Photoemission from single-crystalline Bi-Sr-Ca-Cu-O

An x-ray photoemission experiment was performed on high-$T_c$ superconducting Bi-Sr-Ca-Cu-O single crystals with $T_c\sim 80$ K. It is found that Sr and Ca are interchangeable on the lattice sites of the crystal and that they are electronically distinguishable. This is consistent with the structure model for the crystal with formula unit ${\mathrm{Bi}}_2{(\mathrm{S}\mathrm{r},\mathrm{C}\mathrm{a})}_3{\mathrm{Cu}}_2{\mathrm{O}}_y$. We demonstrate how the distribution of Sr and Ca in the cation sites is determined by x-ray photoemission, which cannot be done by x-ray diffraction.