Interlayer fluxon interaction in Josephson stacks

In a stack of Josephson tunnel junctions both attractive and repulsive interactions may exist between Josephson vortices (fluxons) allocated in different tunnel barriers. We report experimental observations of two modes of coherent fluxon motion in double-junction stacks. At the low-velocity mode, as well as in statics, vortices in neighboring Josephson layers repel each other. In contrast, the high-velocity mode corresponds to an attractive interlayer interaction between vortices of the same polarity.