Electronic Structure of Nd_1.85Ce_0.15CuO_4-δ Irradiated by He⁺ Ions : An X-Ray Absorption Study on the Cu-L₃ and Ce-M_4,5 Edges

Abstract
Polarisation- dependent x-ray absorption spectroscopy at the Cu L₃ and Ce M_4,5 edges have been performed on the epitaxial films Nd_1.85Ce_0.15CuO_4-δ vs. defect concentration induced by He⁺ ion irradiation. The Cu L₃ edge exhibits increasing of integral intensity of both the white line in E||ab and the first peak in E||c by about 15% and 30%, respectively, after suppression of superconductivity. The partial filling of Ce 4f orbitals (~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 3d¹⁰x²-y² orbital and become simultaneously localised in the Ce-O(2) chains. The symmetry of the free states and the mechanism of superconductivity suppression in Nd_1.85Ce_0.15CuO_4-δ under He⁺ ion irradiation are discussed.