Noise and hysteresis in flux-locked TlBaCaCuO SQUIDs

Measurements of TlBaCaCuO-based dc superconducting quantum interference devices (SQUIDs) performed using a practical feedback circuit gave a flat frequency response and an energy sensitivity of 6×10-30 J/Hz at 5 kHz and 77 K with an 80 pH SQUID. Flux-locked loop operation was demonstrated to be strongly effective in eliminating the magnetic field hysteresis of the SQUID. The SQUIDs were operated in the flux-locked loop at temperatures over 90 K and the 1/f noise was found to decrease with increasing temperature near 77 K.