MANY-BODY EFFECTS ON POLARIZATION AND DYNAMICAL CHARGES IN A PARTLY COVALENT POLAR INSULATOR

In a mixed ionic/covalent insulator the dynamical ionic charges are much larger than the static ones. The phenomenon is particularly relevant in ferroelectric perovskites, and is well understood at the mean-field level. Here we use a model Hamiltonian to investigate polarization in a strongly correlated, and partially ionic, insulator. Charge transport is evaluated as a Berry phase. At a critical value of the electron-electron interaction, the model has a transition from a band insulator to a Mott insulator: the static ionic charge is continuous across the transition, whereas the polarization is discontinuous. Above the transition, the anion transports a positive charge. © 1995 The American Physical Society.