Abstract
We study a two-band Peierls-Hubbard model for halogen-bridged mixed-valence transition metal linear chain complexes (MX chains). Electron-electron correlations (both Hubbard and PPP-like expressions) are incorporated using several techniques. A sequence of long period (non-2k F , superlattice) phases is found to occur even at exactly commensurate filling as the strength of on-site electron-phonon coupling is varied. These phases are understood as ordering of “bipolaronic” defects with respect to the commensurate 2k F or other states. Such long period phases may have been observed in MX chains via STM experiments [1]. The superlattice phases are analogous to intrinsic twinning textures. The effect of including the electron-phonon in addition to the electron-electron interaction on the polaron/bipolaron (real space pairing) competition is especially interesting when this class of compounds is viewed as a 1-D analog of high T c superconductors.
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