Finite-Temperature Charge-Ordering Transition and Fluctuation Effects in Quasi One-Dimensional Electron Systems at Quarter Filling(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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Finite-temperature charge-ordering phase transition in quasi one-dimensional (1D) molecular conductors is investigated theoretically, based on a quasi 1D extended Hubbard model at quarter filling with interchain Coulomb repulsion V_⊥. The interchain term is treated within a mean-field approximation, and the 1D fluctuations in the chains are fully taken into account by the bosonization theory. Three regions are found depending on the appearance of the charge-ordered state at a finite temperature when V_⊥ is introduced: (i) a weak-coupling region where the system transforms from a metal to a charge-ordered insulator with a finite transition temperature at a finite critical value of V_⊥, (ii) an intermediate region where this transition occurs by infinitesimal V_⊥ due to the stability of the inherent 1D fluctuation, and (iii) a strong-coupling region where the charge ordered state is already realized in a purely 1D case, of which the transition temperature becomes finite with infinitesimal V_⊥. An analytical formula for the V_⊥ dependence of the transition temperature is derived for each region.
2006-06-15