Mutual phase locking on resonant flux-flow steps in stacked long Josephson junctions

We demonstrate that, independently biasing the junctions in a two stack in the resonant flux-flow regime, we can obtain mutual locking in four different states. Two of them possibly consist of out-of-phase Josephson oscillations and two consist of in-phase oscillations. They are found experimentally to be very stable for a quite large range of applied magnetic field.