Collective charge excitation in a dimer mott insulating system

Charge dynamics in a dimer Mott insulating system, where a nonpolar dimer-Mott (DM) phase and a polar chargeordered (CO) phase compete with each other, are studied. In particular, collective charge excitations are analyzed in three different models where the internal degree of freedom in a dimer is taken into account. Collective charge excitation exists in both the nonpolar DM and polar CO phases, and softens in the phase boundary. This mode is observable in the optical conductivity spectra when light polarization is parallel to the electric polarization in the polar CO phase. Connections between the present theory and the recent experimental results for κ-(BEDT-TTF) ₂Cu₂(CN)₃ are discussed.