We present an effective dipolar-spin model based on the strong coupling analysis, which may explain the possible origin of “spin-liquid insulator.” The issue is related to a dimer Mott insulator reminiscent of an organic triangular lattice system, $\kappa {\text{-ET}}_2{\text{Cu}}_2{\left(\text{CN}\right)}_3$, whose gapless spin-liquid state has been discussed in the context of geometrical frustration of exchange coupling, $J$, between spins on dimer orbitals. It turns out that another degrees of freedom within the insulator, quantum electric dipoles on dimers, interact with each other and modify $J$ significantly through the dipolar-spin coupling, resulting in a possible “dipolar-spin liquid.”

• Received 29 November 2009

DOI:https://doi.org/10.1103/PhysRevB.82.241104