CALCULATED EFFECTS OF CHARGE FLUCTUATIONS ON THE PHONON-DISPERSION OFYBA2CU3O6 AND YBA2CU3O7 .1. THE MODEL

In this paper we present a microscopic model that allows us to study t he effects of charge fluctuations on the phonon dispersion of the high -temperature superconductor YBa2Cu3O7 and its insulating counterpart, YBa2Cu3O6. An ab-initio rigid-ion model with pair potentials calculate d by the Gordon-Kim method from the free-ion charge densities is used as a reference system. Starting from this reference system, charge flu ctuations at the copper- and oxygen ions are introduced into the model . The charge fluctuations are treated as adiabatic degrees of freedom in a non-phenomenological way. The parameters entering the model are e stimated consistently with the reference system from first principles rather than refering to the experimentally determined phonon dispersio n. In addition to the metallic behavior (appropriate to YBa2Cu3O7) obt ained in this way, insulating behavior (appropriate to YBa2Cu3O6) is s imulated by requiring the polarizability function to fulfill a corresp onding long-wavelength sum rule. Screened site-potential changes are d efined that (besides the charge fluctuations) constitute a qualitative measure of the electron-phonon-interaction potential. Furthermore we investigate the long-wavelength limiting behavior of the most importan t quantities occurring in our formalism. We derive formulae that allow us to calculate the contribution of the charge fluctuations to the ma croscopic dielectric constant and the transverse effective charges in the insulating phase.