Photoemission study of a high-Tcsuperconducting Bi-Sr-Ca-Cu oxide

We have used ultraviolet, x-ray, and inverse-photoemission spectroscopies to characterize the electronic structure of a novel bismuth cuprate compound with an average composition ${\mathrm{Bi}}_{2.5}$ ${\mathrm{Sr}}_{1.5}$ ${\mathrm{Ca}}_1$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_{\mathit{y}}$ which becomes superconductor at 84 K. The data indicate that the valence band is shifted ∼2 eV to higher binding energy with respect to band-structure calculations. This together with the presence of shake-up satellites in the Cu 2p spectrum suggests that $U_{\mathrm{dd}}$ is ∼6 eV for Cu 3d electrons in this Bi compound. The electron-electron correlation seems to be much smaller in the Bi-O planes than in the Cu-O planes. The chemical state of the cations is basically ${\mathrm{Cu}}^{2+}$, ${\mathrm{Bi}}^{3+}$, ${\mathrm{Ca}}^{2+}$, and ${\mathrm{Sr}}^{2+}$.