Fluxon dynamics in long annular Josephson tunnel junctions

Single-fluxon dynamics has been experimentally investigated in high-quality $\mathrm{N}\mathrm{b}/\mathrm{A}\mathrm{l}-{\mathrm{AlO}}_x/\mathrm{Nb}$ annular Josephson tunnel junctions having a radius much larger than the Josephson penetration depth. Strong evidence of self-field effects is observed. An external magnetic field in the barrier plane acts on the fluxon as a periodic potential and lowers its average speed. Further, the results of perturbative calculations do not fit the experimental current-voltage profile and, provided the temperature is low enough, this profile systematically shows pronounced deviations from the smooth predicted form. The deviations take the form of fine, hysteretic, almost equally spaced structures.