What role do currents play in the Meissner effect?

[Michaelmoo]

Ok. So I'm really confused. The variation in perspectives by physics authors has really left me worried.

With the meissner effect, are the currents induced in the surface of the super-conductor eddy currents? If not what type of currents are they? Could you provide a justification, as no text I've seen has been able to thus far (that is, why or why not they're eddy currents)?

Could someone also please explain the behavior of these currents, and also a little on their role in the meissner effect (how they lead to magentic field expulsion - diamagnetism).

Thanks in advanced.

EDIT: spelt meissner wrong in the thread title.

Also, the meisner effect describes the magnetic levitation of superconductors below their critical temperature (for those who have never used the term)

 

[xepma]

They are not eddy currents. One way to see this is that eddy currents arise due to a changing magnetic field. But if you place a magnet on a superconductor at room temperature (so there is no superconductivity) and then cool it down so that superconductivity kicks in, the meissner effect still takes place -- even though there is no changing magnetic field.

The technical reason is as follows. A superconducter has the special property that the gauge symmetry of the photon is spontaneously broken. When a gauge symmetry is broken, the corresponding gauge boson (the photon) becomes (effectively) massive. And a force mediator which is massive has the property that it is short ranged. Photons can therefore only penetrate a superconductor for a very short distance (depending on the effective mass of the photon in some sense). "Deep" inside the superconductor there is no electromagnetic field at all. This is precisely the Meissner effect.

 

[Michaelmoo]

Hey. Thanks for your reply. Yes I completely agree with you.

Although, the other argument is that they ARE eddy currents. As the magnet falls due to gravity above the superconductor, it gives rise to a change in magnetic flux and induces perfect eddy currents in the superconductor. This in turn gives rise to a magnetic field that "opposes the motion of the magnet" [lenzes law]; i.e. it expels the magnet and its magnetic field. During the process, the magnet rises (slightly) and the process is repeated (magnet falls due to gravity again etc).

The net result is the momentary (slight) rise and fall of the magnet, resulting in the magnet hovering (although we do not see this hovering).

Could you please address this information? I have a slight feeling it may be wrong as its the only source I've been able to find with this perspective. In case you want to know, the source isn't that reputable and is probably not available in the US.

Also, how did you receive your information? Have you come across this issue at a university level?

Thanks in advance.

 

[Michaelmoo]

anyone?

 

[xepma]

No they are not Eddy currents. Gravity has nothing to with it. You do not need a changing magnetic field for the Meissner effect to take place.

Let me repeat my argument, because maybe I wasn't clear:

Start out with a superconductor at room temperature (so it's not superconducting at the moment). Place a magnet on top of the superconductor. The magnetic field penetrates the superconducter at this moment, since there is no Meissner effect.

Next, we cool down the whole system. At some stage the superconducting material makes a phase transition, and superconductivity kicks in. At this stage, the electromagnetic field will be repelt by the superconductor. As a result the magnet starts to float! It's lifted up. But the magnet wasn't moving at all, so there was no changing magnetic field. It couldn't have been eddy currents that were responsible -- the system was static. All we did was lower the temperature of the system.

The analogy of what you mention is cute, but wrong. It would mean that the magnet has to keep on falling, even if it's just an infinitesimal fall. But that's an unstable situation, and some point the magnet would reach the superconductor. The Meissner effect is a stable configuration. There is no energy loss and the magnet doesn't need to move.

I learned this stuff in my master's and by reading books.

 

[Michaelmoo]

Thanks very much!

Your explanation was very clear yer. And you seem like you know what your on about. Again thanks.

 

[xepma]

No problem!

 

[granpa]

look up 'diamagnetism'. its associated with electron 'lone pairs'

 

[hiyok]

Hey, I completely agree with xepma on his explanantion, which is usually termed as 'Higgs mechanism'. However, his explanantion, though elegant, is too much technical. So, I'd like give an elementary account. Basically, from the current formula given by quantum mechanics, one learns that the electrical current (J) in the presence of an external magnetic field is proportional to the vector potential (A), provided that the system is uniform. Now with the Maxwell equations, which state that (1) the magnetic field is the curl of A and (2) J is proportional to the curl of the magnetic field, one easily sees that, curl×curl×A \propt A, which then prevents the magetic field from penetrating the sample carrying superconducting current.