the observation of a thermal Hall effect in $ ext{α-RuCl}_3$, supporting the presence of Majorana fermion excitations predicted in the Kitaev quantum spin liquid, with implications for topological quantum states.

Contribution

First experimental measurement of thermal Hall conductivity in a Kitaev candidate, revealing signatures consistent with Majorana fermion excitations in a spin liquid.

Findings

Positive $ ext{κ}_{xy}$ develops below 80 K, indicating itinerant excitations.

The $ ext{κ}_{xy}/T$ behavior aligns with theoretical predictions for Majorana fermions.

Thermal Hall effect persists despite magnetic ordering at 7 K, suggesting fractionalized excitations.

Abstract

The Kitaev quantum spin liquid displays the fractionalization of quantum spins into Majorana fermions. The emergent Majorana edge current is predicted to manifest itself in the form of a finite thermal Hall effect, a feature commonly discussed in topological superconductors. Here we report on thermal Hall conductivity $\kappa_{xy}$ measurements in $\alpha$-RuCl$_3$, a candidate Kitaev magnet with the two-dimensional honeycomb lattice. In a spin-liquid (Kitaev paramagnetic) state below the temperature characterized by the Kitaev interaction $J_K/k_B \sim 80$ K, positive $\kappa_{xy}$ develops gradually upon cooling, demonstrating the presence of highly unusual itinerant excitations. Although the zero-temperature property is masked by the magnetic ordering at $T_N=7$ K, the sign, magnitude, and $T$-dependence of $\kappa_{xy}/T$ at intermediate temperatures follows the predicted trend of the itinerant Majorana excitations.