Soft-x-ray emission spectroscopy study of La2CuO4 and three high-Tc superconductors.

Abstract

High-resolution x-ray emission spectroscopy measurements have been performed on sintered samples of ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$, ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$, ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (\ensuremath{\delta}0.1), and compressed polycrystalline ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$. The Cu L\ensuremath{\alpha} and O K\ensuremath{\alpha} emission bands, which reflect the local partial Cu 3d and O 2p states, respectively, are compared to photoelectron spectra and ground-state local-density-approximation (LDA) densities-of-states (DOS) calculations. Our results show that solid-state effects are dominant. In particular, the differences in shape of the O K\ensuremath{\alpha} emission bands follow the trends predicted by LDA, although it must be stressed that they mainly arise from O 2p states at oxygen sites not directly involved in the Cu-O planes. The Cu-side 3d states, on the other hand, vary little from one cuprate to the next and are more strongly localized than expected from LDA predictions. The Anderson-impurity model calculations of Kotani and co-workers are discussed, and it is concluded that they provide a satisfactory explanation of the Cu L\ensuremath{\alpha} emission data, which reflect strong on-site Coulomb interaction. We conclude that the apparently good agreement between LDA-calculated DOS and low-energy photoemission experiments recently reported results from the high sensitivity of the latter to O 2p states. Discrepancies are to be expected in excitation spectra that involve mainly 3d electrons.