Observation of a spatially-coherent resonance mode in stacked Josephson junctions

Underdamped flux-flow dynamics in double-barrier Josephson tunnel junctions is investigated by a spatially-resolving technique. Under the influence of a bias current, Josephson vortices can move simultaneously in both Josephson junctions resulting in the excitation of cavity waves in the junctions. The spatial structure of the cavity resonances is imaged by scanning the sample with a focused electron beam. We find clear indication that the vortices in the two stacked junctions move coherently.