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

Abstract

The thermal conductivity $\hat{I}º$ of the quasi-2D organic spin-liquid candidate EtMe${}_3$Sb[Pd(dmit)${}_2$]${}_2$ (dmit-131) was measured at low temperatures, down to 0.07 K. We observe a vanishingly small residual linear term $\hat{I}{º}_0/T$, in $\hat{I}º/T$ vs $T$ as $T\to 0$. This shows that the low-energy excitations responsible for the sizeable residual linear term ${\hat{I}}^3$ in the specific heat $C$, seen in $C/T$ vs $T$ as $T\to 0,$ 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 $\hat{I}{º}_0/T$ value [Yamashita et al., Science 328, 1246 (2010)]. Our study shows that dmit-131 is in fact similar to $\hat{I}º$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$, another quasi-2D organic spin-liquid candidate where a vanishingly small $\hat{I}{º}_0/T$ and a sizeable ${\hat{I}}^3$ are seen. We attribute heat conduction in these organic insulators without magnetic order to phonons undergoing strong spin-phonon scattering, as observed in several other spin-liquid materials.

This revision includes a response to Yamashita's paper [M. Yamashita, Journal of the Physical Society of Japan 88, 083702 (2019)], data on a new sample, two new figures (Figs. 5 and 8), and a Supplementary Material (available upon request)