Quantum crystals : how the supersolid fever led to the discovery of a giant plasticity

Orateur : Sébastien BALIBAR (Directeur de Recherches au CNRS, Membre de l'Académie des Sciences, Laboratoire de Physique Statistique de l'ENS)

Sometimes one looks for something and one finds something else. The 2004 experiments by Kim and Chan triggered an intense activity on a paradoxical question : could solid helium-4 be elastic and superfluid at the same time ? 9 years later, we have discovered that helium-4 crystals present a giant plasticity in the zero temperature limit if all their impurities are suppressed. Ultrapure helium crystals do not resist to an applied shear stress in one particular direction, like a pile of paper sheets that can glide over each other when submitted to a horizontal shear. This phenomenon is a spectacular example of ”plasticity” because it is a consequence of the motion of crystalline defects called dislocations. It is “giant” because these dislocations move without friction like little violin strings. It disappears if traces of impurities bind to dislocations or if the temperature increases above 0.2 Kelvin and introduces disorder. Plasticity is of great importance in Materials Science. In solid helium where quantum fluctuations are large, it has unprecedented properties : it occurs with a very large amplitude even under the application of extremely small stresses (nanobars), and it is reversible. In classical crystals, rather large stresses are necessary to produce small irreversible strains. Measurements of the damping of the dislocation motion also allowed us to measure the dislocation density and their typical length so that we could rule out the few existing models of supersolidity in solid helium 4. Finally we have understood how our results explain the original experiments by Kim and Chan without invoking supersolidity.

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