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Journal of Applied Physics

A superconducting flip-flop using two interacting weak links

Lide Zhang, Dai Yuan-Dong, Y. H. Kao

Published Aug 1, 1983• 4 citations
Publisher

Abstract

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.

DOI
10.1063/1.332648

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Applied Physics Letters•Aug 26, 1996•1 citation
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.
Superconducting weak link (WL), acting as a Josephson junction (JJ), is one of the widely used elements in superconductor science and quantum circuits. A hysteretic JJ with robust switching between its superconducting and resistive state is an excellent candidate for single-photon detection. However, the ubiquitous fluctuations in the junction strongly influence the stability of the states and, thus, the transition from one to the other. Here, we present an experimental study of switching statistics of critical and retrapping currents of a JJ based on niobium WL in its hysteretic regime. The mean lifetimes of the two metastable states, namely, the zero-voltage superconducting state and finite-voltage resistive state, are estimated from the distributions. Further, close to the hysteresis crossover temperature, observed telegraphic noise in the time domain due to random switching between the states provides their lifetimes directly. We present a thermal model introducing a double-well (bistable) feature with an intriguing quantity with respect to the devices’ temperature states. The effects of temperature fluctuations on the stability of the states are shown. We discuss our results toward further improvement of the efficiency of superconducting WL or nanowire single-photon detectors.
A novel structure of high-speed Josephson logic circuits is proposed. Josephson logic gates have latching characteristics and can hold data as long as bias currents are supplied. Through effective use of these latching characteristics, logic circuits can be constructed with wide operating margins. Dual power supplies, properly phased, separately drive logic circuits divided into two groups. Logic signals are transferred from one logic group to the other or vice versa, and one group is reset into a zero voltage state when the other group is active for logic operation. For combinational circuits, the basic configuration of an astable flip-flop and a delay circuit are presented to prevent the logic circuit from `racing'. As an example of sequential circuits, a bistable flip-flop to store data is constructed without any superconducting loop.
Experimental investigations on the proximity effect bridge (a Josephson device) at zero voltage and at finite voltages in the µV range are reported. The phase-super current relation at zero voltage was measured using an asymmetric superconducting quantum interferormeter circuit. The data are in agreement with the Josephson supercurrent-phase relation I_S=I_C sinδ with deviation less than 5% of the critical current I_c. The supercurrent density in the measured bridges reached as high as 50-100 µA/µm^2. Using microcircuitry techniques, proximity effect bridges were strongly coupled to superconducting microstrip resonators. Selfinduced steps in the I-V characteristics of bridges coupled to resonators were observed in the GHz region at voltages (frequencies) corresponding to the expected modes of the resonators. Two types of steps were seen depending on whether the resonator impedance on resonance was much higher or much smaller than the bridge resistance. A simple two fluid model of the bridge-resonator circuit was developed and the size and shape of self-induced steps were calculated for a generalized Josephson oscillator relation I_S = I_c(l-q + q sin∫ 2e/hV dt) where q = 1 corresponds to the original Josephson relation and q = 1/2 represents the phase slip regime. At low critical currents (I_c < 10 µA) and low voltages (V < 3µV) the size and shape of experimentally observed self-induced steps agree with the q = 1 model. At higher voltages and/or critical currents the step size increasingly deviates from the q = 1 model towards q = 1/2. These observations are interpreted to indicate a progressive reduction of the amplitude of the oscillating Josephson supercurrent in proximity effect bridges from I_c towards I_c /2 as the critical current and/or voltage are increased.
We report here numerical calculations on the properties of superconducting weak links as mixers with external local oscillators (LO). Our calculations are based on the resistively shunted-junction model. We assume that the LO and the signal are ideal current sources of amplitudes ILO and IS and of frequencies ωLO and ωs, respectively. We calculate the IF voltage across the weak link as a function of the bias current, the LO current amplitude ILO, and the normalized signal frequency Ωs = ħωs/(2 e Ic R), where Ic is the critical current and R is the shunt resistance. If ωIF/ωLO and Is/Ic are small, we find a linear IF response. Using this method we find the frequency dependence of the IF response, estimate the conversion efficiency for the matched case in both the absence and the presence of noise, and calculate the required LO power. We have fabricated thin-film constrictions by using electron lithography. We have made constrictions of indium films with thicknesses of ~ 1 200 A with widths as narrow as 2 000 A and lengths as short as 1 000 A and have studied their I-V characteristics in the presence of RF radiation and measured their X-band mixing properties. We have measured an NEP of 10-13 W/Hz1/2 and a dynamic range of 23 dB (3 dB deviation from linearity) and estimate that, with optimum matching, these would become 3 × 10-18 W/Hz1/2 and 74 dB, respectively.
Voltage locking, defined as the production of an identical non-zero DC voltage in the absence of external microwave radiation across each Josephson junction of an array, has been observed in two microbridge Josephson junctions separated by a 2μm wide strip of superconductor. Voltage locking occurs when the bridges are biased with the current flowing in opposing directions thru the bridges and out the strip connecting them. The voltage across each bridge can be pulled over 1μV until each bridge displays an identical non-zero voltage, with the total voltage across both bridges equal to zero. Full locking, as defined above, is observed in excess of 40μV. The voltage and temperature dependence of the locking is described.
We present an analysis and numerical study of a dc superconducting quantum interference device (SQUID) with a damped loop inductance. We find a previously unreported regime of voltage-flux bistability due to the damping resistor when the SQUID is biased below the zero-field critical current. We determine the region where this phenomenon can be found in the parameter space defined by the damping resistance and the inductance parameter. We show that switching between the bistable states can be controlled by external flux pulses and by adding thermal noise to the model, we have determined the maximum noise level at which the bistability can be seen.