Asymmetric ac fluxon depinning in a Josephson junction array: A highly   discrete limit
  Directed motion and depinning of topological solitons in a strongly discrete damped and biharmonically ac-driven array of Josephson junctions is studied. The mechanism of the depinning transition is investigated in detail. We show that the depinning process takes place through chaotization of an initially standing fluxon periodic orbit. Detailed investigation of the Floquet multipliers of these orbits shows that depending on the depinning parameters (either the driving amplitude or the phase shift between harmonics) the chaotization process can take place either along the period-doubling scenario or due to the type-I intermittency. 
Yaroslav Zolotaryuk
nlin.PS