YBCO DC SQUIDS UTILIZING SAPPHIRE STEP EDGE JUNCTIONS

We have tested a number of superconducting quantum interference devices which utilize a step edge grain boundary junction process on sapphire substrates [ l]. We measure white noise levels of about 7 mu Phi/root Hz and low frequency noise around 70 mu Phi(O)/root Hz at 1 Hz in a 16pH device at 77K. By modulating the bias curreat, we achieve only about a factor of 3 to 4 reduction in the amplitude of low frequency noise. We believe excess flux noise constitutes the remainder. A square washer configuration yields flux noise at 100 Hz of about 140 mu Phi(O)/root Hz giving a field sensitivity of 11 pT/root Hz. Measured hysteresis is about- 1 to -5X10(-3) in a low ambient field.