Crystal growth and crystal curvature near roughening transitions in hcp 4He

Abstract

We present a quantitative experimental study of the implications of the roughening transition both on the equilibrium shape and the growth kinetics of hcp 4He crystals. On the one hand, we show that, in close analogy with the Kosterlitz-Thouless transition, the crystal curvature does not vanish continuously at the transition. The corresponding value of the surface stiffness is found in good agreement with recent universal predictions. On the other hand, the growth rate of facets increases near their roughening temperature. For the (0001) facet, the dominant process is the two dimensional nucleation. It allows us to measure the variation of the step energy near the transition. Lastly, the facet mobility also increases when the temperature tends to zero. This shows that the mobility of crystalline steps is limited by their interaction with thermal excitations.