Berlin 2015 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 6: Superconductivity: Cryodetectors
TT 6.2: Vortrag
Montag, 16. März 2015, 09:45–10:00, H 2053
Investigation of low-frequency excess noise in low-Tc dc-SQUIDs — •Sebastian Kempf, Anna Ferring, Andreas Fleischmann, and Christian Enss — Kirchhoff-Institute for Physics, Heidelberg University, Germany.
Dc-SQUIDs are the most sensitive wideband devices for measuring physical quantities that can be naturally converted into magnetic flux. They show exceptional low noise over the full frequency range and are therefore heavily used in a variety of applications. However, they exhibit a low-frequency noise contribution with a spectral density scaling as 1/fα arising either from critical current fluctuations or magnetic flux noise, e.g. due to surface magnetic moments. While the first seems theoretically well understood, the origin of excess magnetic flux noise is still unknown.
Very recently, we have fabricated a set of single SQUIDs as well as N-SQUID series arrays, both employing Nb/Al-AlOx/Nb Josephson junctions, and measured their noise spectra down to mK temperatures. We found that the magnitudes of the 1/fα noise expressed as energy sensitivities є1/f(1 Hz) are as low as 34 h at mK temperatures and that they increase with α. While α ≤ 0.7 for our single SQUIDs, our SQUID arrays show typically true 1/f noise with α ≃ 1. We discuss our current data set in the context of present theories and compare it with data reported earlier. We also discuss an experimental method allowing for a direct measurement of noise spectra of single SQUIDs without the need of subtracting preamplifier noise and show first experimental results obtained with this method.