At low temperatures, helium offers a unique system for the study of quantum mechanical effects on a macroscopic scale. It is the only known condensed matter system, which remains liquid down to the absolute zero temperature. For this reason, helium forms a unique quantum liquid, which can be studied in laboratory conditions. The two stable isotopes of helium, 3He and 4He, obey the two different quantum statistics, 3He being a fermion and 4He a boson. They both undergo a transition into a superfluid state at low temperatures. A new interesting system is obtained by mixing the isotopes. Remarkably, 3He has a finite solubility in 4He even at zero temperature. Dual Fermi-Bose superfluidity of helium mixtures has been predicted to exist, but it has not been observed experimentally. The difficulty is the required extremely low temperature. Adiabatic melting of solid 4He in the presence of liquid 3He is a promising new cooling technique, which is hoped to produce the superfluid transition of helium mixtures. We have been preparing an experiment, which would implement this concept, but due to the complicated experimental setup, we have not yet been able to harvest the full power of the method. The results of this thesis revolve around supporting work for this experiment. In the course of the present studies, some aspects have generated deeper interest in their own right. In this thesis properties of liquid and solid helium mixtures are studied computationally and experimentally. Interactions between 3He atoms dissolved in superfluid 4He are investigated by analyzing experimental data. These interactions determine the transition temperature of the relished superfluid state of helium mixtures. The obtained model for the interactions is further used to calculate other quantities of interest. Retrieving the properties of liquid helium requires novel tools. One such tool, which was introduced to helium research not too long ago, is the quartz tuning fork. It is a mechanical oscillator, whose resonant behavior depends on the surrounding fluid environment. Its complicated geometry presents difficulty in analyzing its characteristics. Numerical methods are utilized in this thesis to understand effects of liquid helium on the quartz tuning fork response.
|Translated title of the contribution||Mikroskooppisia ja makroskooppisia tutkimuksia nestemäisissä ja kiinteissä heliumseoksissa|
|Publication status||Published - 2013|
|MoE publication type||G5 Doctoral dissertation (article)|
- helium-3, helium-4, helium mixtures, superfluid, melting pressure, solubility, osmotic pressure, quartz tuning fork, acoustic emission, second sound