Thermal Conductivity of the Quantum Spin Liquid Candidate EtMe_{3}Sb[Pd(dmit)_{2}]_{2}: No Evidence of Mobile Gapless Excitations

Abstract

The thermal conductivity κ 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 κ_{0}/T, in κ/T vs T as T→0. This shows that the low-energy excitations responsible for the sizable residual linear term γ 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 κ_{0}/T value [Yamashita et al., Science 328, 1246 (2010)SCIEAS0036-807510.1126/science.1188200]. Our study shows that dmit-131 is in fact similar to κ-(BEDT-TTF)_{2}Cu_{2}(CN)_{3}, another quasi-2D organic spin-liquid candidate where a vanishingly small κ_{0}/T and a sizable γ 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.