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Journal of Applied Physics

Behavior of a multiloop dc superconducting quantum interference device

P. Carelli, Vittorio Foglietti

Published Nov 1, 1982• 55 citations
Publisher

Abstract

Measurements of low inductance dc superconducing quantum interference devices (SQUIDs) are presented. The SQUIDs used for such measurements are made using a photolithographic technique similar to that one developed by IBM for the superconducting computer. On the same chip a coil is evaporated with a coupling coefficient k to the SQUID larger than 0.33. The best measured intrinsic energy sensitivity is 4 h. Possible improvements in the design are proposed.

DOI
10.1063/1.330130

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Measurements of low inductance dc superconducing quantum interference devices (SQUIDs) are presented. The SQUIDs used for such measurements are made using a photolithographic technique similar to that one developed by IBM for the superconducting computer. On the same chip a coil is evaporated with a coupling coefficient k to the SQUID larger than 0.33. The best measured intrinsic energy sensitivity is 4 h. Possible improvements in the design are proposed.
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The energy sensitivity of a direct current (dc) superconducting quantum interference device (SQUID) can be improved if it is operated in a two-stage configuration. Employing this technique, a commercial dc SQUID system was modified and made competitive with other sensors especially designed for very low noise applications. We report the noise measurements performed in the temperature range 4.2 K–25 mK. At 4.2 K, the coupled energy sensitivity obtained with the two-stage dc SQUID was approximately ten times better than with a conventional readout electronics. The noise energy decreases linearly until approximately 300 mK, in good agreement with theoretical previsions. At lower temperature the hot-electron effect produces a saturation and the best energy sensitivity measured with open input coil is 35 ℏ.
We optimized the design and operation of a low-Tc direct current superconducting quantum interference device (dc SQUID) with an integrated coupling coil of 1.5 muH inductance taking into account typical effects observed for similar devices. Numerical simulations were performed on a model including the capacitance of the Josephson junctions, thermal noise of the integrated shunt- and damping- resistors as well as a complex frequency dependent impedance of the SQUID loop originating from the integrated coils. The experimentally and numerically determined characteristics and sensitivity are in good agreement. A minimum additional coupled energy resolution of 700 (h/2p) and 250 (h/2p) was measured at a temperature of 4.2 K and 1.5 K, respectively.
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A low-noise coupled DC superconducting quantum interference device (SQUID) especially optimized for low frequency is discussed. Using large Josephson junctions and a low loop inductance the contribution of the critical current fluctuation to 1/f noise can be minimized. To minimize the inductance the Josephson junctions are placed in the center of the washer close to the SQUID loop. A scaled-up copper model of the SQUID and the signal coil is used to analyze the impedance of the SQUID loop affected by the signal coil. An equivalent circuit model describing the effective inductance of the SQUID loop as a function of the frequency is used to design appropriate damping of the resonances. The DC SQUID characteristics are smooth and the noise performance of the SQUID does not markedly suffer from resonances. The contribution of the 1/f noise at 1 Hz is found to be about 5*10/sup -7/ Phi /sub 0// square root Hz.>