SPIN-GAP PHASE IN NEARLY HALF-FILLED ONE-DIMENSIONAL CONDUCTORS COUPLED WITH PHONONS

Asymptotic properties of nearly half-filled one-dimensional conductors coupled with phonons are studied through a renormalization-group meth od. Due to spin-charge coupling via an electron-phonon interaction, th e spin correlation varies with filling as well as the charge correlati on. Depending on the relation between cutoff energy scales of the umkl app process and of the electron-phonon interaction, various phases app ear. We find a metallic phase with a spin gap and a dominant charge-de nsity-wave correlation near half filling between a gapless density-wav e phase (like in the doped repulsive Hubbard model) and a superconduct or phase with a spin gap. The spin gap is produced by phonon-assisted backward scatterings that interfere with the umklapp process construct ively or destructively depending on the character of electron-phonon c oupling.