XANES study of copper mixed valence and mechanism of superconductivity in YBa2Cu3O6.5+δ system

Two high-Tc superconductors YBa2Cu3O6.5+δ are studied with XANES technique. The two superconductor samples have a Tc (zero R) around 91 K and are orthorhombic in structure. CuK-edge XANES measurements are made both, at room and liquid nitrogen temperature (LNT). A mixed valent state for Cu is generally considered essential for their superconducting behaviour. The measured XANES spectra are minutely examined with regard to this question. The results indicate the presence of Cu1+ and Cu2+ at RT but only Cu2+ at LNT. No evidence is found about the presence of Cu3+ state. The results tend to support an intermediate interatomic valent nature of the YBC superconducting system wherein the ground state is derived from a mixture of 3d9, 3d10L, and 3d9L configuration, L meaning the ligand hole. Mounting evidence is quoted in favour of holes being located in the oxygen valence band rather than at the Cu site and the high-Tc superconductivity is ascribed to the pairing of the itinerant configuration 3d9L and its interaction with localized copper 3d electrons. (author)