Josephson diode

A Josephson diode is an electronic device that superconducts electrical current in one direction and is resistive in the other direction. The device is a Josephson junction exhibiting a superconducting diode effect (SDE). It is an example of a quantum material Josephson junction (QMJJ), where the weak link in the junction is a quantum material. The Josephson diode effect can occur in superconducting devices where time reversal symmetry and inversion symmetry are broken.

Josephson diodes can be subdivided into two categories, those requiring an external (magnetic) field and those not requiring an external magnetic field; the so-called “field-free” Josephson diodes. In 2021, the Josephson diode was realised in the absence of applied magnetic field in a non-centrosymmetric material, followed shortly by the first realisation of the zero-field Josephson diode in an inversion-symmetric device.

History
The Josephson diode is named after British physicist Brian David Josephson, who predicted the Josephson effect; and the resistive diode, since it has a similar function. In 2007 a "Josephson diode" was proposed with a design that was similar to conventional p-n junctions in semiconductor, but utilizing hole and electron doped superconductors. This is different from the "Josephson fluxonic diode" that was introduced before the 2000s. It is also different from how the term is currently used, where a Josephson diode is a Josephson junction exhibiting a superconducting diode effect.

In 2020, a superconducting diode effect was shown in an artificial [Nb/V/Ta]n superlattice. A field-free superconducting diode effect was realized in 2021, in a van der Waals heterostructure of NbSe2/Nb3Br8/NbSe2 - a Josephson diode. This heterostructure is a quantum material Josephson junction, in which the weak link (Nb3Br8) is a quantum material that is predicted to be an obstructed atomic insulator / Mott insulator, and is non-centrosymmetric, meaning it distinguishes between electrons with positive and negative momentum. Soon thereafter, a zero-field diode effect was observed in small twist-angle trilayer graphene, a system which possesses in-plane inversion symmetry; in this case, the superconducting state itself is responsible for the breaking of time reversal symmetry and in-plane inversion symmetry.

Superconducting diode effect
The superconducting diode effect is an example of nonreciprocal superconductivity, where a material is superconducting in one direction and resistive in the other. This leads to half-wave rectification when a square wave AC-current is applied. In 2020, this effect was demonstrated in an artificial [Nb/V/Ta]n superlattice. The phenomenon in the Josephson diode is believed to originate from asymmetric Josephson tunneling. Unlike conventional semiconducting junction diodes, the superconducting diode effect can be realized in Josephson junctions as well as junction-free bulk superconductors.

Theories
Currently, the precise mechanism behind the Josephson diode effect is not fully understood. However, some theories have emerged that are now under theoretical investigation. There are two types of Josephson diodes, relating to which symmetries are being broken. The inversion breaking Josephson diode, and the Josephson diode breaking inversion breaking and time-reversal. The minimal symmetry breaking requirement for forming the Josephson diode is inversion symmetry breaking, and is required to obtain nonreciprocal transport. One proposed mechanism originates from finite momentum Cooper pairs. It may also be possible that the superconducting diode effect in the JD originates from self-field effects, but this still has to be rigorously studied.