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Technical Physics

Tunnel josephson junctions as highly sensitive comparators in stroboscopic converters

I. N. Askerzade

Published Jan 1, 2000• 10 citations
Publisher

Abstract

The dynamics of switching to the resistive state of several types of comparators based on tunnel Josephson junctions is analyzed by the method of numerical modeling. The expediency of utilizing the scheme of the balance comparator in the mode of single-quantum pulses for attaining higher performance as the hysteresis of the voltage-current characteristics (VCCs) decreases is demonstrated.

DOI
10.1134/1.1259571

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IEEE Transactions on Magnetics•Mar 1, 1991•40 citations
The dynamics of switching to the resistive state of several types of comparators based on tunnel Josephson junctions is analyzed by the method of numerical modeling. The expediency of utilizing the scheme of the balance comparator in the mode of single-quantum pulses for attaining higher performance as the hysteresis of the voltage-current characteristics (VCCs) decreases is demonstrated.
A Josephson junction modeled by the phenomenological current relation together with internal resistance can exhibit pulses in the voltage across the junction when driven by an oscillating current source. These pulses occur singly, in pairs, triplets, etc. with a repetition rate equal to twice the driving frequency. The inclusion of capacitance and inductance generally degrades the pulse characteristics, but attainable values for tunnel junctions are tolerable, with typical parameter values, the pulses have picosecond widths. The phenomenological model becomes suspect on such time scales, however, because typical gap frequencies are just under 1012Hz. Preliminary studies based on the microscopic tunnel-junction theory nevertheless show results qualitatively similar to those of the Phenomenological model. The effects of resistor noise and current noise have also been studied. It is concluded that well developed ps voltage pulses can be created in physical Josephson junctions.
The switching dynamics of a double-barrier Josephson junction is analyzed as a function of the microscopic properties of its electrodes. In particular, it is found that the nonstationary behavior of the Josephson phase difference is very sensitive to dissipation mechanisms acting inside the intrinsic shunt. The leading factor that determines the dissipation is the local electron density of states N(E) inside the electrodes. The roles of junction geometry, electrode purity, and interface quality are discussed and how they affect the details of N(E), hence the resulting phase dynamics. The microscopic analyses allow optimization of the performance of double-barrier Josephson junction-based rapid-single-flux-quantum circuits in two ways: 1) decreasing the switching time of Josephson elements and 2) reducing the excess wiring. Such an analysis is facilitated with the aid of a lumped circuit representation which generalizes the nonlinear resistive-shunted-junction model.
We have measured the current-voltage $(I\ensuremath{-}V)$ characteristics of small-capacitance single Josephson junctions at low temperatures (T=0.02−−0.6K),(T=0.02--0.6\mathrm{K}),(T=0.02−−0.6K), where the strength of the coupling between the single junction and the electromagnetic environment was controlled with one-dimensional arrays of dc superconducting quantum interence devices (SQUIDs). The single-junction $I\ensuremath{-}V$ curve is sensitive to the impedance of the environment, which can be tuned in situ. We have observed Coulomb blockade of Cooper-pair tunneling and even a region of negative differential resistance, where the zero-bias resistance ${R}_{0}^{\ensuremath{'}}$ of the SQUID arrays is much higher than the quantum resistance ${R}_{K}\ensuremath{\equiv}{h/e}^{2}\ensuremath{\approx}26\mathrm{k}\ensuremath{\Omega}.$ The negative differential resistance is evidence of the coherent single-Cooper-pair tunneling within the theory of current-biased single Josephson junctions. Based on this theory, we have calculated the $I\ensuremath{-}V$ curves numerically in order to compare then with the experimental ones at ${R}_{0}^{\ensuremath{'}}\ensuremath{\gg}{R}_{K}.$ The numerical calculation agrees with the experiments qualitatively. We also discuss the ${R}_{0}^{\ensuremath{'}}$ dependence of the single-Josephson-junction $I\ensuremath{-}V$ curve in terms of the superconductor-insulator transition driven by changing the coupling to the environment.
The use of high-T/sub c/ Josephson junctions for digital applications requires a careful study of the influence of thermal noise on circuit performance. We investigated a balanced current comparator, which contains a basic component of all RSFQ circuits: two overdamped Josephson junctions connected in series. The dependence of the dc-voltage across the Josephson junctions on signal current was measured to determine the threshold uncertainty of switching of the comparator. The experimental data obtained at different sampling pulse frequencies, in a temperature range from 10 to 50 K, were compared with a theoretical model, taking into account the influence of thermal noise as well as of sampling pulse frequency.
The process of reproduction of single flux quantum pulses in a Josephson junction chain composed of a pulse generator, a balanced comparator, and a single Josephson junction is investigated. It is shown that the reproduction of single flux quantum pulses in such a chain is more stable and the mean and the standard deviation of switching time are minimal for the McCumber-Stewart parameter value from 0.5 to 1 and for a normalized clock generator current around 1.5. The influence of input and output inductances on the characteristics of a balanced comparator is investigated. The recommended value of the output inductance leading to smooth pulses and relatively small jitter is around unity.
The sensitivity of a balanced comparator composed of two overdamped Josephson junctions fed by single flux pulses is calculated, taking into account both thermal and quantum fluctuations. The results of the analysis are compared with those of recent experiments. Ultimate resolution of the balanced comparator with feasible junction parameters is estimated to be as high as approximately 50 pA/Hz/sup 1/2/ at 4 K and approximately 10 pA/Hz/sup 1/2/ at T to 0 K. Performance limits of the device are compared with those of its single-junction counterpart.