Far-infrared transmission ofBi2Sr2CaCu2O8films

The far-infrared transmission spectra for high-conductivity ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ films with the radiation polarized in the ab plane have been measured for temperatures above and below $T_c$. The normal-state properties show a Drude-like response. The transmission spectrum can be used to derive both the dc conductivity (90 μΩ cm at $T_c$) and the relaxation rate (165 ${\mathrm{cm}}^{\mathrm{-}1}$ at $T_c$). If an energy gap at 3.5${\mathrm{kT}}_c$ or higher is assumed then the film is in the clean limit. In the superconducting state the transmission drops at low frequency but remains finite.