In 1957, more than 40 years after the discovery of superconductivity, three physicists, Bardeen, Cooper and Schrieffer, finally found the correct explanation to superconductivity in metals. In a theoretical model (that has since been called “BCS”, after their initials), they proposed the following explanation: electrons form a collective quantum state made up of pairs of electrons of opposite spin and momentum. This remarkable state, called a pair condensate, explained all known superconducting properties and made possible the prediction of new ones. It helps predict the behaviour of characteristic lengths. This BCS theory has since been proven by numerous experiments in metals and alloys. But it cannot apply straightforward in the case of some new superconductors such as cuprates or pnictides, and scientists are still working on finding new explanations for these materials.
On a microscopic scale, quantum physics teaches us that the electron behaves like a small wave. When there is a flaw or when one of the atoms of the crystal pattern is vibrating, the wave is disrupted.At a very low temperature, the electrons pair up and merge into one quantum wave that fills the whole material. This unique wave becomes insensitive to the flaws in the material; they are too small to slow the whole wave. The electric resistance has hence disappeared.
The main idea of the BCS theory relies on the quantum nature of electrons. In a metal, electrons are waves. Each of these electrons is relatively independent and follows its own path independent of other electrons. In a superconductor, the majority of these electrons merge in order to form a large collective wave. In quantum physics, we call it “macroscopic quantum wavefunction”, or condensate. When the collective wave is formed, it requires each member to move at the same speed. In a metal, an individual electron is easily diverted by a flaw or an atom that is too big. In a superconductor however, this same electron can be diverted only if, at the same time, all the other electrons of the collective wave are diverted in the exact same manner. The flaw in a single atom surely cannot do that; the wave will not be diverted, and, thus, not slowed down. It superconducts!
>> Watch the historical conference : http://www.conferences.uiuc.edu/bcs50/video/
