Experimental electronic structure of Bi2CaSr2Cu2O8+δ

Highly oriented polycrystalline Bi₂CaSr₂Cu₂O_8+δ has been studied by x-ray photoemission spectroscopy (XPS) to determine the oxidation states of its constituents and the valence electronic structure. Core-level shifts indicate 3⁺ and 2⁺ as main oxidation states for Bi and Cu, respectively. The Ca and Sr core levels exhibit pronounced shifts to lower binding energy compared to the metals. The Ca lines show two main components, indicating either inequivalent sites or the presence of a different phase. The XPS and ultraviolet photoemission spectroscopy valence-band spectra show weak but reproducible metallic emission at the Fermi level (E_F) which we ascribe to states originating in the Bi-O planes. The main Cu d-O p band is between 2 and 8 eV below E_F; this is about 1 eV lower than predicted by band-structure calculations. We argue that this is due to electronic correlations tending to increase the anisotropy in the ground-state charge distribution compared to that obtained in local-density-approximation calculations.