PHONON RENORMALIZATION AND C-AXIS PHONON-PLASMON MIXING IN LA2CUO4

A decomposition of the electronic-density response recently worked out for the description of local, nonlocal, and nonadiabatic electron-pho non interaction effects in the high-temperature superconductors is app lied to the lattice dynamics and phonon-plasmon mixing in La2CuO4. The local part of the density response and the electron-phonon interactio n is approximated by an improved model of rigid ions using effective i onic charges as calculated from a tight-binding analysis of the first- principles electronic band structure. Moreover, covalence effects are simulated by scaling the short-range part of the relevant ab initio pa ir potentials. Such a model serves as a reference system describing th e insulating phase. Phonon dispersion curves are calculated in this mo del and good overall agreement with recent experiments is found. In th e case of the metallic phase, specific nonlocal screening effects in t erms of localized charge fluctuations are additionally introduced at t he ions by a parameter-free procedure. In the framework of the improve d reference system, phonon dispersion curves and phonon-induced change s of the crystal potential are calculated for the metallic phase. Char acteristic changes in the phonon dispersion produced by the insulator- metal transition are discussed and compared with the experiments. Furt her, c-axis phonon-plasmon mixing is investigated. Phonon-plasmon coup ling leads in particular to an additional softening of the symmetrical apical oxygen breathing mode at the Z point (O-Z(z)) being important in context with nonlocal electron-phonon interaction and pair binding. In this way the already large decrease in frequency of this phonon pr edicted in our earlier work in the adiabatic approximation as compared to the calculated frequency of O-Z(z) in the insulating phase is furt her enhanced. The anomalous large renormalization of this mode has bee n confirmed quite recently by the experiments. A final remark is conce rned with the oxygen isotope effects in the high-temperature supercond uctors from the viewpoint of phonon-plasmon coupling.