Optical study of ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_3$ single crystals: Normal and superconducting properties

We report absolute reflectivity measurements on ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_3$ in both normal and superconducting states. The superconducting-gap value was determined to be 2Δ=90±10 ${\mathrm{cm}}^{\mathrm{-}1}$, which gives 2Δ/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$=4.2±0.5, consistent with previous experiments. However, finite absorption below 2Δ frequency was found. No Holstein shift of the phonon-related spectrum was observed. Optical conductivity below 2 eV consists of two contributions: a narrow (≤350 ${\mathrm{cm}}^{\mathrm{-}1}$) Drude-like part (Drude component) and an overdamped midinfrared component. We suggest that while the carriers responsible for the Drude contribution form the superfluid at T<${\mathit{T}}_{\mathit{c}}$, the carriers responsible for the midinfrared one remain normal and account for the finite absorption inside the superconducting gap. The value of the London penetration depth ${\lambda }_{\mathit{L}}$ was determined to be (4550±300) Å. Anomalous temperature dependence of reflectivity at temperatures above ∼120–150 K was observed.