BREAKDOWN OF THE SPIN-PEIERLS STATE IN A ONE-DIMENSIONAL MODEL FOR ELECTRON-LIBRON COUPLING

We study a one-dimensional model for electron-libron coupling in polya niline with a half-filled band in the spin-Peierls (SP) limit of stron g repulsive on-site correlations U. A phonon mean-field treatment prod uces a rich phase diagram with an SP state, but undistorted paramagnet ic and antiferromagnetic states may be more stable for certain paramet ers, implying the possibility of dimerization breakdown with increasin g U. This contrasts universally accepted results for polyacetylene whe re dimerization persists for all U. Our results originate in the relev ance of a steric order parameter for librons which distinguishes the s ituation from that for acoustic phonons.