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 Φ0. 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 −Φ0/2 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.[1][2] 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[3] and later in YBa2Cu3O7–Nb ramp zigzag junctions.[4] 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 (0-π junctions) is the same. such 0–π JJs have been demonstrated in SFS[5] and in underdamped SIFS[6] 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.[7]

See also

References
  1. J. H. Xu; J. H. Miller, Jr.; C. S. Ting (1994). "-vortex state in a long 0- Josephson junction". Phys. Rev. B. 51 (17): 11958–11961. Bibcode:1995PhRvB..5111958X. doi:10.1103/PhysRevB.51.11958. PMID 9977943.
  2. E. Goldobin; D. Koelle; R. Kleiner (2002). "Semifluxons in long Josephson 0--junctions". Phys. Rev. B. 66 (10): 100508. arXiv:cond-mat/0207742. Bibcode:2002PhRvB..66j0508G. doi:10.1103/PhysRevB.66.100508. S2CID 118997443.
  3. C. C. Tsuei; J. R. Kirtley (2002). "d-Wave pairing symmetry in cuprate superconductors --- fundamental implications and potential applications". Physica C. 367 (1–4): 1–8. Bibcode:2002PhyC..367....1T. doi:10.1016/S0921-4534(01)00976-5.
  4. H. Hilgenkamp; Ariando; H.-J. H. Smilde; D. H. A. Blank; G. Rijnders; H. Rogalla; J. R. Kirtley; C. C. Tsuei (2003). "Ordering and manipulation of the magnetic moments in large-scale superconducting -loop arrays" (PDF). Nature. 422 (6927): 50–3. Bibcode:2003Natur.422...50H. doi:10.1038/nature01442. PMID 12621428. S2CID 4398135.
  5. M. L. Della Rocca; M. Aprili; T. Kontos; A. Gomez; P. Spathis (2005). "Ferromagnetic 0- Junctions as Classical Spins". Phys. Rev. Lett. 94 (19): 197003. arXiv:cond-mat/0501459. Bibcode:2005PhRvL..94s7003D. doi:10.1103/PhysRevLett.94.197003. PMID 16090200. S2CID 39682770.
  6. M. Weides; M. Kemmler; H. Kohlstedt; R. Waser; D. Koelle; R. Kleiner; E. Goldobin (2006). "0- Josephson Tunnel Junctions with Ferromagnetic Barrier". Phys. Rev. Lett. 97 (24): 247001. arXiv:cond-mat/0605656. Bibcode:2006PhRvL..97x7001W. doi:10.1103/PhysRevLett.97.247001. PMID 17280309. S2CID 2206595.
  7. A. Dewes; T. Gaber; D. Koelle; R. Kleiner; E. Goldobin (2008). "Semifluxon Molecule under Control". Phys. Rev. Lett. 101 (24): 247001. arXiv:0809.1346. Bibcode:2008PhRvL.101x7001D. doi:10.1103/PhysRevLett.101.247001. PMID 19113654. S2CID 35227367.


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