Characteristics of YBa2Cu3O7_δ Grain Boundary Junction DC-Squids

We report on the characteristics of YBa2Cu307_δ grain boundary junction (GBJ) dc-SQUIDs with a square washer geometry design. The current-voltage characteristics of these SQUIDs are close to those predicted by the resistively shunted junction (RSJ) model. Their flux-voltage characteristics are perfectly periodic from 4.2 to 87K. The noise characteristics have been measured as a function of frequency, temperature, bias current, and applied magnetic flux. At frequencies above about 1kHz kHz the magnetic flux noise energy of the GBJ-SQUIDs is frequency independent and close to the theoretical value estimated from the Nyquist noise in the junction normal resistance. At lower frequencies the noise scales approximately as 1/f. Above about 82-85K the 1/f noise is dominated by flux noise presumably caused by thermally assisted flux motion in the film. Below this temperature it is dominated by voltage noise across the junctions. © 1991 IEEE