Core-level shifts on cleavedYBa2Cu3O7−x(001) surfaces observed in angle-resolved photoemission

We have performed synchrotron-radiation photoemission on the (001) surfaces of superconducting $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ crystals which were prepared by in situ cleaving. The $4\mathrm{a} 4d$ and $\mathrm{Ba} 5p$ core-level line shapes differed substantially when the photoelectrons were collected at normal or grazing emission angles. The normal-emission core-level spectra appear to be composed of two components shifted relative to one another by about 0.9 eV. The more tightly bound component dominates the grazing-emission spectrum. This is interpreted as a surface core-level shift of the Ba atoms to larger binding energies, since the photoelectrons detected at grazing angles are predominantly emitted from the outermost atomic planes. The nature of the shift is discussed.