FAR-INFRARED RESPONSE OF FREE-CARRIERS IN YBA2CU3O7 FROM ELLIPSOMETRIC MEASUREMENTS

Spectroscopic ellipsometry has been employed in the far-infrared range 100-650 cm-1 to determine the frequency-dependent complex dielectric constant and conductivity of a thin film of YBa2CU3O7 as a function of temperature between 20 and 300 K with high sensitivity due to multipl e reflections occurring in the film. We find that in addition to the f ree carriers which form the superconducting condensate there is a zero -frequency-centered oscillator strength which persists at all temperat ures. The temperature dependence of these two contributions to the opt ical response is followed via modeling with two Drude functions. The o ften proposed frequency-dependent scattering rate for a single Drude t erm is inconsistent with our results.