Abstract
It is shown that NMR/NQR-thermometry on Ga single crystals can serve the need of absolute thermometry in a temperature range from below 20 μK to above 1 mK. Thus, the sensitive range includes all phase transition temperatures around 1 mK of solid and fluid 3 He, a necessity for an extension of the 3 He-melting-pressure temperature scale towards lower temperatures. The experiments were performed in magnetic fields of 50 to 200 mT and at temperatures down to 200 μK using single rf-pulses. Special care was taken for the orientation of the magnetic field with respect to crystal orientation which was obtained by in-situ pulsed NMR-experiments. It has been shown that in the investigated parameter range nonlinear effects in NMR due to high spin alignment had no significant influence. In pulsed NMR as well as in specific heat experiments it could be shown that spin-spin interactions are only weak in Ga and do not influence nuclear paramagnetism, at least at temperatures above 100 μK. The advantages of using Ga instead other elements for population difference thermometry are discussed.
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By a rf-combiner two rf-pulses can be added for the two required transitions and hence, the pulses can be applied and measured simultaneously (requirements are two independent frequency generators). We have proven that this technique works. For bulk metallic samples the obtainable Q-values are only around Q ~ 5. Thus, pulsed-NMR yields more then 100 times bigger signals than cw-NMR. Additional tuning of the sample coil to the two different resonance frequencies would certainly improve signal/noise, but requires two independent transmitter electronics and some skill in combining the two signal frequencies into one receiver system.
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Huebner, M., Leib, J. & Eska, G. NQR on Gallium Single Crystals for Absolute Thermometry at Very Low Temperatures. Journal of Low Temperature Physics 114, 203–230 (1999). https://doi.org/10.1023/A:1021810105778
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DOI: https://doi.org/10.1023/A:1021810105778