Grain boundaries and critical currents - a new perspective

More than a decade of research on grain boundaries in high-T-c superconduct ors has brought extensive knowledge about their often peculiar electronic p roperties. However, a comprehensive understanding of the physical principle s underlying these properties is still in development. The universal charac ter of the grain boundary transport properties for many different high-T-c compounds suggests that these are to a large extent controlled by generic a spects of the cuprate materials, such as the grain boundary microstructure, the predominant d(x)2(-y)2 symmetry of the order parameter and the possibi lity of bending of the electronic band structure, leading to the formation of space-charge layers. The latter offers a new perspective to improve grai n boundary interfaces, e.g. by appropriately doping the material. This is d emonstrated by the example of Ca doping of YBa2Cu3O7-delta, a procedure wit h which the grain boundary J(c) has been strongly enhanced.