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 Nd1.85Ce0.15CuO4-delta vs defect concentration induced by He+ ion irradiation. The Cu L-3 edge exhibits an increase in the integral intensity of both the white line in E\\ab (similar to 15%) and the first peak in E\\c (similar to 20%) after superconductivity suppression. At the same time, partial 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 tetravalent 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 superconducting plane forming "impurity" states that seem to be localized within a few unit cells.