4 He Crystals in Reduced Gravity Obtained by Parabolic Flights of a Jet Plane

• Published in: Journal of low temperature physics Vol. 185; no. 3; pp. 295 - 304
• Main Authors: Takahashi, Takuya, Nomura, Ryuji, Okuda, Yuichi
• Format: Journal Article
• Language: English
• Published: New York Springer Nature B.V 01.01.2016
• Subjects: Boundary conditions; Crystals; Deformation; Dilution; Equilibrium; Fluids; Free energy; Jet aircraft; Microgravity; Parabolic flight; Radiation pressure; Relaxation time; Sound waves; Superfluidity
• ISSN: 0022-2291; 1573-7357
• DOI: 10.1007/s10909-016-1592-y

4 He crystals usually sink to the bottom of the container in a superfluid and are deformed into a flat shape by gravity when their size is much larger than the capillary length of 1 mm. When gravity is reduced to zero, the capillary length diverges and the gravity-flattened crystals are expected to relax into an equilibrium crystal shape determined by the interfacial free energy at low enough temperatures where the relaxation time is very short. We performed a reduced gravity experiment on 4 He crystals at ultralow temperatures by developing a specially designed 3 He– 4 He dilution refrigerator compatible with the experimental restrictions in a small jet plane. 4 He crystals relaxed to the equilibrium crystal shape below 600 mK during a reduced gravity period of 20 s produced by a parabolic flight. The equilibrium crystal shape, however, was metastable in most cases, governed by the boundary conditions imposed by the wall. Utilizing acoustic radiation pressure, we deformed the crystal enough to allow it to escape from the metastable shape below 150 mK. After this large deformation, the crystal relaxed to a shape completely different from its initial shape, showing three types of facets, viz., c-, a-, and s-facets, which was concluded to be the lowest energy equilibrium shape.