Anomalous Thermal Conductivity and Magnetic Torque Response in the Honeycomb Magnet $\alpha \text{−}{\mathrm{RuCl}}_3$

We report on the unusual behavior of the in-plane thermal conductivity $\kappa$ and torque $\tau$ response in the Kitaev-Heisenberg material $\alpha \text{−}{\mathrm{RuCl}}_3$. $\kappa$ shows a striking enhancement with linear growth beyond $H=7\mathrm{T}$, where magnetic order disappears, while $\tau$ for both of the in-plane symmetry directions shows an anomaly at the same field. The temperature and field dependence of $\kappa$ are far more complex than conventional phonon and magnon contributions, and require us to invoke the presence of unconventional spin excitations whose properties are characteristic of a field-induced spin-liquid phase related to the enigmatic physics of the Kitaev model in an applied magnetic field.

Figure 1

(a) In-plane thermal conductivity $\kappa (T)$ shown up to 100 K for Samples 1, 2, and 3 at ${\mu }_{0}H=0\mathrm{T}$ (squares) and 14 T (triangles). The solid lines are a guide to the eye. (b) Low-temperature detail of $\kappa (T)$ for a range of $H$ values, shown for Sample 1. (c) $\kappa (T)$ at low $T$ for Sample 2.

Figure 2

(a) $\kappa (T,H)$ for Sample 1, represented by color contours. The horizontal lines correspond to field sweeps measuring $\kappa (H)$ at fixed $T$. (b) Relative thermal conductivity difference $\mathrm{\Delta }\kappa (H)/{\kappa }_0$ shown for the fixed values of $T$ highlighted in panel (a); the curves are presented with constant offsets.

Figure 3

In-plane torque response as a function of $H$, measured at selected values of $T$ with $\varphi =-69°\pm 2°$. Right inset: measurement configuration. Left inset: $\varphi$ dependence of $\tau$ at fields of 4 and 7 T; ${\varphi }_1=-64°$ and ${\varphi }_2=28°$ exhibit 90° symmetry ($\varphi =0°$ is chosen arbitrarily).

Figure 4

(a) $(H,T)$ phase diagram of $\alpha \text{−}{\mathrm{RuCl}}_3$ inferred from the magnitude of $M_{\tau }$ measured at $\varphi \simeq -69°$ (color scale). The white circles indicate $H_{\min }$ as a function of $T$, the cyan circles indicate the position of local minima in $M_{\tau }(H)$, and the green squares indicate the locations of inflection points appearing in $\mathrm{\Delta }\kappa /{\kappa }_0(H)$ [Fig. 2].