Progress in RF-SQUIDs

The author reviews recent developments towards practical RF-SQUIDs (superconducting quantum interference devices) made of high-T/sub c/ as well as conventional superconductors. It is noted that progress has been made in RF-SQUIDs towards practical low-noise sensors. The flux noise values obtained from planar thin-film devices made of both conventional and high-temperature superconductors are comparable to what is currently obtainable with DC-SQUIDs. RF-SQUIDs seem to offer somewhat lower values of low-frequency excess noise than DC-SQUIDs at frequencies below 1 Hz. To obtain optimum sensitivity, it seems advisable for RF-SQUIDs to use a cooled preamplifier. The losses in cooling liquid due to dissipation in the amplifier are negligible in the case of liquid nitrogen, and lead to an increased evaporation of liquid helium on the order of 100 ml/day. Starting from bulk material devices with relatively high flux noise levels, at present the best high-T/sub c/ RF-SQUIDs offer a field sensitivity of less than 200 fT/ square root Hz (at 1Hz). This is already sufficient for a number of biomedical applications.<>