Possible phase locking of vertically stacked Josephson flux-flow oscillators

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Abstract
A system of two vertically stacked Nb/Al‐AlOx/Nb long Josephson junctions is studied experimentally. The magnetic coupling between the junctions is provided by their common electrode of the thickness d∼λL. The current‐voltage characteristics of the stack displays resonant steps which are understood to be the phase‐locked flux‐flow resonances of the two junctions. These modes can be tuned by the magnetic field in a broad voltage range which corresponds to the Josephson frequencies between 150 and 600 GHz. Our observations suggest a possibility of application of the stacked long Josephson junctions as local oscillators for millimeter and submillimeter wave circuits.