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. 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

e

V

{\displaystyle eV}

exceeds twice the value of the superconducting energy gap Δ (in case the gaps in the base electrode and the counterelectrode aren't equal, the onset of current occurs at Δ1 + Δ2.) 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 thermal exiation of quasiparticles above the gap.