Electronic Ferroelectricity in a Dimer Mott Insulator

Motivated from recent experiments in kappa-(BEDT-TTF)_2Cu_2(CN)_3, dielectric and magnetic properties in a dimer-Mott insulator are studied. We derive the effective Hamiltonian where the number of electron per dimer is taken to be one. Charge and magnetic structures in finite temperature are obtained by using the mean-field and Monte-Carlo methods. Magnetic and ferroelectric phases are exclusive with each other. Dielectric fluctuation is remarkable near a phase boundary between dimer-Mott phase and ferroelectric phase. Implications of the present results for kappa-(BEDT-TTF)_2Cu_2(CN)_3 are discussed.