NEW PHASES IN AN EXTENDED HUBBARD-MODEL EXPLICITLY INCLUDING ATOMIC POLARIZABILITIES

We consider the influence of a nearest-neighbor Coulomb interaction in an extended Hubbard model and introduce a new interaction term which simulates atomic polarizabilities. This has the effect of screening th e on-site Coulomb interaction for charged excitations, unlike a neighb or Coulomb interaction which reduces energies of locally neutral excit ations. We find that the spin density to charge density wave phase tra nsition, however, is determined by the unscreened on-site Coulomb repu lsion. The order of this phase transition is affected by polarization. We show that new phases appear, one of which is ferroelectric, when a tomic polarizabilities are explicitly included.