Experimental Evidence for Phase-Locked States in Stacked Long Josephson Junctions

We fabricated and tested samples consisting of two long stacked Josephson junctions with direct access to the intermediate electrode, whose thickness is smaller than London penetration depth λ_L. The electrodes are patterned so that the junctions can be independently biased in the overlap configuration. We report the observation of fluxon-antifluxon phase-locked states in zero external magnetic field and the measurements concerning the range of stability of such states as a function of the bias currents. There is strong evidence for the existence of fluxon-antifluxon pairs even when one of the two junctions is biased on the McCumber curve. We also report the observation of several mutually phase-locked states induced by large rf signals (but not phase-locked to the rf field), that can be interpreted as fluxon-antifluxon oscillating in the stack.