5 - Structural defects in YBa2Cu3O7-δ superconductors

Abstract

The discovery of the high-temperature superconductor produced widespread euphoria among the physics and material science communities. Despite the subsequent worldwide effort in superconductivity research, however, the progress of commercialization of this highly touted material has been rather disappointing. Now it has become clear that the barrier to the practical applications of the perovskite-based oxides is their low critical current density, Jc, especially the low critical transport current density, Jct. For structure-sensitive properties, knowledge of the average, or ideal, structure is not sufficient; it is structural defects that play an important role in determining the properties. It has been learnt that in superconducting materials, some defects destroy superconductivity, while others promote it. For example, defects of an appropriate size can act as flux-pinning centers in the crystal. By preventing the motion of magnetic flux lines, such defects can enhance the ability of the material to carry current. On the other hand, large-angle grain boundaries, which often act as weak links in the cuprate oxides (they subdivide the material into regions of strong superconductivity separated by weak superconducting interfaces), are key elements limiting the use of these materials as bulk superconductors. Thus, an understanding of intragranular defects is critical to point out the way to enhance flux pinning, while an understanding of the interfacial structure of grain boundaries is essential in elucidating the superconducting characteristics of the boundaries and, hence, in seeking possible engineering processes to eliminate the “bad” grain boundaries in the high-temperature superconductors.