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In the previous two articles we have been concerned with what happens to superconducting materials in an external magnetic field. In particular, we have seen that if a material is in its superconducting state, then there is a critical magnetic field (which varies with temperature) such that if an external magnetic field greater than the critical field is applied, then the superconducting material returns to its normal conducting state. If the external field is less then the critical field, then our superconducting material exhibits the Meissner effect. In this article we are concerned with the latter situation. That is, we will consider a superconductor in an external field that is lower than the critical magnetic field. The superconductor exhibits the Meissner effect, and we have a situation similar to that shown in the following figure.
Now we consider what happens as we start to increase the external magnetic field B. We know that once the magnetic field becomes equal to or greater than B_{C}, then our superconductor moves back to the normal conducting state. However, the manner in which it does this depends on whether the superconductor is a “Type 1” or a “Type 2” superconductor. The difference between the two cases can be illustrated by considering first the behaviour of a Type 1 superconductor subject to an increasing external magnetic field:
In the upper graph, we are plotting a quantity known as the diamagnetic moment. Without getting into the technicalities, in crude terms the diamagnetic moment is a measure of the strength of the induced internal magnetic field. The diamagnetic moment is negative because the internal field opposes the external field. While B < B_{C}, the magnitude of the diamagnetic moment increases with increasing external field, in order to preserve a zero total magnetic field inside our superconducting material. For a Type 1 superconductor, once B = B_{C}, the transition to the normal superconducting state is a sudden one. For B >= B_{C}, the diamagnetic moment vanishes and the total magnetic field inside our material becomes equal to the external field. Similarly, the electrical resistivity suddenly reappears at B = B_{C}, and our superconductor has returned to its normal state. With Type 2 superconductors, the graphs for diamagnetic moment and resistivity are somewhat different, as shown below:
In Type 2 superconductors, the critical field (labelled B_{C}_{1} in the above figure) signals a much more gradual return to the normal conducting state. Instead of the diamagnetic moment dropping immediately to zero, the decline is much more gradual, and the normal conducting state is not achieved until a field B_{C}_{2} is reached. At this point, the resistivity returns to its normal value, and the magnetic field inside the conductor is equal to the external field. Type 1 superconductivity is exhibited by most of the elements that exhibit superconductivity. Most alloys that exhibit superconductivity show Type 2 behaviour. Type 2 superconductors form the basis of “superconducting magnets” because of their ability to carry high currents without any losses in the superconducting phase, and also the fact that they remain superconducting even in relatively high magnetic fields (i.e. the one that is being generated in the superconducting magnet). Wikipedia has more details on superconducting magnets.
