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., and C. S. Ting (1994). "π-vortex state in a long 0-π Josephson junction". Physical Review 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-π-junctions". Physical Review 66 (10): 100508. arXiv:cond-mat/0207742. Bibcode:2002PhRvB..66j0508G. doi:10.1103/PhysRevB.66.100508.
  3. ↑ C. C. Tsuei and J. R. Kirtley (2002). "d-Wave pairing symmetry in cuprate superconductors --- fundamental implications and potential applications". Physica C 367: 1. 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 and C. C. Tsuei, (2003). "Ordering and manipulation of the magnetic moments in large-scale superconducting π-loop arrays". Nature (London) 422 (6927): 50–3. Bibcode:2003Natur.422...50H. doi:10.1038/nature01442. PMID 12621428.
  5. ↑ M. L. Della Rocca, M. Aprili, T. Kontos, A. Gomez and P. Spathis (2005). "Ferromagnetic 0-π Junctions as Classical Spins". Physical Review 94 (19): 197003. arXiv:cond-mat/0501459. Bibcode:2005PhRvL..94s7003D. doi:10.1103/PhysRevLett.94.197003. PMID 16090200.
  6. ↑ M. Weides, M. Kemmler, H. Kohlstedt, R. Waser, D. Koelle, R. Kleiner and E. Goldobin (2006). "0-π Josephson Tunnel Junctions with Ferromagnetic Barrier". Physical Review 97 (24): 247001. arXiv:cond-mat/0605656. Bibcode:2006PhRvL..97x7001W. doi:10.1103/PhysRevLett.97.247001. PMID 17280309.
  7. ↑ Dewes, Andreas; Gaber, Tobias; Koelle, Dieter; Kleiner, Reinhold; Goldobin, Edward (December 2008). "Semifluxon Molecule under Control". Physical Review Letters 101 (24). doi:10.1103/PhysRevLett.101.247001.


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