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, $i_{s}$, flows across a gap between regions with a phase difference, $\Delta\phi$, it is related to the critical current, $i_{c}$, by

$$i_{s} = i_{c} \sin \Delta\phi$$ (3.27)

so that the maximum current flows across the gap when there is a phase difference of $\nicefrac{\pi}{2}$, where $i_{s} = i_{c}$.

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.