We fabricated and tested samples consisteing of two long stacked Josephson junctions with direct access to the intermediate electrode, whose thickness is smaller than the London penetration depth $\lambda _L$. The electrodes are patterned so that the junctions can be idependently 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 field9, that can be interpreted as a fluxon-antifluxon pair oscillating in the stack.
Journal of Low Temperature Physics, 1997, Vol 106, Issue 3/4, p. 345-352