The Josephson effect is a term given to the phenomenon of current flow across two superconductors separated by a very, thin insulating barrier. This arrangement, two superconductors connected by a non conducting oxide barrier, is called as a Josephson junction; the current that crosses the barrier is the Josephson current. The terms are named eponymously after British physicist Brian David Josephson, who forecasted the survival of the effect in 1962. It has significant applications in quantum Mechanical Circuits. JOSEPHSON DEVICE Josephson device is a superconductor which is interrupted by a very thin insulating barrier. The superconducting current passes through the barrier without resistance. The happens by virtue of quantum tunnelling. Quantum tunnelling takes place when a particle moves with a space in a manner forbidden by classical physics, due to the potential barrier concerned. In relation to the BCS theory, pairs of electrons move through this barrier continuing the superconducting current. This is known as the de Josephson effect. There are two general types of Josephson junctions: - Over damped and under damped In over damped junctions, the barrier is conducting (i.e. it is a normal metal or superconductor bridge). The effects of junction’s internal electrical resistance will be large compared its small capacitance. An over damped junction will rapidly attain a unique equilibrium state for any known set of conditions. In under damped junctions, the barrier is an insulator. The effects of the junction’s internal resistance will be minimal under damped junctions do not have unique equilibrium states, but are hysteretic. Josephson current is strictly decreased. Hysteresis is clearly visible around 100microvolts. The portion of the curve between 100 and 300 micro volts is current independent and is t he regime where the device can be used as a detector.
Note :Hysteresis is a property of systems that do not immediately follow the forces applied to them, but react slowly, or do not turn wholly to their original state i.e., systems whose states depend on their instantaneous history.
Applications: The Josephson finds numerous important applications. They are as
(1) Its properties are exploited in SQUIDS used to measure flux at the quantum level. This finds application in medicine for measure magnetic flux at the quantum level. This finds application for measurement of small currents in the brain in the heart.
(2) It is also used in Rapid single Flux Quantum integrated circuits, and some other of their properties can be exploited to build photon or particle detectors.
(3) It is used as microwave detectors in the giga and terahertz range.
(4) It finds its application in gradiometer, Oscilloscopes, oscillators, samplers and sensors for biomedical scientific and defence purposes.