FAR-INFRARED SPECTRA OF IMPERFECT YBACUO FILMS ON SI SUBSTRATES

We have measured the far-infrared (FIR) transmittance of low conductiv ity, imperfect YBCO films deposited on Si, al both 300 K and 20 K. Tes t reflectance measurements were also performed at room temperature. Ou r aim was both to study YBCO phonons by transmission measurements and to check if FIR spectra of imperfect YBCO films can be described in th e framework of the BCS theory. In order to describe our measurements, we provide exact expressions for both transmittance and reflectance, a veraged with respect to the interference fringes of the substrate. By modelling the complex dielectric constant of the film, we then study t he YBCO phonons which are evident in the measured spectra. The compari son between spectra of normal and superconducting phases shows that th e superconducting transition affects our spectra only at very low freq uencies. By considering a two fluid model in which only a small fracti on of electrons becomes superconducting and by assuming an energy gap of the order of 130 cm-1, we find that our data can be explained in te rms of the Mattis-Bardeen theory.