First described by Leon Cooper in 1956, an arbitrarily weak attractive interaction near the Fermi surface binds two electrons into a state of lower energy than the Fermi energy.
In conventional superconductors, electron-phonon attraction (weak, on the order of 10⁻³ eV) binds electrons hundreds of nanometres apart into integer-spin composite bosons.
Condensation of Cooper pairs into a single coherent ground state opens an excitation energy gap and underpins the 1957 Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity (1972 Nobel Prize).