TERAHERTZ TRANSMISSION OF A BA1-XKXBIO3 FILM PROBED BY COHERENT TIME-DOMAIN SPECTROSCOPY

The complex transmission coefficient for millimeter and submillimeter waves incident on a Ba0.6K0.4BiO3 thin film (82 nm) has been measured over a frequency range of 200-1200 GHz at temperatures above and below T-c using coherent time-domain spectroscopy. We observe a dramatic change in both the magnitude and phase of the terahertz transmission in the superconducting state caused by a rapid carrier condensation. Both the real (sigma(1)) and imaginary (sigma(2)) parts of the complex conductivity are determined directly from the amplitude and phase of the transmitted electric field without the need for a Kramers-Kronig analysis. By fitting at in the framework of BCS theory, a superconducting gap 2 Delta(0)=6.9 meV=3.8k(B)T(c) is obtained. Below T-c, the sigma(1) is rapidly enhanced for omega/2 pi<500 GHz, which is attributed to the BCS coherence effects. However, the conductivity exhibits monotonic temperature dependence and no clear sigma(1)(T) peak is observed throughout the frequency range measured. The high-frequency penetration depth (similar to 600 nm) is also extracted and discussed. Our results are consistent with a picture of BCS moderate coupling superconductivity in an intermediate to dirty limit.