The organic triangular-lattice 1/2-spin system EtMe${}_3$Sb[Pd(dmit)${}_2$]${}_2$ does not undergo classical antiferromagnetic ordering and is a strong candidate for a quantum spin liquid. It is currently an open issue whether or not the ground state of this system has a broken-symmetry and/or topological structure, and what kind of excitations the ground state has. We have performed NMR measurements on the inner carbons of the Pd(dmit)${}_2$ molecule in order to get reliable information on this issue. By comparing the inner-carbon NMR data with the previously obtained outer-carbon data, we have reached a robust conclusion that these NMR data are not affected by cation molecular motion and definitely reflect the spin state of this system. The temperature dependencies of the spin-lattice relaxation rates suggest that the spin state undergoes an abrupt change at around 1 K, and, thus, the ground state is likely to have a broken-symmetry and/or topological structure. The rates also strongly indicate that the imaginary part of the $q$-integrated dynamic susceptibility (${\lim }_{\omega \to 0}{\sum }_q{\chi }^{\prime \prime }(q,\omega )/\omega$) disappears as $T\to 0$.

• Received 4 August 2011

DOI:https://doi.org/10.1103/PhysRevB.84.094405