EXPERIMENTAL EVIDENCE OF ITINERANT Cu 3d⁹ - OXYGEN HOLE MANY BODY CONFIGURATION IN THE HIGH-T_C SUPERCONDUCTOR <font>YBa₂Cu₃O_~7</font>

Abstract

Cu L 3 x-ray absorption near edge structure (XANES) and Cu L 3 x-ray photoelectron spectroscopy (XPS) of YBa 2 Cu 3 O 6.5+x are compared. The breakdown of one-electron picture of its electronic structure is reported. The data are interpreted by mixing of Cu 3d 9 and of [Formula: see text] (where [Formula: see text] is a hole in the oxygen derived band, ligand hole) many body configuration in the initial state. The localization of Cu 3d 9 configuration is indicated by the bare Coulomb interaction U dd ~6 eV . The conductivity is assigned to the itinerant [Formula: see text] configuration. The experimental evidence that the additional oxygen x, giving higher T c , increases the weight of the [Formula: see text] configuration is reported. The presence of holes on the oxygen atoms is confirmed by the Ols XPS spectra. The Cu 3+ (Cu 3d 8 ) configuration is not observed in L 3 XANES in agreement with valence band XPS giving the energy of the 3d 8 excited state at about 12 eV above the ground state. An energy scheme of the many body configurations in YBa 2 Cu 3 O ~7 is obtained. These experiments give experimental evidence that the high T c superconductivity is due to pairing of holes in the oxygen valence band interacting with localized electrons at the Cu sites.