Mutual Phase Locking of Fluxons in Stacked Long Josephson Junctions: Simulations and Experiment

We report on the experimental observation of reciprocal phase-locking in stacked $Nb-AlO_x-Nb$ Josephson junctions having overlap geometry. When the junctions are independently biased in zero external magnetic field, they each exhibit several Zero Field Steps. Biasing both the junctions on the Zero Field Steps, one with constant current while the other is swept along the step, we have observed phase locking between the fluxons in the two junctions and measured the phase locking range. We have found that the bound state can be very stable, and that the stack exhibits the same features reversing either the polarity of the bias current or the role of the junctions. An analogous investigation of the effect of the magnetic field on the stability of the bound state has been performed. Numerical simulations have shown that the underlying dynamics corresponding to this situation is a bound state of a fluxon-antifluxon pair. Finally the effect of rf radiation on the junctions has been investigated.