X-ray-absorption near-edge-structure study ofLa2−x(Ba,Sr)xCuO4−ysuperconductors

Abstract

Superconductors with the composition ${\mathrm{La}}_{2-x}{(\mathrm{B}\mathrm{a},\mathrm{S}\mathrm{r})}_x\mathrm{Cu}{\mathrm{O}}_{4-y}$ and $x=0.0\mathrm{} \mathrm{to} 0.3$ have been studied through measurements of x-ray-absorption near-edge structure at the Cu $K$ and La $L$ edges. Contrary to recent band-structure calculations, we find that the Cu $3d$ electrons are localized with the copper ions having an integral $3d^9$ configuration (2+ valence) independent of dopant concentration. Variation in the area of the "white lines" at the La $L_{2,3}$ edges with doping indicates that reduction of the valence charge results in an increase in unoccupied states (probably consisting of holes in the O $2p$ band) having $d$ symmetry relative to the La site. These O $2p$ holes also appear to have $d$ symmetry with respect to the Cu sites.