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Superconductor Science and Technology

Static and dynamic transport properties of multi-terminal, multi-junction microSQUIDs realized with Nb/HfTi/Nb Josephson junctions

Silke Wolter, Julian Linek, Thomas Weimann, D. Koelle, R. Kleiner, Oliver Kieler

Published Jun 13, 2022• 5 citations
Publisher

Abstract

Abstract The progressive miniaturization of superconducting quantum interference devices (SQUIDs) used, e.g. for magnetic imaging on the nanoscale or for the detection of the magnetic states of individual magnetic nanoparticles causes increasing problems in realizing a proper flux-bias scheme for reading out the device. To overcome the problem, a multi-terminal, multi-junction layout has been proposed and realized recently for the SQUID-on-tip configuration, which uses constriction-type Josephson junctions (JJ). This geometry is also interesting for SQUIDs based on overdamped superconductor—normal metal—superconductor (SNS) JJ. We fabricated four-terminal, four-junction SQUIDs based on a trilayer Nb/HfTi/Nb process and study their static and dynamic transport properties in close comparison with numerical simulations based on the resistively and capacitively shunted junction model. Simulations and measurements are in very good agreement. However, there are large differences to the transport properties of conventional two-junction SQUIDs, including unusual phase-locked and chaotic dynamic states which we describe in detail. We further extract the current-phase relation of our SNS junctions, which turns out to be purely sinusoidal within the experimental error bars.

DOI
10.1088/1361-6668/ac782b

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Abstract The progressive miniaturization of superconducting quantum interference devices (SQUIDs) used, e.g. for magnetic imaging on the nanoscale or for the detection of the magnetic states of individual magnetic nanoparticles causes increasing problems in realizing a proper flux-bias scheme for reading out the device. To overcome the problem, a multi-terminal, multi-junction layout has been proposed and realized recently for the SQUID-on-tip configuration, which uses constriction-type Josephson junctions (JJ). This geometry is also interesting for SQUIDs based on overdamped superconductor—normal metal—superconductor (SNS) JJ. We fabricated four-terminal, four-junction SQUIDs based on a trilayer Nb/HfTi/Nb process and study their static and dynamic transport properties in close comparison with numerical simulations based on the resistively and capacitively shunted junction model. Simulations and measurements are in very good agreement. However, there are large differences to the transport properties of conventional two-junction SQUIDs, including unusual phase-locked and chaotic dynamic states which we describe in detail. We further extract the current-phase relation of our SNS junctions, which turns out to be purely sinusoidal within the experimental error bars.
We investigated, at temperature 4.2 K, electric transport, flux noise, and resulting spin sensitivity of miniaturized Nb direct current superconducting quantum interference devices (SQUIDs) based on submicron Josephson junctions with HfTi barriers. The SQUIDs are either of the magnetometer-type or gradiometric in layout. In the white noise regime, for the best magnetometer we obtain a flux noise SΦ1/2=250nΦ0/Hz1/2, corresponding to a spin sensitivity Sμ1/2≥29μB/Hz1/2. For the gradiometer we find SΦ1/2=300nΦ0/Hz1/2 and Sμ1/2≥44μB/Hz1/2. The devices can still be optimized with respect to flux noise and coupling between a magnetic particle and the SQUID, leaving room for further improvement towards single spin resolution.
We present a fabrication technology for nanoscale superconducting quantum interference devices (SQUIDs) with overdamped superconductor-normal metal-superconductor (SNS) trilayer Nb/HfTi/Nb Josephson junctions. A combination of electron-beam lithography with chemical-mechanical polishing and magnetron sputtering on thermally oxidized Si wafers is used to produce direct current SQUIDs with 100-nm-lateral dimensions for Nb lines and junctions. We extended the process from originally two to three independent Nb layers. This extension offers the possibility to realize superconducting vias to all Nb layers without the HfTi barrier, and hence to increase the density and complexity of circuit structures. We present results on the yield of this process and measurements of SQUID characteristics.
We analyze electric transport and noise properties at 4.2 K of self-shunted superconductor-normal metal-superconductor (SNS) sandwich-type Josephson junctions, comprising Nb\mathrm{Nb}Nb as the superconductor and Hf\mathrm{Hf}Hf-Ti\mathrm{Ti}Ti as the normal conducting material, with lateral dimensions down to approximately 808080 nm. The junctions are fabricated with an optimized multilayer Nb\mathrm{Nb}Nb technology based on nanopatterning by electron-beam lithography and chemical-mechanical polishing. The dependence of transport properties on the junction geometry (lateral size and barrier thickness ${d}_{\mathrm{Hf}\mathrm{\ensuremath{-}}\mathrm{Ti}}$) is studied, yielding a characteristic voltage Vc{V}_{c}Vc​ up to approximately $100\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{V}$ for the smallest ${d}_{\mathrm{Hf}\mathrm{\ensuremath{-}}\mathrm{Ti}}=17$ nm. The observed small hysteresis in the current-voltage curves of devices with high Vc{V}_{c}Vc​ and large size can be attributed to self-heating of the junctions and fitted with an extended version of the resistively shunted junction model. Measurements of voltage noise of single junctions are consistent with the model including self-heating effects. The potential of our technology for further miniaturization of nanoscale superconducting quantum interference devices and for the improvement of their performance is discussed.
We demonstrate that shunting of superconductor-insulator-superconductor (S-I-S) Josephson junctions by superconductor-insulator-normal metal (S-I-N) structures having pronounced nonlinear I–V characteristics can remarkably modify the Josephson dynamics. In the regime of Josephson generation the phase behaves as an overdamped coordinate, while in the superconducting state the damping and current noise are strikingly small, that is vitally important for application of such junctions for readout and control of Josephson qubits. Superconducting Nb/AlOx/Nb junction shunted by Nb/AlOx/AuPd junction of S-I-N type was fabricated and, in agreement with our model, exhibited nonhysteretic I–V characteristics at temperatures down to at least 1.4 K.
Abstract Miniaturization of superconducting quantum interference devices (SQUIDs) is of major importance for the development of sensitive scanning nanoscale magnetometry tools. The high sensitivity of nanoSQUIDs is restricted, however, to only particular periodic values of the applied magnetic field, making accurate measurements at intermediate values of the field impossible. We present a theoretical investigation of a multi-terminal, multi-junction SQUID (mSQUID) that lifts this limitation by providing electrical means for a continuous shift of the quantum interference pattern with respect to the applied field. Analysis of 4-terminal, 4-junction and 3-terminal, 3-junction mSQUIDs shows that operation at maximum sensitivity can be obtained at any value of the magnetic field by applying control current to the extra terminals. The model describes the variation and the shift of the interference pattern as a function of the control currents, junction asymmetries, and the mSQUID inductance. The mSQUID is also shown to provide a direct measurement of the current–phase relations of superconducting junctions. The derived model provides a quantitative description of the recently developed multi-terminal nanoSQUID-on-tip.
Superconducting quantum interference devices (SQUIDs) that incorporate two superconductor/insulator/superconductor (SIS) Josephson junctions in a closed loop form the core of some of the most sensitive detectors of magnetic and electric fields currently available. SQUIDs in these applications are typically operated with a finite voltage which generates microwave radiation through the ac Josephson effect. This radiation may impact the system being measured. We describe here a SQUID in which the Josephson junctions are formed from strips of normal metal (N) in good electrical contact with the superconductor (S). Such SNS SQUIDs can be operated under a finite voltage bias with performance comparable or potentially better than conventional SIS SQUIDs. However, they also permit a mode of operation that is based on the unusual interplay of quasiparticle currents and supercurrents in the normal metal of the Josephson junction. The method allows measurements of the flux dependence of the critical current of the SNS SQUID without applying a finite voltage bias across the SNS junction, enabling sensitive flux detection without generating microwave radiation.