Far-infrared transmittance and reflectance studies of oriented YBa2Cu3O7-δ thin films

We have measured both the transmittance (scrT) and reflectance (scrR) for several YBa2Cu3O7-δ thin films over the frequency range from 20 to 360 cm-1 at temperatures between 20 and 300 K. The films, grown on MgO substrates by pulsed laser ablation, are highly a-b-plane oriented. We use scrT and scrR to extract the optical conductivity, and find good agreement with results from Kramers-Kronig analysis of reflectance measurements alone. The transmittance is consistent with a two-component dielectric function consisting of Drude (free-carrier) and midinfrared (bound-carrier) absorption. The Drude relaxation rate varies linearly with temperature, and compares well with values obtained from the measured dc resistivity. We use a two-fluid model for the superconducting state and obtain a BCS-like temperature dependence for the superfluid density. A superconducting gap is not observed in our spectra