I'm confused by videos such as this (popular demonstration of 'Quantum Levitation'):


So my current understanding of superconductors is that when in the superconducting state they expel all magnetic field in the bulk of the material (Meissner Effect) and in Type II superconductors flux CAN penetrate via imperfections in the material (flux pinning). From what I've read, the Meissner effect is simply a diamagnetic repulsion resulting in a very weak levitation that obviously could not hold the superconductor upside-down, while flux pinning results in a very strong force that can indeed hold the material upside-down or even withstand a huge amount of pressure.

My understanding of flux pinning is that the superconductor is held in a position of constant magnetic flux density and any movement that would result in the magnetic flux density being changed (e.g. moving the superconductor further away from or toward the magnet) will be strongly resisted. I am also under the impression that for flux pinning to occur, the material has to enter the superconducting state whilst in the desired position.

With that in mind, I am confused as to how, in the video above, the demonstrator is able to manipulate the superconductor in to positions of different magnetic field strength, e.g. toward and away from the magnet, without any issue. Also, the superconductor can be totally removed from the magnetic track with no visible tug or struggle.

I understand the expulsion of magnetic field in the superconductor is actually an exponential drop in magnetic field from the surface characterised by λ, the penetration depth within a given superconductor at which the field drops to 1/e of its surface value. I understand this means that in a thin disk the magnetic field could penetrate the material. I assume this has something to do with the effect shown here but I don't understand how this can be.

Can flux pinning occur in a disk like this? Is this due to the fact that the flux tubes aren't necessarily fixed to defects? If so, how does this still result in a 'levitation'?



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