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IEEE Transactions on Applied Superconductivity

Lumped arrays of shunted bicrystal Josephson junctions

Alexander M. Klushin, K. Ilin, M. Siegel, M. Schubert, G. Wende, H.-G. Mayer

Published Jun 1, 2003• 5 citations
Publisher

Abstract

We report on the design, fabrication, and investigation of lumped arrays of Josephson junctions embedded into coplanar strip-lines (CPS). The arrays were incorporated into the CPS by meandering a bilayer across the grain boundary. Limitations on junctions packing density are discussed. The dependence of the length of the lumped array on spread of critical currents of junctions in the array was found. The investigated arrays with 431 junctions have shown frequency locking with an external irradiation. Voltage steps up to 20 mV at liquid nitrogen temperature were demonstrated.

DOI
10.1109/tasc.2003.813960

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Superconductor Science and Technology•Nov 1, 1999•2 citations
We report on the design, fabrication, and investigation of lumped arrays of Josephson junctions embedded into coplanar strip-lines (CPS). The arrays were incorporated into the CPS by meandering a bilayer across the grain boundary. Limitations on junctions packing density are discussed. The dependence of the length of the lumped array on spread of critical currents of junctions in the array was found. The investigated arrays with 431 junctions have shown frequency locking with an external irradiation. Voltage steps up to 20 mV at liquid nitrogen temperature were demonstrated.
We report on the design, fabrication, and investigation of lumped arrays of, Josephson junctions embedded into coplanar strip-lines (CPS). The arrays were incorporated into the CPS by meandering a bilayer across the grain boundary. Limitations on junctions packing density are discussed. The dependence of the length of the lumped array on spread of critical currents of Junctions in the array was found. The investigated arrays with 431 junctions have shown frequency locking with an external irradiation. Voltage steps up to 20 mV at liquid nitrogen temperature were demonstrated.
Superconductor/normal conductor/superconductor (SNS) Josephson junction series arrays are the subject of research for the realization of a Josephson arbitrary waveform synthesizer (JAWS) based on pulse-driven Josephson junctions. Lumped arrays of SNS Josephson junctions are investigated for this application at the Physikalisch-Technische Bundesanstalt (PTB). The series arrays consisting of Nb/HfTi/Nb SNS junctions are embedded in the center line of a coplanar waveguide transmission line. To increase the ac output voltage, the Josephson junction series array is designed as a meander-like structure. A broadband response of the arrays is required for the complex pulse operation of the JAWS. The arrays are investigated by dc measurements and for test purposes under continuous microwave irradiation at different frequencies. Flat Shapiro steps were measured for meander-like series arrays with 21 parallel lines containing up to 10 752 junctions. A sine wave with a frequency of 2.5 kHz and a peak voltage of 214 mV Peak was synthesized under pulse mode operation.
We have applied our existing focused ion beam based nanoscale planar SNS bridge junction technology to the fabrication of series arrays, with a view to lumped array applications. Single junctions are created in a micrometre-width bilayer track (125 nm Nb on 75 nm Cu) by milling a trench 50 nm wide in the upper superconducting layer. Individual junctions with non-hysteretic, resistively-shunted current-voltage characteristics, critical current I C ~1 mA and characteristic voltage I C R N ~50 µV at 4.2 K can be fabricated routinely. The characteristics of 10-junction series arrays at junction spacings of 0.2 µm to 1.6 µm have been studied at 4.2 K and above. Locking of all the junctions in the array under an applied microwave field is observed as the transition temperature is approached. This effect is achieved at a lower temperature for shorter junction spacings, suggesting a penetration depth-dependent electromagnetic coupling mechanism. Measurements of differential resistance versus current reveal the I C distribution of individual junctions within the array. Spreads of critical current δ I C = I Cmax / I Cmin ~1.5 at 4.2 K are typically observed. The spread in normal state resistance, R N between junctions is negligible in comparison, as the unbroken normal metal layer shunts all of the junctions in the array. This allows locking to be achieved in spite of the appreciable spread in I C .
New microwave circuits for voltage standard application containing 256 shunted bicrystal junctions were designed and investigated. The series array follows a meander line, which prevents the uniform distribution of microwave current along the array. To overcome this drawback, we are placing the meander parallel to a coplanar waveguide line. This provides parallel feeding of microwave power to series connected junctions. A circuit divided in 8 sub-arrays represents a seven-bit digital-to-analog converter. At a frequency f=27332 MHz and for a constant power each of the segments of the array demonstrated the first current step at 78 K.
The application of high temperature superconductor Josephson junctions to voltage metrology is promising for many reasons, beyond the evident advantage of simplified cryogenics and reduced costs of the apparatus. Owing to their intrinsic non-hysteretic behavior, arrays of shunted bicrystal YBCO junctions are particularly interesting for the realization of an AC voltage standard with quantum accuracy. Moreover, shunted bicrystal YBCO junctions arrays are advantageous because of the reduced area of the junctions, the wide range of characteristic voltages, and large critical currents. However, some specific problems arise, like possible effects related to the higher operating temperature. Furthermore, fabrication technology sets tight constraints on the design of structures for the distribution of microwave currents along the array junctions. We investigated the step properties under millimeter wave irradiation at frequencies about 75 GHz of HTS junction and arrays prepared on bicrystal substrates having grain boundaries. High sensitivity techniques were adopted to evaluate the step flatness at nV level and assess the accuracy of the step voltage. A new method that makes use of an open resonator, was implemented to irradiate the array, providing synchronization of a large number of junctions.
A digital-to-analogue converter based on series arrays of high-temperature superconducting Josephson junctions can work properly when the microwave bias current distribution in large arrays is uniform. Unfortunately, the considerable microwave attenuation per overdamped junction with normal resistances RN0.1 results in a strong total attenuation of the microwave signal in a traditional layout. Also, the meander geometry of the array, necessary when using bicrystal junctions, disturbs the uniformity of the rf current, especially in the millimetre waveband. To overcome these drawbacks we coupled the meander array inductively to a parallel transmission line. The mechanism of microwave coupling in which both external and mutual frequency locking takes place was studied numerically. A circuit with a coplanar waveguide feed line was fabricated and successfully tested at an operating frequency of 30 GHz. A microstrip line with a small attenuation for pumping of junction arrays at frequencies up to 120 GHz was demonstrated.