A general two-center six-band Hamiltonian, including copper ${\mathit{d}}_{\mathit{x}}^2$-${\mathit{y}}^2$ and ${\mathit{d}}_{3\mathit{z}}^2$-${\mathit{r}}^2$ as well as oxygen ${\mathit{p}}_{\mathrm{\sigma }}$ and apical ${\mathit{p}}_{\mathit{z}}$ orbitals, is used to calculate the electronic structure of copper-oxide superconductors, within a mean-field approximation. Size effects as well as the role of apical oxygen have been investigated in clusters of different sizes as a function of doping. For square clusters (2n×2n) three low-energy self-consistent solutions have been found. The lowest-energy solution is of the charge- and spin-bag type, being characterized by enhanced Zhang-Rice correlations and a broad density-of-states structure in the Fermi-level (${\mathit{E}}_{\mathit{F}}$) region. Although this structure has mainly nonplanar character just at ${\mathit{E}}_{\mathit{F}}$, its composition at the maximum, just below ${\mathit{E}}_{\mathit{F}}$, is comparable with experimental results. These features may be relevant in the superconducting regime.

• Received 19 October 1993

DOI:https://doi.org/10.1103/PhysRevB.49.9125