Cooper pair definition
There is attraction between the two electrons in the metal through the exchange of phonons. Due to this attraction, the electrons near the Fermi surface combine in pairs to form a so-called "Cooper pair." The formation of the "Cooper pair" causes the energy of the electron gas to fall below the energy of the normal Fermi-Dirac distribution, so that a single energy level appears below the continuous energy band state. The interval between this individual energy level and the continuous energy level is called the energy gap of the superconductor. Today, Cooper plays a huge role in the theory used in superconductivity and interpretation of BCS theory.