Analysis of the core-level photoemission spectra of the superconducting cuprates: evidence for a strongly mixed-valent state

Abstract

We analyze the Cu 2p spectra from a large number of cuprate superconductors and related compounds within the cluster approximation including configuration interaction. We show that the spectra exclude all but a narrow strip of allowed values for the charge-transfer energy (Δ) and the 3d occupancy (n_d). While the spectral features are very different among different compounds, all the compounds for which experimental data are available at the moment have strikingly similar allowed ranges of Δ and n_d. The deduced n_d values unambiguously classify these compounds as strongly mixed valent, making an ionic or near-ionic description highly improbable.