This came up in Joseph's blog post, I figured I might expand on it here.
First thing is that the Meissner Effect is dependent on the incoming magnetic field strength. If it exceeds some particular value then full magnetic penetration will occur and the effect will be lost. Secondly the condition of critical temperature is required here too in order to set up the superconducting state. It should be noted that there is a difference between a superconductor, and a material with an infinite conductivity. A superconductivity requires the Meissner Effect to take place, if not it's the latter.
When a superconductor meets these requirements eddy currents are created on the surface of the material. These happen to take on values that exactly cancel the magnetic field inside the material, resulting in zero magnetic field up to some depth. Due to the mechanisms that creates these currents, they are in fact non-deteriorating. This means that the magnetic fields generated are also non-deteriorating, ie the interior of the material has zero magnetic field always!
Whilst in general the interior of the material has this zero field, around the surface the fields don't exactly cancel. The depth that this occurs at is known as the London penetration depth, and is dependent on both the geometry and composition of the material.
At a certain critical magnetic field it suddenly becomes more energetically favourable for the magnetic fields to penetrate through the material. As a result the Meissner Effect breaks down, and the field jumps right back up in a discontinuous fashion. Again, each material has their own individual critical magnetic field where this happens.
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This is best illustrated with simple sketches of phase diagrams for superconductivity which can be found here.
ReplyDeletehttp://www.supraconductivite.fr/en/index.php?p=supra-levitation-phase-more
Apparently there are two kinds of superconductors: Type 1 and Type 2 (not to be confused with high and low Tc superconductors). For type 1 superconductors, the transition between having the external B fields completely penetrating the material (superconducting phase) to not penetrating (normal metal) is abrupt. But for type 2 superconductors, there is two transitions.
Not penetrating -> penetrating at vortices(more vortices appear as you increase the external field strength) -> completely penetrating (normal metal).
Also there are videos depicting their effects on superconducting trains.
Speaking of cool videos, I think everyone should look at this cool "quantum levitation" video.
ReplyDeletehttp://www.youtube.com/watch?v=Ws6AAhTw7RA
And here is an explanation of how it works:
http://www.quantumlevitation.com/QuantumLevitation/The_physics.html
Basically, it works because the superconductor is very thin, I presume thinner than the penetration depth (or what ever it's called, I've forgotten :P). This leads to the magnetic field being allowed through in discrete quantities. The places where the field is allowed through are called "flux tubes", cool name huh? Anyway, I guess it's energetically favorable for the tubes to be in "weak" areas. They give the example of grain boundaries, I presume they just mean defects, but perhaps this is referring to the boundaries of different crystalline layers or some such. Any thoughts on this one? Any how, this means that the superconductor doesn't want to move, once in place.
Thanks guys for posting the links.
ReplyDeleteDiagram drawing is certainly one thing we've learnt to appreciate throughout our degrees.
Entertaining videos would be another one...