Josephson diode

A Josephson diode (JD) is a special type of Josephson junction (JJ), which conducts (super)current in one direction better than in the opposite direction. In other words, it has asymmetric current-voltage characteristic. Since the Josephson diode is a superconducting device, the asymmetry of the supercurrent transport is the main focus of attention. Opposite to conventional Josephson junctions, the critical (maximum) supercurrents

I

c

+

{\displaystyle I_{c+}}

and

I

c

−

{\displaystyle I_{c-}}

for opposite bias directions are different by absolute values (

I

c

+

≠

|

I

c

−

|

{\displaystyle I_{c+}\neq |I_{c-}|}

).

In the presence of such a non-reciprocity, the bias currents of any magnitude in the range between

I

c

+

{\displaystyle I_{c+}}

and

|

I

c

−

|

{\displaystyle |I_{c-}|}

can flow without resistance in only one direction.

This asymmetry, characterized by the ratio of critical currents

A

{\displaystyle {\mathcal {A}}}

, is the main figure of merit of Josephson diodes and is the subject of new developments and optimizations.

The Josephson diode effect can occur, e.g., 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 to be asymmetric and those not requiring an external magnetic field --- the so-called "field-free" Josephson diodes (more attractive for applications). In 2021, the Josephson diode was realized in the absence of applied magnetic field in a non-centrosymmetric material,

followed shortly by the first realization of the zero-field Josephson diode in an inversion-symmetric device.

History

Since decades the physicists tried to construct Josephson junction devices with asymmetric critical currents.