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arXiv:cond-mat/0203433 · Physical review. B, Condensed matter

Direct demonstration of circulating currents in a controllable π-SQUID generated by a 0 to π transition of the weak links

J. J. A. Baselmans, Bart Jan van Wees, T. M. Klapwijk

Published Jun 5, 2002• 23 citations
arXivHTMLPDF

Abstract

A controllable \ensuremath{\pi}-SQUID is a DC SQUID (direct-current superconducting quantum interference device) with two controllable \ensuremath{\pi} junctions as weak links. A controllable \ensuremath{\pi} junction consists of a superconducting--normal metal--superconducting Josephson junction with two additional contacts to the normal region of the junction. By applying a voltage ${V}_{c}$ over these contacts it is possible to control the state of the junction, i.e., a conventional (0) state or a \ensuremath{\pi} state, depending on the magnitude of ${V}_{c}.$ We demonstrate experimentally that, by putting one junction into a \ensuremath{\pi} state, a screening current is generated around the SQUID loop at an integer external flux. To be able to do this, we have fabricated controllable $\ensuremath{\pi}$ junctions, based on Cu-Nb or Ag-Nb, in a new geometry. We show that at 1.4 K only the Ag-Nb device shows the transition to a \ensuremath{\pi} state as a function of ${V}_{c}$ consistent with theoretical predictions. In a controllable \ensuremath{\pi}-SQUID based on Nb-Ag we observe, apart from a screening current at integer external flux, a phase shift of \ensuremath{\pi} of the ${V}_{\mathrm{SQUID}}\ensuremath{-}B$ oscillations under a suitable current bias, depending on the magnitude of ${V}_{c}.$

DOI
10.1103/physrevb.65.224513

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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.
Abstract We directly observed π-leaps of superconducting phase differences in π-junction-based superconducting quantum interference devices (SQUIDs). The SQUIDs studied here are formed by introducing a π-junction to a conventional-junction (0-junction)-based direct current (DC)-SQUID, which is referred to as the 0-0-π SQUID. Either clockwise or counter-clockwise-circulating currents flow spontaneously in the 0-0-π SQUID because of a π-phase shift of the π-junction. In other words, the 0-0-π SQUID has a bistable state corresponding to the directions of circulating currents. π-leaps are generated by transiting between the two states of the bistable state. π-leaps are an ultra-fast phenomenon and are difficult to observe as they are. We prepared a half-flux quantum (HFQ)-SQUID that comprised two 0-0-π SQUIDs. π-leaps are reflected in a static characteristic, that is, a modulation pattern of the critical current in the HFQ-SQUID. We formed π-junctions with the PdNi layer on 0-junction-based circuits supplied by the National Institute of Advanced Industrial Science and Technology. The modulation pattern of the HFQ-SQUID had a period corresponding to π-leaps as expected, although some microstructures were observed. We demonstrated that the microstructures originated from the asymmetry inside each 0-0-π SQUID by analyzing the relationship between the phase change of 0-0-π SQUIDs and the modulation patterns.
A dc superconducting quantum interference device (SQUID) consisting of a symmetric ring circuit with four Josephson junctions and two independent external currents I1{I}_{1}I1​ and I2{I}_{2}I2​, 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 I2{I}_{2}I2​ modifies the magnetic flux (\ensuremath{\varphi}) periodicity and the value of the maximum measuring current I1m{I}_{1m}I1m​(\ensuremath{\varphi}). For values of I2{I}_{2}I2​>Ic{I}_{c}Ic​ (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.
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The development of superconducting electronics requires careful characterization of the components that makeup electronic circuits. Superconducting weak links are the building blocks of most superconducting electronic components and are characterized by highly nonlinear current-to-phase relations (CPRs), which are often not perfectly known. Recent research has found that the Josephson diode effect (JDE) can be related to the high harmonic content of the current-to-phase relation of the weak links embedded in superconducting interferometers. This makes the JDE a natural tool for exploring the harmonic content of weak links beyond single-harmonic CPR. In this study, we present the theoretical model and experimental characterization of a double-loop superconducting quantum interference device (DL-SQUID) that embeds all-metallic superconductor-normal metal-superconductor junctions. The proposed device exhibits the JDE due to the interference of the supercurrents of three weak links in parallel, and this feature can be adjusted through two magnetic fluxes, which act as experimental knobs. We carry out a theoretical study of the device in terms of the relative weight of the interferometer arms and the experimental characterization concerning flux tunability and temperature.