Low-frequency noise reduction in Y-Ba-Cu-O SQUIDs by artificial defects

We demonstrate, that extremely simple arrangements of few antidots in HTS rf-SQUIDs can significantly reduce the 1/f noise in ambient field down to the level of zero-field noise. The onset field B_on at which the low-frequency noise starts to increase, is shifted from B_on≈8 μT without antidots to B_on≈40 μT with antidots for field cooled measurements and to B_on≈25 μT for zero field cooled experiments. The geometric arrangements of the antidots are obtained from the analysis of the current distribution in the washer and the position of penetrating vortices in the case of zero field cooled experiments