Hot electron effect in the DC SQUID

The authors investigated the temperature dependence of the noise in thin-film DC superconducting quantum interference devices (SQUIDS) down to 20 mK. The white noise measured in the early versions of the SQUIDS did not decrease as the bath temperature was lowered below 150 mK. The authors have attributed this saturation to a hot electron effect in the thin-film AuCu resistors shunting the Josephson junctions. A theoretical investigation showed that the temperature of the electrons in the shunts should be given by T_e=(P/ΣΩ)^1/5, where P is the power dissipated in the shunts, Ω is the shunt volume, and Σ is a proportionality constant. Experimentally, the authors found ε=(2.4±0.6)×10⁹ WK^-5 m^-3. The shunts were redesigned, adding large thin-film cooling fins, to increase their volume substantially. This technique has reduced T_e to about 50 mK, with a corresponding improvement in the sensitivity of the SQUIDS