BCS theory

BCS theory is the microscopic quantum explanation of conventional superconductivity, named for John Bardeen, Leon Cooper, and Robert Schrieffer, who published it in 1957. For nearly half a century after superconductivity was discovered, no one could say why resistance vanished; BCS finally supplied the answer, and the trio won the Nobel Prize. The theory shows how the strange, resistance-free state emerges from the ordinary electrons and ions already present in the metal.

Its core idea is pairing followed by condensation. Electrons attract one another weakly by jostling the crystal lattice, binding into Cooper pairs; these pairs then condense together into a single shared quantum state described by one collective wavefunction. The theory predicts that opening this state costs a minimum amount of energy — an energy gap — so that small disturbances cannot scatter the pairs one at a time. That gap is precisely why the supercurrent flows undisturbed.

BCS theory was a triumph of quantitative prediction: it correctly forecast the size of the energy gap, how the critical temperature depends on the lattice, and subtle features confirmed in later experiments. Its reach is honest but limited — it explains the 'conventional' low-temperature superconductors beautifully, yet the high-temperature superconductors discovered in 1986 do not fit the simple BCS mechanism, and a complete theory of those materials remains an open problem.