Evidence for charge redistribution below Tc in YBa2Cu3O7-δ

We have measured the temperature dependence of the K absorption near-edge fine structure of Cu in polycrystalline YBa2Cu3O7-δ between 47 and 300 K. Temperature dependent differences in the fine structure are resolved within 0.2%. The most pronounced temperature dependence is observed at ≅11 eV, and is attributed to intensity variations of the out-of-plane transitions Cu(2) 1s→4pzπ, and Cu(1) 1s→4pxπ. At Tc their average intensity exhibits a distinct minimum. Below Tc the intensity of the transition Cu(1) 1s→4pxπ is found to increase by ≅ +1%, indicating an increase of electron density at the Cu(1) site. We attribute the experimental findings to a redistribution of charge between chains and planes below the onset of superconductivity. The associated structural distortion is interpreted to increase the symmetry of the Cu(1)O2(1)O2(4) rhombs in the doping block of the crystal. No evidence is found for a bimodal axial lattice instability across the superconducting transition. (orig.)