A small U approach to the 4k_F reflections in organic linear chain compounds

It is shown that a lattice distortion of wavevector 4k_F with an essentially one-dimensional correlation occurs in a one-dimensional metal when the intramolecular Coulomb interaction U is small compared with the band-width 4t; ( U 4t {reversed tilde equals} 1 4). This one-dimensional property suppresses the real phase-transition temperature well below the mean-field transition, where fluctuative 4k_F distortions build up. This accounts for the 4k_F reflections observed in TTF-TCNQ and several other charge transfer compounds. The strong locking of the electrons to the lattice when the band is 1 4 (or 1 2) full, and weak locking when the band is 1 3 (or 2 3) full, are also accounted for.