Role of oxygen in superconductivity of ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ studied by x-ray-absorption near-edge structure

The effect of reduction on the electron states of ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ce}}_{\mathit{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ (0.15<x<0.17) has been revealed by x-ray-absorption near-edge structure on the Cu K edge for a series of samples annealed at various oxygen partial pressures (p${\mathrm{O}}_2$=1–${10}^{\mathrm{-}3}$ atm). We find that the Ce doping-induced states in the near-edge structure systematically increase the magnitude with the decrease of oxygen partial pressure, well correlating with the oxygen pressure dependence of superconducting fraction and ${\mathit{T}}_{\mathit{c}}$ value. The results indicate that the reduction strongly enhances the electron density at the copper sites, through the removal of a small number of defect oxygen atoms that suppress n-type doping in the ${\mathrm{CuO}}_2$ plane.