Far-infrared transmission of ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_3$ thin films

The far-infrared transmission of ${\mathrm{Ba}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{BiO}}_3$ thin films, grown on MgO substrates, was measured in the frequency range from 15 to 200 ${\mathrm{cm}}^{\mathrm{-}1}$, and temperature range from 9 to 100 K. There is a BCS-like peak in the transmission spectrum. The strong-coupling Eliashberg theory gives a better description of the optical data than the weak-coupling Mattis-Bardeen theory. For a ${\mathit{T}}_{\mathit{c}}$=18 K sample the Eliashberg fits give a London penetration depth of 5500±100 Å and an energy gap 2Δ(0)=4.0±0.1${\mathit{kT}}_{\mathit{c}}$ (6.2 meV).