Cooper Pairs |
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| The behavior
of superconductors suggests that electron pairs are
coupling over a range of hundreds of nanometers, three
orders of magnitude larger than the lattice spacing.
Called Cooper pairs, these coupled electrons can take the
character of a boson and condense into the ground state. This pair condensation is the basis for the BCS theory of superconductivity. The effective net attraction between the normally repulsive electrons produces a pair binding energy on the order of milli-electron volts, enough to keep them paired at extremely low temperatures. |
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| The transition of a metal from the normal to the superconducting state has the nature of a condensation of the electrons into a state which leaves a band gap above them. This kind of condensation is seen with superfluid helium, but helium is made up of bosons -- multiple electrons can't collect into a single state because of the Pauli exclusion principle. Froehlich was first to suggest that the electrons act as pairs coupled by lattice vibrations in the material. This coupling is viewed as an exchange of phonons, phonons being the quanta of lattice vibration energy. Experimental corroboration of an interaction with the lattice was provided by the isotope effect on the superconducting transition temperature. The boson-like behavior of such electron pairs was further investigated by Cooper and they are called "Cooper pairs". The condensation of Cooper pairs is the foundation of the BCS theory of superconductivity. | ||||
| Model of Pair Attraction : | ||||
| A visual
model of the Cooper pair attraction has a passing
electron which attracts the lattice, causing a slight
ripple toward its path. Another electron passing in the
opposite direction is attracted to that displacement.
This constitutes a coupling between electrons which can
be depicted in a Feynman diagram. As strange as such an interaction seems, it is experimentally supported by the isotope effect and the evidence for a condensation to a boson -like state at the critical temperature for superconductivity. |
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