Photoemission study ofBi2(Sr,Ca)3Cu2Oy

The electronic structure of ${\mathrm{Bi}}_2{(\mathrm{S}\mathrm{r},\mathrm{C}\mathrm{a})}_3{\mathrm{Cu}}_2{\mathrm{O}}_y$ has been studied by photoemission and electron-energy-loss spectroscopy (EELS). From analysis of the Cu $2p$ core-level photoemission spectra, the $\mathrm{Cu}3d-\mathrm{O}2p$ bond in the Cu${\mathrm{O}}_2$ layer is found to be more ionic than that in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ and $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$, which leads to a weaker superexchange coupling between the ${\mathrm{Cu}}^{2+}$ spins within the layer. Valence-band photoemission and EELS spectra suggest a significant shift of the Bi $6s$ contribution from the deep-lying $\mathrm{Bi}6s-\mathrm{O}2p$ bonding states to the antibonding states in the O $2p$ band region as compared to the results of band-structure calculations on ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$Ca${\mathrm{Cu}}_2$ ${\mathrm{O}}_8$. This is probably due to excess oxygen atoms around Bi, which may raise the Bi $6s$ orbital energy and provide the system with holes.