Investigations of Solid-Liquid Interfaces in Helium at Ultralow Temperatures

This Thesis describes ultralow temperature studies of helium quantum crystals. Owing to the surrounding superfluid, small latent heat of crystallization and correspondingly short relaxation times, which are unreachable in ordinary crystals, helium crystals offer a unique and clean modeling system to study surface phenomena in a solid. The measurements of the crystal shape and growth rates are essential in providing the microscopic understanding of crystal growth. Optical observations are probably the most direct way to quantify the surface of crystals. The results presented in this Thesis were obtained with the help of two very powerful experimental techniques that were successfully adopted for ultralow temperature applications: optical interferometry and high-precision pressure measurements. The optical investigations on He crystals revealed altogether eleven types of facets at temperatures well below 1 mK, while previously only three facet types have been seen. The growth rates of rough and smooth surface states were explored and show significant anisotropy. The measured growth velocities of different facet types indicate that the main growth mechanism is spiral growth in the regime of suppressed mobility. Important thermodynamic parameters of an interface such as the width of an elementary step and the step free energy were directly deduced from the observed growth kinetics. Results suggest that coupling of the interface to the underlying crystal lattice is relatively “strong” in He crystals. Measurements of the spiral growth of the c-facet on He crystals in the presence of a small number of He atoms were also conducted. They show suppression of the crystal growth velocity with the increase of the He atom concentration and indicate “weak” coupling of the interface to the crystal lattice in He.