TUNNELING MEASUREMENTS OF THE ELECTRON-PHONON INTERACTION IN BA1-XKXBIO3

The conductance curves of point-contact tunnel junctions between Ag an d Ba1-xKxBiO3 (x congruent-to 0.4) reveal a BCS behavior with low leak age current at zero voltage and some broadening of the superconducting -gap structure. In the energy range above the superconducting energy g ap, the structure in the voltage dependence of the second derivative d 2V/dI2 of the voltage with respect to the current of the tunnel juncti on has been investigated in detail in magnetic fields up to 10 T. Whil e part of this structure is rapidly changing in a magnetic field, thre e reproducible peaks in d2V/dI2(V) remain stable up to the transition temperature from the superconducting to the normal state with only add itional broadening in the applied magnetic field. An analysis of this structure in terms of strong-coupling effects yields the spectral func tion alpha2F for the electron-phonon interaction. The obtained spectra l weight in the energy region 20-70 meV points to the importance of th e oxygen optical modes in the electron-phonon coupling for the superco nductivity of Ba1-xKxBiO3.