SUPERCONDUCTING ENERGY-GAP IN BA1-XKXBIO3 - TEMPERATURE-DEPENDENCE

The superconducting energy gap of Ba1-xKxBiO3 has been measured by tun neling. Despite the fact that the sample was macroscopically single ph ase with very sharp superconducting transition T-c at 32 K, some of th e measured tunnel junctions made by point contacts between silver tip and single crystal of Ba1-xKxBiO3 had lower transition at 20 K. Local variation of the potassium concentration as well as oxygen deficiency in Ba1-xKxBiO3 at the place where the point contact is made can accoun t for the change of T-c. The conductance curves of the tunnel junction s reveal the BCS behavior with a small broadening of the superconducti ng-gap structure. A value of the energy gap scales with T-c. The reduc ed gap amounts to 2 Delta/kT(c) = 4 divided by 4.3 indicating a medium coupling strength. Temperature dependence of the energy gap follows t he BCS prediction.