A
Josephson junction
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mech ...
is a quantum mechanical device which is made of two superconducting electrodes separated by a barrier (thin insulating tunnel barrier, normal metal, semiconductor, ferromagnet, etc.).
A Josephson junction is a Josephson junction in which the
Josephson phase
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mec ...
''φ'' equals in the ground state, i.e. when no external current or
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
is applied.
Background
The
supercurrent ''I''
''s'' through a
Josephson junction
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mech ...
(JJ) is generally given by ''I''
''s'' = ''I''
''c''sin(''φ''),
where φ is the phase difference of the superconducting wave functions of the two
electrodes, i.e. the
Josephson phase
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mec ...
.
The critical current ''I''
''c'' is the maximum supercurrent that can exist through the Josephson junction.
In experiment, one usually causes some current through the Josephson junction and the junction reacts by changing the Josephson phase. From the above formula it is clear that the phase ''φ'' = arcsin(''I''/''I''
''c''), where ''I'' is the applied (super)current.
Since the phase is 2-periodic, i.e. ''φ'' and ''φ'' + 2''n'' are physically equivalent, without losing generality, the discussion below refers to the interval 0 ≤ ''φ'' < 2.
When no current (''I'' = 0) exists through the Josephson junction, e.g. when the junction is disconnected, the junction is in the ground state and the Josephson phase across it is zero (''φ'' = 0). The phase can also be ''φ'' = , also resulting in no current through the junction. It turns out that the state with ''φ'' = is ''unstable'' and corresponds to the
Josephson energy
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mech ...
maximum, while the state ''φ'' = 0 corresponds to the
Josephson energy
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mech ...
minimum and ''is'' a ground state.
In certain cases, one may obtain a Josephson junction where the critical current is negative (''I''
''c'' < 0). In this case, the first Josephson relation becomes
:
The ground state of such a Josephson junction is
and corresponds to the
Josephson energy
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mech ...
minimum, while the conventional state φ = 0 is unstable and corresponds to the
Josephson energy
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mech ...
maximum. Such a Josephson junction with
in the ground state is called a Josephson junction.
Josephson junctions have quite unusual properties. For example, if one connects (shorts) the superconducting electrodes with the inductance ''L'' (e.g. superconducting wire), one may expect the spontaneous supercurrent circulating in the loop, passing through the junction and through inductance clockwise or counterclockwise. This supercurrent is spontaneous and belongs to the ground state of the system. The direction of its circulation is chosen at random. This supercurrent will of course induce a magnetic field which can be detected experimentally. The magnetic flux passing through the loop will have the value from 0 to a half of
magnetic flux quanta, i.e. from 0 to Φ
0/2, depending on the value of inductance ''L''.
Technologies and physical principles
* Ferromagnetic Josephson junctions. Consider a
Josephson junction
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mech ...
with a ferromagnetic Josephson barrier, i.e. the multilayers Superconductor-Ferromagnet-Superconductor (SFS) or Superconductor-Insulator-Ferromagnet-Superconductor (SIFS). In such structures the superconducting order parameter inside the F-layer oscillates in the direction perpendicular to the junction plane. As a result, for certain thicknesses of the F-layer and temperatures, the order parameter may become +1 at one superconducting electrode and −1 at the other superconducting electrode. In this situation one gets a Josephson junction. Note that inside the F-layer the competition of different solutions takes place and the one with the lower energy wins out. Various ferromagnetic
junctions have been fabricated: SFS junctions with weak ferromagnetic interlayers;
[
] SFS junctions with strong ferromagnetic interlayers, such as Co, Ni,
[
] PdFe
and NiFe
[
] SIFS junctions;
[
][
] and S-Fi-S junctions.
[
]
*Josephson junctions with unconventional order parameter symmetry. Novel superconductors, notably high temperature cuprate superconductors, have an anisotropic
superconducting order parameter which can change its sign depending on the direction. In particular, a so-called d-wave order parameter has a value of +1 if one looks along the crystal axis ''a'' and −1 if one looks along the crystal axis ''b''. If one looks along the ''ab'' direction (45° between ''a'' and ''b'') the order parameter vanishes. By making Josephson junctions between d-wave superconducting films with different orientations or between d-wave and conventional isotropic s-wave superconductors, one can get a phase shift of
. Nowadays there are several realizations of Josephson junctions of this type:
**tri-crystal grain boundary Josephson junctions,
**tetra-crystal grain boundary Josephson junctions,
**d-wave/s-wave ramp zigzag JJs Josephson junctions,
[
]
**tilt-twist grain boundary Josephson junctions,
[
]
**p-wave based Josephson junctions.
*Superconductor-NormalMetal-Superconductor (SNS) Josephson junctions with non-equilibrium electron distribution in N-layer.
[
]
*Superconductor –
quantum dot
Quantum dots (QDs) are semiconductor particles a few nanometres in size, having light, optical and electronics, electronic properties that differ from those of larger particles as a result of quantum mechanics. They are a central topic in nanote ...
-superconductor (S-QuDot-S) Josephson junctions (implemented by
carbon nanotube
A scanning tunneling microscopy image of a single-walled carbon nanotube
Rotating single-walled zigzag carbon nanotube
A carbon nanotube (CNT) is a tube made of carbon with diameters typically measured in nanometers.
''Single-wall carbon na ...
Josephson junctions).
[
]
Historical developments
Theoretically, the first time the possibility of creating a
Josephson junction was discussed by Bulaevskii ''et al.'' ,
[
] who considered a Josephson junction with paramagnetic scattering in the barrier. Almost one decade later, the possibility of having a
Josephson junction was discussed in the context of heavy fermion p-wave superconductors.
[
] Experimentally, the first
Josephson junction was a corner junction made of
yttrium barium copper oxide
Yttrium barium copper oxide (YBCO) is a family of crystalline chemical compounds that display high-temperature superconductivity; it includes the first material ever discovered to become superconducting above the boiling point of liquid nitrogen ...
(d-wave) and
Pb (s-wave) superconductors.
[ The first unambiguous proof of a Josephson junction with a ferromagnetic barrier was given only a decade later.] That work used a weak ferromagnet consisting of a copper-nickel alloy (Cu''x''Ni1−''x'', with ''x'' around 0.5) and optimized it so that the Curie temperature
In physics and materials science, the Curie temperature (''T''C), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can (in most cases) be replaced by induced magnetism. The Cur ...
was close to the superconducting transition temperature of the superconducting niobium
Niobium is a chemical element with chemical symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to pure titanium, and it has sim ...
leads.
See also
*Josephson effect
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum mech ...
* ''φ'' Josephson junction
*Semifluxon In superconductivity, a semifluxon is a half integer vortex of supercurrent carrying the magnetic flux equal to the half of the magnetic flux quantum . Semifluxons exist in the 0-π long Josephson junctions at the boundary between 0 and π region ...
*Fractional vortices In a standard superconductor, described by a complex field fermionic condensate wave function (denoted , \Psi, e^), vortices carry quantized magnetic fields because the condensate wave function , \Psi, e^ is invariant to increments of the phase \ph ...
*Brian D. Josephson
Brian David Josephson (born 4 January 1940) is a Welsh theoretical physicist and professor emeritus of physics at the University of Cambridge. Best known for his pioneering work on superconductivity and quantum tunnelling, he was awarded the Nob ...
References
{{reflist, 30em
Superconductivity
Josephson effect