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IEEE Transactions on Magnetics

Limit characteristics of two-junction magnetic flux detector

Victor I. Danilov, K. K. Likharev, O. Sniguiriev, Eugene S. Soldatov

Published Jan 1, 1977• 8 citations
Publisher

Abstract

The sensitivity of a two-Josephson-junction interferometer as a magnetic flux detector (dc SQUID) is estimated. The minimum value of the basic SQUID characteristic-noise equivalent temperature δT = (δΦ)2/kBL appears to be as low as for the best rf SQUID mode:(\deltaT)_{\min}/(\Deltaf)\cong 3 T/f_{o}. HereTis an operating temperature, fois a characteristic frequency of the Josephson junctions and\Deltafis a post-detector bandwidth. For a typical value of SQUID inductance L this limit corresponds to flux sensitivity of the order of 10-7Φo/Hz1/2.

DOI
10.1109/tmag.1977.1059413

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Applied Physics Letters•Feb 3, 1992•81 citations
The sensitivity of a two-Josephson-junction interferometer as a magnetic flux detector (dc SQUID) is estimated. The minimum value of the basic SQUID characteristic-noise equivalent temperature δT = (δΦ)2/kBL appears to be as low as for the best rf SQUID mode:(\deltaT){\min}/(\Deltaf)\cong 3 T/f{o}. HereTis an operating temperature, fois a characteristic frequency of the Josephson junctions and\Deltafis a post-detector bandwidth. For a typical value of SQUID inductance L this limit corresponds to flux sensitivity of the order of 10-7Φo/Hz1/2.
We numerically calculated and obtained an approximate expression for the thermal noise-limited sensitivity of the pulse-biased SQUID sensor used in the integrated magnetometer. We found that the sensitivity is inversely proportional to the bias frequency and inversely proportional to the critical current of the Josephson junction to the 4/3 power. The sensitivity is also proportional to the temperature to the 4/3 power, and is nearly independent of the bias pulse duration. The sensitivity expression is similar to that of the rf SQUID, however, the coefficient is more than one order of magnitude smaller. The best sensitivity has the same order of magnitude as that of the dc SQUID. The quantum limit is attained at a bias frequency of 0.9 GHz at 4.2 K with the typical junction parameters. This sensitivity is available for the SQUID magnetometer operated in the digital feedback loop.
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.
Noise characteristics of dc SQUIDs in large β regimes have been studied experimentally in the case when a SQUID inductance is resistively shunted. Here, β=2L I 0/Φ0 is the modulation parameter, L is the SQUID inductance, I 0 is the critical current of the Josephson junction and Φ0 is the flux quantum. It is shown experimentally that the performance of the SQUID does not degrade significantly even for large β values; for example, the minimum energy resolution of the SQUID is as small as ε=25h and the flux region where ε<50h is as wide as 0.1Φ0, even for the case β=12, where h is the Planck constant. The experimental results agree quantitatively with the theoretical ones.
Integrated NbN direct current superconducting quantum interference device (DC-SQUID) magnetometers were developed based on high-quality epitaxial NbN/AlN/NbN Josephson junctions for SQUID applications operating at high temperatures. We report the current–voltage and voltage–flux characteristics and the noise performance of the NbN DC-SQUIDs for temperatures ranging from 4.2 to 9 K. The critical current and voltage swing of the DC-SQUIDs decreased by 15% and 25%, respectively, as the temperature was increased from 4.2 to 9 K. The white flux noise of the DC-SQUID magnetometer at 1 kHz increased from 3.9 μΦ0/Hz1/2 at 4.2 K to 4.8 μΦ0/Hz1/2 at 9 K with 23% increase, corresponding to the magnetic field noise of 6.6 and 8.1 fT/Hz1/2, respectively. The results show that NbN DC-SQUIDs improve the tolerance of the operating temperatures and temperature fluctuations in SQUID applications.
The operation of a constant-current quantum interferometer with two Josephson junctions is studied in a system for measuring small changes in a magnetic field with low-frequency modulation of the magnetic flux. Idealized theoretical signal characteristics are found together with their dependence on the operating mode and parameters of a SQUID. These characteristics are used in Pt. II (ref. 12) to determine the minimal detectable change in the magnetic flux, which characterizes the maximal sensitivity of a SQUID.
A superconducting quantum interferometer device (SQUID) electronics with tank frequencies near 150 MHz was constructed to enhance the rf SQUID sensitivity. By these means we characterized our YBa2Cu3O7 thin-film rf SQUIDs with step-edge junctions immersed in liquid N2. We obtained transfer function values of over 40 μV/Φ0, at a 50 Ω input impedance, for both hysteretic and dispersive mode of rf SQUID operation. The rms white and 1/f flux noise levels were significantly lower than at the tank frequency of 20 MHz. The best magnetic field sensitivity at 77 K was 0.9 pT/Hz1/2, and the energy resolution 1.4×10−28 J/Hz, down to 0.3 Hz.