Neuartige Tankschwingkreise für Hochfrequenz-SQUIDs

Abstract

SQUIDs (Superconducting Quantum Interference Devices) are flux-to-voltage transducers, providing an output voltage that is periodic in the applied flux with aperiod of one flux quantum, $\Phi_{0} \equiv$ h/2e $\simeq$ 2.07 x 10$^{-15}$ Wb. They can measure fractions ($\sim$ 10$^{-7}$) of $\phi_{0}$ and are the most sensitive sensors of magnetic flux (and field) known today. SQUID research was intensified due to the discovery of high-temperature-superconductors (HTS). Applications of HTS SQUID magnetometer systems indude e.g. biomagnetic diagnostics, nondestructive evaluation of materials and geomagnetic exploration. The single junction SQUID involves a Josephson junction interfering the current flow around a superconductiong loop and is operated with a radiofrequency (rf SQUID) or highfrequency (hf SQUID) flux bias provided by a tank circuit. At present, the intrinsic and practical limits of their magnetometer performance are far from being attained. The basic issue of this work was to analyze characteristic features and the capability of improvement with the aim to find new design concepts for lower field resolutions. For achieving that aim, the tank circuit is the potentially most effective part of the sensor. In principle, an improved magnetometer performance could be reached by increasing the bias frequency and the quality factor of the tank circuit while its effective coupling strength to the SQUID should be adjusted dose to a minimum treshold value. Since theoperation frequency of hf SQUIDs entered the GHz range, it was important to investigate their hf-characteristics in relation to magnetometer attributes. An intensive inter action of appropriate knowledge from hf and sensor technology has, therefore, been started in this work. Note that due to the experience of this work, the optimization of the tank circuit and SQUID design should not be performed seperately because the coupling between tank circuit and SQUID is a central and critical aspect for the magnetometer performance. A new simulation method has been developed to characterize the basic hf and magnetometer features of hf SQUIDs, especially the coupling between tank circuit and SQUID structure. New measurement techniques have been explored to obtain important hf and SQUID parameters and a complete signal analysis, which are usually not obtainable from standard SQUID electronic readouts. The new simulation and measurement methods have been testet with several existing hf SQUID structures and studies of different coupling arrangements (e.g. S-resonator SQUIDs with indirect or direct coupling, with or without flux focussing pads) and hf-environments (one or two port readout scheme, diode or mixer detector) were performed. The gained know-how supported the inventions ofseveral new hf SQUID concepts. One attempt was to use the SQUID loop with the surrounding flux focussing structure as an integrated hf resonator which renders an external tank circuit superfluous and turned out to function. Other ideas were to use HTS planar high-Q resonators (like stripline resonators or even lumped element resonators) or dielectric high-Q resonators coupled to washer SQUIDs, preferably as endplates in a modular arrangement. At 77 K, field resolutions of 130 fT / $\sqrt{Hz}$ (LC resonator) and 105 fT / $\sqrt{Hz}$ (dielectric resonator) were obtained, one order of magnitude lower compared to the well known S-resonator SQUIDs. Further developments of the new ideas should lead to even more sensitive single junction SQUID magnetometers