X-ray spectral and X-ray electron studies of electronic structure system La2-xSrxCuO4, YBa2Cu3O7-δ, SmBa2Cu3O7-δ

Several superconducting and model systems have been studied by X-ray and X-ray electron techniques. It was found that the main contribution to the upper part of the valence band of the high-temperature superconductors is made by the Cu3d and O2p states. The 4f electrons of La and Sm and the 4d electrons of Y do not make any noticeable contribution to the occupied states near the Fermi level and, hence, cannot be responsiblew for the high-temperature conductivity. The studies revealed an increased negative charge on some of oxygen in the LaO layer of La2-xSrxSuO4 and oxygen atoms in the CuO layer of YBa2Cu3O6.8. The presence of such oxygen may be essential for the occurence of the high-temperature superconductivity