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IEEE All-Russian Microwave Conference (RMC)

Features of the Spectrum of a Niobium Josephson Junction Array Measured with the Use of High-Tc, Josephson Mixer

Mikhail A. Galin, A. I. El’kina, M. Yu. Levichev, Dmitry V. Masterov, Alexey E. Parafin, L. S. Revin

Published Nov 25, 2024• 0 citations
Publisher

Abstract

We performed spectral measurements of a large niobium Josephson junctions array. The array consists of 9996 serially connected Nb/NbSi/Nb junctions placed on a 1×1 cm silicon substrate. The spectrum was analyzed by a heterodyne superconducting receiver based on a high-TcJosephson mixer. It was discovered that the linewidth of Josephson radiation is strongly influenced by configuration and type of a measurement system. We obtain the linewidth down to 0.2 MHz when connecting the array to an autonomous power source and minimizing technical noise. We also observed the shift of the spectral frequency from the Josephson one that can be explained by peculiarities of synchronization of such large multi-oscillator systems.

DOI
10.1109/rmc62880.2024.10846890

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We performed spectral measurements of a large niobium Josephson junctions array. The array consists of 9996 serially connected Nb/NbSi/Nb junctions placed on a 1×1 cm silicon substrate. The spectrum was analyzed by a heterodyne superconducting receiver based on a high-TcJosephson mixer. It was discovered that the linewidth of Josephson radiation is strongly influenced by configuration and type of a measurement system. We obtain the linewidth down to 0.2 MHz when connecting the array to an autonomous power source and minimizing technical noise. We also observed the shift of the spectral frequency from the Josephson one that can be explained by peculiarities of synchronization of such large multi-oscillator systems.
Spectral measurements of a niobium Josephson junction arrays with overwavelength dimensions have been performed using a heterodyne receiver with a mixer based on a high-Tc bicrystal transition. The array is fabricated on a silicon substrate and is composed from 9996 series connected Nb/NbSi/Nb junctions embedded to parallel single-strip lines with the length 7.1 mm. The mixer in the receiver is a Josephson junction formed by a YBaCuO film on a bicrystal boundary of Zr1−xYxO2 substrate. Spectral measurements have been performed in the frequency ranges 150-180 GHz and 225-250 GHz. The aim of the work was the investigation of the influence to the spectral line from some factors of external electrical circuit: the type of power supply and the level of technical noises. Significant decrease of the linewidth from ~ 10 MHz to 0.5-0.8 MHz has been found at the connection of the array to voltage source instead of current source and at the minimizing of technical noises. The shift of spectral frequency from the Josephson one averaged over the array has been observed that is explained by the peculiarities of synchronization of Josephson junctions in large arrays.
We performed the spectral measurements of a niobium based Josephson junction array in order to estimate the linewidth of Josephson emission. The array is formed by 9996 serially connected Nb/NbSi/Nb Josephson junctions occupying the area of 5×7 mm2on a silicon substrate and divided into 7 distinct subarrays. The array was immersed in a liquid helium dewar from where the emission was brought to open space via an oversize waveguide. Firstly, the measurements on a Fourier-transform spectrometer were carried out in a wide frequency range 139–343 GHz at almost all self-induced steps of current-voltage curve. As in our previous work, the observed linewidth of Josephson emission corresponded to the resolution of the spectrometer. Then, we carried out more precise and sensitive measurements using the 211–275 GHz heterodyne receiver based on a Nb/AlOx/Nb mixer with a spectral resolution better than 0.1 MHz. Dependencies of the linewidth on the step number and on the number of connected subarrays are studied. The peaks corresponding to the 2ndharmonic of the Josephson generation are also observed in the spectra. The linewidth of the main harmonic down to 1.5 MHz was observed in these measurements.
We report heterodyne measurements of Josephson microwave radiation emitted by a parallel array of small superconductor-insulator-superconductor (SIS) junctions at submillimeter-wave frequencies. The array consists of five Nb/Al–AlOx/Nb junctions nonevenly distributed in a niobium superconducting stripline, and is optimized for rf coupling in the 450–640 GHz range. We observed Fiske-like resonant steps in its I-V curve in the presence of magnetic field. The device was placed in a waveguide mount, and its radiation was quasioptically coupled out of the cryostat, to a SIS-mixer spectrometer in the same frequency range, with a 4–8 GHz band for spectral analysis. We detected a coherent signal in the spectra when the array was biased on the first and third steps, respectively, at the first harmonic frequency of 242 GHz and at the fundamental frequency of 493 GHz, both being the Josephson frequencies associated with their dc voltages. This strongly suggests that this type of parallel arrays optimized for wideband rf coupling, though strongly discretized, hosts dynamic fluxon regimes similar to long Josephson junctions, which could find applications in superconducting digital electronics or integrated heterodyne submillimeter-wave receivers.
The power and linewidth of the radiation, near 250 GHz, of small distributed Josephson junction array sources have been measured using a single-chip integrated source and receiver. The intermediate frequency (IF), coupled off the chip to a room-temperature spectrum analyzer, permitted a direct measurement of the oscillator linewidth. The one-dimensional arrays were of a novel design with junctions placed in 1/4 wavelength lumps separated by one half wavelength. The measured power ( approximately=2 mu W) and linewidth ( approximately=10 MHz) for the 10 junction sources were in reasonable agreement with those obtained from computer simulations.>
We performed experimental investigations of the interaction between separate discrete niobium Josephson junction (JJ) arrays. It was found out that after tuning the arrays to the equal Josephson frequency the amplification of power is registered. The arrays are straight strip lines with 332 JJs oriented in parallel and located on common silicon substrate as well as on different substrates configured in a stack. The effect of amplification is observed at frequencies of up to 204 GHz while the gain relatively to the sum of power from alone radiated arrays can be as large as 3. These results indicate the mutual synchronization of niobium JJ arrays that would forces the submillimeter generation technique based on large JJ arrays with superwavelength dimensions.
The Josephson frequency conversion in the millimeter and submillimeter wave region is studied with superconducting Nb-Nb point contact junctions at 4.2 K, using the contact both as the local oscillator and the mixer with intermediate frequencies of 0.1 and 9.2 GHz. Frequency conversion signals are only detected in the latter case with a microwave receiver at 9.2 GHz in a range of Josephson frequencies between 145 and 1 600 GHz. The experiments with intermediate frequency fi of 0.1 GHz indicate for this small value of f i that the observed signals are not related to frequency conversion but noise effects. An analysis of the frequency conversion mechanism in a Josephson junction operated as an oscillator-mixer is proposed which includes the noise effects and also the feedback effects induced in a junction driven by a constant current source. We have then investigated the noise properties up to 450 GHz of superconducting point-contacts using the junction as an oscillator-mixer where the Josephson current is mixed with harmonics of an externally applied signal. The emitted intermediate frequency is detected at 9.2 GHz and its linewidth is measured. In the very high frequency range the dc quasiparticle current dominates in a Josephson weak-link and we compare the corresponding theoretical linewidth calculated by Scalapino with our experimental results.