High-energy spectroscopic study of YBa2Cu2.7Fe0.3O6.9with photon energy near the O1sthreshold

Photoemission experiments with $h\nu$ near the O $1s$ threshold exhibit a resonant enhancement of Cu $3d^8$ feature in the valence-band region of Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_{2.7}$ ${\mathrm{Fe}}_{0.3}$ ${\mathrm{O}}_{6.9}$ ($T_c\simeq 40$ K). This first-ever observation of an interatomic resonance provides direct experimental indication of considerable hybridization between Cu $3d$ and O $2p$ states. Comparing the photoemission results with the x-ray absorption spectra (XAS) at O $K$ edge, we show that the 528-eV peak in the XAS of these compounds cannot be attributed to stoichiometry induced holes at $E_F$, contrary to previous interpretation. We discuss various implications of our observations.