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Journal of Applied Physics

Switching dynamics of ac superconducting quantum interference devices and determination of rest currents

Yoshimasa Murayama, A. Asano, Nobuo Kotera

Published Sep 1, 1982• 1 citation
Publisher

Abstract

Switching dynamics have been considered for a simple closed loop composed of a Josephson junction and inductors (an ac superconducting quantum interference device). The rest current which remains in the junction after self-resetting was calculated by either (1) assuming that the junction closes instantaneously after its current is reduced to below Imin, or (2) by means of simulation for t→∞. It is shown from a free energy viewpoint that the introduction of flux quanta is favorable. However, the final state is not the absolute minimum energy, but a metastable state.

DOI
10.1063/1.331523

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Switching dynamics have been considered for a simple closed loop composed of a Josephson junction and inductors (an ac superconducting quantum interference device). The rest current which remains in the junction after self-resetting was calculated by either (1) assuming that the junction closes instantaneously after its current is reduced to below Imin, or (2) by means of simulation for t→∞. It is shown from a free energy viewpoint that the introduction of flux quanta is favorable. However, the final state is not the absolute minimum energy, but a metastable state.
The author numerically demonstrates that a resistive superconducting quantum interference device (RSQUID) with two nonhysteretic Josephson junctions works as a relaxation oscillator. Sequential switching of the Josephson junctions transfers positive and negative flux quanta in the RSQUID loop one by one. Differently from a conventional two-junction superconducting quantum interference device, the dissipative RSQUID loop does not maintain the quantized flux, and hence, finite flux can be accumulated in the RSQUID loop during sequential switching of the Josephson junctions. When the accumulated flux reaches a critical value, the corresponding loop current prevents subsequent junction switching. That is, the oscillation stops. After the loop current decays, sequential switching of the junctions resumes. In addition to the waveforms of relaxation oscillation, dependence of relaxation oscillation on device parameters is presented.
We experimentally investigate the stochastic phase dynamics of planar Josephson junctions (JJs) and superconducting quantum interference devices (SQUIDs) defined in epitaxial InAs/Al heterostructures, and characterized by a large ratio of Josephson energy to charging energy. We observe a crossover from a regime of macroscopic quantum tunneling to one of phase diffusion as a function of temperature, where the transition temperature T^{*} is gate-tunable. The switching probability distributions are shown to be consistent with a small shunt capacitance and moderate damping, resulting in a switching current which is a small fraction of the critical current. Phase locking between two JJs leads to a difference in switching current between that of a JJ measured in isolation and that of the same JJ measured in an asymmetric SQUID loop. In the case of the loop, T^{*} is also tuned by a magnetic flux.
We propose a new type of Josephson switching device which utilizes quantum transitions in a superconducting quantum interference device (SQUID) loop. This device is driven by a DC bias current and RF magnetic field. The output voltage is determined by the transition rate and the AC-Josephson relationship. We investigate the desirable device parameters by simulation. Two parameters, the maximum step height and the overlapped current region for the 0th and 1st steps, are plotted as functions of frequency and power of the RF-field activation. We also derive the relationship between the gain and device parameters. The analytical relationship is in good agreement with the numerical results. Besides the simple switching operation, a flip-flop operation is also successfully simulated. The results indicate that the device with appropriate parameters is suitable for switching applications.
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.
We investigate the switching current of a Cooper pair transistor with tunable Josephson energy. The junctions are fabricated in a superconducting quantum interference device (SQUID) geometry which allows for an in situ tunable effective Josephson energy by application of a magnetic field. We find a 2e2e2e-periodic switching current versus gate charge. As the magnetic field is increased the switching current stays 2e2e2e-periodic but the magnitude is suppressed. At a magnetic field of half a flux quantum through the SQUID's the switching current is minimum. We can theoretically model the experimental data by assuming a switching current which is proportional to the ideal critical current squarred. We show that such a dependence is expected in the limit where the effect of thermal fluctuations on the system is strong.