The Microscopic Theory of Superconductivity–Verifications and Extensions

Abstract

The microscopic theory of superconductivity was formulated in 1957 by Bardeen, Cooper, and Schrieffer. In the BCS model, the ground state of super electrons is built up by pairs kept together by an attractive interaction. An exchange of phonons can give such an attraction. The electrons in the superconducting condensate are correlated within a macroscopic distance. To excite electrons, or break pairs, a certain gap energy is needed. A number of experimental confirmations were discussed already in the original paper. The BCS theory initiated many experiments. Some of them provided a stimulus to logical extensions of the theory. These have mainly been concerned with a more realistic, energy-and momentum-dependent interaction than the original approximation. Lifetime aspects are also of importance. In this short survey we describe the results of a few experiments that test the concepts of pairing, energy gap and phonon-mediated attraction. The main emphases are on the powerful tunneling method and the variation of the superconducting transition temperature with different physical and chemical parameters.