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IEEE Transactions on Magnetics

Seven-channel RF SQUID with 1/f noises only at very low frequencies

Hiroshi Ohta, M. Takahata, Yoshio Takahashi, K. Shinada, Yasuharu Yamada, Takao HANASAKA, Y. Uchikawa, M. Kotani, Toshiaki Matsui, Bokuji Komiyama

Published Mar 1, 1989• 11 citations
Publisher

Abstract

A seven-channel RF SQUID (superconducting quantum interference device) with anodized short weak links has been fabricated. The devices exhibit a sensitivity of 23.6 fT/ square root Hz and a 1/f noise onset of 0.05 Hz at a white noise level of 1.8*10/sup -4/ phi /sub 0// square root Hz. The intrinsic flux noise is estimated from an observed step pattern to be 4.9*10/sup -5/ phi /sub 0// square root Hz. It is shown that Arnold theory explains very well the behavior of Josephson junctions with a critical current density of around 10/sup 4/ A/cm/sup 2/.>

DOI
10.1109/20.92461

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We describe the noise performance of thin-film YBa 2 Cu 3 O 7 RF SQUIDS fabricated on MgO substrates with single step-edge Josephson junctions. A substantial range of noise levels has been observed for devices having similar electrical parameters (inductance, critical current, RF frequency). The best devices have white-noise levels below 10 -4 phi 0 Hz -1/2 when operated at a frequency of 170-200 MHz and have 1/f noise corners of a few hertz. Measurements of the V RF versus L RF characteristics of one SQUID, indicate that intrinsic device noise dominates the measured total noise of these devices. The white noise of all devices exceeds the predictions of the theory of Kurkijarvi at al. by a large factor, apparently because of non-ideal behaviour of step-edge Josephson junctions.
A high-sensitivity microwave SQUID (superconducting quantum interference device), suitable for RF attenuation measurements, is constructed taking advantage of the low density of states of current carriers in high-T/sub c/ materials. The dynamics, the noise properties, and the sensitivity of the microwave SQUID are theoretically analyzed. The ultimate energy resolution of the waveguide SQUID is inversely proportional to the characteristic frequency omega /sub c/=R/sub q//L of the loop. omega /sub c/ is high for YBaCuO junctions at 4.2 K because of small leakage. A flux sensitivity of 4*10/sup 4-6/ phi /sub 0// square root Hz has been achieved, which is in good agreement with the theoretical predictions.>
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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.