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Japanese Journal of Applied Physics

Quasiparticle-Injected Superconducting Weak Links and Their Application to dc SQUID

Kazuyoshi Kojima, Shigetoshi Nara, Koichi Hamanaka

Published Jan 1, 1987• 2 citations
Publisher

Abstract

Quasiparticle-injected superconducting weak links of lead alloy were fabricated under various conditions. The resistance of the weak links depends on the indium concentration of the weak link; a larger indium concentration gave a larger normal resistance. By applying the weak links of this large normal resistance to dc superconducting quantum interference devices (SQUID), a larger output voltage (∼30 µV), comparable to that of a conventional resistively shunted dc SQUID, could be obtained.

DOI
10.1143/jjap.26.81

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Quasiparticle-injected superconducting weak links of lead alloy were fabricated under various conditions. The resistance of the weak links depends on the indium concentration of the weak link; a larger indium concentration gave a larger normal resistance. By applying the weak links of this large normal resistance to dc superconducting quantum interference devices (SQUID), a larger output voltage (∼30 µV), comparable to that of a conventional resistively shunted dc SQUID, could be obtained.
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.
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 have fabricated and characterized the noise performance of well-coupled Nb-Pb alloy edge junction dc superconducting quantum interference devices (SQUIDs) with a resistively shunted inductance. The extra damping resistor removes the resonances in the current-voltage characteristics resulting in a very low value of the extrinsic energy sensitivity in flux-locked-loop operation. Numerical simulations, with the junction capacitance included, indicate that the damping resistance does not degrade noise performance until it is significantly less than the junction-shunting resistance. We also tested different SQUID designs and demonstrated that a large SQUID inductance, proposed earlier to be desirable when resistively shunted, can seriously affect the low-frequency flux noise.
The design, fabrication, and noise properties of an ultra-low-noise tunnel junction dc SQUID (superconducting quantum interference device) are presented. The two resistively shunted Pb-alloy tunnel junctions are each 3.6 μm diam. The SQUID has an inductance L of ≈11 pH, a critical current per junction, I0 of ≈100 μA, and a resistance per shunt R of ≈2 Ω. At 2.3 K, eI0R≈kBT, and simple theory predicts that the minimum intrinsic energy sensitivity should be ≈2h. The actual minimum value of the measured high-frequency (70 kHz) flux noise at 4.2 K is φn=1.3×10−7φ0 Hz−1/2. This corresponds to an intrinsic energy sensitivity, φ2n/2L, of 3.2×10−33 J Hz−1 or 5h. Lowering the bath temperature to 1.8 K gave no improvement in performance.
This paper concerns a new three terminal superconducting weak link device, having a third electrode attached to the weak link part for injecting quasiparticles. The structure, the fabrication procedure and the result of the measurement of its static I-V characteristics are presented. As a result, a current gain larger than that of any other Josephson switching devices is obtained. It is found that the larger current gain depends strongly upon the input resistance between the third electrode and the weak link part. The switching speed of the device is also theoretically discussed.
The noise characteristics of a large- beta DC SQUID (superconducting quantum interference device) have been studied experimentally in the case where the SQUID inductance is resistively shunted. Energy resolutions of the SQUIDs, epsilon , have been measured for various beta values up to beta =19. It is shown experimentally that the energy resolution of these large- beta SQUIDs is as small as that of the conventional SQUID with beta =1. For example, an energy resolution of 25h was obtained for beta =12 (L= 340 pH and I/sub 0/=37 mu A), where h is Planck's constant. The experimental results agree quantitatively with theoretical ones. It is also shown that the present resistivity shunted inductance scheme is very useful in developing SQUIDs operating at T=77 K with high-T/sub c/ superconductors, where the value of beta will necessarily be large in order to avoid noise effects. Numerical simulations of the performance of the high-T/sub c/ SQUID are presented.>