CORE AND VALENCE XPS SPECTRA OF CLEAN, CLEAVED SINGLE-CRYSTALS OF YBA2CU3O7

Small single crystals of YBa2Cu3O7 with sharp superconducting transitions at 91 K were cleaved at 300 K in UHV (10-9 torr) and their XPS spectra were measured using small spot (300 μm dia.) Al Kα radiation. The cleaved surfaces showed no initial contamination, as determined by a lack of C(1s) intensity. Only one Ba core level feature is found to be characteristic of the bulk superconductor (Ba(5p 3 2), Ba(4d 5 2), and Ba(3d 5 2) at 12.0 eV, 87.0 eV, and 777.4 eV respectively). A second Ba feature at ~1 eV higher binding energy, found on some cleaved surfaces, represents a surface Ba phase. For the intrinsic surface the O(1s) spectrum is dominated by a peak at 528.0 eV and contains at least two unresolved components. Any large intensity at higher binding energy (530 to 533 eV are often reported) represents reactions at the surface. A relatively large intensity is observed at EF for high quality single crystals. This intensity and the Ba, Y and Cu core level features are stable for a few hours following the cleave at 300 K. There no evidence for oxygen loss at 300 K as judged by the constant O(1s) intensity. These results are compared to results in the literature, where it is often claimed that the higher binding energy feature relates to the intrinsic superconductor. We believe this claim is erroneous, as is the claim that oxygen is lost rapidly from the superconductor in vacuum if the sample is not maintained at 20 K. Finally, the Cu(2p 3 2) spectrum of the intrinsic superconductor's surface is very similar to that of Cu2+ in CuO, whereas data reported in literature often show a strong Cu1+ component. © 1990.