Spin-liquid state in an organic spin-1/2 system on a triangular lattice, EtMe3Sb[Pd(dmit)2]2

The series of [ Pd( dmit) 2] salts ( dmit = 1,3- dithiole- 2- thione- 4,5- dithiolate) is a Mott insulator on an anisotropic triangular lattice and has an antiferromagnetic exchange constant J similar to 200 K. While it has been revealed that most of them undergo antiferromagnetic transitions, the nature of the spin state of EtMe3Sb[ Pd( dmit) 2] 2 which has a nearly isotropic triangular lattice is still unknown. We investigated the spin state by means of C-13 NMR measurements down to 1.37 K for polycrystalline samples in which the carbon atoms in both ends of the Pd( dmit) 2 molecule were selectively enriched with the C-13 isotope. Both behaviours of the spin - lattice relaxation rate and the spectra indicate no spin ordering/ freezing at least down to 1.37 K, which is lower than onehundredth of the exchange interaction. Furthermore, the spin - lattice relaxation rate does not show an exponential temperature dependence and seems to retain a finite value at low temperatures. Thus, the ground state of this systemis possibly the almost- gapless spin- liquid state.