In
superconductivity
Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlik ...
, a semifluxon is a half integer
vortex
In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in t ...
of
supercurrent
A supercurrent is a superconducting current, that is, electric current which flows without dissipation in a superconductor. Under certain conditions, an electric current can also flow without dissipation in microscopically small non-superconductin ...
carrying the
magnetic flux equal to the half of the
magnetic flux quantum
The magnetic flux, represented by the symbol , threading some contour or loop is defined as the magnetic field multiplied by the loop area , i.e. . Both and can be arbitrary, meaning can be as well. However, if one deals with the superconduct ...
. Semifluxons exist in the 0-π
long Josephson junctions at the boundary between 0 and π regions. This 0-π boundary creates a π discontinuity of the Josephson phase. The junction reacts to this discontinuity by creating a semifluxon. Vortex's supercurrent circulates around 0-π boundary. In addition to semifluxon, there exist also an antisemifluxon. It carries the flux and its supercurrent circulates in the opposite direction.
Mathematically, a semifluxon can be constructed by joining two tails of conventional (integer) fluxon (kink of the sine-Gordon equation) at the 0-π boundary.
Semifluxon is a particular example of the ''fractional vortex pinned at the phase discontinuity'', see
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 \p ...
for details.
For the first time the semifluxons were observed at the tricrystal grain boundaries in d-wave superconductors
and later in YBa
2Cu
3O
7–Nb ramp zigzag junctions.
In these systems the phase shift of π takes place due to d-wave order parameter symmetry in YBa
2Cu
3O
7 superconductor. The observations were performed using low temperature scanning SQUID microscope.
Later, researchers succeeded to fabricate 0-π junctions using conventional low-T
c superconductors and ferromagnetic barrier, where the physics is completely different, but the result (0-π junctions) is the same.
such 0–π JJs have been demonstrated in SFS
and in underdamped SIFS
junctions.
Further, physicists were able to demonstrate a molecule made of two interacting semifluxons arranged antiferromagnetically. It has a degenerate ground state up-down or down-up. It was shown that one can readout the state of such a semifluxon molecule by using on-chip SQUIDs. One can also switch between the up-down or down-up states of the molecule by applying the current.
See also
*
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 mec ...
*
π Josephson junction
*
φ Josephson junction
*
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 \p ...
References
Josephson effect
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