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Applied Physics Letters

Performance of high-Tc superconducting quantum interference devices with resistively shunted inductances

Dae Joon Kang, W.E. Booij, Mark G. Blamire, Edward J. Tarte

Published Dec 28, 1998• 5 citations
Publisher

Abstract

The voltage modulation depths, ΔV of high-Tc direct current superconducting quantum interference devices (SQUIDs) with resistors connected in parallel with their inductances were investigated. Both the junctions and resistors in the SQUIDs were fabricated using focused electron-beam irradiation. The effect of varying the resistor value (using focused ion-beam trimming) and the screening parameter βL (by varying the temperature and hence the junction critical current) were studied. Significant enhancement of ΔV relative to an equivalent unshunted SQUID for βL values up to 50 was observed, and the most effective shunt resistor value was found to be approximately equal to the junction resistance.

DOI
10.1063/1.122939

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•Aug 17, 2018•0 citations
The voltage modulation depths, ΔV of high-Tc direct current superconducting quantum interference devices (SQUIDs) with resistors connected in parallel with their inductances were investigated. Both the junctions and resistors in the SQUIDs were fabricated using focused electron-beam irradiation. The effect of varying the resistor value (using focused ion-beam trimming) and the screening parameter βL (by varying the temperature and hence the junction critical current) were studied. Significant enhancement of ΔV relative to an equivalent unshunted SQUID for βL values up to 50 was observed, and the most effective shunt resistor value was found to be approximately equal to the junction resistance.
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.
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We have investigated asymmetrically shunted Nb/Al-AlOx_xx​/Nb direct current (dc) superconducting quantum interference devices (SQUIDs). While keeping the total resistance RRR identical to a comparable symmetric SQUID with R−1=R1−1+R2−1R^{-1} = R_1^{-1} + R_2^{-1}R−1=R1−1​+R2−1​, we shunted only one of the two Josephson junctions with R=R1,2/2R = R_{1,2}/2R=R1,2​/2. Simulations predict that the optimum energy resolution εεε and thus also the noise performance of such an asymmetric SQUID can be 3--4 times better than that of its symmetric counterpart. Experiments at a temperature of 4.2\,K yielded ε≈32 ℏε\approx 32\,\hbarε≈32ℏ for an asymmetric SQUID with an inductance of 22 pH22\,\rm{pH}22pH. For a comparable symmetric device ε=110 ℏε= 110\,\hbarε=110ℏ was achieved, confirming our simulation results.
Micron-size superconducting quantum interference devices (μ\muμ-SQUIDs) are highly-sensitive magnetic flux sensors where the critical current or the voltage is modulated by the magnetic flux. The so-called voltage read-out mode is nevertheless often hindered by thermal hysteresis. μ\muμ-SQUIDs with improved thermal evacuation and small critical current can feature a phase-dynamic regime over a narrow bias-current and temperature range. Guided by a dynamic thermal model, we could widen these current and temperature ranges by using a shunt resistor. A significant increase in these ranges and, more strikingly, in the voltage oscillations magnitude is observed when the shunt is made of an adequate inductance in series with the resistor. An increase in inductance above a threshold value leads to relaxation oscillations in voltage. The large voltage modulation thus obtained offers an improved flux resolution in the voltage read-out mode down to 1.3 K.