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Journal of Applied Physics

Characterizing, modeling, and optimizing high-T c superconducting quantum interference devices

Vittorio Foglietti, R. H. Koch, Jian Sun, Robert B. Laibowitz, W. J. Gallagher

Published Jan 1, 1995• 10 citations
Publisher

Abstract

We have measured the current-voltage, flux-voltage, and noise characteristics of 77 K superconducting quantum interference devices made with step edge junctions and multilevel edge junctions on SrTiO3 substrates and with step edge junctions on LaAlO3. We find in each case that the I-V curves can have excess currents of up to 50%–80%, that the responsivity, ∂V/∂Φ, can be substantially smaller than expected from the resistively shunted junction (RSJ) model, but that the white noise voltage spectral density is about as expected. We discuss the extent to which the reduction of the responsivity is correlated with the excess current. We note that the observed unchanged magnitude of the voltage noise and the reduced responsivity would both be expected if we based the RSJ predictions on a model in which the excess current is not considered part of the Josephson current. Finally and perhaps fundamentally, we find that simulations made assuming a nonsinusoidal current-phase relationship give rise to an excess current, a reduction of the responsivity, and little change in the voltage noise spectral density, at least qualitatively as is observed.

DOI
10.1063/1.359333

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We have measured the current-voltage, flux-voltage, and noise characteristics of 77 K superconducting quantum interference devices made with step edge junctions and multilevel edge junctions on SrTiO3 substrates and with step edge junctions on LaAlO3. We find in each case that the I-V curves can have excess currents of up to 50%–80%, that the responsivity, ∂V/∂Φ, can be substantially smaller than expected from the resistively shunted junction (RSJ) model, but that the white noise voltage spectral density is about as expected. We discuss the extent to which the reduction of the responsivity is correlated with the excess current. We note that the observed unchanged magnitude of the voltage noise and the reduced responsivity would both be expected if we based the RSJ predictions on a model in which the excess current is not considered part of the Josephson current. Finally and perhaps fundamentally, we find that simulations made assuming a nonsinusoidal current-phase relationship give rise to an excess current, a reduction of the responsivity, and little change in the voltage noise spectral density, at least qualitatively as is observed.
We have fabricated Y1Ba2Cu3O7−x step-edge junction dc superconducting quantum interference devices (SQUIDs) and characterized their noise performance. The current-voltage characteristics of our SQUIDs are of resistively shunted junction type with critical current densities jc of about 104 A/cm2 and maximum flux to voltage transfer functions δV/δΦ of 20 μV/Φ0 at 77 K. We compare model predictions for 1/f noise in dc SQUIDs with experimental data and show that the frequency dependent noise in our devices results from critical current fluctuations of the Josephson junctions. A bias current reversing technique is effective in suppressing this noise contribution. A flux noise Φn of 80 μΦ0/Hz1/2 at 1 Hz and 77 K was measured on a dc SQUID with an inductance Ls of 60 pH.
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We report the development of a fabrication process of high-resistance step-edge Josephson junctions on sharp argon ion beam etched substrate steps. For a reproducible fabrication, the redeposition of material at the step during the etch process has to be avoided. This is achieved by an alternating angle of ion incidence parallel to the edge of the photoresist mask. The reproducible straight step profile exhibits an angle around and is reflected in reproducible electrical properties of the junctions. Critical current and asymptotic normal state resistance both scale with the junction width w . The current-voltage ( I - V ) characteristics depend on the ratio of film thickness t to step height h . For > 0.85 they are flux-flow-like; for < 0.8 they correspond to the resistively shunted junction (RSJ) model. Lower ratios of correspond to higher specific resistances resulting at 77 K typically in asymptotic normal state resistances between 5 and per junction, depending on w . At 77 K we achieve products of more than . The junctions are applied to dc superconducting quantum interference devices (dc-SQUIDs) with inductances between 25 and 35 pH. At 77 K transfer functions of more than are achieved. noise of the step-edge dc-SQUIDs is observed, but using bias current modulation it is entirely suppressed resulting in a spectral density of flux noise of measured at 1 Hz and 77 K for a SQUID with an inductance of 30 pH.
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Partly resistive superconducting quantum interference devices (RSQUIDs) are used in noise thermometry in the liquid helium temperature range. Here we analyze one type of RSQUID that has not been thoroughly investigated until now: the direct current biased with two Josephson junctions and a resistive part connected in series with the junctions and closed by a superconducting ring. The resistive shunted model of the Josephson junctions is used for the analysis. Two cases of the RSQUID ring inductance were analyzed: (i) negligibly small and (ii) small but finite inductance. The expressions for the low frequency output signal, the current–voltage (I–V) characteristics, and the current flowing through the resistive part are obtained. In the dependence of the output frequency versus the dc current passed through the resistive part often an offset is observed experimentally. This phenomenon can be explained as an influence of an asymmetry in the critical currents of the junctions.
Direct current-superconducting quantum interference devices (SQUIDs) characterized by asymmetric Josephson junctions have never been used for applications. In order to demonstrate their potential advantages, a throughout numerical analysis of different asymmetric configurations has been carried out. A damping resistance has been included in the SQUID circuit and the thermal noise associated with junction and damping resistances has been considered in the numerical model. Magnetic modulations and flux noise spectral densities have been computed as a function of many parameters (bias current, asymmetry, SQUID inductance, and damping resistance) and the performance of symmetric and asymmetric devices have been compared. The results show that, by properly optimizing the SQUID design, asymmetric SQUIDs are characterized by higher magnetic flux to voltage transfer coefficient and lower flux noise. As a result, asymmetric configurations can be very useful in all the applications where high sensitivity is required.