Characteristics of YBa2Cu3O7- delta grain boundary junction DC-SQUIDs

The characteristics of YBa₂Cu₃O_{7- delta} grain boundary junction (GBJ) DC-SQUIDs with a square washer geometry design are discussed. 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 87 K. The noise characteristics have been measured as a function of frequency, temperature, bias current, and applied magnetic flux. At frequencies above about 1 kHz the magnetic flux noise energy of 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 82-5 K 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.