4He crystals in superfluid under zero gravity

What is it about?

We report the experimental realization of the equilibrium crystal shape (ECS) of 4He co-existing with the superfluid under zero gravity below 200 mK, utilizing the parabolic flight of a small jet plane. The huge growth coefficient causes a macroscopic size 4He crystal to be very sensitive to the Earth gravity and be significantly deformed from ECS. Thus, a gravity-free condition has to be achieved at very low temperatures to realize ECS in 4He. We developed a 3He-4He dilution refrigerator which meets strict requirements of experiments onboard a small jet plane and successfully observed a unique ECS of 4He under zero gravity.

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Why is it important?

The ECS concept was first proposed by J. Gibbs in the end of 19th century and then advanced by G. Wulff at the beginning of 20th century. Though the idea is so old and well established, no experimental realization of ECS has been attained in a macroscopic scale due to the complexity of the crystal growth in the ordinary crystals. The 4He crystal which is grown in the superfluid at very low temperature has been expected to be the unique exception, whose huge growth coefficient is ideal to realize ECS in a short relaxation time. Our finding in the novel multi-extreme conditions, zero gravity and ultra-low temperatures, made a new progress on the old but fundamental concept of ECS of the quantum crystal 4He.