The core photoemission spectrum of a copper +2 impurity embedded in a square lattice of oxygen -2 has been calculated using an extended Anderson impurity Hamiltonian including a Cu $3d$ hole-O $2p$ hole repulsive interaction. Comparison with experimental data shows that the ground state of the system is a mixed-valent insulator in the absence of hole doping, in spite of the Cu $3d$ level being within the oxygen $2p$-derived band. Doping of an extra hole induces a transition to a weakly mixed-valent metallic state with drastic modifications in the calculated core-level spectrum.

• Received 3 January 1989

DOI:https://doi.org/10.1103/PhysRevB.39.11570