Numerical Study of Finite-Temperature Phase Transitions in Quasi-One-Dimensional Molecular Conductors

We have theoretically investigated the charge ordering, the dimer Mott, and the spin-Peierls phase transitions in quarter-filled quasi-one-dimensional organic conductors, such as DCNQI$_{2}X$ and TMTTF$_{2}X$, by considering the extended Hubbard model with electron-lattice couplings and inter-chain Coulomb interaction. We apply the stochastic-series-expansion quantum Monte Carlo method to the effective one-dimensional model obtained by the adiabatic and inter-chain mean-field approximation. Temperature dependences of the order parameters and the susceptibilities are calculated for the charge ordering, the dimer Mott, and the spin-Peierls transitions. The results show a competition between the charge-ordered and dimer Mott insulating states, consistent with our previous work [1], and both of them undergo the spin-Peierls transition at low temperatures. There, two types of spin-Peierls phases with spin gap appear in competition with showing different orderings of period four in lattice distortion and charge disproportionation. [1] H. Seo, Y. Motome, and T. Kato, J. Phys. Soc. Jpn. {\bf 76}, 013707 (2007).