FINITE-SIZE STUDIES OF PHASES AND DIMERIZATION IN ONE-DIMENSIONAL EXTENDED PEIERLS-HUBBARD MODELS

We study extended Hubbard and Peierls-Hubbard models with up to N=10 sites and half-filled bands with modified boundary conditions. For the extended Hubbard model, the finite-size dependence of the transition from the charge-density-wave regime (CDW) to the spin-density-wave (SDW) regime and of the condensation transition is critically examined, clarifying some previous results. For the Peierls-Hubbard model and N10 with modified periodic boundary conditions, we always see a decrease of dimerization with U. Extrapolating these results to infinite N, we find that dimerization is enhanced by U for small electron-phonon coupling ±If<0.75, whereas it decreases with U for 0.75<±If1. By extending this model by a nearest-neighbor repulsion V, the amplitude of dimerization reaches a maximum at the same value U2V, where the CDW-to-SDW transition occurs in the extended Hubbard model. © 1990 The American Physical Society.