Determination of valence of Cu in superconducting La2-x(Sr,Ba)xCuO

Abstract

The measurement of Cu K-edge x-ray absorption near-edge structure of monovalent, divalent, and trivalent copper oxides has been used to identify the presence of different valence of copper in the ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$(Sr,Ba${)}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$ (x=0.0--0.3) system. The results indicate the coexistence of Cu ii and Cu iii states in these compounds. Theoretical calculations based on a cluster of ${\mathrm{CuO}}_{4}$${\mathrm{O}}_{2}$${\mathrm{La}}_{8}$LaSr predict the presence of nearly degenerate electronic ground states representing nominally Cu ii and Cu iii configurations.