By studying soliton (fluxon) motion in long annular Josephson junctions it is possible to avoid the influence of the boundaries and soliton-soliton collisions present in linear junctions. A new experimental design consisting of a niobium coil placed on top of an annular junction has been used to insert individual fluxons or antifluxons into the junction in a controllable way. The dynamical behavior of different numbers of trapped fluxons was investigated. In addition, we were able to change the junction parameters by changing temperature. In some of the zero-field steps, the experiments reveal a small jump to higher values of the voltage at the top of the steps. This phenomenon can be caused by a crossover from a nonbunched state to a bunched state of the fluxon motion. By performing direct numerical simulations using the perturbed sine-Gordon equation with parameter values determined from the experiments we have been able to confirm the above explanation. ©1996 American Institute of Physics.
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