Excess low-frequency noise in YBCO rf SQUIDs in weak magnetic fields

Abstract

Thin-film HTS SQUIDs operated at 77 K and exposed to weak magnetic fields exhibit significant excess low-frequency noise arising from thermally-activated hopping of flux trapped in the superconducting film. We report an investigation of the dependence of this phenomenon on SQUID design and fabrication, measurement conditions and magnetic field history. The level of excess noise was directly related to the amount of flux penetrating the SQUID, and consequently was worse in large SQUIDs than in small SQUIDs due to the greater flux focussing of the larger SQUID. In SQUID fabrication, good film quality (high J_c) was found to be essential to minimize low frequency noise and careful patterning was required to avoid degrading the film. The method of cooling the SQUID was found to strongly affect the level of excess noise, with cooling in the magnetic field in which the SQUID was to be operated being preferable to zero-field cooling. The excess noise was typically 10 pTHz^−1/2 at 1 Hz for 150 pH rf washer SQUIDs having a white noise floor of about 1 pTHz^−1/2 operated in an applied field of 50 μT.