K. Juntunen, J. Muhonen, E. Pentti, A. Salmela, A. Sebedash, J. Tuoriniemi.
The research efforts at the YKI-cryostat have been shared between two main projects: studies of self-cooled 3He-4He mixtures close to the melting pressure and investigations on nuclear magnetism in pure metals, most recently in lithium. The ultimate goal of the helium mixture experiment is to search evidence for superfluidity of dilute 3He.
The demagnetization experiment on lithium was carried to the end at the turn of the years 2003–2004. The ordering temperature of the nuclear spin system was established to be 300 nK at zero field. The wealth of data collected during the last year were further analyzed to construct the ordering phase diagram and to better understand the behavior of the spin system.
The mixture experiment will utilize a completely new cooling method entitled “adiabatic melting”. The mixture in the experimental chamber will be pressurized at a low temperature until the 4He component will solidify at about 2.5 MPa. 4He can be transferred in to and out of the cell through a superleak filling line, even though ordinary capillaries will be blocked by solid helium under the conditions of the experiment. 3He will be expelled from the 4He crystal and float on top of the solid phase. There, it will be cooled across the superfluid transition temperature by our copper nuclear refrigeration stage. Subsequently, the 4He crystal will be melted by removing some of the 4He through the superleak, and the 3He will be diluted by the remaining liquefied 4He being released from the solid. This produces further cooling due to the absorbed heat of mixing. The cooling ratio increases as the temperature goes down and the minimum achievable temperature is determined by the residual heat leak to the liquid.
The main experimental chamber was cooled down to the microkelvin regime for the first time in the midst of the report year. No serious flaws were observed and the cell could be refrigerated down to about 0.2 mK, lower than required for successful adiabatic melting, whether or not it was filled with liquid helium mixture. Some of the diagnostic devices were still not operable and the superleak filling line was not installed, so that the final cooling performance had yet to remain to be demonstrated. The missing items were worked on at the end of the year and the completed setup will be ready to be cooled down soon next year.