The electronic structure of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.02},$ ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.94}$ and ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ has been studied by using soft-x-ray absorption and emission spectroscopies. The unoccupied and occupied states are reflected by O $1s$ absorption and O $K\alpha$ emission spectra. An absorption prepeak appearing at 528.5 eV in the O $1s$ absorption spectra of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.94}$ and ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ indicates the transferring of spectral weight from states in the upper Hubbard band to the upper edge of the valence band. The absorption-emission spectra of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.94}$ and ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$ 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 ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.94}$ and ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8.$ 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 ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.02},$ 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 ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.02},$ ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.94},$ and ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8.$ 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 ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ and ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_{8-\delta }.$

• Received 17 March 1999

DOI:https://doi.org/10.1103/PhysRevB.61.9140