On the characteristic voltage of highly oxygenated YBCO grain boundary junctions

Early in the study of cuprate grain boundary junctions (GBJs) it was observ ed that the junctions' characteristic voltage (IcRn) tended to scale, at le ast approximately, as J(c)(1/2). 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 enhanc ed oxidation techniques can increase J(c). by up to a factor of 10, IcRn re mains saturated at similar to 1.2 mV (at 4.2 K) for 24 degrees GBJs on stro ntium titanate bicrystals. Only with oxygen removal through inert gas annea ling is scaling observed. These results can be explained by a model where t he GBJ consist of a thin barrier layer, created by the localized cation dis order, sandwiched between two interface layers of non-uniform YBCO of highl y variable oxygen deficiency. The barrier layer is largely unaffected by th e enhanced oxidation techniques leading to the plateau in IcRn. Inhomogenei ties in oxygen content and order within the interface layers are dependent on the quality of the grain boundary growth process and annealing history a nd in turn are responsible for the variations in IcRn leading to the previo usly observed scaling. Ozone anneals and electromigration reduces the oxyge n inhomogeneities and electromigration reduces the oxygen inhomogeneities w ithin the interface region, facilitating the measurement of the intrinsic I cRn of the grain boundary barrier layer.