Bcs theory of superconductivity pdf

Rapid progress in the understanding of superconductivity gained momentum in the mid-1950s. The key ingredient was Leon Neil Cooper’s calculation of the bcs theory of superconductivity pdf states of electrons subject to an attractive force in his 1956 paper, “Bound Electron Pairs in a Degenerate Fermi Gas”. In 1957 Bardeen and Cooper assembled these ingredients and constructed such a theory, the BCS theory, with Robert Schrieffer.

The theory was first published in April 1957 in the letter, “Microscopic theory of superconductivity”. December 1957 article, “Theory of superconductivity”. It is believed that BCS theory alone cannot explain this phenomenon and that other effects are in play. Cooper showed such binding will occur in the presence of an attractive potential, no matter how weak.

In conventional superconductors, an attraction is generally attributed to an electron-lattice interaction. The BCS theory, however, requires only that the potential be attractive, regardless of its origin. In the BCS framework, superconductivity is a macroscopic effect which results from the condensation of Cooper pairs. An electron moving through a conductor will attract nearby positive charges in the lattice.

This deformation of the lattice causes another electron, with opposite spin, to move into the region of higher positive charge density. The two electrons then become correlated. Because there are a lot of such electron pairs in a superconductor, these pairs overlap very strongly and form a highly collective condensate. In this “condensed” state, the breaking of one pair will change the energy of the entire condensate – not just a single electron, or a single pair. Thus, the collective behavior of the condensate is a crucial ingredient necessary for superconductivity.

Extensions of BCS theory exist to describe these other cases, although they are insufficient to completely describe the observed features of high-temperature superconductivity. This state is now known as the BCS state. In the normal state of a metal, electrons move independently, whereas in the BCS state, they are bound into Cooper pairs by the attractive interaction. The BCS formalism is based on the reduced potential for the electrons’ attraction. This ansatz was later shown to be exact in the dense limit of pairs.

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