As would be expected, the current in superconductor, is called a supercurrent. This current will theoretically flow forever requiring no voltage to support it after the first jump start. When a supercurrent flows into a set up called a Josephson junction, which is simply a break in the superconducting wire filled with either a non-superconducting metal or an insulator (it can also be done by thinning the superconducting wire significantly but this is harder to explain), it is still able to pass across the insulating gap. The eponymous Brian Josephson, at just age 22, was able to predict the behaviour of cooper pairs in the superconductor and as a result predict the voltage and current across the weak insulator. The effect of current flowing through the insulator had been attributed to the insulator’s breakdown but it took Josephson to realise that in fact cooper pairs were able to quantum tunnel just like their regular electron brethren and it is the effect that now holds his name.
This study has aimed to study the Josephson effect when an external magnetic field is applied to the junction. The junction used was that of aluminium (Al) as the superconductor and indium arsenic (InAs) nanowires for the intersecting junction. The nanowires were connected to their superconducting leads by electron beam lithography so that the wires were at right angles to the leads. While the magnetic field was absent the set up perfectly matched previously recorded Al and InAs nanowire experiments.
However, when a current was applied perpendicular to the base substrate a remarkable change was observed. The current through the junction remained constant as the magnetic field strength was raised until 15 mT was hit. At about this point the current starts rapidly rising with field strength, reaching double the current at about 23 mT. When the field is increased past this the current starts dying down again. The set up of the wires and leads at right angles to each other meant the magnetic field’s effect could be tested when orthogonal and parallel to both, neither or just one of the junction components. A model has been created to explain the observations but it is quantitatively lacking, predicting a critical magnetic field of about 150 mT, ten times higher than the observed critical field strength. More work is being done to try and explain this discrepancy by including more experimentally relevant features in the model and hopefully successful results will come soon.