FAR-INFRARED TRANSMISSION OF BA1-XKXBIO3 THIN-FILMS

The far-infrared transmission of Ba1-xKxBiO3 thin films, grown on MgO substrates, was measured in the frequency range from 15 to 200 cm-1, a nd temperature range from 9 to 100 K. There is a BCS-like peak in the transmission spectrum. The strong-coupling Eliashberg theory gives a b etter description of the optical data than the weak-coupling Mattis-Ba rdeen theory. For a T(c) = 18 K sample the Eliashberg fits give a Lond on penetration depth of 5500 +/- 100 angstrom and an energy gap 2 DELT A(0) = 4.0 +/- 0.1kT(c)(6.2 meV).