Bunching of fluxons by Cherenkov radiation in Josephson multilayers

A single magnetic fluxon moving at a high velocity in a Josephson multilayer (e.g., high-temperature superconductor such as BSCCO) can emit electromagnetic waves (Cherenkov radiation), which leads to the formation of stable dynamic states consisting of several bunched fluxons. We find such bunched states in numerical simulations in the simplest cases of two and three coupled junctions. At a given driving current, several different bunched states are stable and move at velocities that are higher than the corresponding single-fluxon velocity. These and some of the more complex higher-order bunched states and transitions between them are investigated in detail.