Observation of Strong Electron Correlations in YBa2Cu3O7-δ by hν Dependent Photoelectron Spectroscopy

After a review of the understanding of resonant photoemission phenomena we present photoemission spectra from single phase YBa2Cu3O7-δ which have been measured in the photon energy range 17-130 eV, at T=80 K, T=300 K and intermediate temperatures using synchrotron radiation. Intense satellites associated with the Cu 3d and O 2p bands are observed to undergo resonances at the Cu 3p and O 2s core level excitation thresholds respectively indicating the presence of holes in the Cu and O hybrid bands on both Cu and O sites. The strong correlation effects for the valence electrons suggest that one electron calculations may not be appropriate for a detailed description of the electronic properties of this material.