Asymmetry in melting and growth relaxation of 4He crystals in superfluid after manipulation by acoustic radiation pressure

What is it about?

Using acoustic radiation pressure, we selectively drove both the melting and growth of 4He crystals in superfluid in desired surface orientations and discovered that the melting was much slower than the growth for the vicinal surface. This discovery is opposite to the fundamental knowledge in crystal growth physics and mineralogy. The melting had a complicated behavior in multiple stages showing anomalous shapes, indicating the important role of superflow on the crystallization phase transition. This has solved one of the fundamental problems in crystal growth and will also shed light on the hydrodynamics of Bose condensed systems which are actively discussed.

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Why is it important?

We succeeded in manipulating the crystal-superfluid interface of 4He in desired orientations by acoustic radiation pressure and discovered that the relaxation times to the equilibrium state after the manipulation were very different between in melting and growth. The melting relaxation was much slower than the growth relaxation for the c-facet and its vicinal surface. This asymmetry is opposite to the ordinary knowledge in crystal growth physics and mineralogy; melting is usually much faster than crystal growth. This asymmetry was apparent in anisotropic surfaces as the facets and vicinal surfaces, but did not show up on the isotropic rough surfaces. Furthermore, the melting relaxation had a complicated behavior in multiple stages showing the anomalous shapes, such as needle or irregular shapes, depending on temperature, while growth relaxation was simply back to the initial flat surface in a single stage. This indicates the important role of the superflow induced by anisotropic interface motion on the relaxation processes. We were able to explain the asymmetry and anomalous shapes semi-quantitatively by considering the effect of high velocity superflow.

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