Boundary-limited and glassy-like phonon thermal conduction in EtMe${}_3$Sb[Pd(dmit)${}_2$]${}_2$

In molecular-based quantum-spin-liquid candidate EtMe${}_3$Sb[Pd(dmit)${}_2$]${}_2$ with two-dimensional $S$=1/2 triangular lattice, a finite residual linear term in the thermal conductivity, ${\kappa }_0\mathrm{/}T\equiv \kappa \mathrm{/}T(T\to 0)$, 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 ${\kappa }_0\mathrm{/}T$, the phonon thermal conductivity ${\kappa }_{ph}$ is comparable to that of other organic compounds. In these crystals, the phonon mean free path ${\mathrm{\ell }}_{ph}$ saturates at low temperatures, being limited by sample size. On the other hand, in the second group with zero ${\kappa }_0\mathrm{/}T$, ${\kappa }_{ph}$ is one order of magnitude smaller than that in the first group, comparable to that of amorphous solids. In contrast to the first group, ${\mathrm{\ell }}_{ph}$ shows a glassy-like non-saturating behavior at low temperatures. These results suggest that the crystals with long ${\mathrm{\ell }}_{ph}$ are required to discuss the magnetic excitations by thermal conductivity measurements.