INCOMMENSURATE GROUND-STATES OF A COMMENSURATE PEIERLS-HUBBARD HAMILTONIAN

The possibility of long-period ("superlattice") ground states is systematically investigated in a commensurate (3/4-filled), one-dimensional two-band Peierls-Hubbard Hamiltonian. This model has been proposed to describe halogen-bridged mixed-valence linear-chain compounds and is a one-dimensional analog of a Hamiltonian proposed to model cuprate and bismuthate oxide superconductors. Conventional superlattice phases are driven by competing length scales (for example, from incommensurate filling or an external incommensurate potential). However, these long-period ground states are intrinsic to a competition between long-range repulsive Coulomb forces and the attractive on-site electron-phonon interaction, even at exactly 3/4 filling where a period of four lattice constants might be naively expected by Peierls's argument.