Experiments on the faceting and supersolidity in helium crystals

This Thesis describes investigations on the solid-liquid interface of helium. Helium crystals represent a unique model system for the studies of the crystalline phenomena, such as faceting and crystal growth, and their surface can be studied, in principle, down to the zero temperature quantum limit (0 K = -273.15°C). Helium also becomes extremely pure at low temperatures as all the impurities except the isotopical ones freeze out. In this Thesis the solid-liquid interfaces of both helium-3 and helium-4 have been studied using a Fabry-Pérot interferometer and a high-accuracy pressure gauge. The optical studies on the faceting of helium-3 crystals have revealed that the quantum motion of the interface, which keeps the solid-liquid interface rough (not faceted) down to 0.1 K, becomes damped at low temperatures. The quantum fluctuations of the interface become more and more damped due to the Fermi-degeneracy of the liquid which creates a bottle-neck for the spin transport through the moving interface. As a result, facets start to appear and, finally, at the lowest temperatures below 0.001 K the solid-liquid interface becomes so localized that it resembles the surface of classical crystals. The melting curve of high-quality helium-4 crystals has been measured between 0.01-0.32 K with an accuracy of 0.1 Pa without finding any entropy signature which the possible supersolid transition could cause. The entropy below 0.3 K was attributed to phonons in solid and in liquid and the upper limit of the non-phonon entropy was set to 5 · 10-8 R. The measurements on the thermal expansion of liquid helium-4 between 0.02-0.72 K in constant volume pointed out that the rotons start to contribute to the thermodynamics of the solid-liquid interface of helium-4 above 0.3 K. In order to study the role of defects in the possible supersolidity, also the stacking faults were studied on helium-4 crystals