Flux-flow Oscillator Research Articles

Fluxon dynamics in long Josephson junctions in the presence of a temperature gradient or spatial nonuniformity

Fluxon dynamics in nonuniform Josephson junctions was studied both experimentally and theoretically. Two types of nonuniform junctions were considered: the first type had a nonuniform spatial distribution of critical and bias currents and the second had a temperature gradient applied along the junction. An analytical expression for the I-V curve in the presence of a temperature gradient or spatial nonuniformity was derived. It was shown that there is no static thermomagnetic Nernst effect due to Josephson fluxon motion despite the existence of a force pushing fluxons in the direction of smaller self-energy (from the cold to the hot end of the junction). A phenomenon, the {open_quotes}zero crossing flux flow step{close_quotes} (ZCFFS) with a nonzero voltage at a zero applied current, was observed in nonuniform long Josephson junctions. The phenomenon is due to the existence of a preferential direction for the Josephson vortex motion. ZCFFS{close_quote}s were observed at certain magnetic fields when the critical current in one direction but not the other becomes zero. Possible applications of nonuniform Josephson junctions in flux flow oscillators and as a superconducting diode are discussed. {copyright} {ital 1997} {ital The American Physical Society}

High frequency applications of intrinsic Josephson junctions in mesa structures on Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+y/ single crystals

We report on high frequency experiments with mesa structures patterned on Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+y/ single crystals (BSCCO). Due to the intrinsic Josephson effect between adjacent CuO/sub 2/ bilayers these mesas form stacks of 100 to 500 Josephson junctions. The ac-Josephson effect was investigated in the frequency range between 6 GHz and 120 GHz. We focus our interest on possible applications of intrinsic Josephson junctions as flux-flow oscillators and high-frequency mixers. These components form the essential parts of an integrated mm-wave receiver in BSCCO technology.

A dc SQUID based low-noise 4 GHz amplifier

The dc SQUID based RF amplifier (SQA) looks very attractive as an IF amplifier for integration with a SIS mixer and a flux-flow oscillator (FFO) in a fully superconducting submillimeter wave receiver suitable for space applications. Important advantages of the SQA are its low noise, extremely low power consumption, and complete compatibility with the fabrication process currently used for SIS mixers. Single stage amplifiers with a novel signal coupling circuit have been developed and tested in the frequency range 3.6-4.1 GHz. The 1 /spl mu/m/sup 2/ area Nb-AlO/sub x/-Nb junctions shunted by Ti resistors are grouped in a double washer dc SQUID. Two samples with slightly different designs showed the following gain, noise temperature and 3 dB bandwidth: 10.0/spl plusmn/1 dB, 5.0/spl plusmn/1.5 K, 3.86-3.90 GHz, and 6.8/spl plusmn/1 dB, 22/spl plusmn/7 K, 3.89-4.05 GHz, respectively.

An integrated 500 GHz receiver with superconducting local oscillator

An integrated quasioptical receiver consisting of a planar double-dipole antenna, SIS mixer and superconducting Flux-Flow Oscillator (FFO) with matching circuits has been designed, fabricated and tested in the frequency range 420-530 GHz. The integrated receiver is very suitable for space applications because of its low size, mass and power consumption. All components of the receiver are integrated on a 4 mm/spl times/4 mm/spl times/0.2 mm crystalline quartz substrate using a single Nb-AlO/sub x/-Nb trilayer. The successful operation of the integrated receiver comprising a number of new crucial elements has been demonstrated. A DSB noise temperature as low as 140 K at 500 GHz and a tuning range of more than 100 GHz have been obtained. A comparison of the FFO with conventional external LO has been performed.

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

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.

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

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.

Exponentially tapered Josephson flux-flow oscillator.

We introduce an exponentially tapered Josephson flux-flow oscillator that is tuned by applying a bias current to the larger end of the junction. Numerical and analytical studies show that above a threshold level of bias current the static solution becomes unstable and gives rise to a train of fluxons moving toward the unbiased smaller end, as in the standard flux-flow oscillator. An exponentially shaped junction provides several advantages over a rectangular junction including: (i) smaller linewidth, (ii) increased output power, (iii) no trapped flux because of the type of current injection, and (iv) better impedance matching to a load. \textcopyright{} 1996 The American Physical Society.

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.

Superconductor–insulator–superconductor tunnel strip line: Features and applications

The superconductor–insulator–superconductor (SIS) tunnel junction based on Nb material has been investigated theoretically at frequencies of 200–1400 GHz by modeling it as a segment of a superconducting microstrip transmission line and using the Mattis–Bardeen theory of anomalous skin effect. The modeling was used to predict the performance of a flux-flow oscillator (FFO) and a SIS frequency down-converter (mixer). It is shown that in a FFO the internal loss (quasiparticle and surface) limits flux flow to the frequency band of 260–770 GHz. A SIS junction of micron size employed as a mixer for frequency down-conversion demonstrates distributed behavior at frequencies above 550 GHz. We suggest that electrically long SIS junctions of resonant size can be used as self-compensating mixing elements with attractive performance in Nb as well as NbN mixers.

Linewidth of submillimeter wave flux-flow oscillators

A reliable technique for wide band measurements of the spectral linewidth of superconducting oscillators integrated on-chip with superconductor-insulator-superconductor (SIS) detectors has been developed. The spectral linewidth of flux-flow oscillators (FFO) based on the unidirectional and viscous flow of magnetic vortices in a long overdamped Josephson tunnel junction was measured in the frequency range 250–580 GHz, and a linewidth as low as 200 kHz was obtained at 450 GHz. Also stable frequency locking of a FFO to very high (≤60th) harmonics of an external microwave reference source has been demonstrated. The proposed technique may improve the sensitivity, frequency resolution, and stability of the fully superconducting integrated submillimeter wave receiver.

Coupling of Josephson flux-flow oscillators to an external RC load

We investigate by numerical simulation the behaviour of the power dissipated in a resistive load capacitively coupled to a Josephson flux-flow oscillator and compare the results to those obtained for a d.c. coupled purely resistive load. Assuming realistic values for the parameters R and C, both in the high- and in the low- case the power is large enough to allow the operation of such a device in applications.

First implementation of a superconducting integrated receiver at 450 GHz

An integrated quasioptical receiver consisting of a planar double dipole antenna, superconductor-insulator-superconductor mixer and a superconducting local oscillator (LO) with matching circuits has been designed, fabricated and tested in the frequency range 360–490 GHz. A flux-flow oscillator (FFO) based on unidirectional and viscous flow of magnetic vortexes in a long Josephson tunnel junction, is employed as a local oscillator. All components of the receiver are integrated on a 4 mm×4 mm×0.2 mm crystalline quartz substrate using a single Nb–AlOx–Nb trilayer. The lowest DSB noise temperature of 470–560 K has been achieved within a frequency range of 425–455 GHz.

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.

Properties of autonomous and injection locked flux flow oscillators

Flux flow oscillators (FFO) have been experimentally investigated at frequencies up to 850 GHz. At 440 GHz the received power in an on-chip integrated SIS mixer is 5 /spl mu/W. The first experimental measurements of the FFO linewidth as a function of applied dc bias current and magnetic field have been performed both for two autonomous FFOs and with one of the FFOs injection-locked to a narrow-band external microwave source. By beating the two autonomous FFOs an integral spectral linewidth as low as 750 kHz is measured at 280 GHz. Mechanisms leading to a broadening of the FFO linewidth are discussed. Experimentally it is shown that the FFO can be used as harmonic generator. >

Fabrication of all-NbCN flux-flow oscillators with a tapered slotline antenna and off-chip detection of their radiation power

All-NbCN flux-flow oscillators (FFO) with tapered slotline antenna (TSA) were fabricated on MgO substrates. A five-step microstrip transformer was used to couple an FFO, which has a low impedance, to a TSA having a high impedance, The microstrip transformer is designed to have a return loss more than 20 dB at the center frequency. We have measured submillimeter wave radiation from the oscillators using an off-chip silicon-composite bolometer. Radiation from a fabricated FFO was detected in a bias voltage range from 1.18 mV to 1.48 mV, which corresponds to a frequency range from 570 GHz to 710 GHz. >

Integrated sub-MM wave receivers

The concept of a fully integrated superconducting receiver looks very attractive for sub-mm space astronomy where low weight, power consumption and volume are required. The possibility to integrate on a few chips the different planar components: a SIS mixer, a superconducting local oscillator (LO), an intermediate frequency amplifier followed by superconducting circuits for digitizing and processing of down converted signals, is discussed. A first implementation of a real integrated quasioptical receiver for frequencies up to 500 GHz is described. The one-chip receiver comprises a double dipole antenna, parallel biased SIS array mixer and Josephson Flux Flow Oscillator (FFO) with matching circuits. The results of extensive investigations of the integrated receiver as well as design and investigation of novel superconducting elements are presented. >

Spectral linewidth of autonomous and injection-locked flux-flow oscillators.

Spectral linewidth of autonomous and injection-locked flux-flow oscillators.

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.

An integrated superconducting sub-mm wave receiver for linewidth measurements of Josephson flux-flow oscillators

By using an integrated superconducting sub-mm wave receiver, the composite linewidth of two Josephson flux-flow oscillators (FFO) was measured to be less than 2.1 MHz in the band 280-330 GHz. The output power coupled to a 10 /spl Omega/ microstripline was about 0.5 /spl mu/W, and could be adjusted by changing the current bias of the oscillator junction. The receiver consists of two identical FFO's and an SIS (superconductor-insulator-superconductor) mixer. The composite linewidth of the oscillators was determined by mixing the two signals in the SIS mixer to a lower intermediate frequency (if). The FFO's are connected to the SIS mixer via centerfed interdigital capacitors. One FFO is well coupled to the SIS mixer by using a 3-step Chebychev microstripline transformer and a fairly large interdigital capacitor, while the second FFO has about 15-20 dB weaker coupling. A strip inductor, rf-terminated by a radial stub, resonates out the SIS junction capacitance at about 340 GHz. Radial stubs were also used as rf chokes for the if-line (4 GHz). The FFO's and the SIS element are made of Nb/Nb-oxide/PbBi tunnel junctions. The receiver circuit was optimized by careful CAD modeling (HP Microwave Design System).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Integrated flux-flow oscillators for submillimeter wave receivers

A superconducting Flux-Flow Oscillator (FFO) integrated on the same chip with a small Josephson junction detector has been experimentally investigated in the frequency range 100–450 GHz. Both the emitted power and the frequency of the FFO can be varied by adjusting the dc bias current and/or the applied dc magnetic field. Microwave powers as high as 0.3 μW have been measured at 375 GHz. The spectral width of the FFO is about 1 MHz as estimated from harmonic mixing experiments. Also a fully integrated superconducting submillimeter wave receiver using the FFO as local oscillator has been successfully tested. The circuit included coupling transformers, a superconducting variable attenuator and a detector junction with tuned-out capacitance.