X-RAY-PHOTOEMISSION-SPECTROSCOPY STUDY OF THE SURFACE DETERIORATION OF BI2SR2CACU2O8 AND BI1.7PB0.3SR2CACU2O8 SINGLE-CRYSTALS AT 26-K

We examine the time evolution of the O 1s core-level and valence-band spectra of high-quality Pb-free and -doped Bi2Sr2CaCu2O8(001) single crystals cleaved at 26 K in ultrahigh vacuum. A component line at 533.5 eV of the O 1s spectrum is unambiguously identified as extrinsic and separated by the doublet structure related to the normal-state electronic structure of this compound. Evidence is given of the surface physisorption of carbon monoxide at low temperature (< 30 K) that is removed at room temperature without loss of stoichiometric oxygen. The effects of pollution on the valence-band features are monitored and correlated with those of the core spectra. Spectral changes due to electron etching are also addressed.