Core-level changes induced by oxygen in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

The characterization of the surface electronic structure as a function of the oxygen content by means of photoemission is presented for sintered powders, films, and single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. Core-level line shapes of O, Ba, and Cu are strongly influenced by the oxygen stoichiometry, which is varied by heating cycles in vacuo and in ${\mathrm{O}}_2$ atmosphere. The evolution of the core levels following oxygen in- and out-diffusion was followed by x-ray-diffraction measurements. The results indicate that the observed spectral changes are related to the oxygen deficiency in the bulk more than to extrinsic contamination of the surface and point to the importance of charge redistribution and oxygen ordering in the basal plane.