We extend our formalism of an ab initio description of electronic screening in solids with a strong ionic component of bonding by introducing electronic density polarization effects of dipole type in addition to charge transfer polarization processes. The latter have been found earlier to be very important for the lattice dynamics and the electron-phonon interaction in the high-temperature superconductors. The dipole deformations are calculated with the Sternheimer method and the influence on the polarizability induced by the crystalline environment and by self-interaction corrections is studied. We further derive detailed expressions for the various coupling coefficients appearing in the dynamical matrix in our formalism and apply this theory in a first step in view of the ultimate goal, namely, the high-temperature superconductors, to the calculation of complete dispersion curves for the alkali halide NaCl and the alkaline-earth oxide MgO taking into account for the first time dipole polarization and charge transfer effects together. The agreement of our calculated results with the experiments is good. As a main effect we find a renormalization of the LO modes and a reduction of the LO-TO splitting by both types of polarization processes. The dipole polarization, however, dominates the electronic screening by far. Finally, we investigate the macroscopic dielectric constant and the transverse effective charges and discuss the results in context with the Clausius-Mossotti relation.

No abstract

We investigate the experimentally observed anomalous softening of certain high-frequency oxygen bondstretching modes during the insulator-metal transition via the underdoped phase in La 2 CuO 4 . The calculations show that the screening processes producing the softening behavior are well described in terms of charge fluctuations on the outer shells of the ions composing the crystal. In order to discriminate between the charge response in the insulator and the metal suitable models are used being consistent with rigorous sum rules for the density response in the long-wavelength limit of the various phases. In the case of the underdoped phase a response sum rule halfway between that of an insulator and a metal is proposed expressing a loss in the partial density of states for the Cu orbitals at the Fermi level.

No abstract

Interlayer phonons are useful probes for getting more insight into the charge response across the CuO2 planes of the high-temperature superconductors (HTSC), which is a matter of controversial in the literature, because the type of charge response is reflected by characteristic signatures of the phonons. In this context we investigate the question of whether c-axis phonons are expected to show metallic or insulating response behaviour. Formerly obtained qualitative results for La2CuO4 are made quantitative by generalizing our previously developed description of the charge response and screening in the HTSC in terms of charge fluctuations (CF) taking additionally dipole fluctuations (DF) into account. Calculations are performed of the c-axis phonon dispersion for different values of the c-axis plasmon frequency in order to model different anisotropies of the material. In this way the effect of the charge response caused by both CF and DF is investigated from the static, adiabatic case to the dynamic, nonadiabatic regime and compared with experimental results from neutron scattering and infrared spectroscopy. From the interpretation of the data, phonon-plasmon mixing becomes very likely. The three dimensionality due to direct Coulomb interaction and dielectric coupling by DF between the layers is shown to be important. Moreover, we calculate for characteristic c-axis modes in the extended model including both CF and DF the displacement-induced self-consistent changes of the crystal potential that is a suitable measure for the strength of electron-phonon coupling.