HIGH-SENSITIVITY MICROWAVE RF SQUID OPERATING AT 77-K

High-T(c) RF SQUIDS with bias frequencies in the microwave range exhibit low noise and high energy resolution compared with conventional 20 MHZ RF SQUIDS. Especially interesting for applications is the absence of any low-frequency excess noise above about 1 Hz. Due to a relatively low flux focusing factor of the SQUID body, the field sensitivity of bare SQUIDs is only of the order of 1 pT Hz-1/2. In this work, several modifications of the SQUID geometry were studied in some detail to decrease the flux noise further and to enhance the field sensitivity.