Collective charge excitation in low dimensional organic salts

Electronic ferroelectricity is known as phenomena where electric polarization is attributed to the charge order without inversion symmetry. This is seen in some transition metal oxides, e.g. LuFe$_{\mathrm{2}}$O$_{\mathrm{4}}$, and charge transfer salts. Quasi 2-dimesional organic salt kappa-(ET)$_{\mathrm{2}}$Cu$_{\mathrm{2}}$(CN)$_{\mathrm{3\thinspace }}$is one of the electronic ferroelectricities. Two ET molecules construct a dimer and are arranged on a triangular lattice. Recently, it is reported that a dielectric anomaly is experimentally observed around 30K. An origin of this dielectric anomaly is thought to be an ?electronic? dipole generated by a localized hole in one side of the ET molecules in dimers. Motivated by the experimental results, we study charge dynamics in dimer-Mott insulating system with internal charge degree of freedom in a dimer. We adopt the three kinds of models, extended Hubbard model, V-t model and its effective pseudo-spin model. We analyze these models by utilizing the exact diagonalization method and spin wave approximation, and focus on the collective charge excitation. In the ground state, paraelectric dimer-Mott phase and ferroelectric charge ordered phase compete with each other. We find the low-energy intra-dimer charge excitations which show a strong light polarization dependence. The collective excitation mode which is observable by light being parallel to the electric polarization shows a softening and a remarkable frequency dispersion around the phase boundary. This collective charge excitation of the ?electronic? dipole explains the recently observed peak structure in optical conductivity for the THz region.