A. V. Ustinov

POSSIBLE PHASE LOCKING OF VERTICALLY STACKED JOSEPHSON FLUX-FLOW OSCILLATORS

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.

NUMERICAL STUDY OF FLUXON DYNAMICS IN A SYSTEM OF TWO-STACKED JOSEPHSON JUNCTIONS

The dynamics of magnetic fluxons in a system of two vertically stacked long Josephson junctions is investigated numerically. The model is based on the approach by S. Sakai, P. Bodin, and N. F. Pedersen [J. Appl. Phys. 73, 2411 (1993)] and is described by two strongly coupled sine‐Gordon equations. In agreement with recent experimental data, we confirm numerically the effect of splitting of the fluxon travelling mode into two separated modes with different characteristic velocities. The simulated current‐voltage characteristics indicate stable phase‐locked flux‐flow resonances of two junctions. These results support a possibility of application of the stacked long Josephson junctions as a system of coherent oscillators for millimeter and sub‐millimeter wave bands.

Multi-fluxon effects in long Josephson junctions

Experiments with fluxons moving in long Josephson junctions of different geometries are reviewed. A controllable way of introducing fluxons one by one into annular Josephson junctions has been realized. This allows a comparison with existing fluxon chain perturbation theory. Several new millimeter-wave stimulated regimes found in linear junctions are discussed. Large asymmetric zero-crossing steps of constant voltage have been observed in the I-V curves. These steps may have a potential application in Josephson voltage standards. Large narrowband radiation emitted from the flux-flow oscillator in the 62-77-GHz frequency range has been detected.<<ETX>>

Tunable phase locking of stacked Josephson flux‐flow oscillators

A tuning technique for mutual phase locking of two vertically stacked long Josephson junctions is suggested and successfully tested. The technique is based on passing a current through the middle electrode between two tunnel barriers. Mutually synchronized oscillation modes in the Nb/Al–AlOx/Nb stack are found to be tunable in the frequency range from 180 to 420 GHz. Presented results extend the possibility of using stacked long Josephson junctions as coherently operating oscillators for millimeter and submillimeter wave bands.

Fluxon pinning in annular Josephson junctions by an external magnetic field

The behavior of long annular Josephson junctions with various numbers n of trapped fluxons is studied in the presence of a uniform external magnetic field parallel to the plane of the junction’s tunnel barrier. We report on measurements of the magnetic dependencies of the critical current and current–voltage characteristics with different n. Experimental results are explained by the model assuming that a barrier-parallel magnetic field produces potential wells for fluxons and antifluxons at the opposite locations. We also present numerical simulations using the perturbed sine-Gordon equation with an additional magnetic-field-induced term. Good agreement is found between the measured critical current versus magnetic-field dependencies and the numerical simulations.

NUMERICAL ANALYSIS OF THE COHERENT RADIATION EMISSION BY TWO STACKED JOSEPHSON FLUX-FLOW OSCILLATORS

The numerical investigation of the radiation emission by a system of two magnetically coupled, long Josephson junctions is reported. Time‐dependent synchronized voltage response in the flux‐flow regime is analyzed for the case of in‐phase and out‐of‐phase oscillations in the junctions. Simulations show that Josephson junctions operating in the in‐phase flux‐flow mode may generate rf radiation power by a factor of more than 4 larger than that of a single Josephson junction. The radiation in the out‐of‐phase flux‐flow mode is characterized by nearly completely suppressed amplitudes of odd harmonics and considerably damped even harmonics as compared to that of a single barrier junction. The dependence of the radiation power on the parameter spread between the junctions is investigated. The advantages of using stacked Josephson junctions as oscillators for the sub‐mm wave band are discussed.

Flux flow and resonant modes in multi-junction Josephson stacks

Magnetic field dependent current-voltage characteristics of stackled Nb/(Al−AlOx/Nb)n long Josephson junctions are investigated experimentally. The thickness of their common superconducting electrodes provides the magnetic coupling between the junctions. For stacks of n=7 Josephson junctions the current-voltage characteristics display collective flux-flow behaviour of Josephson vortices. In the interior layers Josephson vortices move simultaneously under the influence of the bias current. The flux-flow behaviour is modulated by a complicated structure of cavity-like resonances which show broad range of characteristic frequencies. The measurements can be qualitatively explained by the Kleiner model for the resonances in stacks. Mutual locking of junctions in the stack is indicated by pronounced cavity resonances with large voltage spacing.

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.<<ETX>>

Multi-fluxon zero-field modes in long Josephson tunnel junctions

The higher order zero field steps of long inline and overlap Josephson junctions have been investigated experimentally by means of low temperature scanning electron microscopy. The results indicate that several stable states are possible when more than one fluxon is present in the junction, corresponding to a different spacing among the fluxons during their propagation. These dynamic states should differ by the spectrum of the emitted radiation. Numerical simulations show that the fluxon interaction with plasma oscillations appears to be responsible for the observed behavior.

Fluxon dynamics in discrete Josephson transmission lines with stacked junctions

The experimental and numerical study of parallel one-dimensional arrays consisting of double-layer stacked small Josephson tunnel junctions is reported. In addition to several features already known for classic single-layer arrays, we find a new resonant state which corresponds to the locking of the fluxon oscillations to a frequency of small-amplitude oscillations of the superconducting phase in middle islands of small stacks. Numerical simulations of a simplified circuit agree well with experiments and confirm the suggested qualitative mechanism. In these experiments we find no indication for the so-called multiple-fluxon propagation in double-junction arrays.