Introduction

As soon as the cuprates were discovered, measurements of all types were made on the samples then at hand. Photoelectron spectroscopy was employed early to learn something of the electronic structure of these materials. As there were many more XPS spectrometers than ultraviolet, many XPS papers appeared in the first year of the “cuprate era.” One of the early goals was to seek evidence of Cu3+ ions in those cuprates that were superconducting, thereby demonstrating that the holes introduced by doping resided, at least partially, on the Cu sites. Cu 2p spectra existed for a series of compounds in which the Cu ions had a known valence, mostly +1 and +2, but at least one, NaCuO2, in which it was trivalent. The Cu 2p apparent binding energy increased as the valence increased, so the binding energy could be used to determine the valence to about one significant figure. (The shift is not linear with valence. The binding energy difference between Cu2+ and Cu3+ is rather small (Karlsson et al., 1992).) The search was not successful. Core-level spectra from all the other elements in the cuprates were studied, not just the Cu 2p. Valence-band spectra were measured for comparison with densities of states from band calculations. Considerable agreement was found, but not enough to say definitively that LDA calculations did or did not give an accurate picture of the electronic structure of the cuprates.