Electronic structure of Nd1.85Ce0.15CuO4-delta, irradiated by He+ ions: An x-ray absorption study on the Cu-L-3 and Ce-M-4,M-5 edges

Polarisation-dependent x-ray absorption spectroscopy at the Cu L-3 and Ce M-4,M-5 edges have been performed on the epitaxial films Nd1.85Ce0.15CuO4-delta vs. defect concentration induced by He+ ion irradiation. The Cu L-3 edge exhibits increasing of integral intensity of both the white line in E parallel to ab and the first peak in E parallel to c by about 15% and 30%, respectively, after suppression of superconductivity. The partial filling of Ce 4f orbitals (similar to 15%) takes place although the Ce ions remain formally tetravalent, Ce+4. Thus, XAS experiments give a clear evidence that the excess electrons leave Cu 3d(x)(10)2(-y)2 orbital and become simultaneously localised in the Ce-O(2) chains. The symmetry of the free states and the mechanism of superconductivity suppression in Nd1.85Ce0.15CuO4-delta under He+ ion irradiation are discussed.