F. Gao et al., TERAHERTZ TRANSMISSION OF A BA1-XKXBIO3 FILM PROBED BY COHERENT TIME-DOMAIN SPECTROSCOPY, Physical review. B, Condensed matter, 52(5), 1995, pp. 3607-3613
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 ch
ange 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 con
ductivity are determined directly from the amplitude and phase of the
transmitted electric field without the need for a Kramers-Kronig analy
sis. By fitting at in the framework of BCS theory, a superconducting g
ap 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 th
e BCS coherence effects. However, the conductivity exhibits monotonic
temperature dependence and no clear sigma(1)(T) peak is observed throu
ghout the frequency range measured. The high-frequency penetration dep
th (similar to 600 nm) is also extracted and discussed. Our results ar
e consistent with a picture of BCS moderate coupling superconductivity
in an intermediate to dirty limit.