Asymmetry in melting and growth relaxation of⁴He crystals in superfluid after manipulation by acoustic radiation pressure

Abstract

The relaxation dynamics of the crystal-superfluid interface of 4 He after deformation induced by acoustic radiation pressure was investigated for various crystal orientations. The melting relaxation after growth was approximately 10 times slower than the growth relaxation after melting for vicinal surfaces and facets, while both relaxation times were consistent with each other for rough surfaces. The asymmetry in the time constant between the melting and growth of vicinal surfaces and facets can be qualitatively explained as the effect of superflow induced by local rapid interface motion, such as a quick rounding of facet edges of the 4 He crystal. Rough surfaces move more isotropically and no significant local rapid interface motion is induced; therefore, their relaxation is likely to be symmetric with a minimal effect of superflow. While the growth relaxation was simply back to the initial shape in a single stage, the melting relaxation was much more complex with multiple stages and the exhibition of various anomalous shapes depending on temperature. Anomalous shapes such as needle-like shapes during melting have a larger curvature and higher energy and thus should have disappeared more quickly than the growth shape with a smaller curvature, but they were considerably stable and disappeared slowly. This counter-intuitive asymmetry suggests the significant role of superflow in the relaxation process. 4 He quantum crystals in superfluid is the exceptional material to be manipulated by small driving forces such as heat, gravity, and flow [6-13], because they grow extremely fast at low temperatures and play a very important role in understanding the fundamental aspects of crystal growth physics [14][15][16][17]. In previous reports, we have demonstrated that acoustic radiation pressure, which is the small second-order acoustic force usually not relevant to the crystallization process [18,19], induces both growth and melting of 4 He crystals and is a very useful tool to manipulate the interface in a desired orientation, even to a large extent [17,[20][21][22].