Grain-Boundary Josephson Junctions in the High-Temperature Superconductors

Abstract

Defects can determine the electrical transport properties of solids. This statement also holds true for the high-temperature superconductors. In these materials defects play an important role concerning two aspects. On the one hand, high transport critical current densities in these materials can only be achieved by the presence of a high density of defects providing pinning centers for the magnetic flux lines. It is well-known that ideal defects for flux pinning should have a diameter equal to about the coherence length. Furthermore, their density should be high enough to provide sufficient flux pinning at high magnetic fields. Due to the short coherence length of the cuprate superconductors their order parameter is depressed considerably by local defects. In this way point defects can be effective pinning cites for magnetic flux lines increasing the transport critical current density in single crystalline materials. On the other hand, extended defects in the cuprate superconductors in most cases form weak links reducing the transport critical current density. Extended defects in these materials seem to be organized in planes as, for example, stacking faults or small and large-angle grain boundaries. This shows that defects can both increase and decrease the transport critical current density depending on their detailed nature. Grain boundaries are probably the most important defects in the cuprate superconductors. On the one hand, they are known to cause disappointingly low transport critical current density in polycrystalline materials thereby limiting high current density applications.