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superconducting tunnel junction
The superconducting tunnel junction (STJ) — also known as a superconductor–insulator–superconductor tunnel junction (SIS) — is an electronic device consisting of two superconductors separated by a very thin layer of insulating material. Current passes through the junction via the process of quantum tunneling. The STJ is a type of Josephson junction, though not all the properties of the STJ are described by the Josephson effect.
These devices have a wide range of applications, including high-sensitivity detectors of electromagnetic radiation, magnetometers, high speed digital circuit elements, and quantum computing circuits.
==Quantum tunneling==
All currents flowing through the STJ pass through the insulating layer via the process of quantum tunneling. There are two components to the tunneling current. The first is from the tunneling of Cooper pairs. This supercurrent is described by the ac and dc Josephson relations, first predicted by Brian David Josephson in 1962.〔B. D. Josephson, "Possible new effects in superconductive tunnelling," ''Physics Letters'' 1, 251 (1962), 〕 For this prediction, Josephson received the Nobel prize in physics in 1973. The second is the quasiparticle current, which, in the limit of zero temperature, arises when the energy from the bias voltage exceeds twice the value of superconducting energy gap Δ. At finite temperature, a small quasiparticle tunneling current — called the subgap current — is present even for voltages less than twice the energy gap due to the thermal promotion of quasiparticles above the gap.
If the STJ is irradiated with photons of frequency , the dc current-voltage curve will exhibit both Shapiro steps and steps due to photon-assisted tunneling. Shapiro steps arise from the response of the supercurrent and occur at voltages equal to , where is Planck's constant, is the electron charge, and is an integer.〔S. Shapiro, "Josephson currents in superconducting tunneling: The effect of microwaves and other observations" ''Physical Review Letters'' 11, 80 (1963), 〕 Photon-assisted tunneling arises from the response of the quasiparticles and gives rise to steps displaced in voltage by relative to the gap voltage.〔M. Tinkham, ''Introduction to Superconductivity'', 2nd edition, Dover Publications, 1996〕
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