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Journal of Applied Physics

Flux-flow-type Josephson oscillator for millimeter and submillimeter wave region. II. Modeling

Tadao Nagatsuma, Keiji Enpuku, Keiji Yoshida, F. Irie

Published Dec 1, 1984• 115 citations
Publisher

Abstract

A theoretical study is made of a travelling-wave-type oscillator, which utilizes a flux flow in a long Josephson junction for use as a local oscillator in the integrated superconducting receiver system. An internal electromagnetic field of the oscillator junction in the flux-flow state is investigated both numerically and analytically. It is shown that the voltage amplitude of the internal oscillation increases gradually in the direction of the flux flow and reaches a maximum value at the junction end. An equivalent circuit of the oscillator is also obtained, which gives dependences of the emitted radiation on frequency, magnetic field, and load. It is shown that the output power attains the value of the order of 10−6 W in the frequency range between 100 and 500 GHz, and that the output power and the radiation frequency can be controlled by both the bias voltage and the applied magnetic field. These theoretical results explain quantitatively the experimental ones with a Pb-alloy long junction of length 24 λJ.

DOI
10.1063/1.333849

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Flux-flow type Josephson oscillator for millimeter and submillimeter wave region

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Journal of Applied Physics•Feb 15, 1988•70 citations
A theoretical study is made of a travelling-wave-type oscillator, which utilizes a flux flow in a long Josephson junction for use as a local oscillator in the integrated superconducting receiver system. An internal electromagnetic field of the oscillator junction in the flux-flow state is investigated both numerically and analytically. It is shown that the voltage amplitude of the internal oscillation increases gradually in the direction of the flux flow and reaches a maximum value at the junction end. An equivalent circuit of the oscillator is also obtained, which gives dependences of the emitted radiation on frequency, magnetic field, and load. It is shown that the output power attains the value of the order of 10−6 W in the frequency range between 100 and 500 GHz, and that the output power and the radiation frequency can be controlled by both the bias voltage and the applied magnetic field. These theoretical results explain quantitatively the experimental ones with a Pb-alloy long junction of length 24 λJ.
An oscillator which utilizes the effect of the vortex motion in long Josephson tunnel junctions, i.e., flux flow, has been presented in millimeter and submillimeter wave region. An electromagnetic wave generated by the oscillator is detected with a small tunnel junction as a detector with a refined coupling configuration. Quantitative evaluation of the detected power showed that the detected power attained the value of 10−6 W in the frequency range between 100 and 400 GHz, which is far superior to previous results. Frequency and magnetic field dependences of the present system were also measured, which showed that the output power was able to be controlled by the dc magnetic field. The present oscillator will be promising as the local oscillator in the integrated Josephson receiver systems.
Numerical as well as experimental studies have been made of the oscillation stability for the travelling-wave-type oscillator, which utilizes a vortex motion in a long Josephson junction, i.e., a flux flow. For the Josephson oscillator, a steep current step in a dc I-V characteristic improves the oscillation stability. Two kinds of effects which degrade the steepness of the current step have been investigated for the long junction of the overlap geometry. One is the self-field effect which makes the current step inclined, and the other is the resonant motion of the vortices which induces a staircaselike structure on the current step. We have discussed the method for the reduction of these effects and proposed a novel junction geometry without these effects, i.e., an overlap junction with a projection on one junction side. With the new geometry, we have obtained current steps whose slope is about one hundred times steeper than those obtained with the conventional overlap geometry and whose height attains about a half of the maximum critical current over the wide voltage region 0.1≤Vdc≤1.0 mV. The linewidth calculated from the current step is expected to be a few kHz.
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=145±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.
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.
A Flux-flow oscillator based on a long Josephson junction is a local oscillator in fully superconducting integrated receivers (SIRs). The low frequency voltage fluctuations define the free-running FFO radiation linewidth, an important parameter for employing the SIR as a submm spectrometer. The DC voltage of the oscillator is independently controlled by both the dissipative DC bias current I B and the loss-free external magnetic field at the two FFO ends. It is mainly the natural wideband current noise of the Josephson barrier, which is converted into the fluctuations of voltage by the differential resistance Rd on the current I B . We have developed and experimentally demonstrated the method for suppression of the voltage fluctuations by convey of the bias current noise into the anticorrelated noise of the magnetic field. The experiments were made with Nb/AlN/Nb FFOs, having a specially designed shunting structure at the end, where flux quanta enter the junction.
Power emission from a flux-flow-type Josephson oscillator with a thin-film coupling method is studied experimentally. The thickness of one electrode of the oscillator is made thin enough to allow electromagnetic fields generated by the oscillator to be emitted through the thin-film electrode. Radiation power from the oscillator is detected with superconductor-insulator-superconductor detectors, which are fabricated on top of the oscillator. Experimental results show that sufficient power can be obtained from the oscillator by using the thin-film coupling method when the thickness of the electrode is comparable to the London penetration depth. The results obtained are in reasonable agreement with theoretical predictions. The thin-film coupling method will be useful when the oscillator is connected to external circuits, such as an impedance matching circuit.