Question

Can anyone explain, what is cooper pair(BCS Theory of Superconductors)??

Answer 1

@ujjwal @Taufique @experimentX

Answer 2

read this one : http://hyperphysics.phy-astr.gsu.edu/hbase/solids/coop.html

Answer 3

yeah i can... but i want a discussion .

Answer 4

Roughly speaking, all "super" phenomena -- superconductivity, superfluidity -- are the results of Bose condensation, in which particles following Bose-Einstein statistics fall into the many-body ground state. Now, if you have studied basic QM, you know that the spacing between energy levels for a confined system is greatest between the ground and first excited state. So once a many-body system of bosons is in its ground state, it becomes unsually hard to get it out of that state. In part because of the energy gap between the ground state and the first excited state, but also because the excitation has to be something that is spread out over the entire are of the system in some way to be effective.

What that means is that a Bose condensate will be unusually resistant to absorbing energy and changing its state, and hence will persist in doing whatever it was doing in the ground state indefinitely. (Generally the Bose condensate is considered to co-exist with a certain fraction of "normal" fluid, and I don't know whether the condensate or the normal fluid carries any currents, e.g. of electricity, momentum or heat.)

You can easily get a Bose condensation in He-4, because it's a boson. Whether a particle is a boson or not depends on how many elementary particles it has. Every elementary particle is a fermion, and has spin 1/2. But when you make a composite particle, you add the spins to form a total spin, and if you get an integer the composite particle is a boson. The two protons and two neutrons and two electrons in He-4 add up to make a boson.

But what about the electrons in a conductor? They're all fermions! However, it is known that there can be attractions between the electrons mediated by the crystal lattice of positively-charged ion cores. Roughly speaking, a free electron here will pull positively-charged ions toward it, and these will, in turn, pull on another free electron over there. It's sort of the reverse of how electrons can "glue" nuclei together in a molecule: in a crystal, the ions can produce attractive forces between the electrons. Actually, they are distortions in the lattice that do the job, and so we usually talk about phonon-mediated attractions between the electrons (or really quasielectrons).

The current theory is that under certain circumstances, these phonon-mediated attractions can pull a pair of electrons close enough together that they behave like a single composite particle, which, since we're adding spin 1/2 to 1/2, is an object with integer spin -- a boson! So these Cooper pairs of electrons can undergo a Bose condensation, and give us superconductivity. Why not triples or quadrupoles of electrons? No special reason, except that the attraction is quite weak, so it will be most effective (against the naturally disruptive forces of random thermal motion) for pairs.