Spin Liquid State in an Organic Mott Insulator with a Triangular Lattice

${}^1\mathrm{H}$ NMR and static susceptibility measurements have been performed in an organic Mott insulator with a nearly isotropic triangular lattice, $\kappa \mathrm{\text{−}}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\mathrm{\text{−}}\mathrm{T}\mathrm{T}\mathrm{F}{)}_2{\mathrm{C}\mathrm{u}}_2(\mathrm{C}\mathrm{N}{)}_3$, which is a model system of frustrated quantum spins. The static susceptibility is described by the spin $S=1/2$ antiferromagnetic triangular-lattice Heisenberg model with the exchange constant $J\sim 250\mathrm{K}$. Regardless of the large magnetic interactions, the ${}^1\mathrm{H}$ NMR spectra show no indication of long-range magnetic ordering down to 32 mK, which is 4 orders of magnitude smaller than $J$. These results suggest that a quantum spin liquid state is realized in the close proximity of the superconducting state appearing under pressure.