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Photoemission studies have been performed on all classes of high temperature superconductors except the Tl-related compounds. Particular attention was paid to the surface cleanliness. Comparison with band calculation shows that the one-electron picture cannot adequately explain the electronic structure of this type of materials. Most important, Cu satellites were observed both in the valence band and the Cu 2p core level for all the samples studied, signaling the importance of the d-d correlation effects. The Cu 3d character of these satellites in the valence band was verified using resonance photoemission. The results have been interpreted in terms of a cluster model derived from the two band Anderson Hamiltonian, which in the past has been used successfully to describe the electronic structure of highly correlated systems. No clear satellite structure was observed in the O 1 s core spectrum, which is consistent with the bandlike nature of the oxygen states. Examples of changes in the electronic structure, which could be related to Tc, (such as substituting Y by Pr in the Y-Ba-Cu-O system and altering the number of Cu-O layers in the Bi-Ca-Sr-Cu-O system), are also discussed

We present the results of photoemission studies of "valence disordered" monoclinic BaBiO3 in the photon energy range 15-120 eV. The line-shapes of the valence band photoemission spectra and the Ba contributions to the valence band are very similar to the line shapes of the total density of states and Ba partial density of states, respectively. Oxygen resonance is observed, demonstrating the existence of empty O 2p states. These results support a more covalent rather than a simple ionic picture for the electronic states of BaBiO 3.

We present the results of photoemission studies of "valence-disordered" monoclinic BaBi03 in the photon energy range 15 -120 eV. The hne shapes of the valence-band photoemission spectra and the Ba contributions to the valence band are similar to the line shapes of the total density of states and Ba partial density of states, respectively. The Bi 5d core levels show only one peak for each spin-orbit component. Oxygen resonance is observed, demonstrating the existence of empty 0 2p states. These results support a more covalent rather than a simple ionic picture for the electronic states of BaBi03. Our data suggest that the charge Auctuation between the two unequal Bi sites is small. We also discuss the origin of the semiconducting character of the "valence-disordered" Ba-Bi03.