Effects of electron-electron and electron-phonon interactions in narrow-band systems.

Abstract

Different ordered states, CDW, BOW, and SDW, are investigated theoretically for both interacting and noninteracting cases as well as for different band filling systems. For noninteracting case, we find that the BOW is always accompanied by the CDW and vice versa in one-dimensional system for commensurability > 2. The strong electron-molecular vibration coupling drives both CDW and BOW, and plays thus an important role in the stabilization of the CDW state for these non-half-filled materials. Within the simply extended Peierls-Hubbard model, the experimentally observed lattice distortion of MEM(TCNQ)₂ can be precisely understood with our model calculation. In addition to the on-site repulsion U, the nearest-neighbor Coulomb interaction V is shown to play a vital role in the strongly correlated system; and a critical value V(c) is found for the quarter-filled system. With the understanding of the dominant broken symmetries in quasi-one-dimensional quarter-filled band, some implications for superconducting materials are discussed. The effect of band filling on the 4k(F) BOW instability is studied by the extended Peierls-Hubbard model; it is found that a strongly systematic tendency of the 4k(F) is dependent on the band filling. By studying the pair binding energy in some small clusters, we point out that the electron (or hole) pairing is not due to the Coulomb interaction, at least in the small U region.