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Soviet Journal of Low Temperature Physics

A high-temperature SQUID

B. I. Verkin, V. V. Kartsovnik, P. P. Pavlov, V. A. Pavlyuk, Yu. A. Tavrin

Published Jul 1, 1988• 0 citations
Publisher

Abstract

Electrodynamic phenomena are studied in an interferometer made of high-Tc bulk ceramic Y1Ba2Cu3Oy. with a single quantization circuit closed with a weak coupling. The measured spectral density of noise flux in SQUID operating as a negative-feedback magnetometer was 3.5·10−4Φ0 Hz−1/2 for T = 77 K and 6.5·10−5Φ0 Hz−1/2 for T = 40 K.

DOI
10.1063/10.0031985

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Electrodynamic phenomena are studied in an interferometer made of high-Tc bulk ceramic Y1Ba2Cu3Oy. with a single quantization circuit closed with a weak coupling. The measured spectral density of noise flux in SQUID operating as a negative-feedback magnetometer was 3.5·10−4Φ0 Hz−1/2 for T = 77 K and 6.5·10−5Φ0 Hz−1/2 for T = 40 K.
A method for the determination of the noise spectral density in a high-temperature microwave SQUID operating in the hysteresis regime is developed. Under these conditions, the reflection coefficient serves as an output signal. It is shown that if a directional coupler used for extracting the reflected wave is placed as close to the SQUID loop as possible, the magnetometer can be designed as a microwave integrated circuit with a noise flux spectral density SΦ1/2<10−5Φ0/Hz0.5,where Φ0S_\Phi ^{1/2} < 10^{ - 5} \Phi _0 /{\text{Hz}}^{{\text{0}}{\text{.5}}} ,{\text{where }}\Phi _{\text{0}} SΦ1/2​<10−5Φ0​/Hz0.5,where Φ0​ , is the magnetic flux quantum.
The operation of YBa2Cu3O7 rf superconducting quantum interference devices (SQUIDs) using a dielectric SrTiO3 resonator as tank circuit was tested. At 77 K, we attained for such resonators unloaded quality factors Q0 up to 7000 at resonant frequencies near 1 GHz, when measuring without electromagnetic shielding. For a washer SQUID with an area of 4×4 mm2 and an inductance of ≂100 pH, which was properly coupled to the resonator, we measured an approximately 60 μV peak-to-peak SQUID signal and a white flux noise spectral density of SΦ1/2=1×10−5Φ0/√Hz, corresponding to a field sensitivity of BN=105 fT/√Hz. With an additional single-layer flux transformer with a diameter of the pickup loop of 22 mm in flip-chip arrangement, the field sensitivity was improved to 30 fT/√Hz.
The single-chip superconducting quantum interference device (SQUID) we made using niobium Josephson junctions integrates a two-junction SQUID and a superconducting feedback circuit. The switching probability, its derivative to input flux, and the flux spectral density measured by varying the sinusoidal bias frequency and amplitude agree well with thermal noise theory. The best sensitivity and cutoff frequency were 6.2×10−6 Φ0■√Hz and 1.1 kHz for a bias frequency at 10 MHz. The dynamic range was 2.5×105. These data show the SQUID to be suited to practical biomagnetic application.
The spectral density SΦ(f ) of the low-frequency 1/f noise of high transition temperature dc superconducting quantum interference devices (SQUIDs) with narrow linewidths was independent of B0, the magnetic field in which they were cooled, up to a threshold value, about 33 μT in the best case. Above this threshold, which is associated with the entry of flux vortices into the film, the noise increased rapidly. By contrast, for large square washer SQUIDs, SΦ(f ) scaled linearly with B0. Estimates indicate that the 1/f flux noise produced by the pickup loop of a directly coupled magnetometer is negligible.