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The theory of superconductivity was formulated by Bardeen, Cooper, and Schrieffer (1957) and is called the BCS theory. It very successfully describes the superconducting properties of weak superconductors, such as aluminum, which are weak because of the small strength of the electron-phonon interaction. Further refinements of the theory have led to the strong coupling theory of Eliashberg (1960) which describes well the properties of strong superconductors such as lead. The distinction between weak and strong is roughly given by the value of the electron-phonon mass enhancement factor λ, as shown by McMillan (1968). We shall first discuss the BCS theory. It must rank as one of the great successes of many-body formalism, since the theory provides detailed agreement with experiments. This agreement is a refreshing change from most comparisons between many-body theory and experiment, where we often get lost in vertex corrections, correlations, or computer calculations. The beauty of BCS is that it is, mathematically, a simple theory which is exceedingly accurate. The reason for this is that the basic coupling forces are weak, and mean field theory works well.
KeywordsFermi Surface Tunnel Junction Normal Metal Tunneling Current Electron Tunneling
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