SYMMETRY OF THE FREE STATES OF AN ELECTRON-DOPED ND2-XCEXCUO4-DELTA SUPERCONDUCTOR DETERMINED BY X-RAY-ABSORPTION SPECTROSCOPY

Polarization-dependent x-ray-absorption spectroscopy at the Cu L-3 and Ce M-4,M-5 edges has been performed on epitaxial films of Nd1.85Ce0.1 5CuO4-delta vs defect concentration induced by He+ ion irradiation. Th e Cu L-3 edge exhibits an increase in the integral intensity of both t he white line in E\\ab (similar to 15%) and the first peak in E\\c (si milar to 20%) after superconductivity suppression. At the same time, p artial filling of Ce 4f orbitals (similar to 16%) takes place, though the shape of Ce M-4,M-5 edges remains the same as for the formally tet ravalent Ce ion. The mechanism of superconductivity suppression in Nd1 .85Ce0.15CuO4-delta under He+ ion irradiation is discussed. The Cu L-3 -edge measurements vs Ce doping indicate that the major part (similar to 70%) of the excess electrons fill Cu 3d(x2-y2) states in Nd1.85Ce0. 15CuO4-delta. The remaining part extends beyond the CuO2 superconducti ng plane forming ''impurity'' states that seem to be localized within a few unit cells.