PHOTOEMISSION-STUDIES OF ZN, CO, AND GD SUBSTITUTED YBA2CU3O7-DELTA

Zn and Co substitute for Cu in YBa2Cu3O7-delta at the plane and chain sites, respectively. We report a high resolution angle-resolved photoemission study on Zn and Co substituted YBa2Cu3O7-delta single crystals to determine the effects on electronic structure. The valence states near E(F) along GAMMA-S are virtually unaffected by the substitutions for samples that are superconducting. However, the spectral weight near E(F) disappears for a higher Co-doped, non-superconducting crystal. The results are compared with earlier studies on pure YBa2Cu3O7-delta with controlled oxygen stoichiometry. Measurement on Gd-123 crystals with reduced oxygen stoichiometries show gradual reduction of spectral weight for a band along GAMMA-Y(X), in contrast to the results for the bands along GAMMA-S. The differences are probably due to a different admix of Cu-O chain and plane characters in these bands. We also measured Gd 4f resonance spectra, which exhibit complex structure, indicating the presence of Gd atoms in chemically inequivalent (probably bulk and near surface) sites.