SCREENING AND PHONON-PLASMON SCENARIO AS CALCULATED FROM A REALISTIC ELECTRONIC BAND-STRUCTURE BASED ON LDA FOR LA2CUO4

The investigations performed in this work primarily deal with the ques tion whether or no phonon-plasmon coupling along the c-axis in metalli c La2CuO4 will occur in case a realistic electronic bandstructure base d on the local density approximation (LDA) is employed for the calcula tion of the electronic Polarizability Pi and the dielectric matrix eps ilon. Phonon-plasmon mixing will appear in a significant way if the fr equency of the free, uncoupled plasmon, as determined from the zeros o f the dielectric matrix epsilon, 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 bandstru cture along the c-axis must be sufficiently weak (small bandwidth). Qu alitatively, we recently have studied phonon-plasmon mixing by using a n Ii-band tight binding model for the electronic bandstructure of the CuO plane, while the electronic bandstructure along thee-axis was intr oduced from the outside applying suitable interlayer couplings in para metrized form. In the present calculation the electronic bandstructure employed for the calculation of Pi and epsilon , respectively, result s 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 mod el). No additional assumptions concerning the interlayer coupling are introduced in contrast to the 11-band model. Besides the nonadiabatic regime along the Lambda similar to (0, 0, 1) direction where dynamical screening and phonon-plasmon mixing comes into play, the phonon dispe rsion is also calculated for the main symmetry directions using the ad iabatic approximation, i.e. assuming static screening. Besides other f eatures, from this calculation the growing importance of the itinerant component of the charge response in the CuO plane to achieve high-T-c values can be extracted. The charge fluctuations and the self-consist ent changes of the crystal potential at the ion sites as induced by th e symmetrical apical oxygen breathing mode at the Z point (O-Z(z)), be ing important for nonlocal electron-phonon interaction and pair bindin g, are calculated for the phonon-like and plasmon-like modes. Furtherm ore, a comparison is made with recent inelastic neutron scattering res ults for certain phonon anomalies. Finally, a possible solution concer ning the apparent inconsistency of the current interpretation of the n eutron c-axis data with the infrared c-axis optical spectra in La2CuO4 is shortly outlined. Based on this discussion, the possibility for th e application of a typical (LDA) bandstructure result for the descript ion of the c-axis response in LaCuO the other HTSCs is estimated.