THE FAR-INFRARED ENERGY-GAP OF BA1-XKXBIO3 THIN-FILMS

The far infrared transmission spectra of Ba1-xKxBiO3 (x = 0.4) thin fi lms, grown by molecular beam epitaxy on MgO substrates, were measured in the frequency range of 15 cm(-1) to 200 cm(-1), and temperature ran ge from 9K to 86K. In the normal state the transmission is independent of frequency. The transmission in the superconducting state approache s zero at zero frequency then rises with frequency to a peak whose pos ition scales with the critical temperature and reduces as the temperat ure increases. The far infrared transmission curves are better describ ed by strong coupling Eliashberg theory rather than weak coupling Matt is-Bardeen theory. From the Eliashberg calculation, which is based on the alpha(2)F(omega) data from tunneling measurements, the London pene tration depth lambda(L)(0) of a T-c = 18 K sample is found to be 5500 +/- 100 Angstrom and the energy gap is 2 Delta(0) = 4.0kT(c) = 50.1 cm (-1) (6.2 meV). These results show that the strong coupling dirty limi t gives distinctly different electrodynamics than the weak coupling di rty limit.