Increase of O 2p unoccupied electronic states within the ab plane of YBa2Cu3O6.8 due to a superconducting transition

Temperature-dependent O K-edge absorption spectra of single-crystalline YBa2Cu3O6.8 films were obtained across Tc (77 K), using linearly polarized x-rays. When the incident x rays were polarized parallel to the a(b) axis, absorption within 1.4 eV just above the Fermi level increased as the temperature was lowered from above Tc (120 K) to below Tc (68 K). The increase disappeared upon breaking the superconducting state by applying a dc current. When polarization was parallel to the c axis, lowering the temperature below Tc had no effect on the absorption. The increase implies that, in the superconducting state, the 2p orbitals on the O(2) and O(3) sites in the CuO2 plane have more holes than in the normal conducting state.