Phonon renormalization and c-axis phonon-plasmon mixing in La2CuO4.

Abstract

A decomposition of the electronic-density response recently worked out for the description of local, nonlocal, and nonadiabatic electron-phonon interaction effects in the high-temperature superconductors is applied to the lattice dynamics and phonon-plasmon mixing in ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$. The local part of the density response and the electron-phonon interaction is approximated by an improved model of rigid ions using effective ionic charges as calculated from a tight-binding analysis of the first-principles electronic band structure. Moreover, covalence effects are simulated by scaling the short-range part of the relevant ab initio pair potentials. Such a model serves as a reference system describing the insulating phase. Phonon dispersion curves are calculated in this model and good overall agreement with recent experiments is found. In the case of the metallic phase, specific nonlocal screening effects in terms of localized charge fluctuations are additionally introduced at the ions by a parameter-free procedure. In the framework of the improved reference system, phonon dispersion curves and phonon-induced changes of the crystal potential are calculated for the metallic phase. Characteristic changes in the phonon dispersion produced by the insulator-metal transition are discussed and compared with the experiments. Further, c-axis phonon-plasmon mixing is investigated. Phonon-plasmon coupling leads in particular to an additional softening of the symmetrical apical oxygen breathing mode at the Z point (${\mathit{O}}_{\mathit{Z}}^{\mathit{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 predicted in our earlier work in the adiabatic approximation as compared to the calculated frequency of ${\mathit{O}}_{\mathit{Z}}^{\mathit{z}}$ in the insulating phase is further enhanced. The anomalous large renormalization of this mode has been confirmed quite recently by the experiments. A final remark is concerned with the oxygen isotope effect in the high-temperature superconductors from the viewpoint of phonon-plasmon coupling.