In physics, a fluxon is a quantum of electromagnetic flux. In the context of superconductivity, in type II superconductors, fluxons (also known as Abrikosov vortices) can form when the applied field lies between B c 1 {\displaystyle B_{c_{1}}} and B c 2 {\displaystyle B_{c_{2}}} . The fluxon is a small whisker of normal phase surrounded by superconducting phase, and supercurrents circulate around the normal core. The magnetic field through such a whisker and its neighborhood, which has size of the order of London penetration depth λ L {\displaystyle \lambda _{L}} (~100 nm), is quantized because of the phase properties of the magnetic vector potential in quantum electrodynamics, see magnetic flux quantum for details. In the context of long Superconductor-Insulator-Superconductor Josephson tunnel junctions, a fluxon (a.k.a. Josephson vortex) is made of circulating supercurrents and has no normal core in the tunneling barrier. Supercurrents circulate just around the mathematical center of a fluxon, which is situated with the (insulating) Josephson barrier. Again, the magnetic flux created by circulating supercurrents is equal to a magnetic flux quantum Φ 0 {\displaystyle \Phi _{0}} (or less, if the superconducting electrodes of the Josephson junction are thinner than λ L {\displaystyle \lambda _{L}} ).
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