Magnetodielectric phenomena in charge-spin coupled system in electronic ferroelectricity

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$_{2}$O$_{4}$, and charge transfer salts. Quasi 2-dimesional organic salt kappa-(ET)$_{2}$Cu$_{2}$(CN)$_{3 }$is one of the kappa-ET salts of which two ET molecules construct a dimer and are arranged on a triangular lattice. Recently, it is reported that the increase of dielectric constant is experimentally observed below 60K. This indicates the presence of ferro (or anti-ferro) electric transition. An origin of the electric polarization is thought to be generated by a localized hole in one side of ET molecules in dimers [1,2]. We present a theory of magneto-dielectric phenomena in electronic ferroelectricity, in particular for this kappa-ET salt [3].We treat the electric dipole as a pseudo spin and construct an effective model Hamiltonian with the inter-dimer transfer integrals and the Coulomb interactions on a triangular lattice. We analyze this model by utilizing mean-field approximation and classical Monte-Carlo method, and investigate magnetodielectric phenomena which originate from spin-charge coupling and geometrical frustration. [1] T. Sasaki et al. (unpublished). [2] C. Hotta. (unpublished). [3] M. Naka and S. Ishihara.