Exact diagonalization study of the hole distribution in CuO3 chains within the four-band dp model

We consider the hole distribution in CuO3 chains as a function of the total hole number 0 less than or equal to n less than or equal to 2 per chain unit within the standard dp model containing one orbital per site. The nonequivalency of the apical and the chain oxygen sites is taken into account explicitly. Using slightly modified standard CuO2 plane parameter sets, we compare the results of exact diagonalization studies of periodic and open CuO3 chains with experimental data for YBa2Cu3O7 - delta (YBCO) and Ca(Sr)(2)CuO3, and with theoretical results obtained within the following often used approximations: the Hartree-Fock and the local ansatz approximations, as well as with the exclusion of double occupancies at Cu sites and also that at oxygen ones (spinless fermion picture). We have found out that the ratio of the hole densities on the apical and the chain oxygen sites is sensitive to the magnitude of the nearest-neighbor Coulomb interaction V-pd and the difference of their site energies Delta(pp). Adopting values V(pd)similar to 1 eV and V(pp)similar to 0.5V(pd), we estimate Delta(pp)=1.5 to 2.5 eV for YBCO from the comparison with O Is x-ray absorption spectroscopy and O-17 nuclear magnetic resonance data. Using experimental values of the corresponding binding energies and the components of the electric field gradient tensor, the site energy of the apical oxygen 2p(z) states relative to the planar copper Cu(2) 3d(x2-y2) states is estimated as similar to 5 to 6 eV. With increasing hole doping and/or the strength of V-pd, the holes are increasingly localized at the apical oxygen sites. Various chain aspects of proposed scenarios for the PrBa2Cu3O7 problem are briefly discussed.