Synchrotron-radiation photoemission study of the high-$T_c$ superconductor $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$

Photoemission measurements have been performed on the high-$T_c$ superconductor Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.85}$ using synchrotron radiation of 20- 110 eV. It is found that the valence band is shifted by 1.1 to 1.6 eV towards higher binding energy relative to band calculations. The effective intraatomic Coulomb energy between $d$ electrons ($U_{\mathrm{dd}}$) was evaluated from the energy position of the valence-band satellite due to the two-hole bound state: $U_{\mathrm{dd}}=5-6\mathrm{}$ eV, which is comparable to the valence-band width ($W=6-7\mathrm{}$ eV). This strongly suggests that electron correlation plays a key role in characterizing the electronic properties of this oxide superconductor.