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Very malleable solids

Computer simulations help explain the flexibility of helium crystals

In solid helium, the defects (blue, white and green spheres) dislocate along the crystal layers

Maurice de Koning / Unicamp In solid helium, the defects (blue, white and green spheres) dislocate along the crystal layersMaurice de Koning / Unicamp

Crystals composed of atoms of the chemical element Helium-4 behave in a surprising way. When they are cooled to temperatures near absolute zero (-273°C), these crystals, created in a laboratory, acquire unexpected plasticity. They go from being rigid like a rock to becoming as malleable as modeling clay.

Discovered in 2013, this property — the huge plasticity of solid helium — is still little understood. Computer simulations of the behavior of the helium atoms are beginning to reveal how it comes about. “Our simulations suggest that small defects in the crystal structure play an important role in determining the ability of solid helium to deform plastically,” says physicist Maurice de Koning, of the University of Campinas (Unicamp), one of the authors of the study published in July 2016 in Physical Review Letters.

Helium is the lightest chemical element after hydrogen. At room temperature and atmospheric pressure, it is a gas. Helium becomes a liquid only when cooled to temperatures near absolute zero. This change of state occurs both for the variety with an atomic mass of three, in which the atom consists of two protons and one neutron, and for the variety with an atomic mass of four, containing two protons and two neutrons. If, in addition to being cooled, it is also placed under high pressure, helium-4 freezes, becoming a crystalline solid. In the crystal, the atoms arrange themselves in a relatively uniform geometric pattern made up of piled, flat layers.

Like all crystals, however, this structure has defects: the absence of atoms in some points or atoms dislocated from their ideal positions. Some of these defects take the form of long, fine lines. When a given force is applied to the crystal, these defects can migrate along a single layer, causing the layers to move with respect to one another, deforming the material. “The easier these defects can move, the lesser the force needed to deform the material,” explains De Koning.

Applying the laws of quantum mechanics, De Koning and two colleagues from the United States ran a computer simulation to determine what happens in a helium-4 crystal containing 8,000 atoms (a millimeter-sized crystal grown in a laboratory contains billions of atoms). The virtual crystal was made up of perfect layers, except for two defects, each a few dozen atoms in length.

The simulations indicate that, even at such low temperatures, fluctuations in the positions of the helium atoms cause the defects to move around all the time, spontaneously, even when no force is acting on the crystal. “Their mobility is extremely high,” says De Koning. “At low enough temperatures, you do not need to apply much force to cause them to start to move.”

“Their calculations agree precisely with observations [in tests with real crystal samples],” says physicist Sébastien Balibar of the Paris Normal Superior School, in France, whose team discovered the giant plasticity of helium. According to Balibar, the displacement of defects in the crystal is the same phenomenon that causes conventional metal alloys, such as steel, to become malleable at high temperatures. “While the defects in these materials require high temperatures and strong forces to move, quantum effects cause the defects in solid helium to move rapidly under the exact opposite conditions,” he observes.

CCES – Center for Computational Engineering and Sciences (nº 2013/08293-7); Grant Mechanism Research, Innovation and Dissemination Center (RIDC); Principal Investigator Munir Salomão Skaf (Unicamp); Investment R$ 14,009,150.98 (for the entire project).

Scientific article
BORDA, E. J. L.; CAI, W. E DE KONING, M. Dislocation structure and mobility in hcp 4He. Physical Review Letters. July 22, 2016