Dry demagnetization cryostat for sub-millikelvin helium experiments: refrigeration and thermometry

摘要

We demonstrate successful "dry" refrigeration of quantum fluids down to$T=0.16$\,mK by using copper nuclear demagnetization stage that is pre-cooledby a pulse-tube-based dilution refrigerator. This type of refrigerationdelivers a flexible and simple sub-mK solution to a variety of needs includingexperiments with superfluid $^3$He. Our central design principle was toeliminate relative vibrations between the high-field magnet and the nuclearrefrigeration stage, which resulted in the minimum heat leak of $Q=4.4$\,nWobtained in field of 35\,mT. For thermometry, we employed a quartz tuning fork immersed into liquid$^3$He. We show that the fork oscillator can be considered as self-calibratingin superfluid $^3$He at the crossover point from hydrodynamic into ballisticquasiparticle regime.