Characteristics of DC SQUIDs Patterned on Highly Granular Y–Ba–Cu–O Thin Films

Measurements have been made of the properties of thin granular film DC SQUIDs fabricated from the high Tc superconductor Y–Ba–Cu–O, paying special attention to peculiar effects arising from the unique granularity of the film. The output voltage modulation of the SQUIDs was found to be very much lower than expected from the theory, and we present here several possible explanations for it, based on such factors as the asymmetry of the junctions and high kinetic inductance of the film. Multiple periodicities and phase jumps on VΦ-characteristics were observed. Normal operations of the SQUIDs were observed till ∼65 K. Estimating from the best data, the equivalent flux noise was below 3×10-4 Φ0/Hz1/2 in the frequency range above 600 Hz at around 50 K.