We report core-level spectroscopic measurements of several cuprate perovskites comparing those which exhibit superconductivity above 90 K to others which do not. Both x-ray photoemission and x-ray absorption results show that in the former Cu is more oxidized, as in CuO (nominally Cu2+), than in the latter. Trivalent Cu is clearly excluded. The O 1s core level is at higher binding energy in the superconductors, implying the presence of O 2p holes. The rare-earth valence in Ce- and Pr-based perovskites is more than 3+, apparently related to their failure to superconduct.
Physical Review B (Condensed Matter) 36(7), pp.3895-3898