X-ray photoemission spectroscopy of the 90 K superconductor Ba 2 YCu 3 0 7 _ δ

The x-ray photoemission spectroscopic (XPS) data from different pelletized samples of the 90 K superconductor Ba 2 YCu 3 0 7 _ δ (where δ ~ 0.2) have been obtained. The valence band spectrum (recorded at 300 and 170 K), which is composed of contributions from both the Cu 3d and O 2p levels, is compared with the full potential linearized augmented plane wave (FLAPW) calculated electronic density-of-states (DOS) reported by Massidda et al. and Mattheiss and Hamann. The experimental data indicate a relatively low DOS at the Fermi level. Detailed measurements of the core level Cu 2p, O Is, Ba 3d, 4d and Y 3d spectra of the superconducting and related standard materials, are presented. Data for the superconducting material were recorded in the freshly prepared form as well as after scraping in situ. The Cu 2p core level, satellite, and Auger spectra for the various samples were carefully examined in order to assess the possibility of the presence of Cu3+ ions in Ba 2 YCu 3 0 7 _ δ It is observed that surface reaction in air to form carbonates and hydroxides occurs readily in the superconducting material.