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Physical review. B, Condensed matter

Variable superconducting quantum-interference device: Theory

H. J. Fink, V. Grünfeld, S. M. Roberts

Published Jul 1, 1987• 5 citations
Publisher

Abstract

A dc superconducting quantum interference device (SQUID) consisting of a symmetric ring circuit with four Josephson junctions and two independent external currents ${I}_{1}$ and ${I}_{2}$, which enter the circuit at different points, has a phase boundary between the fully superconducting and the normal states that is different from that of a conventional SQUID. The control current ${I}_{2}$ modifies the magnetic flux (\ensuremath{\varphi}) periodicity and the value of the maximum measuring current ${I}_{1m}$(\ensuremath{\varphi}). For values of ${I}_{2}$>${I}_{c}$ (critical current of Josephson junction) the fully resistive state exists over flux intervals centered around (n+(1/2)${\ensuremath{\varphi}}_{0}$, where n is an integer and ${\ensuremath{\varphi}}_{0}$ is the fluxoid quantum. A superconducting circuit without Josephson junctions is suggested which should have a phase boundary similar to that found here. Minimization procedures which are usually employed in analyzing SQUID's are discussed.

DOI
10.1103/physrevb.36.74

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A bistable superconducting quantum interference device (SQUID) exclusively comprising Josephson junctions (JJs) is proposed. The design integrates two 2Φ2 \Phi2Φ-JJs within the loop structure, while the state of the SQUID is monitored through a conventional 2π2 \pi2π-periodic JJ\mathbf{J J}JJ (0\mathbf{0}0-JJ). This configuration eliminates the need for geometric inductors, leveraging the nonlinear phase dynamics for bistability. Extensive simulations confirm robust bistable operation under controlled external flux bias, positioning the proposed SQUID as an area-efficient building block for superconducting memory cells, sensors for various applications, and SFQ-compatible logic elements.
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We have fabricated and studied a promising kind of direct current superconducting quantum interference device (dc-SQUID) in which the magnitude and sign of the critical current of the individual Josephson junctions can be controlled by additional voltage probes connected to the junctions. We show that the amplitude of the voltage oscillations of the SQUID as a function of the applied magnetic field can be tuned and that the phase of the oscillations can be switched between 0 and π in the temperature range of 0.1–4.2 K using a suitable control voltage. This is equivalent to the external application of (n+1/2) flux quantum.