IEEE Transactions on Applied Superconductivity

Coherent flux-flow in vertically stacked long Josephson tunnel junctions

Alexey Vladimirovich Ustinov, H. Kohlstedt, Christoph Heiden

Published Jun 1, 1995• 10 citations

Abstract

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.>

DOI: 10.1109/77.403158

Related papers

Possible phase locking of vertically stacked Josephson flux-flow oscillators

Alexey Vladimirovich Ustinov, H. Kohlstedt, Christoph Heiden

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.

Applied Physics LettersSep 12, 199449 citations

Tunable phase locking of stacked Josephson flux-flow oscillators

E. Goldobin, H. Kohlstedt, Alexey Vladimirovich Ustinov

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.

Applied Physics LettersJan 8, 199622 citations

Dynamics and Applications of Long Josephson Junctions

Yongming Zhang

The first object of this work is to theoretically study the effects of tunnel junction parameters on flux-flow Josephson oscillators (FFOs) by numerical simulation and to experimentally study the rf properties of FFOs by integrating them together with an SIS (superconductor-insulator-superconductor) mixer/detector on a chip. By using a CAD modeled integrated sub-mm wave receiver, the composite linewidth of two FFOs was measured to be less than 2.1 MHz in the band 280-330 GHz. The receiver consists of two identical FFOs and an SIS mixer. The linewidth was determined by mixing the two signals in the SIS mixer and analyzing the intermediate frequency. The output power coupled to a 10 .OMEGA. microstripline was about 0.5 µW, and could be adjusted by changing the current bias of the oscillator. The FFOs are connected to the SIS mixer via interdigital capacitors. The FFOs and the SIS element are made of Nb/NbOx/PbBi junctions by the "reactive ion beam oxidation" technique with the critical current density jc controlled by the oxidation time. Lift-off stencils for defining the junction sizes and the top-electrodes were made by using electron-beam writing on PMMA/Copolymer double layer resist. The jc of a long Josephson junction (LJJ) determines the fluxon motion. For LJJs which had jc > 1 kA/cm2, velocity-matching (VM, where the phase velocity of propagating fluxons equals that of the electromagnetic wave in the LJJ) steps could be tuned linearly from about 0.2 to 1.4 mV by an applied magnetic field. Coupled radiation at around 100 GHz (about 3 nW), 261 GHz (4.6 nW), and 330 GHz (0.43 µW), from flux-flow oscillations, was observed. From the step's steepness, the junction quality factor QJ is determined. QJ is between 4 to 7 in the voltage range of 0.4 to 1 mV. Coupled radiation (> 3 nW) at around 100 GHz corresponding to resonant soliton oscillation in low jc samples (250 to 1000 A/cm2) was detected. Simulations show that, for a uniform bias feed, the height of the VM step .gammam increases almost linearly with the external magnetic field .beta.e and the quasiparticle loss .alpha., but is less dependent on the surface loss .beta.. An approximate expression .gammam = a + .alpha(1+b).beta.e agrees with simulations. The maximum ac output voltage decreases with increasing dampings .alpha. and .beta.. It is important to include the .beta.-term in the mathematical model. The second part of this work is devoted to rf properties of long YBa2Cu3O7-.delta. (YBCO) grain boundary junctions made on 0- 32 °. [001]-tilt YSZ bicrystals. Josephson flux-flow resonances and Fiske resonances were observed. The first resonances could be tuned from about 0.5 to 2.0 mV in a 30 µm long junction at 16 K, and we determined the ratio between barrier thickness and relative dielectric constant td/.epsilon.r=2.9 Å and the loss .alpha.= 1.3 at 16 K. Simulations support the experimental observation that Josephson vortices can propagate unidirectionally in an overdamped, homogeneous junction up to .alpha.Ã? 2. From the Fiske resonances at 256-638 GHz, their Q-values, and the Steward-McCumber parameter, we find td/.epsilon.r=4 Å, the London penetration depth .gamma.L=145Â±5 nm and the surface resistance RS ~20 m.OMEGA. at 280 GHz. We conclude that the boundary is dielectric and the YBCO grains are well superconducting within 5-20 nm from the boundary. The dielectric between the grains could be part of a tunnel barrier and/or a matrix for filamentary conduction. Josephson flux-flow transitors based on long bicrystal junctions were made with an Au loop used as the control line. The line modulated the critical current Ic. Current gain larger than unity was obtained below 70 K. The gain increased with the junction length and varied roughly like the square root Ic when adjusting the temperature. A transresistance larger than 5 .OMEGA. was obtained at 15 K. Better performance is expected from junctions with high current densities.

Jan 1, 19937 citations

Coupling of long Josephson junction oscillators at millimeter-wave frequencies

Matteo Cirillo, Fortunato Santucci, P. Carelli +2 more

The authors report on results of phase-locking experiments in a system composed of two long Josephson junctions oscillating at millimeter-wave frequencies. The effects of the coherent behavior of the two junctions are visible on the current-voltage characteristic of the long junctions and in the power enhancement effects observed on the current-voltage characteristic of a detector junction. It is shown that is possible to couple two long junctions in such a way that the enhanced radiation can be transmitted to other devices. Results from another mode of operation of the long Josephson junction as an oscillator are also presented. This mode is based on magnetic-field-induced singularities in long junctions (Fiske steps) generated by the interaction of the cavity modes of the junction seen as an open-ended transmission line and the Josephson oscillations. This mode has given very promising results above 200 GHz, where a power of 20 nW was coupled in a 30- Omega normal state resistance tunnel junction.>

IEEE Transactions on Applied SuperconductivityMar 1, 19933 citations

On-chip radiation detection from stacked Josephson flux-flow oscillators

Alexey Vladimirovich Ustinov, S. V. Shitov, N. N. Iosad +1 more

Radiation measurements with a double-junction stacked Josephson flux-flow oscillator on-chip coupled to an SIS detector are reported. Impedance matching between the oscillator and the detector has been achieved using a broad band coupling circuit. Radiation power in the detector up to 10 nW has been measured in the frequency range between 170 and 410 GHz. Coherent radiation from two stacked junctions has been observed both at the fundamental Josephson frequency and at its second harmonic. The distribution of the radiation power between first two harmonics allows us to distinguish between mutually coupled in-phase and out-of-phase flux-flow modes in the junctions. Coherent operation of stacked Josephson junction oscillators in the millimeter and sub-millimeter wave band is demonstrated.

IEEE Transactions on Applied SuperconductivityJun 1, 19972 citations

On-chip radiation detection from stacked Josephson flux-flow oscillators

S. V. Shitov, Alexey Vladimirovich Ustinov, N. N. Iosad +1 more

Radiation measurements with double-junction stacked Josephson flux-flow oscillator on-chip coupled to a superconductor–insulator–superconductor detector are reported. Impedance matching between the oscillator and the detector has been achieved using a broad band coupling circuit. Radiation power in the detector up to 10 nW has been measured in the frequency range between 170 and 410 GHz. Coherent radiation from two stacked junctions has been observed both at the fundamental Josephson frequency and at its second harmonic. The distribution of the radiation power between the first two harmonics allows us to distinguish between mutually coupled in-phase and out-of-phase flux-flow modes in the junctions. Coherent operation of stacked Josephson junction oscillators in the millimeter and sub-millimeter wave band is demonstrated.

Journal of Applied PhysicsDec 15, 199610 citations