The electronic structure of copper oxides has been investigated by photoelectron (x-ray photoemission, ultraviolet photoemission), Auger electron, and bremsstrahlung isochromat spectroscopies. The experimental results are compared with one-electron band-structure calculations as well as with a cluster configuration interaction model. It is demonstrated that the results for ${\mathrm{Cu}}_2$O agree well with band theory, whereas those for CuO clearly show strong deviations which we argue are due to electron-correlation effects in the open-shell $d$ bands. From the comparison to cluster calculations we extract values for the $\mathrm{Cu} d-d$ and $\mathrm{O} p-p$ Coulomb interactions, the O to Cu charge transfer energy, and the degree of $\mathrm{Cu} d-\mathrm{O} 2p$ hybridization. From this we demonstrate that CuO is a charge-transfer gap insulator.

• Received 25 July 1988

DOI:https://doi.org/10.1103/PhysRevB.38.11322