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 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 fluxon at the 0-π boundary. Semifluxon is a particular example of the fractional vortex pinned at the phase discontinuity, see Fractional vortices for details.
For the first time the semifluxons were observed at the tricrystal grain boundaries in d-wave superconductors and later in YBa2Cu3O7–Nb ramp zigzag junctions. In these systems the phase shift of π takes place due to d-wave order parameter symmetry in YBa2Cu3O7 superconductor. The observations were performed using low temperature scanning SQUID microscope.
Later, researchers succeeded to fabricate 0-π junctions using conventional low-Tc superconductors and ferromagnetic barrier, where the physics is completely different, but the result 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.