BCS Cooper Pairs (1 Viewer)

[I97]

After reading this http://www.physics.usyd.edu.au/~khachan/PTF/Superconductivity.pdf i'm a bit confused over which way i should go about answering a possible question where they may potentially ask to draw a diagram.
Basically it says "Note that the explanation and pictorial representation of a Cooper pair presented here comes directly from
BCS theory. However, current HSC textbooks tend to distort this picture with unphysical situations such
as the Cooper pair being within one or two atomic spacings and traveling in the same direction – each of
these situations is false. "
I understand that the HSC BCS has been watered down but which one is preferred in a HSC context.

Also another question regarding the Meissner effect. In different sources i've read different things as to whether or not perfect eddy currents cause the levitation. The students guide says "Meissner effect is separate to the induction of eddy currents since a change in magnetic flux is needed to induce eddy currents" which makes sense in a way since the magnetic flux is expelled. This is supported by http://lrrpublic.cli.det.nsw.edu.au...xplorer/physics/lo/superc_12/superc_12_02.htm which shows this logic in their diagram.
But then both physics in focus and jacaranda base their explanations over the induced currents producing a just as strong magnetic field in the opposite direction which leads to a cancellation of the external magnetic field. So again which one is correct, or perhaps 'preferred' in the HSC?

Thanks

 

[PhysicsMaths]

After reading this http://www.physics.usyd.edu.au/~khachan/PTF/Superconductivity.pdf i'm a bit confused over which way i should go about answering a possible question where they may potentially ask to draw a diagram.
Basically it says "Note that the explanation and pictorial representation of a Cooper pair presented here comes directly from
BCS theory. However, current HSC textbooks tend to distort this picture with unphysical situations such
as the Cooper pair being within one or two atomic spacings and traveling in the same direction – each of
these situations is false. "
I understand that the HSC BCS has been watered down but which one is preferred in a HSC context.

Also another question regarding the Meissner effect. In different sources i've read different things as to whether or not perfect eddy currents cause the levitation. The students guide says "Meissner effect is separate to the induction of eddy currents since a change in magnetic flux is needed to induce eddy currents" which makes sense in a way since the magnetic flux is expelled. This is supported by http://lrrpublic.cli.det.nsw.edu.au...xplorer/physics/lo/superc_12/superc_12_02.htm which shows this logic in their diagram.
But then both physics in focus and jacaranda base their explanations over the induced currents producing a just as strong magnetic field in the opposite direction which leads to a cancellation of the external magnetic field. So again which one is correct, or perhaps 'preferred' in the HSC?

Thanks

This is what you should talk about. In reality, the electrons move in opposite directions to obey the law of conservation of momentum, but you must talk about them moving in the same direction in HSC physics.

Also, magnetic levitation can be caused by both eddy currents and cooper pairs - you will include or disclude eddy currents depending on the question. For example, if you were to explain magnetic levitation occurring within a magnet that was originally placed on the superconducting disk, you would state that the meissner effect was responsible for the levitation. However, if the experiment showed the superconducting disk cooled below its critical temperature, and a magnet approaching the disk, you could mention both the meissner effect and eddy currents (due to the magnet experiencing a changing flux)

 

[phamtom44]

No, the currents associated with the Meissner effect ARE not eddy currents. It is best to simply refer to them as "surface currents" at a HSC level.
Basically all you need to know regarding the Meissner effect at a HSC level is that once the superconductor's temperature is below its critical temperature, you get electrons forming cooper pairs allowing the conduction of electrons with 0 resistance. When you apply a magnetic field 'surface currents' are produced in the superconductor and these produce a magnetic field in the exact opposite direction, expelling the field that would penetrate it. (Type 1 superconductors totally exclude the magnetic field, Type 2 totally exclude when the applied B field is low, but only partially exclude when it is high). The repulsive force due to the magnetic field is sufficient to balance out the weight force and the magnet levitates.

The reason why these aren't eddy currents is because by Faraday's law, the conductor has to experience a changing flux in order for the eddy current to be induced.

If you conducted your experiment at school, you would've noted that even if you placed the magnet on top of the superconductor when it wasn't superconducting and then cooled it down with liquid nitrogen until it was below the critical temperature, the magnet still levitates even though there isn't a changing magnetic field or any relative motion between the magnet and the superconductor.

Because the dot point for BCS is "discuss the BCS theory" you could discuss the disadvantages as well as advantages of this theory, some of which you mentioned like
- It implies that the electrons that constitute the cooper pair are close together when they are actually very far apart. (If they were close together, the repulsive force) between two electrons would be larger than the attraction from the temporarily deformed lattice and so they would just repel each other.
- Doesn't integrate quantum physics/can't explain type 2 superconductivity.