Cooling fins to limit the hot-electron effect in dc SQUIDs

Abstract

The generally accepted noise theory of the dc SQUID predicts that the energy resolution scales as the electron temperature in the Josephson junction shunt resistors. As in metals at low temperature the electron-phonon coupling becomes very weak, the electron gas of the thin film shunt resistors undergoes a Joule heating due to the bias current and its temperature can be significantly higher than that of the thermal bath. This heating, the hot-electron effect, causes a deviation from the linear behaviour of noise versus temperature and a saturation of the SQUID noise, typically at temperatures of about 200 mK. This effect can be reduced considerably by increasing the effective volume available for the electron-phonon interaction by attaching "large" cooling fins to the shunt resistors. Our measurements have been performed on two thin film devices made with the same design of a dc SQUID but without the Josephson junctions: one device with standard shunt resistors, the other with shunt resistors with cooling fins. From these measurements one can expect for the SQUID with cooling fins an improvement of the noise saturation temperature of at most a factor 2, from 200 mK to about 100 mK.