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تسجيل مستخدم جديدBoundary-limited and glassy-like phonon thermal conduction in EtMe$_3$Sb[Pd(dmit)$_2$]$_2$
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Minoru Yamashita
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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, $kappa_0/Tequivkappa/T (T rightarrow 0)$, 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 $ell_{ph}$ saturates at low temperatures, being limited by 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, $ell_{ph}$ shows a glassy-like non-saturating behavior at low temperatures. These results suggest that the crystals with long $ell_{ph}$ are required to discuss the magnetic excitations by thermal conductivity measurements.
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EtMe$_3$Sb[Pd(dmit)$_2$]$_2$, an organic Mott insulator with nearly isotropic triangular lattice, is a candidate material for a quantum spin liquid, in which the zero-point fluctuations do not allow the spins to order. The itinerant gapless excitatio
ns inferred from the thermal transport measurements in this system have been a hotly debated issue recently. While the presence of a finite linear residual thermal conductivity, $kappa_0/T equiv kappa/T (T rightarrow 0)$, has been shown [M. Yamashita {it et al.} Science {bf 328}, 1246 (2010)], recent experiments [P. Bourgeois-Hope {it et al.}, Phys. Rev. X {bf 9}, 041051 (2019); J. M. Ni {it et al.}, Phys. Rev. Lett. {bf 123}, 247204 (2019)] have reported the absence of $kappa_0/T$. Here we show that the low-temperature thermal conductivity strongly depends on the cooling process of the sample. When cooling down very slowly, a sizable $kappa_0/T$ is observed. In contrast, when cooling down rapidly, $kappa_0/T$ vanishes and, in addition, the phonon thermal conductivity is strongly suppressed. These results suggest that possible random scatterers introduced during the cooling process are responsible for the apparent discrepancy of the thermal conductivity data in this organic system. The present results provide evidence that the true ground state of EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ is likely to be a quantum spin liquid with itinerant gapless excitations.
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