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Applied Physics Letters

Experiments on the interaction between long Josephson junctions and a coplanar strip resonator

A. Davidson, N. F. Pedersen

Published Apr 20, 1992• 7 citations
Publisher

Abstract

Experiments are reported on a new geometry designed to couple long Josephson junction fluxon oscillators to a resonant cavity. The junctions were made with a niobium-aluminum oxide-niobium trilayer process with a critical-current density of around 1000 A/cm2. Various numbers of such junctions were incorporated directly as part of a coplanar strip half-wave resonator, with fundamental mode of about 34 GHz. Both the current density and oscillation frequency are higher than in previous experiments. Evidence for phase locking of multiple junctions is presented.

DOI
10.1063/1.107129

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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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We have designed and tested a coplanar waveguide configuration which allows for coupling and detection of microwave radiation to planar and stacked Josephson junctions structures. The design is particularly versatile for experiments such as phase-locking of Josephson oscillations to external signals and detection of low frequency components in the waveform of a pumped junction. Using this design we have measured subharmonic phase-locking in long Josephson junctions and performed a first quantitative check of the existing theoretical models.

IEEE Transactions on Applied Superconductivity•Jun 1, 1999•1 citation
Experiments are reported on a new geometry designed to couple long Josephson junction fluxon oscillators to a resonant cavity. The junctions were made with a niobium-aluminum oxide-niobium trilayer process with a critical-current density of around 1000 A/cm2. Various numbers of such junctions were incorporated directly as part of a coplanar strip half-wave resonator, with fundamental mode of about 34 GHz. Both the current density and oscillation frequency are higher than in previous experiments. Evidence for phase locking of multiple junctions is presented.
Experiments on Josephson soliton oscillators phase locked to cavity modes of a coplanar waveguide resonator are discussed. The junctions were made using a trilayer Nb/AlO/sub x//Nb structure with a current density of about 1000 A/cm/sup 2/. Strong coupling between the junctions and the cavity was observed at 34 and 100 GHz. Subharmonic structure on the current-voltage characteristics indicate weaker coupling up to 300 GHz. These results are important for designing integrated soliton local oscillators for superconductor-insulator-superconductor (SIS) mixers in the millimeter band.
Josephson soliton oscillators integrated in a resonator consisting of two closely spaced coplanar superconducting microstrips have been investigated experimentally. Pairs of long 1-D Josephson junctions with a current density of about 1000 A/cm/sup 2/ were made using the Nb-AlO/sub x/-Nb trilayer technique. Different modes of half-wave resonances in the thin-film structure impose different magnetic field configurations at the boundaries of the junctions. The DC I-V characteristic shows zero-field steps with a number of resonator-induced steps. These structures are compared to RF-induced steps generated by phase-locking of the soliton motion to an external microwave source. The two different resonant configurations identified may be interpreted as a coupling to the microwave field at the end boundary and as a more homogeneous coupling distributed over the junction length.>
Arrays of phase locked Josephson junctions are attractive for use as sub-mm wavelength voltage controlled oscillators for heterodyne mixers and free space radiators. We have performed phase locking experiments on 40 junction, parallel biased arrays of Josephson junctions coupled through transmission lines of varying delays. The longest junction to junction delay implemented was 5.4 ps for a design frequency of 90 GHz, while the shortest was 1.45 ps, for a design frequency of 345 GHz. The resonant frequency f/sub 0/ of the arrays was found to be almost independent of the transmission line delay with f/sub 0/, scaling with the plasma frequency for arrays fabricated at critical current densities of 10/sup 3/ and 4.5/spl times/10/sup 3/A/cm/sup 2/. The data indicate that at the resonant frequency, strong phase locking is present among the junctions for junction separations significantly less than one half wavelength even in arrays many wavelengths long.>
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.>
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.
We have designed and tested a coplanar waveguide configuration which allows for coupling and detection of microwave radiation to planar and stacked Josephson junctions structures. The design is particularly versatile for experiments such as phase-locking of Josephson oscillations to external signals and detection of low frequency components in the waveform of a pumped junction. Using this design we have measured subharmonic phase-locking in long Josephson junctions and performed a first quantitative check of the existing theoretical models.