Thermal Conductivity of the Quantum Spin Liquid Candidate EtMe3Sb[Pd(dmit)2]2: No Evidence of Mobile Gapless Excitations

The thermal conductivity. 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 -> 0. This shows that the low-energy excitations responsible for the sizable residual linear term. in the specific heat C, seen in C/T vs T as T -> 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)(2)Cu-2(CN)(3), another quasi-2D organic spin-liquid candidate where a vanishingly small kappa(0)/T and a sizable gamma 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.