Far-infrared transmission studies on a superconducting BaPb1-xBixO3 thin film: Effects of a carrier scattering rate

We measured transmission spectra of a BaPb1-xBixO3 (x similar to 0.25) thin film in the frequency region between 5 and 130 cm(-1). At low temperature, there is a BCS-Like peak in the transmission spectrum near 21 cm(-1), corr esponding to 2 Delta(0)/k(B)T(c)=3.2+/-0.3, and a suppression at lower freq uencies. The transmission measurement in superconducting state covering the frequency region up to 13 Delta was found to be useful in probing the scat tering rate of carriers. The value of scattering rate, 1/tau, was estimated to be about 280 cm(-1). This suggests that BaPb1-xBixO3 belongs to the mod erately dirty regime (1/tau similar to 14x2 Delta) rather than the extremel y dirty regime of the Mattis-Bardeen theory. [S0163-1829(99)00613-X].