Screening and Phonon±Plasmon Scenario as Calculated from a Realistic Electronic Bandstructure Based on LDA for La2CuO4

The investigations performed in this work primarily deal with the question whether or no phonon±plasmon coupling along the c-axis in metallic La2CuO4 will occur in case a realistic electronic bandstructure based on the local density approximation (LDA) is employed for the calculation of the electronic polarizability P and the dielectric matrix e. Phonon±plasmon mixing will appear in a significant way if the frequency of the free, uncoupled plasmon, as determined from the zeros of the dielectric matrix e, will fall into the frequency range of the phonon modes of appropriate symmetry coupling to the plasmon. For such a possibility to arise the dispersion of the electronic bandstructure along the c-axis must be sufficiently weak (small bandwidth). Qualitatively, we recently have studied phonon±plasmon mixing by using an 11-band tight binding model for the electronic bandstructure of the CuO plane, while the electronic bandstructure along the c-axis was introduced from the outside applying suitable interlayer couplings in parametrized form. In the present calculation the electronic bandstructure employed for the calculation of P and e, respectively, results from a complete tight-binding representation of the first-principles LAPW data including La 5d, Cu 3d, 4s, 4p and O 2p states (31-band model). No additional assumptions concerning the interlayer coupling are introduced in contrast to the 11-band model. Besides the nonadiabatic regime along the L …0; 0; 1† direction where dynamical screening and phonon±plasmon mixing comes into play, the phonon dispersion is also calculated for the main symmetry directions using the adiabatic approximation, i.e. assuming static screening. Besides other features, from this calculation the growing importance of the itinerant component of the charge response in the CuO plane to achieve high-Tc values can be extracted. The charge fluctuations and the self-consistent changes of the crystal potential at the ion sites as induced by the symmetrical apical oxygen breathing mode at the Z point (OZ), being important for nonlocal electron±phonon interaction and pair binding, are calculated for the phonon-like and plasmon-like modes. Furthermore, a comparison is made with recent inelastic neutron scattering results for certain phonon anomalies. Finally, a possible solution concerning the apparent inconsistency of the current interpretation of the neutron c-axis data with the infrared c-axis optical spectra in La2CuO4 is shortly outlined. Based on this discussion, the possibility for the application of a typical (LDA) bandstructure result for the description of the c-axis response in LaCuO the other HTSCs is estimated.