CALCULATION OF OPTICAL CONDUCTIVITY AND ELECTRON-ENERGY LOSS SPECTRA OF YBA2CU3O7 IN THE 10-160 EV RANGE

We calculate the high frequency macroscopic dielectric function epsilon(omega) of YBaCuO by treating the solid as a dense gas of polarizable cells. The cell polarizability is constructed from independent atomic contributions within a local-density based random phase approximation (LDRPA) and improvements thereof. In this way, interband transitions are treated very schematically but transitions from the core-like Ba 5p6 shell are treated quite well. This is important, because the intense Ba 5p-5d like giant dipole resonance provides about half of the optical conductivity around 20 eV and contributes strongly to the all-valence electron plasma frequency. The calculated conductivity is in quite good agreement with experiment in magnitude and shape above 15-20 eV. The calculated EELS is similar in magnitude and shape to the experimental one but shifted to higher energies: in particular, the valence electron plasmon occurs around 33(26) eV in the calculated (experimental) EELS. This is reasonable, since the real solid involves lowlying charge transfer excitations and interband transitions which are absent in the atomic model of the YBaCuO unit cell. In the atomic model, the corresponding intensity is taken into account by higher frequency atomic transitions.