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BCS theory describes a superconducting instability caused by attractive interactions between opposite-momentum fermions near a Fermi surface.

John Bardeen, Leon Neil Cooper and John Robert Schrieffer developed BCS theory in 1957, using attraction mediated by phonons.^[ref]

Weak-coupling Cooper pairs overlap. The Cooper instability reorganizes an unpaired Fermi surface; it is not Bose-Einstein condensation of compact, pre-existing molecules. The original paper explicitly distinguishes these pictures.^[ref]

Two complementary clues made the microscopic problem concrete. The superconducting isotope effect connected the transition to phonons.^[ref]

A 1957 transmission experiment used thin lead and tin films as samples in far-infrared and millimeter-microwave measurements. The measured transmission was converted to complex electrical conductivity, normalized to the normal-state value. Comparing film thickness, resistance, annealing and the two metals tested the response's reproducibility. Weak low-photon-energy absorption followed by an onset supported a superconducting energy gap; its extraction required a thin-film electrodynamic model.^[ref]

The later Giaever superconducting tunnelling experiment provided another way to resolve the superconducting energy gap through quantum tunnelling.^[ref] The derivations below explain what the model predicts, including the distinction between a single quasiparticle's minimum energy and the threshold for making two of them.

A physicist whose joint measurements with R. E. Glover III used infrared and microwave transmission to probe superconductivity. The dissipative and reactive parts of electrical conductivity changed differently with frequency. Comparing tin and lead films exposed a common scaled response and an excitation threshold, providing a quantitative way to investigate the superconducting energy gap.^[ref]

An experimental researcher whose work with M. Tinkham measured how thin superconducting films transmit radiation of different frequencies. Extracting their complex electrical conductivity revealed the onset of absorption above an energy threshold. That threshold supplied evidence for a superconducting energy gap, connecting optical response with the spectrum of allowed electronic excitations.^[ref]