Thermal conductivity of the quantum spin liquid candidate EtMe3Sb[Pd(dmit)2]2: No evidence of mobile gapless excitations

The thermal conductivity $\kappa$ of the quasi-2D organic spin-liquid candidate EtMe3Sb[Pd(dmit)2]2 (dmit-131) was measured at low temperatures, down to 0.07 K. We observe a vanishingly small residual linear term $\kappa_0$/T, in $\kappa$/T vs T as T $\rightarrow$ 0. This shows that the low-energy excitations responsible for the sizeable residual linear term $\gamma$ in the specific heat C, seen in C/T vs T as T $\rightarrow$ 0, are localized. We conclude that there are no mobile gapless excitations in this spin liquid candidate, in contrast with a prior study of dmit-131 that reported a large $\kappa_0$/T value [Yamashita et al., Science 328, 1246 (2010)]. Our study shows that dmit-131 is in fact similar to $\kappa$-(BEDT-TTF)2Cu2(CN)3, another quasi-2D organic spin-liquid candidate where a vanishingly small $\kappa_0$/T and a sizeable $\gamma$ are seen. We attribute heat conduction in these insulators without magnetic order to phonons undergoing strong spin-phonon scattering.