Soft x-ray absorption studies on high-Tcsuperconducting oxides

Abstract

We report measurements of the Cu $2p$, O $1s$, and La $3d$ absorption edges in ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_{4-y}$ and a Sr-doped, superconducting sample of ${\mathrm{La}}_{1.8}{\mathrm{Sr}}_{0.2}\mathrm{Cu}{\mathrm{O}}_{4-y}$. The Cu $2p$ absorption edge in the superconducting sample is considerably broader and exhibits additional structure compared to the normal ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_{4-y}$. This additional structure is consistent with models for the superconductivity, which involve difference valence states of the Cu atoms or different spin configurations at the Cu sites. The oxygen edge exhibits fine structure which changes with sample preparation and is presumably due to oxygen defects in the samples.