Quantum spin liquid in the spin-1/2 triangular 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. Among them, EtMe3Sb[Pd(dmit)(2)](2) has a nearly regular- triangular lattice. We have investigated the spin state of this salt by C-13-NMR and static susceptibility measurements. The temperature dependence of the susceptibility is described as that of a regular-triangular antiferromagnetic spin-1 / 2 system with an exchange interaction J=220-250 K. The C-13-NMR measurements reveal that there is no indication of either spin ordering/ freezing or an appreciable spin gap down to 1.37 K, which is lower than 1% of J. This result strongly suggests that this system is in the quantum spin-liquid state with no appreciable spin gap, which has been long sought after.