Study on grain boundary Josephson junctions in BaPb1−xBixO3 thin films

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Abstract
Several characteristics have been experimentally investigated for the grain boundary Josephson junctions inherently formed at every grain boundary of polycrystalline sputtered BaPb1−xBixO3 superconducting thin films, where x=0.2, 0.25, and 0.3. The origin of the boundary Josephson junction barrier is ascribed to the low carrier density, the structure sensitiveness, and surface states originating from the Bi ions in the grain boundary region, all of which this superconducting oxide posesses at the same time. The grain boundary Josephson junction exhibits tunneling‐type I‐V characteristics, which progressively change to those of a weak link in the temperature range near the superconducting transition temperature. The gap voltage observed is less than the value given by the Bardeen–Cooper–Schrieffer theory, except for x=0.3. The temperature dependence of the maximum Josephson current deviates from the Ambegaokar–Baratoff theory in the same temperature range. All these tendencies are interpreted in terms of the McMillan tunneling model for a proximity sandwich, based on a model of a parabolic grain boundary potential barrier. Qualitative agreements are attained for the composition dependence of these characteristics. Relations between the Bi content and the junction barrier properties are also discussed, partly in relation to the strong coupling tendency in this oxide system.