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Josephson JunctionJosephson junctions, first developed by B. D. Josephson, are quantum-mechanical circuit elements of superconducting devices. They are most commonly used in SQUIDs and Rapid Single Flux Quantum integrated circuits. A Josephson junction is the interface between two superconducting materials separated by a non-superconducting barrier. A current may flow freely within the superconductors, but the barrier prevents the current from flowing freely between them. However, the supercurrent may tunnel through the barrier, depending on the quantum phase of the superconductors. The amount of supercurrent that may tunnel through the barrier is restricted by the size and substance of the barrier. The maximum value the supercurrent may attain is called the critical current of the Josephson junction, and is an important phenomenological parameter of a junction. Josephson junctions have two basic electrical properties. The first is that the junctions have an inductive reactance. That is, similarly to inductors, the voltage difference across a junction is related to the time rate of change of the current. The second is that a constant voltage across a junction will produce an oscillating current through the barrier, and vice versa. Thus, Josephson junctions convert a direct-current voltage to an alternating-current current. There are two general types of Josephson junctions: overdamped and underdamped. In overdamped junctions, the barrier is conducting (normal metal or superconductor bridge). The effects of the junction's internal electrical resistance will be large compared to its small capacitance. An overdamped junction will quickly reach a unique equilibrium state for any given set of conditions. The barrier of an underdamped junction is an insulator. The effects of the junction's internal resistance will be minimal. Underdamped junctions do not have unique equilibrium states, but are hysteretic.
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