ROLE OF BONDING REDUCED SCREENING, AND STRUCTURE IN THE HIGH-TEMPERATURE SUPERCONDUCTORS

In this paper we have studied the role of bonding, reduced screening, and structure in high-temperature superconductors (HTSC) using La2CuO4 as a prototype material. In addition to the ionic and metallic compon ent of bonding recently investigated, we estimate the influence of cov alence effects on the structural parameters and the phonon spectrum. A lso, the effect of a reduced dispersion of the electronic band structu re on the polarizability and the phonon frequencies is considered and (hypothetical) mechanisms are investigated, which would favor ferroele ctricity in the HTSC. In a further topic we deal with reduced screenin g, introduced by nonadiabatic effects related to the quasi-two-dimensi onal band structure and its consequence for the electron-phonon intera ction, the phonon dispersion, and the pairing mechanism. We then focus our interest on features related to the tetragonal-to-orthorhombic ph ase transition in La2CuO4. Structural parameters are calculated and th e influence of the transition on the phonon dispersion as well as on t he phonon-induced charge fluctuations is discussed. Finally, we presen t, on the basis of nonlocal electron-phonon interaction of the charge- fluctuation type, a theoretical interpretation of a universal relation ship between T(c) and the hole content in the HTSC.