Electronic structure of YBa2Cu3Ox and YBa2Cu4O8 studied by soft-x-ray absorption and emission spectroscopies

The electronic structure of YBa2CU3O6.02, YBa2Cu3O6.94 and YBa2Cu4O8 has been studied by using soft-xray absorption and emission spectroscopies. The unoccupied and occupied states are reflected by O 1s is absorption and O K alpha emission spectra. An absorption prepeak appearing at 528.5 eV in the O 1s absorption spectra of YBa2Cu3O6.94 and YBa2Cu4O8 indicates the transferring of spectral weight from stares in the upper Hubbard band to the upper edge of the valence band. The absorption-emission spectra of YBa2Cu3O6.94 anaYBa(2)Cu(4)O(8) show the unbroken density of states crossing the Fermi level reflecting the metallic character of these compounds. No significant shape changes are observed in the resonant excited O K alpha spectra of YBa2Cu3O6.94 and YBa2CU4O8. However, some small variations in spectral profile are attributed to the changes in the relative number of excited oxygen atoms from inequivalent sites. Strong spectral changes are observed in the resonant O K alpha spectra of YBa2Cu3O6.94 which is attributed to the strong covalent chemical bonds between inequivalent oxygen atoms and different charge-state copper atoms. The analysis of x-ray absorption and resonant x-ray emission spectra provides the character of unoccupied states of YBa2CU3O6.02, YBa2CU3O6.94, and YBa2Cu4O8. The experimental findings suggest an existence of an appreciable amount of doping-induced holes at O(1) and O(4) sites and an importance of O(1)-Cu(1)-O(4) ribbons for superconducting properties of YBa2Cu3O7-delta and YBa2CU4O8-delta.