Josephson effect
Flow of electric current between two pieces of superconducting material (see superconductivity) separated by a thin layer of insulating material. This flow was predicted by the British physicist Brian Josephson in 1962, based on the BCS theory (see John Bardeen). According to Josephson, pairs of electrons can move from one superconductor to the other across the insulating layer (tunneling). The locus of this action is called a Josephson junction. The Josephson current flows only if no battery is connected across the two conductors. A major application of this discovery is in superfast switching devices used in computers, which can be 100 times faster than ordinary semiconducting circuits.
Josephson effect [′jō·səf·sən i‚fekt]
Josephson effect
The passage of paired electrons (Cooper pairs) through a weak connection (Josephson junction) between superconductors, as in the tunnel passage of paired electrons through a thin dielectric layer separating two superconductors.
Quantum-mechanical tunneling of Cooper pairs through a thin insulating barrier (on the order of a few nanometers thick) between two superconductors was theoretically predicted by Brian D. Josephson in 1962. Josephson found that a current of paired electrons (supercurrent) would flow in addition to the usual current that results from the tunneling of single electrons. Josephson predicted that if the current did not exceed a limiting value (the critical current), there would be no voltage drop across the tunnel barrier. This zero-voltage current flow is known as the dc Josephson effect. Josephson also predicted that if a constant nonzero voltage were maintained across the tunnel barrier, an alternating supercurrent would flow through the barrier in addition to the dc current produced by the tunneling of unpaired electrons. This phenomenon is known as the ac Josephson effect. See Tunneling in solids
Josephson pointed out that the magnitude of the maximum zero-voltage supercurrent would be reduced by a magnetic field. In fact, the magnetic field dependence of the magnitude of the critical current is one of the more striking features of the Josephson effect. Circulating supercurrents flow through the tunnel barrier to screen an applied magnetic field from the interior of the Josephson junction just as if the tunnel barrier itself were weakly superconducting. The screening effect produces a spatial variation of the transport current, and the critical current goes through a series of maxima and minima as the field is increased.
Josephson junctions, and instruments incorporating Josephson junctions, are used in applications for metrology at dc and microwave frequencies, frequency metrology, magnetometry, measurement of absolute temperatures below about 1 K, detection and amplification of electromagnetic signals, and other superconducting electronics such as high-speed analog-to-digital converters and computers. A Josephson junction, like a vacuum tube or a transistor, is capable of switching signals from one circuit to another; a Josephson tunnel junction is the fastest switch known. Josephson junction circuits are capable of storing information. Finally, because a Josephson junction is a superconducting device, its power dissipation is extremely small, so that Josephson junction circuits can be packed together as tightly as fabrication techniques permit. All the basic circuit elements required for a Josephson junction computer have been developed. See Low-temperature thermometry, Superconducting devices, Superconductivity
Josephson Effect
the passage of a superconduction current through a thin layer of dielectric separating two superconductors (the so-called Josephson contact), predicted on the basis of the theory of superconductivity by the English physicist B. Josephson in 1962 and detected by the American physicists P. Anderson and J. Rowell in 1963.
Conduction electrons pass through the dielectric (usually a metal oxide film about 10 angstroms [Å] thick) because of a tunneling effect. If the current through a Josephson contact does not exceed a certain value, called the critical current of the contact, there is no voltage drop at the contact (the so-called direct-current Josephson effect; see Figure 1, c). If a current greater than the critical current is passed through the contact, there is a voltage drop V at the contact, which then radiates electromagnetic waves (alternating-current Josephson effect; see Figure 1, d). The radiation frequency vand the voltage at the contact are related such that v = 2eV/h, where e is the electron charge and h is Planck’s constant. The radiation arises because in passing through the contact the paired electrons that create the superconduction current acquire excess energy 2eV (in relation to the ground state of the superconductor). The only possibility for an electron pair to return to the ground state is to emit electromagnetic energy quanta hv = 2eV.
An analogous effect is observed when the superconductors are joined by a thin jumper (bridge or point contact) or when there is a fine metal film in the normal state between them. Such Josephson contact systems are called loosely coupled superconductors. On the basis of the Josephson effect, superconducting interferometers have been built with two loose parallel couplings between the superconductors. The special quantum nature of the superconducting state leads to interference of the superconduction currents that have passed through the loose connections. The critical current here depends on the external magnetic field, making it possible to use such a device for the extremely accurate measurement of magnetic fields—to between 8 x 10-7 and 8 x 10-8 amperes per m (10-8-10-9 oersted). It is also possible to use loosely coupled superconductors as low-power generators that can be easily switched over to a wide range of frequencies, or as sensitive detectors, amplifiers, and other instruments for the superhigh frequencies and the far infrared ranges.
REFERENCES
Langenberg, D. N., et al. “Effekty Dzhozefsona.” Vspekhi fizicheskikh nauk, 1967, vol. 91, issue 2, p. 317.Kulik, I. O., and I. K. lanson. Effekt Dzhozefsona v sverkhprovodiashchikh tunnel’nykh strukturakh. Moscow, 1970.
L. G. ASLAMAZOV
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