X-ray-absorption studies of electron doping and band shifts in R2-xCexCuO4-δ (R=Pr, Nd, Sm, Eu, and Gd)

We have measured the polarized x-ray-absorption spectra at the Cu K edge of Nd2-xCexCuO4-δ single crystals and the unpolarized spectra of R2-xCexCuO4-δ (R=Pr, Nd, Sm, Eu, and Gd) ceramic samples. The results indicate that up to the T '-phase stability limit (and including the narrow region of superconductivity), Ce doping continuously donates electrons into the Cu 3d orbitals. Our results evidence a Cu1+ content in these T '-phase materials which increases almost linearly with the Ce-doping level x, and an electron doping fraction from each Ce atom to Cu which is quite close to unity (except in the R=Gd series). It is found that the size variation of the rare-earth elements R causes a number of feature changes in the Cu K edge, including an about 0.5 eV upward edge shift. These effects are explained by the ligand field shift of the unoccupied Cu 4p and 3d bands. A cluster model description has been developed to correlate the spectral features of the Cu K-edge spectra to different 3d configurations. The experimental results can be well described by the following model parameters: Cu 1s-3d Coulomb interaction Ucd∼6.5 eV, Cu 1s-O 2p Coulomb interaction UcL∼1.1 eV, d-ligand hybridization V∼2.7 eV, CuO charge-transfer energy Δ∼1.3 eV (at x=0), and Δ∼0.5 eV (at x=0.2).