What is The real cause of meissner effect in superconductors?

[cartik]

As i have learnt, the lenz's law is not the cause of the meissner effect.This can also be proved by some experiments.I was wondering what may be the real cause for the meissner effect?
As i don't know the cause, i have no idea, in which part of physics i must post this query.

 

[marcusl]

This is an appropriate Forum for the question. The answer depends on a combination of solid state physics (which is based on quantum mechanics) and thermodynamics/statistical mechanics. When electrons condense into Cooper pairs below the transition temperature, it turns out that bulk energy is lowered by excluding magnetic flux from the interior. I don't know of a simple, intuitive way to explain this. Perhaps someone else knows...

 

[Enthalpy]

All I can tell is that if a flux goes through the type I superconductor above its critical temperature, this flux is expelled when the material is cooled. This would not happen through d(phi)/dt alone, which would just freeze the existing flux.

If the type I superconductor is a loop, the non-zero flux passing through the enclosed vacuum does get trapped and freezed as the material is cooled. Used for Squid gravity gradiometers.

And type II superconductors don't expel the flux...