Electronic structure and hole-hole coupling in YBa2Cu3O7-x systems

The electronic structure of YBa2Cu3O7-x for x=0, has been determined by means of an approximation to the self-energy for Hubbard systems with two channels for the localization (p and d). This self-energy contains the unrestricted Hartree-Fock terms and the dynamically screened exchange. The calculated electronic structure presents a different ionization state for the atoms of the CuO2 sheets with respect to the CuO3 chain. For increasing x values, electron transferences between O-O, Cu-Cu, and Cu-O atoms are produced. These electronic transferences provide holes to the CuO2 sheets in symmetries dx2-y2 of Cu(2) and px and py of O(2) and O(3). These holes suffer interatomic screening, which produces coupling for determined densities of holes and widths of the bands. The main parameter for obtaining superconductivity is the lower limit of the frequency interval for which the interatomic Wpd(ω) is negative. This frequency has to be sufficiently less than a critical cutoff frequency. We discuss the conditions of the electronic structure for obtaining high-Tc superconductivity