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IEEE Transactions on Magnetics

Performances of a flux-flow-type Josephson amplifier

T. Hashimoto, Keiji Enpuku, Keiji Yoshida

Published Mar 1, 1989• 9 citations
Publisher

Abstract

The authors have been developing an analog amplifier utilizing a flux flow phenomena in a Josephson line, i.e. the flux-flow-type Josephson amplifier (FFA). The FFA converts a signal current I/sub s/ to a voltage V/sub dc/ across the Josephson line, and the I/sub s/-V/sub dc/ characteristic determines the performance of the FFA, e.g. the gain and saturation level. Studies have been made to improve the I/sub s/-V/sub dc/ relation. It is shown that the transresistance can be improved significantly by reducing the width of the Josephson line and increasing the kinetic inductance of the line. The kinetic inductance can be increased by thinning the line electrode and by using an electrode with large London penetration depth, such as NbN. The transresistance obtained is as large as 6.8 Omega for NbN/Pb-alloy Josephson lines with a thickness of 5 nm and a width of 4 mu m. It is also shown that the linear operating range of the amplifier is as large as 1 mV.>

DOI
10.1109/20.92526

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Dynamics and Applications of Long Josephson Junctions

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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.

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In order to realize efficient coupling between a flux-flow type Josephson oscillator (FFO) and a stripline impedance transformer, the authors have made a theoretical study of the inductive coupling between FFO and a stripline. It is shown that by choosing the output impedance of the FFO Z o smaller than the characteristic impedance of the coupling section Z 1 , the frequency dependence of the output power at the end of the coupling section can be extremely flat over a large frequency band. A model simulation employing a NbN/Pbin Josephson junction and a Nb/NbN stripline demonstrates that in the case of optimum design Z 0 =0.6 Z 1 , the bandwidth of the coupling section, which is defined by the frequency range where the deviation is within +or-5%, is shown to be as large as 80% of the centre frequency.

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Flux flow amplifiers and logic operating at millimeter wave frequencies

J. Martens, T. E. Zipperian, VINCENT M. HIETALA +4 more

Superconducting flux flow transistors (SFFTs) have recently been used in a W-band amplifier operating from 91 to 98 GHz and in an invertor configured as an oscillator operating at 86 GHz. These SFFTs consist of three weakened YBaCuO links in parallel. The links are about 1- mu m wide, 3- mu m long, and 20-nm thick (relative to an electrode thickness of about 200 nm). The links are biased above their local critical field so that vortices flow through the links in accordance with the Lorentz-type force supplied by the bias. The 'gate' is a control line located proximate to the link system and supplies a local magnetic field. Above a critical current, flux flow begins and the device enters the active region characterized by a transresistive response to control current and a finite output resistance. A flux flow invertor circuit can consist of 2 SFFTs: both devices are in parallel with the current bias and the input is a control current to one device.

IEEE Transactions on Electron Devices•Jan 1, 1992•0 citations
The authors have been developing an analog amplifier utilizing a flux flow phenomena in a Josephson line, i.e. the flux-flow-type Josephson amplifier (FFA). The FFA converts a signal current I/sub s/ to a voltage V/sub dc/ across the Josephson line, and the I/sub s/-V/sub dc/ characteristic determines the performance of the FFA, e.g. the gain and saturation level. Studies have been made to improve the I/sub s/-V/sub dc/ relation. It is shown that the transresistance can be improved significantly by reducing the width of the Josephson line and increasing the kinetic inductance of the line. The kinetic inductance can be increased by thinning the line electrode and by using an electrode with large London penetration depth, such as NbN. The transresistance obtained is as large as 6.8 Omega for NbN/Pb-alloy Josephson lines with a thickness of 5 nm and a width of 4 mu m. It is also shown that the linear operating range of the amplifier is as large as 1 mV.>
A linear amplifier using a flux-flow phenomenon in a long Josephson junction has been designed and tested by connecting a load resistance. The present experiment has demonstrated that a current gain of 500/W s (1/µm) is obtained for the load resistance of 9 mΩ when an input signal is applied with a control line of width W s (µm). An equivalent circuit of the amplifier has also been presented, which shows that the device is most suitable for current amplification with large gain in low impedance circuits.
In order to realize efficient coupling between a flux-flow type Josephson oscillator (FFO) and an external stripline, the authors carried out experiments to investigate the inductive coupling scheme in a strong coupling regime. By adopting NbN films with large penetration depths as electrodes for strong coupling, they were able to realize coupling sections with a coupling coefficient alpha =M/ square root L/sub 1/L/sub 2/ as large as 0.7, with M, L/sub 1/, and L/sub 2/ being mutual inductance, self-inductances of, the FFO, and the stripline, respectively. Using this design for the coupling, the authors succeeded in detecting AC fields at frequencies around 150 GHz, which were emitted from FFO to the stripline via the coupling section.
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.
In order to realize efficient coupling between a flux-flow type Josephson oscillator (FFO) and a stripline impedance transformer, the authors have made a theoretical study of the inductive coupling between FFO and a stripline. It is shown that by choosing the output impedance of the FFO Z o smaller than the characteristic impedance of the coupling section Z 1 , the frequency dependence of the output power at the end of the coupling section can be extremely flat over a large frequency band. A model simulation employing a NbN/Pbin Josephson junction and a Nb/NbN stripline demonstrates that in the case of optimum design Z 0 =0.6 Z 1 , the bandwidth of the coupling section, which is defined by the frequency range where the deviation is within +or-5%, is shown to be as large as 80% of the centre frequency.
Superconducting flux flow transistors (SFFTs) have recently been used in a W-band amplifier operating from 91 to 98 GHz and in an invertor configured as an oscillator operating at 86 GHz. These SFFTs consist of three weakened YBaCuO links in parallel. The links are about 1- mu m wide, 3- mu m long, and 20-nm thick (relative to an electrode thickness of about 200 nm). The links are biased above their local critical field so that vortices flow through the links in accordance with the Lorentz-type force supplied by the bias. The 'gate' is a control line located proximate to the link system and supplies a local magnetic field. Above a critical current, flux flow begins and the device enters the active region characterized by a transresistive response to control current and a finite output resistance. A flux flow invertor circuit can consist of 2 SFFTs: both devices are in parallel with the current bias and the input is a control current to one device.