Large Thermal Hall Effect in $\alpha$-RuCl$_3$: Evidence for Heat Transport by Kitaev-Heisenberg Paramagnons

TL;DR

This study reports a significant thermal Hall effect in $ ext{α-RuCl}_3$, indicating heat transport by fractional quasiparticles consistent with Kitaev physics, especially in the paramagnetic phase above magnetic ordering temperature.

Contribution

The paper provides experimental evidence of heat transport by Kitaev-Heisenberg paramagnons through thermal Hall measurements in $ ext{α-RuCl}_3$, supporting the existence of fractional quasiparticles.

Findings

Observation of positive thermal Hall conductivity $_{xy}$ increasing with magnetic field

Strong temperature dependence of $_{xy}$ with a maximum around 30 K

Significant positive thermal magnetoresistance in longitudinal heat conductivity

Abstract

The honeycomb Kitaev model in a magnetic field is a source of a topological quantum spin liquid with Majorana fermions and gauge flux excitations as fractional quasiparticles. We present experimental results for the thermal Hall effect of the material $\alpha$-RuCl$_{3}$ which recently emerged as a prime candidate for realizing such physics. At temperatures above long-range magnetic ordering $T\gtrsim T_N\approx8$ K, we observe with an applied magnetic field $B$ perpendicular to the honeycomb layers a sizeable positive transversal heat conductivity $\kappa_{xy}$ which increases linearly with $B$. Upon raising the temperature, $\kappa_{xy}(T)$ increases strongly, exhibits a broad maximum at around 30 K, and eventually becomes negligible at $T\gtrsim 125$ K. Remarkably, the longitudinal heat conductivity $\kappa_{xx}(T)$ exhibits a sizeable positive thermal magnetoresistance effect. Thus, our findings provide clear-cut evidence for longitudinal and transverse magnetic heat transport and underpin the unconventional nature of the quasiparticles in the paramagnetic phase of $\alpha$-RuCl$_{3}$.