Superconductors Cooper pairs

The quantum of magnetic flux is only 2.07 x 10-15 Wb, which is approximately equal to the amount of the earth s magnetic field enclosed by a ring of 10p,m in diameter. The Josephson effect is observed when two superconductors are separated by a very thin insulating layer (about 20 nm). Single electrons and Cooper pairs can tunnel through such a layer. The characteristics of the Josephson junction are now used to define the volt and have enabled the uncertainty in the maintained standard to be reduced to 0.1 p,V. 

If transition temperatures other than those allowed by superconducting pure metals are required, two metallic layers can be deposited to form a bi-layer TES. In most cases, only one of the two metals is a superconductor. In this case, the Cooper pairs diffuse from... 

In 1957, Bardeen, Cooper, and Schrieffer published their theory of superconductivity, known as the BCS theory. It predicts that under certain conditions, the attraction between two conduction electrons due to a succession of phonon interactions can slightly exceed the repulsion that they exert directly on one another due to the Coulomb interaction of their like charges. The two electrons are thus weakly bound together forming a so-called Cooper pair. It is these Cooper pairs that are responsible for superconductivity. In conventional superconductors, these electrons are paired so that their spin and orbital angular momenta cancel. They are described by a wave function, known as an order parameter. In this case the order parameter has symmetry similar to that of the wave function of s electrons and represents a singlet state. 

Cooper pairs -m superconductors [CERAMICS AS ELECTRICAL MATERIALS] (Vol 5)... 

The process of Andreev reflection involves a spin-up electron of energy E coupling with a spin-down electron of energy —E to form a Cooper pair in the superconductor. If the normal metal is replaced by a ferromagnet with a finite polarization P, not all electrons of one spin species will be able to find a corresponding electron of the opposite spin species in order to form Cooper pair. Hence, the probability of Andreev reflection will be reduced by a factor of (l-P), where we define the polarization P by... 

Fig. 11. Schematic diagram for (a) the dynamic Jahn-Teller effect and (b) the Moskalenko-Suhl-Kondo mechanism. In the former, the distorted Cg molecule undergoes the tunneling motion between three equivalent configurations. This results in the formation of the orbital-singlet state. In the latter, as shown by the black arrows, the Cooper pairs are transferred from one Fermi sphere to another, which is the pair-transfer process, a remarkable feature of multiband superconductors, and stabilizes the superconductivity. Also, as shown by the white arrows, the Cooper pairs are scattered coherently within each Fermi sphere, which is the pair-scattering process in usual superconductors.

In 1962 a postgraduate student, Brian Josephson, working in the University of Cambridge, and later to win a Nobel Prize, predicted that Cooper pairs should be able to tunnel through a thin (approximately 1 nm) insulating barrier from one superconductor to another with no electrical resistance [46]. This quantum tunnelling was confirmed by experiment and is known as the Josephson effect . The superconducting electronic devices exploit Josephson junctions. 

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