In condensed matter physics, a Cooper pair or BCS pair is a pair of electrons (or other fermions) bound together at low temperatures in a certain manner first described in 1956 by American physicist Leon Cooper. Cooper showed that an arbitrarily small attraction between electrons in a metal can cause a paired state of electrons to have a lower energy than the Fermi energy, which implies that the pair is bound. In conventional superconductors, this attraction is due to the electron–phonon interaction. The Cooper pair state is responsible for superconductivity, as described in the BCS theory developed by John Bardeen, Leon Cooper, and John Schrieffer for which they shared the 1972 Nobel Prize.
Although Cooper pairing is a quantum effect, the reason for the pairing can be seen from a simplified classical explanation. An electron in a metal normally behaves as a free particle. The electron is repelled from other electrons due to their negative charge, but it also attracts the positive ions that make up the rigid lattice of the metal. This attraction distorts the ion lattice, moving the ions slightly toward the electron, increasing the positive charge density of the lattice in the vicinity. This positive charge can attract other electrons. At long distances this attraction between electrons due to the displaced ions can overcome the electrons' repulsion due to their negative charge, and cause them to pair up. The rigorous quantum mechanical explanation shows that the effect is due to electron–phonon interactions.
The energy of the pairing interaction is quite weak, of the order of 10−3eV, and thermal energy can easily break the pairs. So only at low temperatures are a significant number of the electrons in a metal in Cooper pairs. The electrons in a pair are not necessarily close together; because the interaction is long range, paired electrons may still be many hundreds of nanometers apart. This distance is usually greater than the average interelectron distance, so many Cooper pairs can occupy the same space. Electrons have spin-1⁄2, so they are fermions, but a Cooper pair is a composite boson as its total spin is integer (0 or 1). This means the wave functions are symmetric under particle interchange, and they are allowed to be in the same state. The tendency for all the Cooper pairs in a body to 'condense' into the same ground quantum state is responsible for the peculiar properties of superconductivity.
The BCS theory is also applicable to other fermion systems, such as helium-3. Indeed, Cooper pairing is responsible for the superfluidity of helium-3 at low temperatures. It has also been recently demonstrated that a Cooper pair can comprise two bosons. Here the pairing is supported by entanglement in an optical lattice.
Relationship to superconductivity
Cooper originally considered only the case of an isolated pair's formation in a metal. When one considers the more realistic state of many electronic pair formations, as is elucidated in the full BCS Theory, one finds that the pairing opens a gap in the continuous spectrum of allowed energy states of the electrons, meaning that all excitations of the system must possess some minimum amount of energy. This gap to excitations leads to superconductivity, since small excitations such as scattering of electrons are forbidden. The gap appears due to many-body effects between electrons feeling the attraction.
Herbert Fröhlich was first to suggest that the electrons might act as pairs coupled by lattice vibrations in the material. This was indicated by the isotope effect observed in superconductors. The isotope effect showed that materials with heavier ions (different nuclear isotopes) had lower superconducting transition temperatures. This can be explained by the theory of Cooper pairing: since heavier ions are harder to move they would be less able to attract the electrons resulting in a smaller binding energy for Cooper pairs.
The theory of Cooper pairs is quite general and does not depend on the specific electron-phonon interaction. Condensed matter theorists have proposed pairing mechanisms based on other attractive interactions such as electron–exciton interactions or electron–plasmon interactions. Currently, none of these alternate pairing interactions has been observed in any material.
It should be mentioned that Cooper pairing does not really involve individual electrons pairing up to form "quasi-bosons". The paired states are energetically favored, and electrons go in and out of those states preferentially. This is a fine distinction that John Bardeen makes:
- "The idea of paired electrons, though not fully accurate, captures the sense of it." 
The mathematical description of the second-order coherence involved here is given by Yang.
- Cooper, Leon N. (1956). "Bound electron pairs in a degenerate Fermi gas". Physical Review 104 (4): 1189–1190. Bibcode:1956PhRv..104.1189C. doi:10.1103/PhysRev.104.1189.
- Nave, Carl R. (2006). "Cooper Pairs". Hyperphysics. Dept. of Physics and Astronomy, Georgia State Univ. Retrieved 2008-07-24.
- Kadin, Alan M. (2005). "Spatial Structure of the Cooper Pair". Journal of Superconductivity and Novel Magnetism 20 (4): 285. arXiv:cond-mat/0510279. doi:10.1007/s10948-006-0198-z.
- Feynman, Richard P.; Leighton, Robert; Sands, Matthew (1965). Lectures on Physics, Vol.3. Addison–Wesley. pp. 21–7, 8. ISBN 0-201-02118-8.
- Cooper Pairs of Bosons
- Nave, Carl R. (2006). "The BCS Theory of Superconductivity". Hyperphysics. Dept. of Physics and Astronomy, Georgia State Univ. Retrieved 2008-07-24.
- J. Bardeen, "Electron-Phonon Interactions and Superconductivity", in Cooperative Phenomena, eds. H. Haken and M. Wagner (Springer-Verlag, Berlin, Heidelberg, New York, 1973), p. 67.
- C. N. Yang, "Off-Diagonal Long-Range Order." Rev. Mod. Phys. 34, 694 (1962)