Verlag des Forschungszentrums Jülich
JUEL-3747
Different layouts of superconducting planar resonators have been tested. For the optimization of rf SQUID's sensitivity, several factors have been taken into account: an appropriate frequency range, a good high-frequency coupling coefficient between resonator and SQUID, a large magnetic focusing factor. The resonance frequency and coupling coefficient dependence on layout design were systematically studied. A good understanding of the planar resonator for rf SQUID was obtained.
The rf SQUID has two operation mode: hysteretic mode and nonhysteretic mode. By measuring the resonance frequencies quality factors of rf SQUID at different external magnetic fields, the operation mode of HTS rf SQUIDs was determined, and the rf SQUID parameters coupling coefficient k2 , hysteresis parameter [beta]L (Ic), the normal resistance Rn were measured indirectly. The critical current Ic and the Rn showed a good agreement with those measured directly after opening the SQUID loop. By measuring the temperature dependence of SQUID's flux noise, the optimal working conditions were obtained at temperatures where the resonance frequency modulation by external magnetic field was nearly maximal.
Some predictions of the recently developed theory of rf SQUID at large thermal fluctuations were checked experimentally by: determining the operation mode of HTS rf SQUID at [beta]L >1; comparing the theoretically extracted rf SQUID parameters with those measured directly; comparing the theoretical flux noise with noise measured experimentally. In all these three aspects a good agreement between theory and experiment was found. My experiments showed that at large thermal fluctuations the rf SQUID can operate in nonhysteretic mode at [beta]L exceeding 3 for SQUID with loop inductance Ls=260pH.
Using a phase-sensitive detector and a amplitude detector, the role of phase signal in nonhysteretic rf SQUID's flux-voltage ([Theta]-V) transfer function was investigated. The experiment showed that at optimal operating conditions the contribution of phase signal to the [Theta]-V transfer function is small.
For the practical application of rf SQUIDs, a first-order axial electronic gradiometer was constructed. The gradiometer has a near field resolution of about 35 fT/Hz. Using the signal either of the lower magnetometer or the gradiometer high quality heart signal could be collected in a magnetically shielded room of moderate shielding factor.
Zeng, Xianghui
Experimental study of HTS rf SQUIDs at large thermal fluctuations
107 S., 2000
This dissertation describes experimental work on the superconducting YBa2Cu3O7-x (YBCO) planar resonators, the properties of high-Tc superconductor (HTS) rf superconducting quantum interference devices (SQUIDs) at large thermal fluctuations, and the practical application of HTS rf SQUIDs.
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