Browsing by Author "Kaivola, Matti, Professor"

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Experimental studies on dilute helium mixtures at low temperatures
(2012) Salmela, Anssi
School of Science | Doctoral dissertation (article-based)
When cooled to almost zero temperature, helium offers a very versatile subject for the experimental study of quantum matter. The two stable isotopes of helium, He-3, and He-4, and their mixtures, are the only macroscopic condensed matter systems that do not solidify at normal pressure even upon cooling to the absolute zero temperature. The cause for this is the large zero point energy compared to the interatomic bonds of the light noble gas. At extremely low temperatures the properties of the chemically identical isotopes differ considerably due to differences in quantum statistics. An especially interesting system is the dilute phase of the mixture of the isotopes, in which He-3 atoms form a rarefied fermi gas, whose properties can be continuously adjusted by varying the pressure and concentration, while the bosonic He-4 component remains in the background in the superfluid state. Also the He-3 component of the dilute mixture has been theoretically predicted to undergo a superfluid transition at very low temperature. This thesis presents new methods for the experimental study of helium mixtures at very low temperatures, as well as results on measurements on the properties of helium fluids at previously unstudied temperature and pressure range. Adiabatic melting is a cooling method in which the cooling, based on the mixing of helium isotopes, is utilized in conditions where the heat capacity of the refrigerated system is falling fast, permitting a very low final temperature. In the first millikelvin-range trial of the new method, the mixture was cooled to 300 microkelvin temperature. This is not a particularly outstanding result compared to the traditional cooling methods, but the experiment was still encouraging as the limitations were found to be technical and no fundamental obstacles for improvements were discovered. The properties of the helium sample were studied by a sensitive pressure gauge, and by small immersed mechanical oscillators. Using these, the saturation solubility of the mixture was determined over the widest possible pressure range from zero to the melting pressure of the mixture. In addition the osmotic pressure of the mixture for several concentrations at the melting pressure was measured at millikelvin-range temperatures. Traditional vibrating wires and quartz tuning forks were used as the oscillators. Quartz tuning forks were found to be feasible sensors, though acoustic phenomena related to the high resonance frequency complicated the response.