OPTICAL-RESPONSE OF BA1-XKXBIO3 - EVIDENCE FOR AN UNUSUAL COUPLING MECHANISM OF SUPERCONDUCTIVITY

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].