13C NMR study of the spin-liquid state in the triangular quantum antiferromagnet EtMe3Sb[Pd(dmit)2]2

The family of layered organic salts X[Pd(dmit)(2)](2) are Mott insulators and form scalene-triangular spin-1/2 systems with antiferromagnetic exchange interactions J. Among them, EtMe3Sb[Pd(dmit)(2)](2) has a nearly regular-triangular spin system with J - 220-250 K. Our previous study proposed that a quantum spin-liquid state is realized in this system because this system was revealed to have neither spin ordering/freezing nor an appreciable spin gap down to 1.37 K. In the present study, to confirm the absence of spin ordering/freezing in the low-temperature limit, we cooled this system down to 19.4 mK, which is 4 orders of magnitude smaller than J, and measured C-13-NMR spectra. The obtained spectra do not show any critical broadening. Therefore, it is concluded that the spin system of EtMe3Sb[Pd(dmit)(2)](2) does not have spin ordering/freezing in the low-temperature limit; that is, the spin system is in a quantum-fluctuating spin state.