4He crystals in superfluid under zero gravity

What is it about?

Although it is hard to imagine that ordinary crystal change in shape when gravity is reduced for a short period, we observed that 4He crystal in superfluid quickly responded to the gravity change from 1G to 0G. Crystal growth rate of the 4He crystal depends strongly on temperature and this response was observable only at very low temperature below 630 mK. We achieved this condition by developing a special 3He evaporation refrigerator compatible with the parabolic flight of a jet plane. In the zero gravity condition, we demonstrate by a direct visualization that a huge 4He crystal relaxes to a shape for minimization of the surface free energy and that a wall-free 4He crystal floats and drifts with a rotation in superfluid.

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Why is it important?

The equilibrium shape of 4He quantum crystals in a superfluid was observed in a zero gravity environment utilizing the parabolic flight of a jet plane. A very fundamental question in crystal shape is what is the equilibrium shape minimizing the total surface energy. Relaxation to the equilibrium shape, however, is a very slow process in ordinary classical crystals and thus the crystal shape observed is the quenched shape in most cases. A 4He crystal growing from the superfluid is an exceptional material which relaxes very quickly, although 4He crystal is easily deformed by the gravity on the ground. In this paper, we succeeded in visualizing the undeformed 4He crystals under zero gravity. Several facets emerged and enlarged on the surface when the gravity was rapidly reduced to zero. We also observed that 4He crystals floated and drifted in the superfluid. This demonstration of a gravity-free 4He crystal will be the first step to settle important issues regarding the physics of crystal shapes.