Thin film Niobium SQUIDs at 20 MHz and 9 GHz

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Abstract
Niobium thin film SQUIDs containing various types of weak link configurations have been studied at 20 MHz and at 9 GHz. Structures seen in the microwave response of these devices are similar to the hysteretic steps seen in the normal to superconducting transition of soft superconducting microbridges which have been attributed to the generation of phase slip sites in the weak link. The quasiparticle diffusion length for niobium associated with the phase slip sites is nearly temperature independent, and is found to be less than a micrometer in length, approximately an order of magnitude shorter than the corresponding value for tin. The temperature range of operation and the noise characteristics of these devices operated as rf-SQUIDs at 20 MHz appear to vary inversely with the ratio of the quasiparticle diffusion length to the zero temperature coherence length inferred from the microwave data. The minimization of this ratio should result in better SQUID response at all excitation frequencies.