Josephson Junctions: High-T _c

With the advent of ceramic superconductors, attempts were very quickly made to translate the techniques known from classical superconducting electronics to this new class of materials. In classical superconductivity, Josephson junctions are generally fabricated in a multilayer technology, incorporating at least two superconducting layers (niobium or NbN) and between them either insulating layers for tunnel junctions (Al ₂O ₃, MgO, AlN, etc.) or normal conducting layers for proximity effect devices or combinations thereof for special junction types, e.g., SINIS junctions. For high- T _c devices this concept turned out to be very difficult with the materials preparation techniques available at that time and with the very short coherence length of the high- T _c superconductors (HTSs). It was recognized very early that grain boundaries in HTSs behave like Josephson junctions. Much research was done on how to prepare grain boundaries in HTSs in a controlled manner and to elucidate the relationship between grain boundary structure and critical current density, j _c, for the supercurrent. This led to the experimental verification of the prediction for a d-wave symmetry of the wave functions describing the superconducting state in a HTS. The results also made clear that grain boundary junctions are unlikely to create the necessary reproducibility needed for superconducting electronic devices. Nevertheless, as a research tool or for devices with a very small number of junctions like SQUIDs, the grain boundary junctions are very valuable.

Subsequently a new junction type has been demonstrated,...