The presence or absence of itinerant gapless excitations in the organic quantum spin liquid (QSL) candidate EtMe₃Sb[Pd(dmit)₂]₂, probed by a finite residual linear term in the thermal conductivity κ₀ ∕T ≡ κ ∕T (T → 0), has been controversial hot topic. We find that κ₀ ∕T strongly depends on the cooling process. A finite κ₀ ∕T is observed in a slow-cooled sample, but not in a rapid-cooled sample [1]. These results provide evidence that the true ground state of EtMe₃Sb[Pd(dmit)₂]₂ is a QSL with itinerant gapless excitations.

The organic Mott insulator EtMe3Sb[Pd(dmit)₂]₂ possesses a nearly isotropic triangular lattice of localized spin-1/2 on the Pd(dmit)₂ dimers. This geometrical frustration effect caused by the triangular lattice and a proximity effect of the Mott transition are believed to enhance the zero-point fluctuation not to allow the spins to order. While the presence of a finite linear residual thermal conductivity, κ₀ ∕T ≡ κ ∕T (T → 0) has been shown [2], recent experiments [3, 4, 5] have reported the absence of κ₀ ∕T in some samples. (Fig. 1). However, in the experiments reporting the absence of κ₀ ∕T, the phonon thermal conductivity is strongly suppressed [3]. In sharp contrast, a large phonon thermal conductivity with a phonon mean free path reaching almost the boundary limit is observed in the experiments showing the presence of the κ₀ ∕T. Therefore, it is not clear if the absence of κ₀ ∕T shows absence of the gapless spin excitations or if the gapless spin excitations are simply suppressed when the phonons are strongly scattered.

Fig. 1. Temperature dependence of the thermal conductivity divided by the temperature of EtMe₃Sb[Pd(dmit)₂]₂. The data of the previous reports [2, 3] are also plotted.

Here we show that both the magnitude of the phonon thermal conductivity and the presence/absence of κ₀ ∕T strongly depend on the cooling process of the sample. When cooling down very slowly, a sizable κ₀ ∕T is observed. In contrast, when cooling down rapidly, κ₀ ∕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 EtMe₃Sb[Pd(dmit)₂]₂ is likely to be a quantum spin liquid with itinerant gapless excitations.

Fig. 2. Low-temperature data of κ ⁄ T plotted as a function of T. The dashed lines show an extrapolation of the data.

References

• [1] M. Yamashita et al., Phys. Rev. B 101, 140407(R) (2020) * Editor’s suggestion.
• [2] M. Yamashita et al., Science 328, 1246 (2010).
• [3] M. Yamashita, J. Phys. Soc. Jpn. 88, 083702 (2019).
• [4] P. Bourgeois-Hope et al., Phys. Rev. X 9, 041051 (2019)
• [5] J. M. Ni et al., Phys. Rev. Lett. 123, 247204 (2019).

Authors

• M. Yamashita, Y. Sato^a, T. Tominaga^a, Y. Kasahara^a, S. Kasahara^a, H. Cui^a, R. Kato^a, T. Shibauchi^c, and Y. Matsuda^a
• ^aKyoto University
• ^bRIKEN
• ^cThe University of Tokyo