Electronic correlations in YBa2Cu3O7- from Auger spectroscopy

Cu L3VV Auger spectra of the superconducting compound YBa2Cu3O7−δ have been measured above and below the critical temperature Tc=90 K and compared with those of CuO. In the superconductor a localized peak whose intensity increases with temperature is found. Such a structure, lacking in copper oxide, is attributed to electronic correlations in the Cu 3d8 configuration