Magnetic thermal conductivity far above the N\'eel temperatures in the Kitaev-magnet candidate $\alpha$-RuCl$_{3}$

TL;DR

This study reveals a high-temperature peak in thermal conductivity of $ ext{α-RuCl}_3$, attributed to itinerant spin excitations linked to Kitaev couplings, indicating long-distance magnetic coherence above the Néel temperature.

Contribution

It demonstrates that magnetic excitations contribute to thermal conductivity well above the magnetic ordering temperature in a Kitaev candidate material.

Findings

High-temperature peak in thermal conductivity around 100 K

Magnetic excitations have a mean free path of about 20 nm at 100 K

Magnetic thermal conductivity is robust against structural phase transitions

Abstract

We have investigated the longitudinal thermal conductivity of $\alpha$-RuCl$_{3}$, the magnetic state of which is considered to be proximate to a Kitaev honeycomb model, along with the spin susceptibility and magnetic specific heat. We found that the temperature dependence of the thermal conductivity exhibits an additional peak around 100 K, which is well above the phonon peak temperature ($\sim$ 50 K). The higher-temperature peak position is comparable to the temperature scale of the Kitaev couplings rather than the N\'eel temperatures below 15 K. The additional heat conduction was observed for all five samples used in this study, and was found to be rather immune to a structural phase transition of $\alpha$-RuCl$_{3}$, which suggests its different origin from phonons. Combined with experimental results of the magnetic specific heat, our transport measurement suggests strongly that the higher-temperature peak in the thermal conductivity is attributed to itinerant spin excitations associated with the Kitaev couplings of $\alpha$-RuCl$_{3}$. A kinetic approximation of the magnetic thermal conductivity yields a mean free path of $\sim$ 20 nm at 100 K, which is well longer than the nearest Ru-Ru distance ($\sim$ 3 \AA), suggesting the long-distance coherent propagation of magnetic excitations driven by the Kitaev couplings.