Effects of oxygen stoichiometry on the electronic structure ofYBa2Cu3Ox

Synchrotron-radiation photoemission was used to compare the electronic structure of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ ($x=6.8\mathrm{}$) which has a superconducting transition at 93 K, and semiconducting $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ ($x=6.2\mathrm{}$) which was prepared by vacuum annealing the $x=6.8\mathrm{}$ material. The decrease in oxygen stoichiometry not only affects the intensity of two oxygen-related valence-band features, but also shifts the Fermi level upward and alters the shape of the Cu $3d$ satellite peak. The latter effect indicates a partial reduction to the ${\mathrm{Cu}}^{1+}$ valence state in the vacuum-annealed sample. The superconducting sample could also be modified in situ by an intense soft-x-ray beam, leading to oxygen desorption and to photoemission spectra much like the annealed sample.