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Oh, now I feel better. This article starts off with a bang, doesn’t it, Billy?

Oh, Billy, I couldn’t keep the secret any longer, it took a lot of effort to me so far. But I am sure you are the rip-the-plaster-off sort of person. Now, if this surprising piece of news hasn’t knocked you out, I will justify my affirmation. This is the last chapter but one of our dissertation about superconductivity and there is still so much to do. Then, sleeves up… and working!

Let’s consider, Billy, a parallelepiped of irreversible type II superconducting material, infinitely extended into two dimensions, so that only one gives non-trivial results. This is the simplest case, which enables us to better understand the concept. Let’s focus on this transversely finite-sized face and apply a magnetic field parallel to its longitudinal direction, increasing its magnitude progressively. Once the amplitude of the magnetic field has overcome the threshold Hc1, the first vortex enters the material. However, the sample is provided with plenty of defects, that act like energetically favourable sites. Therefore, as soon as the vortex meets a pinning centre, it gets blocked in its specific position. This occurs for each incoming vortex, so that in short time, the slab presents a really inhomogeneous distribution of vortices, since they are concentrated mainly near the two sides and no vortex is located in the central part.

As the external field grows further, the Lorentz-like force might become larger than the pinning one, then a vortex gets discarded from the corresponding pinning centre and starts to wander again, in search for the next nearest defect. This is likely situated in a more internal region of the slab, since all the other pinning centres are already occupied. At the same time, since vortices move always from the side to the centre and the more external pinning centres are already engaged, the outer vortices constitute a thin peripheral barrier, that hinders the access of new vortices and attempt to prevent any further “intrusion”. We can say, Billy, that the slab tends to counteract flux penetration through a shielding reaction. Thus, while the applied field is larger and larger, additional vortices penetrate into the sample, always starting from the sides and eventually migrating towards the centre, defect by defect, but at increasing difficulty. Finally, the material will exhibit a strongly non-uniform vortex distribution, much denser on the sides rather than in the central part.

This is the result of the material repulsive behaviour, that we can define as a diamagnetic response. In other words, the sample tries to prevent as much as possible the penetration of the applied field, represented here by the normal core vortices, attempting to reproduce the Meissner effect, but without completely succeeding in it.
However, Billy, pay attention: we are still studying the mixed state, not the superconducting one, where Meissner effect actually takes place, so we have not justified the opening sentence yet.

So far, we have studied the behaviour of the slab when the applied field gets gradually increased. Now, beginning from the strongly inhomogeneous vortex distribution we have just realized, let’s perform the opposite operation: let’s reduce the external field. Please Billy, notice that we are proceeding with the same sample of before, extending the experimental sequence.
Here, the coup de theatre.
When the applied field gets decreased, the flux distribution can’t be described simply by reversing the previous behaviour. For certain, a reduction of the applied field determines a reduction in the number of vortices, since they are the carriers of the magnetic flux density. However, as the penetration of vortices was contrasted when the field was increasing, once the vortices have settled in the material, on the contrary, the sample tries in every way to retain the vortices inside itself. It struggles to avoid that vortices run away, in order to maintain the previous level of magnetization as much as possible.

It exerts such an intense attractive force on the vortices, that even when the applied field has been reduced so much that it has vanished, there are still some vortices in the superconductor.

But, hey hey Billy, what is happening? Vortices with no applied field? If the applied field is null, it means that its value is certainly below Hc1, then we are in the superconducting state! But how can vortices survive if we are in the superconducting state? What about the Meissner effect and the expulsion of the magnetic flux density? Have I made fun of you so far? Eh eh Billy, that’s just it, old sport! The Meissner effect doesn’t take place in irreversible type II superconductors. And we have finally justified the very first sentence.
This fact has enormous consequences, both from the theoretical and the practical point of view. Firstly, such a result implies that the Meissner effect is not the best proof for showing superconductivity any longer. Moreover, this is possible if and only if we realize a procedure similar to the one we have performed. This means that the chronological order of the experiment is extremely important and the magnetization of the superconductor depends on its history, that in technical jargon we call a “hysteretical behaviour”.

In addition, the fact that the sample is provided with vortices, then with a certain residual magnetic force, even when the applied field is absent, indicates that we have turned the superconductor into a… magnet! But not an ordinary magnet, Billy… a supermagnet! A supermagnet is 10 times stronger than the magnets you have on your whiteboard at home! Imagine the Geomag structures you could build up with supermagnets! However, there is always this tiny little detail: you need a refrigerant system that is able to cool down temperature below at least 77 K…

We are almost there, Billy! Next time we will finally reach our goal! Get ready!

Pubblicato da Giulia Maffeis

Crede che esistano due cose infinite: l'universo e il suo amore per la fisica, ma riguardo la prima nutre ancora dei dubbi.