## Josephson Tunnelling

If two superconducting regions are kept totally isolated from each other the phases of the electron-pairs in the two regions will be unrelated. If the two regions are brought together then as they come close electron-pairs will be able to tunnel across the gap and the two electron-pair waves will become coupled. As the separation decreases the strength of the coupling increases. The tunnelling of the electron-pairs across the gap carries with it a superconducting current as predicted by B.D. Josephson[14] and is called Josephson Tunnelling'' with the junction between the two superconductors called a Josephson Junction''.

Like a superconductor this gap has a critical current. If a supercurrent, , flows across a gap between regions with a phase difference, , it is related to the critical current, , by

 (3.27)

so that the maximum current flows across the gap when there is a phase difference of , where .

The Josephson Tunnelling Junction is a special case of a more general type of weak link between two superconductors. Other forms include constrictions and point contacts but the general form is of a region between two superconductors which has a much lower critical current and through which a magnetic field can penetrate.

Dr John Bland, 15/03/2003