Browsing by Author "Wang, Y. Y."

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    Electron-energy-loss and optical-transmittance investigation of Bi2Sr2CaCu2O8
    Wang, Y. Y.; Feng, G. F.; Ritter, Alfred L. (American Physical Society, 1990-07)
    The energy-loss function Im(-1/ε) of Bi2Sr2CaCu2O8 has been measured over the range Eloss=0.8 to 80 eV by transmission electron-energy-loss spectroscopy (EELS) (nonimaging). The energy and momentum resolution were 0.1 eV and 0.04 Å-1, respectively. The low-energy spectra (Eloss≤3 eV) were studied as a function of momentum transfer (0.1 Å-1≤q≤0.3 Å-1). A well-defined peak in the loss function at Eloss∼1 eV is observed to disperse with momentum proportional to q2. This excitation is analyzed in terms of both an intracell, charge-transfer exciton model and the free-carrier (plasmon) model. The derived effective mass of the exciton mtot/m≃1.0 is far too small for a localized exciton. Using the free-carrier model and random-phase-approximation expressions for the dispersion coefficient, the carrier density and carrier effective mass can be determined separately. From our data and similar measurements by Nücker et al. [Phys. Rev. B 39, 12 379 (1989)], it is found that the effective mass roughly scales with carrier density. A heuristic model is introduced based on the assumption that low-energy gaps exist in portions of the Fermi surface due to structural instabilities. The model suggests how the effective mass could appear to scale with carrier density and why a single Drude term (with frequency-independent effective mass) does not describe the mid- to far-infrared optical spectra. Finally, the optical transmittance of the EELS sample was measured and the spectra analyzed in terms of the free-carrier model.
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    Optical excitations in Bi2Sr2CuO6 and Bi2Sr2CaCu2O8: Evidence for localized (excitonic) and delocalized charge-transfer gaps
    Wang, Y. Y.; Ritter, Alfred L. (American Physical Society, 1991-01)
    The optical properties of Bi2Sr2CuO6 (2:2:0:1) and Bi2Sr2CaCu2O8 (2:2:1:2) have been investigated by transmission electron-energy-loss spectroscopy. At low energy, E(loss) less-than-or-equal-to 10.0 eV, common features are observed in the spectra of these two materials at E(loss) = 0.0, approximately 2.7, 3.6, and 4.6 eV. The effective number of charges associated with these excitations has been estimated using the optical sum rule, and from the ratio of effective charges in the two materials, the origin of this feature has been inferred based on the relative number of Cu-O2 and Bi-O layers per formula unit. The effective-charge ratio for the free carriers at zero energy loss, N2:2:1:2/N2:2:0:1 = 5.0, could not be used to determine whether the carriers were in the Cu-O2 or Bi-O planes because the oxygen doping in the two materials was not known. But at E(loss) = 2.7 and 3.6 eV, the effective-charge ratio is 2.6 indicating that these transitions are associated with the Cu-O2 planes since the ratio is close to 2/1. The effective-charge ratio is 0.9 for E(loss) = 4.6 eV suggesting that this excitation is localized in the Bi-O planes. If the two excitations in the Cu-O2 layers of 2:2:0:1 and 2:2:1:2 are identified with the delocalized and localized charge-transfer reactions [Mark S. Hybertsen, Michael Schluter, and Niels E. Christensen, Phys. Rev. B 39, 9028 (1989)], then the three-band Hubbard parameters E = E(p) - E(d) and U(pd) (E is the energy difference between the Cu 3d and O 2p levels and U(pd) is the Coulomb repulsion between two holes occupying adjacent Cu and O sites) are 1.8 and 0.9 eV, respectively, with an uncertainty of +/- 0.5 eV.