Superconducting weak links

This review covers experimental results and theoretical ideas on the properties of superconducting weak links, i.e., weak electrical contacts between superconducting electrodes which exhibit direct (non-tunnel-type) conductivity. When the dimensions of such weak links are sufficiently small, the Josephson effect is observed in them, in other words, a single-valued and $2\pi$ -periodic relationship exists between the supercurrent $I_s$ and the phase difference $\sigma$ of the electrodes. With increasing dimensions, this relationship has a tendency to deviate gradually from the Josephson behavior. This deviation varies, depending on whether the weak link material is a superconductor or a normal metal. The various known types of weak links are described, and special mention is made of those weak links which are most suitable for physical investigations and have various practical applications. The data on the nonstationary (ac) processes in weak links, when the phase difference varies with time, are analyzed. In conclusion the existing concepts about the processes in weak links are briefly summarized and the most urgent outstanding problems are outlined.