Riekki, Tapio
(Helsingfors universitet, 2016)
Helium has two stable isotopes: more common 4He with four nucleons, and the very rare 3He with three nucleons. At sufficiently low temperature, helium can become superfluid that has no viscosity. This transition is quantum mechanical in nature, and since bosonic 4He and fermionic 3He follow different quantum statistics, there is a significant difference in the transition temperature between them. It is about 2 K for pure 4He, but for pure 3He it is three orders of magnitude lower, around 1 mK.
3He – 4He mixtures also have several interesting properties at very low temperatures, such as the finite solubility of 3He in 4He even at absolute zero limit. However, at kelvin range, where our experiment took place, the notable feature is the shifting of the supefluid transition temperature of 4He to a lower temperature due to addition of 3He.
Bulk superfluid helium can support two different sound modes: first sound is ordinary pressure (or density) wave, whereas second sound is a temperature (or entropy) wave, unique to superfluid systems. In inviscid superfluid systems, temperature fluctuations can propagate as second sound wave, but in normal systems, on the other hand, this is not possible, as all temperature fluctuations are strongly damped. First sound and second sound do not usually exist independent of each other, rather pressure variations are accompanied by variations in temperature, and vice versa.
In this thesis, we studied experimentally the coupling between first and second sound in dilute 3He - superfluid 4He mixtures, at saturated vapor pressure, at temperatures between 2.2 K and 1.7 K, and at 3He concentrations ranging from 0 % to 11%, using a quartz tuning fork mechanical oscillator. Second sound that is coupled to first sound can create anomalies in the resonance response of the quartz tuning fork, so-called second sound resonances. We learned that there exists a temperature and concentration region, where these anomalies disappear, which would indicate two sound modes decoupling from each other. We also present a hydrodynamical model that correctly predicts the decoupling behavior.