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Complex inductance, excess noise, and surface magnetism in dc SQUIDs.
S Sendelbach1, D Hover, M Mück
1Department of Physics, University of Wisconsin, Madison, Wisconsin 53706, USA.
Physical Review Letters
|October 2, 2009
Summary
We studied the complex inductance of direct current superconducting quantum interference devices (dc SQUIDs) at millikelvin temperatures. Inductance fluctuations correlate with flux noise, suggesting surface spin reconfiguration impacts device performance.
Area of Science:
- Superconducting quantum devices
- Low-temperature physics
- Quantum sensing
Background:
- Direct current superconducting quantum interference devices (dc SQUIDs) are sensitive magnetic flux detectors.
- Understanding noise sources in dc SQUIDs is crucial for improving sensor performance.
- Millikelvin temperatures present unique challenges and phenomena in superconducting devices.
Purpose of the Study:
- To characterize the complex inductance of dc SQUIDs at millikelvin temperatures.
- To investigate the temperature and frequency dependence of SQUID inductance.
- To identify the underlying physical mechanisms responsible for observed inductance fluctuations.
Main Methods:
- Experimental cooling of dc SQUIDs to millikelvin temperatures.
- Precise measurement of complex inductance as a function of temperature.
- Analysis of inductance fluctuations and their power spectral density.
- Correlation analysis between inductance noise and flux noise.
Main Results:
- The dc SQUID inductance exhibits complex, history-dependent behavior with temperature.
- Inductance fluctuations of the order of 1 femtohenry (fH) were observed.
- A 1/f power spectrum characterizes inductance fluctuations at fixed temperatures.
- Inductance noise is strongly correlated with conventional 1/f flux noise.
Conclusions:
- The observed inductance behavior is attributed to the reconfiguration of surface spin clusters.
- Correlated fluctuations in effective magnetic moments and relaxation times of surface spins influence SQUID inductance.
- This work provides insights into noise mechanisms in sensitive superconducting devices at ultra-low temperatures.
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