L2,3 XANES of the High Tc Superconductor YBa2Cu3O~7 with Variable Oxygen Content

The copper L2,3 soft x-ray absorption spectra of the single phase high temperature (Tc=91 K) superconductor YBa2Cu3O~7 have been measured by using synchrotron radiation at LURE. The spectra show at the absorption threshold a white line at 931.1 eV due to Cu 2p3d10 final states for the Cu 3d9 initial state, and a broad high energy shoulder with the maximum at 933 eV assigned to Cu 2p3d10L and to Cu 2p3d10L4s1 final states (L indicates a hole in the oxygen 2p derived valence band), i.e. to the initial states 3d9L and 3d10L. The intensity of this last feature has been found to decrease with decreasing oxygen content x in YBa2Cu3O6.5+x. No evidence of Cu 2p3d9 final states has been found. This experiment gives direct evidence that additional oxygen x does not give Cu3+ ions with Cu 3d8 configuration but gives 3d9L electronic conliguration (i.e. Cu2+O- pairs) and no gap is observed between the 3d9 (Cu+) and 3d10L configuration (i.e. Cu+O- pairs). Therefore in the metallic phase which exhibits superconductivity a continous set of itinerant states 3d9L and 3d10L is observed