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Applied Physics Letters

Controlled-junction superconducting quantum interference device via phonon injection

G. D. Hutchinson, Hua Qin, D. G. Hasko, Dae Joon Kang, D. A. Williams

Published Dec 29, 2003• 6 citations
Publisher

Abstract

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.

DOI
10.1063/1.1638627

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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.
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.
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.
A thin-film dc superconducting quantum interference device (SQUID) consisting of quasiparticle-injected superconducting weak links was fabricated and successfully operated. This SQUID was made on a silicon substrate and it consists of a ∼100-nm-thick lead alloy with two injector junctions. The characteristics of this dc SQUID are controllable by varying the injecting rate of quasiparticles; therefore, it can be operated at arbitrary temperatures below the transition temperature of the superconducting material.
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.
Nano-Superconducting Quantum Interference Devices (nano-SQUIDs) are usually fabricated from a single layer of either Nb or Al. We describe here a simple method for fabricating suspended nano-bridges in Nb/Al thin-film bilayers. We use these suspended bridges, which act as Josephson weak links, to fabricate nano-SQUIDs which show critical current oscillations at temperatures up to 1.5 K and magnetic flux densities up to over 20 mT. These nano-SQUIDs exhibit flux modulation depths intermediate between all-Al and all-Nb devices, with some of the desirable characteristics of both. The suspended geometry is attractive for magnetic single nanoparticle measurements.
The predominantly dx2−y2-pairing symmetry in most high-Tc superconductors provides the opportunity to fabricate Josephson junction circuits in which part of the junctions are biased by a phase difference of the superconducting order parameter of π. We present fabrication and measurements of an all high-Tc dc superconducting quantum interference device (dc SQUID) realized with thin-film technology, of which the Josephson junctions consist of one standard junction and one junction with a π-phase shift. The characteristics of the π-SQUID are compared with the properties of a standard high-Tc SQUID.