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 d etermined to be 2DELTA = 90 +/- 10 cm-1, which gives 2DELTA/k(B)T(c) = 4.2 +/- 0.5, consistent with previous experiments. However, finite ab sorption below 2DELTA frequency was found. No Holstein shift of the ph onon-related spectrum was observed. Optical conductivity below 2 eV co nsists of two contributions: a narrow (less-than-or-equal-to 350 cm-1) Drude-like part (Drude component) and an overdamped midinfrared compo nent. We suggest that while the carriers responsible for the Drude con tribution form the superfluid at T<T(c), the carriers responsible for the midinfrared one remain normal and account for the finite absorptio n inside the superconducting gap. The value of the London penetration depth lambda(L) was determined to be (4550 +/- 300) angstrom. Anomalou s temperature dependence of reflectivity at temperatures above approxi mately 120-150 K was observed.