Electronic structure of high-T<SUB>c</SUB> superconductors from soft-x-ray absorption

A series of high-Tc superconductors was studied using soft x-ray absorption spectroscopy at the Cu-LII,III thresholds. From a comparison with CuO and NaCuO2, it is shown that these superconductors contain mostly 3d9 and 3d9L (L=oxygen ligand hole) configurations in the ground state, with negligible amounts of Cu(III) 3d8. The spectral weights of the 3d9L components are found to be higher for the YBa2Cu3O7-type superconductors than for the La2CuO4-derived compounds and exceed the values expected from oxygen stoichiometry