BCS Theory

- Overview

BCS theory, in physics, is a comprehensive theory developed in 1957 by American physicists John Bardeen, Leon N. Cooper, and John R. Schrieffer (whose initials provide the BCS name) to explain superconducting materials the behavior of. When superconductors cool to temperatures near absolute zero, they suddenly lose all resistance to the flow of electricity. 

Cooper discovered that electrons in a superconductor were assembled in pairs, now called Cooper pairs, and that the motions of all Cooper pairs within a single superconductor were correlated; they constituted a system that functioned as a single entity. Applying a voltage to a superconductor causes all Cooper pairs to move, creating an electric current. When the voltage is removed, current continues to flow indefinitely because the pairs meet no resistance. For the current to stop, all Cooper pairs must stop at the same time, which is an extremely unlikely scenario. When a superconductor warms, its Cooper pairs separate into individual electrons, and the material becomes normal, or non-superconducting. 

BCS theory explains many other aspects of superconductor behavior. The theory provides a way to experimentally measure the energy required to separate a Cooper pair into individual electrons. The BCS theory also explains the isotope effect, whereby the temperature at which superconductivity occurs decreases if heavier atoms of the elements that make up the material are introduced.

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