Finite-Temperature Phase Transitions in Quasi-One-Dimensional Molecular Conductors

Phase transitions to symmetry-broken states in quarter-filled quasi-one-dimensional molecular conductors, such as DCNQI$_2X$, TMTTF$_2X$, and EDO-TTF$_2X$, are studied theoretically. We consider extended Hubbard chains with on-site, intra-chain, and inter-chain Coulomb interactions, coupled to the lattice degree of freedom by Peierls-type and Holstein-type electron-lattice interactions. We apply the numerical quantum transfer-matrix method to an effective one-dimensional model, treating the inter-chain term within mean-field approximation. Finite-temperature properties are investigated for the charge ordering, the dimer-type Mott transition (bond dimerization), and the spin-Peierls transition (bond tetramerization), by computing the temperature dependences of the order parameters together with those of the charge and spin susceptibilities. A coexistent state of charge order and bond dimerization exhibiting dielectricity is predicted in a certain parameter range, even when intrinsic dimerization is absent. Reference: H. Seo, Y. Motome and T. Kato, preprint(cond-mat/0611499) to be published in J. Phys. Soc. Jpn. {\bf 76} (2007) No. 1.