On the characteristic voltage of highly oxygenated YBCO grain boundary junctions

Early in the study of cuprate grain boundary junctions (GBJs) it was observed that the junctions’ characteristic voltage (IcRn) tended to scale, at least approximately, as J 1/2 c . To re-examine the cause of this effect we have measured IcRn as a function of the more complete GBJ oxygenation that can be achieved by ozone annealing and electromigration. While these enhanced oxidation techniques can increase Jc by up to a factor of 10, IcRn remains saturated at ∼1.2 mV (at 4.2 K) for 24◦ GBJs on strontium titanate bicrystals. Only with oxygen removal through inert gas annealing is scaling observed. These results can be explained by a model where the GBJ consist of a thin barrier layer, created by the localized cation disorder, sandwiched between two interface layers of non-uniform YBCO of highly variable oxygen deficiency. The barrier layer is largely unaffected by the enhanced oxidation techniques leading to the plateau in IcRn. Inhomogeneities in oxygen content and order within the interface layers are dependent on the quality of the grain boundary growth process and annealing history and in turn are responsible for the variations in IcRn leading to the previously observed scaling. Ozone anneals and electromigration reduces the oxygen inhomogeneities and electromigration reduces the oxygen inhomogeneities within the interface region, facilitating the measurement of the intrinsic IcRn of the grain boundary barrier layer.