Quantum spin liquid in the spin-1∕2triangular antiferromagnetEtMe3Sb[Pd(dmit)2]2

Abstract

The family of layered organic salts $X{[\mathrm{Pd}{(\text{dmit})}_{2}]}_{2}$ are Mott insulators and form scalene-triangular spin-$1∕2$ systems. Among them, $\mathrm{Et}{\mathrm{Me}}_{3}\mathrm{Sb}{[\mathrm{Pd}{(\text{dmit})}_{2}]}_{2}$ has a nearly regular-triangular lattice. We have investigated the spin state of this salt by $^{13}\mathrm{C}\text{\ensuremath{-}}\mathrm{NMR}$ 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 $J=220\ensuremath{-}250\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The $^{13}\mathrm{C}\text{\ensuremath{-}}\mathrm{NMR}$ measurements reveal that there is no indication of either spin ordering/freezing or an appreciable spin gap down to $1.37\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, 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.