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Superconductor Science and Technology

Arrays of Josephson junctions closed into a superconducting loop as a source of coherent radiation

Marian Darula, P. Seidel, F Busses, Stefan Benacka

Published May 1, 1994• 6 citations
Publisher

Abstract

We have studied via computer simulations the dynamical properties of a circuit consisting of two arrays of Josephson junctions closed into a superconducting loop. Due to an internal circuit resonance the phase locking of Josephson junctions is enhanced, which makes possible the application of such a circuit as a source of coherent microwave radiation. The optimal circuit parameters and the influence of thermal noise are discussed. It is shown that the critical spread in junction parameters is about twice that of linear Josephson junction arrays. The possibility of tuning such a device is demonstrated.

DOI
10.1088/0953-2048/7/5/023

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Journal of Applied Physics•Aug 15, 1993•21 citations
We have studied via computer simulations the dynamical properties of a circuit consisting of two arrays of Josephson junctions closed into a superconducting loop. Due to an internal circuit resonance the phase locking of Josephson junctions is enhanced, which makes possible the application of such a circuit as a source of coherent microwave radiation. The optimal circuit parameters and the influence of thermal noise are discussed. It is shown that the critical spread in junction parameters is about twice that of linear Josephson junction arrays. The possibility of tuning such a device is demonstrated.
Performance of various microwave receiving devices, using the Josephson junctions closed with superconducting loops and/or united in arrays, is discussed. These composite nonlinear elements enable us to eliminate problem of their matching with the signal source, to increase the saturation power and to come close to ultimate limits of the device noise performance. For internally-pumped devices, connection of several junctions in a closed-loop (ring) array turns out to be most effective, with coherent junction self-oscillations. For externally-pumped devices, the use of low-inductance (non-hysteretic) SQUIDs is of the greatest interest, enabling one to avoid the difference in junction dynamics due to parameter random spread.
Abstract When the impedance of a Josephson junction is too low, the elements are connected in series. We connected two arrays with N series elements of Josephson junctions each (denoted 2N‐JJ loop) and tried to find conditions for it to operate coherently by investigating voltage‐current curves. the computer simulation showed that coherent operation is easier with this circuit than with a series connection of the elements when the elements have different junction parameters. Pb‐alloy tunnel‐type junctions were produced which have N = 2 and 3, and we found that voltage steps generated by application of rf current with frequency F have a voltige interval corresponding to NF. This agrees with the simulation results and indicates coherent operation.
Two-dimensional arrays of Josephson junctions have been shown to operate in a state in which the microwave oscillations of the junctions are phase-coherent. However, the performance of an array is strongly dependent on both the details of the load-coupling circuit and the characteristic parameters of the junctions. We have measured two-dimensional arrays of resistively shunted Josephson tunnel junctions which are capacitively coupled to a nearby detector junction. Changing the critical current of the junctions in our arrays, while keeping the junction parasitics and array coupling roughly the same, results in an unexpected change in device operation. We have also investigated variations of the coupling circuit. Numerical simulations of the detection circuit enable us to estimate the power output from the array and to optimize the parameters of the system.
Mutual phase locking of Josephson oscillations in a two-junction cell and multijunction arrays is analysed. The locking is due to ac currents of Josephson frequency ω, which flow through some special coupling circuit. For the most important case of almost identical junctions, the locking frequency range is shown to be proportional to the imaginary part of the complex conductivity Y(ω) of the coupling circuit. The power and linewidth of coherent oscillations, as well as the coherence stability with respect to the junction intrinsic noise and parameter spread, have been calculated. The maximum locking range and, hence, the maximum parameter tolerances are shown to take place in the closed-loop-type (ring) structures with long-range junction interactions.
A high temperature superconducting YBa2Cu3Oy array of five Josephson junctions designed with additional coupling lines has been developed to demonstrate the effects of frequency locking and impedance matching for applications such as oscillators, mixers, and detectors. The Josephson self-radiation power was directly detected by a superheterodyne receiver, and Shapiro steps were also measured. The Josephson self-radiation properties reveal good quality of phase locking and microwave coupling with external circuits. The maximum self-radiation power of our array is about 50 pW which is several ten times higher than that of a single Josephson junction, and its peak point exactly satisfies the Josephson current-voltage relation. The Shapiro-step measurements show that the behavior of current-voltage curve depends on the effective inductance of coupling lines which affects the total impedance of Josephson junction array and microwave coupling. The Josephson oscillation frequency was obtained up to about 880 GHz which is 73% of the maximum available frequency calculated from the characteristic voltage of the Josephson junctions. Experimental results show that this type of Josephson junction array can improve the Josephson self-radiation power and increase the maximum detectable frequency.
The dynamic properties of a superconducting quantum interference device (SQUID) containing arrays of Josephson junctions in a superconducting loop, and in particular the case of a four-junction SQUID, is analyzed theoretically via computer simulations. It is shown that phase locking of Josephson junctions determines the dynamic behavior of the SQUID. In the case of a stable phase-lock state hysteretic I-V curves as well as unusual voltage-flux dependencies appear. The influence of a small spread in the Josephson junction parameters upon the stability of the phase-lock state is investigated in parameter space.