Superconducting tunnel junction

All currents flowing through the STJ pass through the insulating layer via the process of quantum tunneling.

The second is the quasiparticle current, which, in the limit of zero temperature, arises when the energy from the bias voltage

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.

Shapiro steps arise from the response of the supercurrent and occur at voltages equal to

[2] Photon-assisted tunneling arises from the response of the quasiparticles and gives rise to steps displaced in voltage by

[3] The device is typically fabricated by first depositing a thin film of a superconducting metal such as aluminum on an insulating substrate such as silicon.

Oxygen gas is then introduced into the chamber, resulting in the formation of an insulating layer of aluminum oxide (Al

After the vacuum is restored, an overlapping layer of superconducting metal is deposited, completing the STJ.

To create a well-defined overlap region, a procedure known as the Niemeyer-Dolan technique is commonly used.

This technique uses a suspended bridge of resist with a double-angle deposition to define the junction.

The thin aluminum layer is proximitized by the thicker niobium, and the resulting device has a superconducting critical temperature above 4.2 K.[4] Early work used lead-lead oxide-lead tunnel junctions.

A high frequency signal from an astronomical object of interest is focused onto the STJ, along with a local oscillator source.

This photon-assisted tunneling changes the current-voltage curve, creating a nonlinearity that produces an output at the difference frequency of the astronomical signal and the local oscillator.

[7] These receivers are so sensitive that an accurate description of the device performance must take into account the effects of quantum noise.

A photon absorbed in the superconductor breaks Cooper pairs and creates quasiparticles.

STJ devices have been employed as single-photon detectors for photon frequencies ranging from X-rays to the infrared.

SQUIDs are the world's most sensitive magnetometers, capable of measuring a single magnetic flux quantum.

The STJ is the primary active element in rapid single flux quantum or RSFQ fast logic circuits.

[10] When a high frequency current is applied to a Josephson junction, the ac Josephson current will synchronize with the applied frequency giving rise to regions of constant voltage in the I–V curve of the device (Shapiro steps).

Because frequency can be measured with very high precision, this effect is used as the basis of the Josephson voltage standard, which implements the SI definition of the volt.

Illustration of a thin-film superconducting tunnel junction.
Illustration of a thin-film superconducting tunnel junction (STJ). The superconducting material is light blue, the insulating tunnel barrier is black, and the substrate is green.
Energy diagram of a superconducting tunnel junction.
Energy diagram of a superconducting tunnel junction. The vertical axis is energy, and the horizontal axis shows the density of states . Cooper pairs exist at the Fermi energy , indicated by the dashed lines. A bias voltage V is applied across the junction, shifting the Fermi energies of the two superconductors relative to each other by an energy eV, where e is the electron charge. Quasiparticle states exist for energies greater than Δ from the Fermi energy, where Δ is the superconducting energy gap. Green and blue indicate empty and filled quasiparticle states, respectively, at zero temperature.
Sketch of the current-voltage curve of a superconducting tunnel junction.
Sketch of the current–voltage (I–V) curve of a superconducting tunnel junction. The Cooper pair tunneling current is seen at V = 0, while the quasiparticle tunneling current is seen for V > 2Δ/e and V < -2Δ/e.