Coupled fluxon modes in stacked Nb/AiOx/Nb long Josephson junctions

The current-voltage characteristics of N vertically stacked Nb-based long Josephson junctions (N=1,2,3) are measured. For a single junction in the stack, we observe the splitting of the flux-flow mode into M modes, each of them being characterized by different magnetic field dependence and by different cavity-resonance (Fiske-mode) frequency. Our results suggest that a fluxon moving in one junction in the stack may experience M=N different Swihart velocities due to the coupling to adjacent junction(s) through the common electrode(s) of the thickness smaller than the London penetration depth. In the twofold stacks, we find an indication for a coherent mode corresponding to coupled fluxon arrays moving in both junctions.