Effects of Ca substitution on the electronic structure ofNd2−x−yCexCayCuO4−δ

Abstract

The effects of Ca substitution on the core levels and valence electronic states of ${\mathrm{Nd}}_{2\ensuremath{-}x\ensuremath{-}y}{\mathrm{Ce}}_{x}{\mathrm{Ca}}_{y}{\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ have been studied using x-ray-photoelectron spectroscopy. It is found that the fraction of monovalent Cu derived from the Cu-$2p$ spectra apparently increases with increasing Ca concentration. Ca doping leads to an enhancement in the density of states at the Fermi level for superconducting ${\mathrm{Nd}}_{1.85\ensuremath{-}y}{\mathrm{Ce}}_{0.15}{\mathrm{Ca}}_{y}{\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}.$ This could arise from Ca doping making it easier to lose oxygen from the samples and lead to an increase in electron carriers.