REENTRANT CRITICAL-CURRENT BEHAVIOR AS A COMMON FEATURE OF SINGLE GRAIN-BOUNDARY JOSEPHSON-JUNCTIONS IN BICRYSTALS OF COPPERLESS OXIDE SUPERCONDUCTORS

Critical current and electrical transport properties of single grain boundaries in bulk bicrystals of the copperless oxide superconductors BaPb1-xBixO3 and Ba1-xKxBiO3 were measured in order to investigate quasiparticle tunneling and Josephson properties of these materials, which hold promise for the creation of low temperature microwave electronics and cryogenic particle detectors, An unusual and remarkable feature of the temperature dependencies of critical current is the non-monotonous behavior with sharp maximum at temperatures well below the critical temperature, This anomalous low temperature re-entrant behavior appears to be a new universal feature of single grain boundaries in copperless oxide superconductors. Our studies of grain boundary transport properties in BaPb1-xBixO3 and Ba1-xKxBiO3 bicrystals lead us to the conclusion that re-entrant behavior of critical current originates from changes in oxygen electronic states due to heterovalent substitution in copperless materials.