X-ray photoemission measurements of Y-, Sm-, Eu-, and Yb-based high-temperature superconductors show a clear correlation between Cu 2p satellite features, which previous work has shown are associated with materials which exhibit superconductivity, and a high-binding-energy peak in the O 1s spectrum. In samples with varying oxygen content the intensity of the Cu 2p satellites changes, reflecting the Cu valence which varies between 1 + and 2 + . The high-binding-energy O peak appears to track the Cu valence. This high-binding-energy peak may be indicative of the presence of O 2p holes, suggested in some models as necessary for superconductivity. The Ba 3d peak position also changes systematically with the Cu valence.
