Excess low-frequency noise in YBCO thin-film devices

Excess low-frequency noise degrades the performance of many of the high T/sub c/ superconducting devices that have been proposed or demonstrated. We report measurements of noise in grain-boundary junctions and flux-flow amplifier devices that have been fabricated in several different laboratories. The system is calibrated in terms of absolute noise temperature, and its voltage noise sensitivity is below 0.1 nVHz/sup -1/2/. Device characteristics and noise have been measured as a function of junction area, critical current, temperature, bias current and magnetic field. Noise levels in a fixed frequency band of 40-60 kHz are reported, as well as the frequency dependence between 0.1 Hz and 60 kHz. Noise temperatures exceeding the device physical temperature at certain levels of bias current are attributed to fluctuations in critical current and normal tunnelling resistance. Noise due to magnetic flux motion is also seen. We compare results with available theory and other relevant data in the literature, and assess the practical effects of noise on the performance of high T/sub c/ devices such as SQUIDs, flux flow amplifiers and bolometers.<>