Oscillations in the magnetothermal conductivity of α−RuCl3 : Evidence of transition anomalies

Abstract

The two-dimensional layered insulator $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ is a candidate material for a quantum spin-liquid state, which may be realized when a magnetic field suppresses the antiferromagnetic order present at low temperature. Oscillations in the field dependence of the thermal conductivity, observed for an in-plane magnetic field $B$ up to a critical field ${B}^{★}$, have been attributed to exotic charge-neutral fermions, viewed as evidence of a quantum spin-liquid state between the critical field ${B}_{c}\ensuremath{\simeq}7$ T at which the antiferromagnetic phase ends and ${B}^{★}$. Here we report measurements of the thermal conductivity of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ as a function of magnetic field up to 15 T applied in two distinct in-plane directions: parallel and perpendicular to the Ru-Ru bond. We find that the number of oscillations between ${B}_{c}$ and ${B}^{★}$ is the same for the two field directions even though the field interval between ${B}_{c}$ and ${B}^{★}$ is different. In other words, the period of the oscillations is controlled by the transition fields ${B}_{c}$ and ${B}^{★}$. We conclude that these are not true oscillations---coming from putative fermions in a spin-liquid state---but anomalies associated with a sequence of magnetic transitions.