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Physical Review B

Micro-SQUIDs with controllable asymmetry via hot-phonon controlled junctions

Gareth Podd, G. D. Hutchinson, D. A. Williams, D. G. Hasko

Published Apr 2, 2007• 29 citations
Publisher

Abstract

We describe the fabrication and measurement of a micrometer-sized direct-current superconducting quantum interference device (dc-SQUID) in which the critical currents of each of the constriction-type Josephson junctions can be controlled independently and in situ via a process of nonequilibrium (hot-phonon) irradiation from a nanofabricated gated structure. The control mechanism is based on hot phonons which are injected into the superconducting microbridges from close proximity, but electrically isolated, normal-metal constrictions. We have also developed a one-dimensional computer model to analyze the behavior of micro-SQUID devices including situations in which we modify the asymmetry of the device. We show from the model that the experimental results are consistent with a change in effective length of the microbridge junctions with respect to the coherence length of the film. The experimental data, and its interpretation in relation to the micro-SQUID model, confirm that this technique, based on hot-phonon irradiation for controlling the critical current in Dayem bridge Josephson junctions, is compatible with the Josephson effect and a feasible method for post-fabrication parameter control in superconducting circuits using Dayem bridge Josephson junctions.

DOI
10.1103/physrevb.75.134501

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We describe the fabrication and measurement of a micrometer-sized direct-current superconducting quantum interference device (dc-SQUID) in which the critical currents of each of the constriction-type Josephson junctions can be controlled independently and in situ via a process of nonequilibrium (hot-phonon) irradiation from a nanofabricated gated structure. The control mechanism is based on hot phonons which are injected into the superconducting microbridges from close proximity, but electrically isolated, normal-metal constrictions. We have also developed a one-dimensional computer model to analyze the behavior of micro-SQUID devices including situations in which we modify the asymmetry of the device. We show from the model that the experimental results are consistent with a change in effective length of the microbridge junctions with respect to the coherence length of the film. The experimental data, and its interpretation in relation to the micro-SQUID model, confirm that this technique, based on hot-phonon irradiation for controlling the critical current in Dayem bridge Josephson junctions, is compatible with the Josephson effect and a feasible method for post-fabrication parameter control in superconducting circuits using Dayem bridge Josephson junctions.
The optimal design of a direct current superconducting quantum interference device (DC-SQUID) with high sensitivity requires two identical Josephson junctions, which is difficult to achieve. The design of a DC-SQUID using controlled Josephson junction technology provides a mechanism for modifying the characteristics of the device. Here we report on the nano-fabrication and measurement of a DC-SQUID utilizing two Dayem bridge weak-link Josephson junctions fabricated with a width ∼100 nm and a length of ∼200 nm. These junctions are controlled via the irradiation of hot phonons from nearby titanium/chromium normal metal constrictions which are current biased. The devices were measured at a temperature of 4.2 K and control over the critical current oscillations was observed.
The direct current superconducting quantum interference device (DC-SQUID), using controlled Josephson junction technology, provides a mechanism to modify the characteristics of the device post-fabrication. We report on the fabrication and measurement of a micron sized DC-SQUID using two Dayem bridge weak-link Josephson junctions with integrated “heaters.” The weak link critical current is controlled by hot phonons from the current biased titanium, normal metal heater. By using the heaters, control over the critical current oscillations of the SQUID was observed at 4.2K.
We present measurements of DC superconducting quantum interference devices based on Nb/graphene/Nb Josephson junctions. The superconducting proximity effect in graphene can be used to build Josephson junctions whose critical current can be controlled by field-effect gates. These junctions combine the tunability of semiconductor Josephson junctions with the high critical currents and low contact resistances of metal SNS junctions. By using local gates, the SQUID junction critical currents can be modified individually and this allows the sensitivity and symmetry of the SQUID to be controlled in-situ. We compare the critical current of the SQUID with simulations that include a non-sinusoidal current phase relation in the junctions, as expected for ballistic graphene junctions. We also investigate the transfer function of the device in both symmetric and asymmetric configurations and find a highest transfer function of 300 μV/Φ0. Graphene Josephson junctions have the potential to add functionality to existing technologies; for example, to make SQUID magnetometers with tunable sensitivity or superconducting qubits with fast electrical control.
Local control of superconducting circuits by high-impedance electrical gates offers potential advantages in superconducting logic, quantum processing units, and cryoelectronics. Recent experiments have reported gate-controlled supercurrent in Dayem bridges made of metallic superconductors, mediated by direct current leakage, out-of-equilibrium phonons, or possibly other mechanisms. However, a time-domain characterization of this effect has been lacking. Here, we integrate Dayem bridges made of niobium on silicon into coplanar waveguide resonators and measure the effect of the gate voltage at steady state and during pulsed operation. We consider two types of arrangements for the gate: a side-coupled gate and a remote injector. In both cases, we observe sizable changes in the real and the imaginary part of the constriction's impedance for gate voltages of the order of 1 V. However, we find striking differences in the time-domain dynamics, with the remote injector providing a faster and more controlled response. Our results contribute to our understanding of gate-controlled superconducting devices and their suitability for applications.
We report on the design, the fabrication and the performance of an integrated magnetic nano-sensor based on niobium dc-SQUID (Superconducting QUantum Interference Device) for nanoscale applications is presented. The nano-sensors are based on nanometric niobium constrictions (Dayem bridges) inserted in a square loop having a side length of 200 nm. Measurements of voltage-flux characteristic, flux to voltage transfer factor and noise performances are reported. In small signal mode, the sensors have shown a magnetic flux noise spectral density of 1.5 μΦ 0 /Hz 1/2 corresponding to a spin sensitivity in unit of Bohr magneton of 60 spin/Hz 1/2 . Supercurrent decay measurements of these devices are also reported. Such measurements provide useful information for applications which employ the SQUID as a trigger where the sensor works on the zero voltage state. The experimental data, have shown an intrinsic current fluctuation less than 0.2% of the critical current at liquid helium temperature, corresponding to an intrinsic sensor magnetic flux resolution of a few mΦ 0 . In view of the nano-SQUID employments in the detection of small spin populations, the authors calculated the spin sensitivity and the magnetic response relative to the single spin, as a function of its position within the SQUID hole. The results show that the SQUID response depends strongly on the spin position.
The interplay of alternative materials, large-scale integration technologies, and innovative devices plays a pivotal role on the way to fault-tolerant quantum computing. In this context, gate-controlled superconductivity (GCS) emerges as a key enabling technology, promising to advance superconducting logic and quantum electronics, complementing conventional approaches based on Josephson effects and quantum interference. Here, we demonstrate all-metallic superconducting ZrN nanostructures on silicon, fabricated entirely using a subtractive, monolithic dry-etch approach with a 300 mm CMOS-compatible process flow. For Dayem-bridge weak links with decreasing width down to 27 nm, we observe Tc of 7.3 K, width-dependent switching characteristics, and a progressive approach of the tunneling limit. In laterally gated Dayem-bridge and nanowire devices, we observe GCS with full suppression of the critical current, where required gate voltages <5 V promise compatibility with CMOS interfaces. Asymmetries in polarity-dependent power injection via field emission support GCS models that attribute the effect to quasiparticle relaxation through phonon emission. Additional asymmetries in the GCS effect suggest an influence of the substrate condition after dry etching. Field emission features in the gated nanowire device imply superimposed current paths along the gate edge and corners. Finally, we analyze magnetic interference in Dayem-bridge weak links and superconducting quantum interference devices, where quantum interference is governed by Josephson effects and inductance loop asymmetries. Our results contribute to the scaling efforts in hybrid superconducting electronics and quantum-processing units.