Hj. Kaufmann et al., OPTICAL-RESPONSE OF BA1-XKXBIO3 - EVIDENCE FOR AN UNUSUAL COUPLING MECHANISM OF SUPERCONDUCTIVITY, Physical review. B, Condensed matter, 58(14), 1998, pp. 9479-9484
We have analyzed optical reflectivity data for Ba1-xKxBiO3 in the far-
infrared region using Migdal-Eliashberg theory and found it inconsiste
nt with standard electron-phonon coupling: Whereas the superconducting
state data could be explained using moderate coupling, lambda=0.7, th
e normal state properties indicate lambda less than or equal to 0.2. W
e have found that such behavior could be understood using a simple mod
el consisting of weak standard electron-phonon coupling plus weak coup
ling to an unspecified high energy excitation near 0.4 eV. This model
is found to be in general agreement with the reflectivity data, except
for the predicted superconducting gap size. The additional high energ
y excitation suggests that the dominant coupling mechanism in Ba1-xKxB
iO3 is ndt standard electron-phonon. [S0163-1829(98)08938-3].