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Journal of Applied Physics

Tunneling Cryotron Flip-Flop

Juri Matisoo

Published May 1, 1968• 5 citations
Publisher

Abstract

We have constructed tunneling cryotron flip-flops, have observed current transfer from one leg to the other, and have measured the time required for current transfer to be 2 nsec. The results are in agreement with the model previously proposed.1 The tunneling cryotron flip-flop consists of two devices in parallel, linked by a wholly superconducting path. A current is fed to the parallel devices; the current is steered from one device to the other by application of a control pulse. The measurements have been made on a loop of 1.16 in. in length (L=1.08×10−10H), containing two 10 mil diam junctions, having zero-field critical currents of 21 mA and 29 mA, respectively (in operation these are biased to near maxima on the gain curves, at 33 mA and 42 mA). The measuring apparatus is similar to that previously described.1 In operation, a current of 20 mA is transferred from one device to the other by the application of a 2 mA control pulse, a gain of 10. The time required for this current to transfer, Δttransfer, is 1.4 nesc≤Δttransfer≤2.0 nsec, with an uncertainty of at most 0.4 nsec. According to the previously proposed model of a constant gap voltage driving the loop inductance and which does not include the junction transition times (Ref. 1), the transfer time for 20 mA of current should be 2 nsec. One implication of this excellent agreement is that the junction transition time is appreciably smaller than the previously measured upper bound of 800 psec.2 Additional operating details will be discussed.

DOI
10.1063/1.1656623

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Journal of Applied Physics•May 1, 1968•1 citation
We have constructed tunneling cryotron flip-flops, have observed current transfer from one leg to the other, and have measured the time required for current transfer to be 2 nsec. The results are in agreement with the model previously proposed.1 The tunneling cryotron flip-flop consists of two devices in parallel, linked by a wholly superconducting path. A current is fed to the parallel devices; the current is steered from one device to the other by application of a control pulse. The measurements have been made on a loop of 1.16 in. in length (L=1.08×10−10H), containing two 10 mil diam junctions, having zero-field critical currents of 21 mA and 29 mA, respectively (in operation these are biased to near maxima on the gain curves, at 33 mA and 42 mA). The measuring apparatus is similar to that previously described.1 In operation, a current of 20 mA is transferred from one device to the other by the application of a 2 mA control pulse, a gain of 10. The time required for this current to transfer, Δttransfer, is 1.4 nesc≤Δttransfer≤2.0 nsec, with an uncertainty of at most 0.4 nsec. According to the previously proposed model of a constant gap voltage driving the loop inductance and which does not include the junction transition times (Ref. 1), the transfer time for 20 mA of current should be 2 nsec. One implication of this excellent agreement is that the junction transition time is appreciably smaller than the previously measured upper bound of 800 psec.2 Additional operating details will be discussed.
Transitions from the zero-voltage to the voltage state have been directly measured on large (≃104 μm2) Josephson junctions. By adjusting the current load, the switching times (10–90%) were varied from 60 to 400 psec. Good agreement was found between theoretical and experimental results. The times required to transfer current from junctions into superconductive inductances (10−11−10−10 H) were also investigated. The switching times ranged from 300 to 550 psec and agreed basically with our simple theoretical model. To explain the details of the voltage developed during current transfer, self-pumped junction resonances had to be invoked. Although these times are expected to be smaller in miniaturized devices, they represent to our knowledge the shortest times ever reported for circuits with a dissipation of less than 10−5 W in continuous operation.
Low-temperature superconducting circuits have become important for many scientific applications. However, there are presently no high current-capacity switches (~1 mA) with low power dissipation for sub-Kelvin operation. One candidate for a sub-Kelvin switch is the cryotron, a device in which the superconductivity of a wire is suppressed with a magnetic field. Here, we demonstrate a cryotron switch suitable for sub-Kelvin temperatures. In the closed state, the maximum device current is about 900 μA. The device is switched to its open state with 2 mA of control current and has a leakage of approximately 500 nA. The transition between the closed and open states of the device is faster than 200 ns, where the measurement is limited by the speed of our measurement apparatus. We also discuss low-temperature applications for our cryotron such as a single-pole, double-throw switch.
Experimental results are presented showing that two superconducting indium microbridges coupled via a resistor and a capacitor can exhibit bistable states under a constant current bias, with one bridge at a finite voltage while the other is at zero voltage. The voltage states of the two microbridges can be interchanged by applying an external trigger current to the circuit. Current–voltage characteristics are compared with analog simulation as well as numerical calculation and found in good agreement. The effect of bias current, coupling resistor, capacitor, bridge resistance, and critical current values on the circuit properties are investigated. Our estimate for the flip-flop switching time is in the neighborhood of 0.1 nsec.
Using of traditional logic elements in digital electronics has ensured the development of a wide range of electronic devices for many years. In parallel, research and development of logical elements are carried out on the basis of other non-traditional physical phenomena or effects, in particular logic elements based on the phenomenon of superconductivity, the prospect of which is the use of very small energy consumption and ultrahigh performance. For superconducting logic elements, using various Josephson cryoelectronic structures, known as Josephson cryotrons, based on the stationary and non-stationary effects of Josephson. On the basis of the Josephson cryotrons, one can create Josephson elements of computer memory, and Josephson elements of digital logic. The main requirements for the Josephson cryotrons are a stable operating mode and high speed or short switching time. Information about the mode of operation and the speed give us the transition characteristics of the cryotrons - the time dependence of the voltage on the cryotron while changing its logical state, which can be obtained either experimentally, or theoretically. This work aims at a search for new ways of increasing the switching rate of digital logical operators by employing physical structures other than the traditional semiconductor-based schemes. We propose the principles of designing digital logical operators based on Josephson cryotrons, whose operation utilizes the stationary and dynamical Josephson effects, and describe the operational principles of the logical elements “AND” and “OR” bases on tunneling Josephson junctions “superconductor-insulator-superconductor”. Our proposed mathematical models for the commutation processes in such logical elements allowed us to calculate their transition characteristics during the switching and to determine the main parameters in such models. It was shown that the logical elements “AND” and “OR” can be implemented on individual cryotrons and that their logical state can be controlled by input signals in a form of current pulses. Such logical elements meet all the requirements for digital logical elements and have switching time of about 2-3 ps, which indicates their significantly increased switching rate.
We theoretically study the behavior of the critical current of a thermally biased tunnel Josephson junction with a particular design, in which the electrodes of the junction are enclosed in two different superconducting loops pierced by independent magnetic fluxes. In this setup, the superconducting gaps can be modified independently through the magnetic fluxes threading the loops. We investigate the response of the device as a function of the magnetic fluxes, by changing the asymmetry parameter, i.e., the ratio between the zero-temperature superconducting gaps $\ensuremath{\delta}={\mathrm{\ensuremath{\Delta}}}_{10}/{\mathrm{\ensuremath{\Delta}}}_{20}$ and the temperatures of the two rings. We show a magnetically controllable steplike response of the critical current, which emerges even in a symmetric junction, $\ensuremath{\delta}=1$. Finally, we discuss the optimal working conditions and the high response of the critical current to small changes in the magnetic flux, reporting good performance of a magnetic flux-to-critical current transducer, with a high transfer function that depends on the operating point, the temperature gradient, and the quality of the junction.
Two types of superconducting memory cells are now being considered: the continuous-film memory and cryotron-switched storage loops. The former suffer from low cell operating range compared to cell-to-cell variation. The cryotron cells, although having a wider operating range, must be switched very rapidly to have a detectable output signal. A recent development in cryogenic sensing called ``Current Stretch'' has allowed more freedom in the design of superconducting memory cells. With current stretching the information is transformed to a persistent input current in a cryotron amplifier. The signal produced when the cell is switched is then proportional to the magnetic flux change rather than the rate of change of flux. This feature eliminates the usual memory requirements of fast-access current risetime, pickup noise cancellation, fast cell switching, and the need for a separate sense line with cryotron-accessed memories. A new class of cell which takes advantage of this technique will be described. It consists of a cryotron shunted across a loop, one side of which has a lower inductance and critical current than the other. If the cryotron is activated while current is injected into the cell, a persistent current is created in the loop. Cell-flux changes are detected through one of the cryotron access trees using current stretching. The operating tolerance of these cells is only limited by their geometry. They have no metal-to-metal contacts, use only three metal layers including the ground plane, need no sense line, no noise cancellation, and can be switched arbitrarily slowly. Cells can be fabricated as small as 10 mil2 and operating ranges as large as 40% have been observed. The cell speed is on the order of 50 nsec with a single crossover access cryotron.