Quantum spin liquid in the spin-$1∕2$ triangular antiferromagnet $\mathrm{Et}{\mathrm{Me}}_3\mathrm{Sb}{\left[\mathrm{Pd}{\left(\text{dmit}\right)}_2\right]}_2$

The family of layered organic salts $X{\left[\mathrm{Pd}{\left(\text{dmit}\right)}_2\right]}_2$ are Mott insulators and form scalene-triangular spin-$1∕2$ systems. Among them, $\mathrm{Et}{\mathrm{Me}}_3\mathrm{Sb}{\left[\mathrm{Pd}{\left(\text{dmit}\right)}_2\right]}_2$ has a nearly regular-triangular lattice. We have investigated the spin state of this salt by ${}^{13}\mathrm{C}\text{−}\mathrm{NMR}$ 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 $J=220-250\mathrm{K}$. The ${}^{13}\mathrm{C}\text{−}\mathrm{NMR}$ measurements reveal that there is no indication of either spin ordering/freezing or an appreciable spin gap down to $1.37\mathrm{K}$, 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.