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A Four-Band Hubbard Model for Doped CuO5 and Related Clusters. Populations of Holes on Apex Oxygens Determined by the Full VB CI Method

Four-band Hubbard Hamiltonians for CuO5 and related clusters were exactly diagonalized by the valence-bond (VB) full configuration interaction (CI) technique. It was found that O2p holes are mainly introduced into the pxy=orbitals of the CuO2 plane, while they can be distributed over the 2pz-orbitals of apex oxygens under the specific condition that the orbital energy gap between apex and plane oxygens exceeds a certain threshold (1.5 eV). Implications of these results are discussed in relation to high-Tc superconductivity for copper oxides.