Quantum electric dipoles in spin liquid dimer Mott insulator

We present an effective dipolar-spin model based on the strong coupling analysis, which may explain the possible origin of the ``spin liquid Mott insulator'' of the organic triangular lattice system, kappa-ET$_{2}$Cu$_{2}$ (CN)$_{ 3}$, whose gapless spin liquid state had been discussed in the context of geometrical frustration of exchange coupling, $J$, between spins on dimer orbitals. We find out that another degrees of freedom within the insulator, quantum electric-dipoles on dimers, interacts with each other by the Coulomb interaction and brings about a significant suppression of$ J$ through the dipolar-spin coupling.