High energy-resolution electron energy-loss spectroscopy study on Sr-doping dependence of the electronic structure of La(2-x)Sr(x)CuO₄

Electron energy-loss spectra of oxide superconductors of La(2-x)Sr(x)CuO₄ (χ = 0.08, 0.15 and 0.18) and an insulator of La₂CuO₄ (x = 0.0) have been measured from their perfect crystalline areas (180 nmφ) over an energy range of 0-60 eV. Each spectrum showed two large peaks at about 13 and 30 eV. Those peaks were assigned to the interband transition of O 2p → La 5d/4f and the volume plasmon due to the oscillation of all valence electrons, respectively. The loss functions of carrier-doped materials (x = 0.08, 0.15 and 0.18) showed a sharp peak at 0.8-0.9 eV, which was not observed in the loss function of x = 0.0. The dielectric functions of the materials are derived from the loss-functions by Kramers-Kronig analysis. The imaginary part of the dielectric functions showed that two new transition peaks at 1.5 and 2.6 eV were formed for x = 0.08, 0.15 and 0.18 by Sr-doping. The transitions at 1.5 and 2.6 eV were assigned to the transitions from O 2p to the mid-gap state and Cu 3d band, respectively. The peaks observed at 0.8-0.9 eV in the loss-functions of x = 0 08, 0.15 and 0.18 were assigned not to a free carrier plasmon but to a plasmon strongly coupled with the new interband transition at 1.5 eV. Reflectance spectra calculated from the dielectric functions of x = 0.08, 0.15 and 0.18 show a steep decrease at about 1 eV corresponding to the peak at 0.8-0.9 eV in the loss-functions. The reflectance spectra agree well with that of optical measurements. Thus, the so-called plasma edge in reflectance spectra was explained to be formed by the interband transition at 1.5 eV.