Coherent flux-flow in vertically stacked long Josephson tunnel junctions

Vertical stacks of two Nb/Al-AlO/sub x//Nb long Josephson junctions with nearly identical parameters have been studied experimentally. The magnetic coupling between the junctions was provided by their common electrode of thickness d/spl sime//spl lambda//sub L/. The I-V characteristics displayed flux-flow resonances of the two junctions. Two different mutually phase-locked regimes were found: the lower velocity anti-phase mode, and the high velocity in-phase mode. These modes, while staying locked, could be tuned by the magnetic field in a broad voltage range which corresponds to the Josephson frequencies of 150-600 GHz for the lower mode and 280-400 GHz for the upper mode. Our observations suggest applications of the stacked long Josephson junctions as local oscillators for millimeter and sub-millimeter wave circuits.<
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