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 b oundaries in bulk bicrystals of the copperless oxide superconductors B aPb1-xBixO3 and Ba1-xKxBiO3 were measured in order to investigate quas iparticle tunneling and Josephson properties of these materials, which hold promise for the creation of low temperature microwave electronic s and cryogenic particle detectors, An unusual and remarkable feature of the temperature dependencies of critical current is the non-monoton ous behavior with sharp maximum at temperatures well below the critica l temperature, This anomalous low temperature re-entrant behavior appe ars to be a new universal feature of single grain boundaries in copper less oxide superconductors. Our studies of grain boundary transport pr operties in BaPb1-xBixO3 and Ba1-xKxBiO3 bicrystals lead us to the con clusion that re-entrant behavior of critical current originates from c hanges in oxygen electronic states due to heterovalent substitution in copperless materials.