High-temperature superconductivity in the presence ofO 2p−Cu 3dholes: A spectroscopic study

Abstract

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 ${\mathrm{Cu}}^{2+}$), 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.