Berlin 2015 – scientific programme
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TT: Fachverband Tiefe Temperaturen
TT 6: Superconductivity: Cryodetectors
TT 6.5: Talk
Monday, March 16, 2015, 10:30–10:45, H 2053
SQUID-based noise thermometer for sub-Millikelvin refrigerators — •Marco Schmidt1, Jörn Beyer1, Monique Klemm1, Sassan Alivaliollahi2, and Henry Barthelmess2 — 1Physikalisch-Technische Bundesanstalt, Abbestraße 2-12, 10587 Berlin — 2Magnicon GmbH, Barkhausenweg 11, 22339 Hamburg
The magnetic field fluctuation thermometer (MFFT) is a high-accuracy SQUID-based noise thermometer suitable for sub-Kelvin thermometry. A highly sensitive low-Tc SQUID magnetometer detects inductively the magnetic field fluctuation above a metal surface. The fluctuations are generated by the thermal activated noise currents inside the metal body that is thermally anchored to the temperature stage to be measured. The spectral shape is independent of temperature as the electrical conductivity is constant and the geometry is fixed. The magnetic noise power spectral amplitudes at any frequencies are directly proportional to temperature. Hence, only one reference measurement at a known temperature is required for calibration. A complete MFFT thermometer system for the temperature range of ca. 4 K down to <10 mK is commercially available. We have now developed an integrated MFFT with an extended range of operation down to <1 mK. For this purpose the sensitivity of the SQUID sensor has been increased, the metal body geometry modified and the magnetic shielding of the MFFT module improved. These modifications make it possible to obtain a thermometer noise temperature of <10 µK. We discuss the rationale for our MFFT configuration and present numerical simulations and experimental results.