Infrared optical response of YBa2Cu3O7 and PrBa2Cu3O7. An ellipsometric study

Ellipsometry was used to study (110) and (001) oriented films of YBa2Cu3O7 and PrBa2Cu3O7 in the mid- and near-infrared spectral regions. Below a photon energy of 0.1 eV, the in-plane component of the dielectric tensor of YBa2Cu3O7 is dominated by a Drude term with a squared plasma energy of (3.0±0.3) eV2. This ''oscillator strength'', and the Lorentzian broadening energy of (0.104±0.005) eV at room temperature, are confirmed by the changes induced in Pr-substituted material, and by low-temperature measurements in the near-infrared. The deviation from the Drude behavior observed above 0.1 eV is accounted for by a broad absorption band with an oscillator strength of (3.6±0.1) eV2 in YBa2Cu3O7 which shifts to higher energies and takes over almost all of the oscillator strength of the Drude term when Y is substituted by Pr. The response to electric fields perpendicular to the planes is much weaker, with an upper bound of 0.63 eV2 for the squared plasma energy. The in-plane loss function computed from themeasured dielectric function follows the Drude-like lineshape, modified by the bound-state absorption band, down to the lowest energy reached in our measurements (0.058 eV). (orig.)