X-ray-absorption near-edge-structure study of La 2 − x ( B a , S r ) x Cu O 4 − y superconductors

Superconductors with the composition La2−x(Ba,Sr)xCuO4−y and x=0.0 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 3d9 configuration (2+ valence) independent of dopant concentration. Variation in the area of the "white lines" at the La L2,3 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.