ABSTRACT
Many important properties of superconductors depend solely on the existence of a complex order parameter which is gauge covariant in the same way as the wavefunction for a charged particle. V L Ginzburg– L D Landau theory in particular depends on this idea alone. Weak links showing the usual Josephson effects may be constructed both between pairs of cuprate superconductors and between cuprate and conventional superconductors. The appearance of Josephson effects between conventional and cuprate superconductors and the appearance of Shapiro steps at the usual voltages nħω/2e both confirm that cuprate superconductors involve an order parameter for paired electrons. The types of weak link most fully investigated include tunnel junctions, sns junctions, scanning tunneling microscope (stm) and other point contacts with conventional superconductors, grain boundary junctions and electron-beam damage junctions. The short coherence length in the cuprates means that grain boundaries behave like weak links.
