A microscopic theory of superconductivity was developed in 1957 by John Bardeen, Leon Cooper & Robert Schrieffer, which is known as the BCS theory. The central feature of the BCS theory is that two electrons in the superconductor are able to form a bound pair called a Cooper pair if they somehow experience an attractive interaction between them. electrons normally repel one another because of their like charges. This may be thought of in the following way and is illustrated in Figure.


When an electron passes through the positive ion core in the crystal lattice then the electron makes attraction with the positive ion. This attraction distorts the ion motion in the lattice. Now suppose another electron interacts with the distorted lattice. This time due to this interaction the energy of the second electron is lowered. Now the two electrons interacts through the lattice distortion and results in lowering of energy. This lowering of energy indicates the attractive force exist between the two electrons. This force is stronger if the two electrons have equal and opposite moment and spin. This interaction is called electron-lattice interaction or electron-phonon-electron interaction.

The superconductivity occurs when an attractive interaction between two electrons due to phonon exchange dominates the usual repulsive interaction. This is the fundamental postulate of BCS theory.

Cooper pair formation is a result of electron-phonon-electron interaction. During scattering, any change in the momentum of the electrons of the pair is accompanied by an equal and opposite change in momentum of the other electron of the cooper pair. The cooper pair can, therefore, move through the material without encountering any resistance. This makes it possible to achieve an extremely high conductivity in superconducting materials.

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