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The electronic properties of quarter-filled organic materials showing spin-Peierls transition are investigated theoretically. By studying the one-dimensional extended Peierls-Hubbard model analytically as well as numerically, we find that there is a competition between two different spin-Peierls states due to the tetramized lattice distortion in the strongly correlated regime. One is accompanied by lattice dimerization which can be interpreted as a spontaneous Mott insulator, while the other shows the existence of charge order of Wigner crystal-type. Results of numerical density matrix renormalization group computations on sufficiently large system sizes show that the latter is stabilized in the ground state when both the on-site and the inter-site Coulomb interactions are large.Quasi-one-dimensional (1D) organic conductors exhibit a variety of electronic states with spatially inhomogeneous charge, spin, and lattice structures. A typical example is the family of TMTSF 2 X and TMTTF 2 X, 1) where X denotes different anions. Recently, the existence of charge ordering (CO) has been identified in several of such quasi-1D compounds, which has renewed interest in these systems. It was first found in DI-DCNQI 2 Ag, a member of the R 1 R 2 -DCNQI 2 X family, with X= Ag or Li, and R 1 and R 2 taking different substitution groups to modify the DCNQI molecule itself, in which the charge pattern has been identified as a Wigner crystal-type one. 2, 3) Independently, analogous CO was predicted theoretically to also exist in TMTTF 2 X based on the result of mean field calculations, 4) and soon after it was confirmed experimentally. 5,6) In the two families menioned above, the 1D π-band is quarter-filled in terms of electrons or holes. The wave vector along the chain direction for the CO state mentioned above is 4k F , corresponding to the period of two molecules, which suggests the origin of this phenomenon to be the long range nature of Coulomb interaction as theoretically studied in the past. 7) Further studies have been performed on explicit models appropriate for the description of the electronic properties of DCNQI/TMTTF molecules, i.e., 1D extended Hubbard models of quarter filling with on-site and nearest-neighbor Coulomb interactions, U and V , respectively. 4,[8][9][10][11] In those models, the CO insulating ground state is actually stable, in general, when both U and V are appreciably large compared to the transfer integrals.

Mechanisms of a variety of charge and lattice ordered phases observed in halogen-bridged binuclear metal complexes are theoretically studied by applying the exact diagonalization and strong-coupling expansion methods to one-and two-band extended Peierls-Hubbard models. In R 4 [Pt 2 (pop] containing charged MMX chains, three electronic phases are suggested by experiments. We find that the variation of the electronic phases originates not only from competition between site-diagonal electron-lattice and electron-electron interactions but also from competition between short-range and long-range electron-electron interactions. On the other hand, in Pt 2 (RCS 2 ) 4 I (R~CH 3 , n-C 4 H 9 ) containing neutral MMX chains, a site-off-diagonal electron-lattice interaction and the absence of counter ions are found to be crucial to produce the recently found, ordered phase. The optical conductivity spectra are also studied, which directly reflect the electronic phases. Their dependence on the electronic phase and on model parameters is clarified from the strong-coupling viewpoint.

Real-time dynamics of charge density and lattice displacements is studied during photoinduced ionic-to-neutral phase transitions by using a one-dimensional extended Peierls-Hubbard model with alternating potentials for the one-dimensional mixed-stack charge-transfer complex, TTF-CA. The time-dependent Schrödinger equation and the classical equation of motion are solved for the electronic and lattice parts, respectively. We show how neutral domains grow in the ionic background. As the photoexcitation becomes intense, more neutral domains are created. Above threshold intensity, the neutral phase is finally achieved. After the photoexcitation, ionic domains with wrong polarization also appear. They quickly reduce the averaged staggered lattice displacement, compared with the averaged ionicity. As the degree of initial lattice disorder increases, more solitons appear between these ionic domains with different polarizations, which obstruct the growth of neutral domains and slow down the transition.

The dynamical behavior of an acoustic polaron in typical non-degenerate conjugated polymer, polydiacetylene, is numerically studied by using Su-Schrieffer-Heeger's model for the one dimensional electron-lattice system. It is confirmed that the velocity of a polaron accelerated by a constant electric field shows a saturation to a velocity close to the sound velocity of the system, and that the width of a moving polaron decreases as a monotonic function of the velocity tending to zero at the saturation velocity. The effective mass of a polaron is estimated to be about one hundred times as heavy as the bare electron mass. Furthermore the linear mode analysis in the presence of a polaron is carried out, leading to the conclusion that there is only one localized mode, i.e. the translational mode. This is confirmed also from the phase shift of extended modes. There is no localized mode corresponding to the amplitude mode in the case of the soliton in polyacetylene. Nevertheless the width of a moving polaron shows small oscillations in time. This is found to be related to the lowest odd symmetry extended mode and to be due to the finite size effect.