Cooper pairs - a central mechanism in the BCS theory of superconductivity: the electrons in the lattice, because of lattice effects, may form bound systems despite their mutual repulsion and fermionic exclusiveness. The bound systems consist of two elections with the opposite momenta: the Cooper pairs.

• Members of a pair are not necessarily (and in general will not be) physically near each other - they are related in momentum space.

• Consisting of two half-integer spin fermions, the Cooper pairs are composite bosons, and they subsequently may undergo Bose-Einstein condensation, resulting in, for example, superconductivity.

• This is also the mechanism of superfluidity of Helium 3.