OPTICAL STUDY OF BA0.6K0.4BIO3 SINGLE-CRYSTALS - NORMAL AND SUPERCONDUCTING PROPERTIES

We report absolute reflectivity measurements on Ba0.8K0.4BiO3 in both normal and superconducting states. The superconducting-gap value was determined to be 2DELTA = 90 +/- 10 cm-1, which gives 2DELTA/k(B)T(c) = 4.2 +/- 0.5, consistent with previous experiments. However, finite absorption below 2DELTA frequency was found. No Holstein shift of the phonon-related spectrum was observed. Optical conductivity below 2 eV consists of two contributions: a narrow (less-than-or-equal-to 350 cm-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<T(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(L) was determined to be (4550 +/- 300) angstrom. Anomalous temperature dependence of reflectivity at temperatures above approximately 120-150 K was observed.