Polarization-dependent x-ray-absorption spectroscopy at the Cu $L_3$ and Ce $M_{4,5}$ edges has been performed on epitaxial films of ${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ vs defect concentration induced by ${\mathrm{He}}^{+}$ ion irradiation. The Cu $L_3$ edge exhibits an increase in the integral intensity of both the white line in $\mathbf{E}\parallel \mathrm{ab}$ $(\sim 15\%)$ and the first peak in $\mathbf{E}\parallel c$ $(\sim 20\%)$ after superconductivity suppression. At the same time, partial filling of Ce $4f$ orbitals $(\sim 16\%)$ takes place, though the shape of Ce $M_{4,5}$ edges remains the same as for the formally tetravalent Ce ion. The mechanism of superconductivity suppression in ${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ under ${\mathrm{He}}^{+}$ ion irradiation is discussed. The Cu $L_3$-edge measurements vs Ce doping indicate that the major part $(\sim 70\%)$ of the excess electrons fill Cu ${3d}_{x^2-y^2}$ states in ${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}.$ The remaining part extends beyond the ${\mathrm{CuO}}_2$ superconducting plane forming “impurity” states that seem to be localized within a few unit cells.

• Received 21 May 1997

DOI:https://doi.org/10.1103/PhysRevB.57.8671