Superconducting state

From the wide range of experimental studies on superconductivity, those which throw most light on the changes which occur amongst the electrons of a metal when it is cooled through the superconducting transition are isolated. The character of the electron condensation into Cooper pairs is studied first for a single pair of electrons interacting through an attractive potential, related to the electron lattice interaction, and is then treated in such a way as to include the many electron aspects of the interaction, a process found to be necessary in the introductory study. The results of such theoretical treatments are assessed in the light of the experimental facts discussed previously, and the shortcomings of the Bardeen--Cooper-Schrieffer treatment are discussed. The Ginzburg--Landau equations are derived, and several examples of their use in such circumstances are given. Special attention is devoted to the existence of the type II superconductors, and a discussion of flux pinning and creep in such materials is included. The unique description of the superconducting state as a macroscopic quantum state described by one single wavefunction throughout the superconductor leads to the possibility of varying degrees of coupling between neighbouring superconductors, and the arising Josephson effects are discussed along with examples of their application