Observation of strong electron correrlations in YBA2CU3O7-delta by H-NU-dependent photoelectron-spectroscopy

We present photoemission spectra from single-phase YBa2Cu3O7-δwhich 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 3 d and O 2 p bands are observed to undergo resonances at the Cu 3 p and O 2 s 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.