Excess noise in dc SQUIDs from 4.2K to 0.022K

Four types of excess noise have been identified in dc SQUIDs operated in the temperature range 0.022 to 4.2 K. At temperatures between about 2 and 4 K, the spectral density of the low frequency flux noise of a wide variety of thin-film dc SQUIDs scales as 1/f^mwhere m = 1.0 ± 0.1. In SQUIDs with Nb loops the noise originates as an "apparent flux noise", whereas in those with Pb or PbIn loops the noise is substantially lower and originates in critical current fluctuations. When any of these devices is cooled to temperatures below about 0.5 K, the spectral density of the excess flux noise scales as 1/f^m, with m = 0.66 ± 0.08 in most cases, and the noise always originates as an apparent flux noise. At the lowest temperatures, the white noise saturates at an effective temperature of about 150 mK; this excess noise probably arises from self-heating in the resistance shunting each tunnel junction.