BCS theory of superconductors is
named after the three physicists who discovered it - Bardeen, Cooper, and
Schrieffer.
In many superconductors, the attractive interaction between two
negatively charged electrons is caused indirectly by the interaction between the electrons
and the vibrating crystal lattice. A quantised vibration of the crystal lattice is called a
phonon.
BCS theory considers superconductivity as a macroscopic quantum
mechanical effect. It proposes that electrons with opposite spin can become paired, forming
Cooper pairs.
A negatively charged electron moving through a conductor will cause
a slight increase in concentration of positive charges in the lattice around it. The elctron sort
of drags positive charges towards it. This increase in turn can attract another negative
electron. This means that in effect two negatively charged electrons are then held together
with a certain binding energy. This is known as a Cooper pair.
If this binding energy
is higher than the energy provided by collisions due to the oscillation of atoms in the
conductor (which is true at low temperatures because the lattice oscillation is very small),
then the electron pair will stick together and resist all kicks, thus not experiencing resistance.
Hence you get superconductivity.