BCS THEORY ON SUPERCONDUCTIVITY

BCS theory on superconductors.

This theory was proposed by three scientists Bardeen, Cooper and Schrieffer in 1957.
This theory explains about zero resistance of a superconductor.
In normal substances the flow of free electrons is opposed by the vibration of ions or atoms in the lattice due to the collision between them. Hence the normal conductors possess resistance.
In the case of superconductors below the critical temperature (Tc), the atom or the ion is distorted by the free electron during the collision.
The result produces a mechanical wave called phonon.
During this collision the free electron exchanges an amount of its momentum with the lattice ion.
Hence the momentum of free electron in reduced. It moves with less momentum.
If another free electron collides with the distorted lattice ion (phonon), the second electron gains an amount of momentum from the phonon. Hence the second free electron moves with greater
momentum.
But the change in momentum for the two free electrons is equal and they move with opposite momentum and spin. This pair of electrons is called Cooper pair.
The motion of Cooper pair of electrons is not effected by adding impurities and any change in Thermal characteristics. Hence there will be no deviation in the flow of Cooper pair. This is the reason for zero resistance of superconductors.

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