Boundary-limited and Glassy-like Phonon Thermal Conduction in EtMe3Sb[Pd(dmit)2]2

In molecular-based quantum-spin-liquid candidate EtMe3Sb[Pd(dmit)(2)](2) with two-dimensional S = 1/2 triangular lattice, a finite residual linear term in the thermal conductivity, kappa(0)/T kappa/T(T -> 0), has been observed and attributed to the presence of itinerant gapless excitations. Here we show that the data of kappa measured in several single crystals are divided into two groups with and without the residual linear term. In the first group with finite kappa(0)/T, the phonon thermal conductivity kappa(ph) is comparable to that of other organic compounds. In these crystals, the phonon mean free path l(ph) saturates at low temperatures, being limited by the sample size. On the other hand, in the second group with zero kappa(0)/ T, kappa(ph) is one order of magnitude smaller than that in the first group, comparable to that of amorphous solids. In contrast to the first group, l(ph) shows a glassy-like non-saturating behavior at low temperatures. These results suggest that the crystals with long l(ph) are required to discuss the magnetic excitations by thermal conductivity measurements.