Oscillations of the thermal conductivity observed in the spin-liquid state of $\alpha$-RuCl$_3$

TL;DR

This study reports quantum-like oscillations in the thermal conductivity of the spin-liquid candidate $ ext{α-RuCl}_3$ under magnetic fields, revealing novel quantum phenomena in an insulator with potential implications for understanding quantum spin liquids.

Contribution

It provides the first observation of oscillations in thermal conductivity in a spin-liquid candidate, suggesting quantum oscillation phenomena in an insulator with a large energy gap.

Findings

Oscillations are periodic in 1/H and prominent within 7.3-11 T.

Oscillation amplitude decreases exponentially above 0.5 K.

Oscillations are linked to in-plane magnetic field components.

Abstract

In the class of materials called spin liquids, a magnetically ordered state cannot be attained even at milliKelvin temperatures because of conflicting constraints on each spin (for e.g. from geometric or exchange frustration). The resulting quantum spin-liquid (QSL) state is currently of intense interest because it exhibits novel excitations as well as wave-function entanglement. The layered insulator $\alpha$-RuCl$_3$ orders as a zigzag antiferromagnet below $\sim$7 K in zero magnetic field. The zigzag order is destroyed when a magnetic field $\bf H$ is applied parallel to the zigzag axis a. Within the field interval (7.3, 11) Tesla, there is growing evidence that a QSL state exists. Here we report the observation of oscillations in its thermal conductivity below 4 K. The oscillation amplitude is very large within the interval (7.3, 11) T and strongly suppressed on either side. Paradoxically, the oscillations are periodic in 1/\emph{H}, analogous to quantum oscillations in metals, even though $\alpha$-RuCl$_3$ is an excellent insulator with a gap of 1.9 eV. By tilting $\bf H$ out of the plane, we find that the oscillation period is determined by the in-plane component $H_a$. As the temperature is raised above 0.5 K, the oscillation amplitude decreases exponentially. The decrease anticorrelates with the emergence above $\sim$2 K of an anomalous planar thermal Hall conductivity measured with $\bf H\parallel a$. To exclude extrinsic artifacts, we carried out several tests. The implications of the oscillations are discussed.