COMPARATIVE NOISE MEASUREMENTS IN YBCO STEP-EDGE AND BI-CRYSTAL GRAIN-BOUNDARY JUNCTIONS

A system has been developed for the absolute measurement of excess noi se in high-T(c) superconducting devices. Grain-boundary Josephson junc tions of step-edge and bi-crystal type, fabricated in four different l aboratories, have been characterised and compared in the frequency ran ge 0.1 Hz-60 kHz, and at temperatures between 35 K and 80 K. All junct ions tested had bias-dependent noise peaks ranging in amplitude (at 60 kHz) from 0.2 to 5 nV Hz-1/2, which generally increased with decreasi ng temperature. The noise power spectral density varied with frequency as 1/f. Higher levels of noise, and a more complex current dependence , were found for small-area bi-crystal junctions, and for step-edge ju nctions generally. The results are compared with theory, and the impli cations for the development of low-noise devices (e.g. SQUID's) are di scussed.