What causes quantum coherence in superconductivity and superfluidity?

[sci-guy]

I have a question about quantum coherence phenomena. I understand (basically) how laser light results from light waves that are in sync. I also see how ferromagnetism results from orderly alignment of electron spin amongst iron atoms (though I've posted in another thread asking whether this is a genuine example of "quantum coherence"). And in doing some basic research on superconductivity, I kind of see how electrons move freely in the ordered lattice of supercooled conductors (e.g. http://hyperphysics.phy-astr.gsu.edu/hbase/solids/coop.html#c3 ). I haven't been able to find any explanation for superfluidity (not even sure one exists).

My question is about superconductivity (and superfluidity, if its understood):

Whereas laser light and ferromagnetism seem to result from 'quantum coherence' in terms of their energy (i.e. coherent wave forms), superconductivity (and I would assume, superfluidity) appear to result from quantum structural order (i.e. the lattice in superconductors that allows free flow of electrons). Why is this any more of an example of quantum coherence than any crystal solid, like diamond (or IS crystal an example of quantum coherence? Even snowflakes?) What exactly does the coherence refer to in superconductivity and superfluidity?

 

[sci-guy]

I think I found an answer regarding superfluidity, which not only explains the phenomenon but also clearly indicates what's "coherent":

Most commonly demonstrated in liquid helium, superfluidity occurs when the helium is cooled and some helium atoms have reached their lowest possible energy. At this point, these atoms' quantum wave functions begin to overlap so that they form a Bose-Einstein condensate, in which all the atoms behave as one large atom, and their quantum nature is manifested on the macroscopic scale.

-- http://www.physorg.com/news/2010-10-physicists-superfluid.html

So it is a wave coherence, like laser light (and ferromagnetism, if that's an accepted example).

Seeing that, I'm more doubtful that common crystals are examples of quantum coherence. There must be something more specific about the structure of superconductors that makes them examples of wave coherence. Anyone?

 

[A. Neumaier]

I think I found an answer regarding superfluidity, which not only explains the phenomenon but also clearly indicates what's "coherent":



So it is a wave coherence, like laser light (and ferromagnetism, if that's an accepted example).

Seeing that, I'm more doubtful that common crystals are examples of quantum coherence. There must be something more specific about the structure of superconductors that makes them examples of wave coherence. Anyone?

Read about ''coherent states'' and ''Bogol(i)ubov transformation''

 

[sci-guy]

Thanks -- I found this:

http://cnx.org/content/m22750/1.3/

It says that all Coopers pairs in a superconductor can be defined by a single wave function (because they act as bosons, not fermions). So the structural lattice is just a facilitator, not the real cause of coherence.

 

[A. Neumaier]

Thanks -- I found this:

http://cnx.org/content/m22750/1.3/

It says that all Coopers pairs in a superconductor can be defined by a single wave function (because they act as bosons, not fermions). So the structural lattice is just a facilitator, not the real cause of coherence.

The lattice simply acts as the generator of an external potential in which the electrons move.