Compared to classical solids, the quantum solid helium-4 can undergo a giant plastic deformation that is also reversible.
If you bend a piece of metal into a new shape, it will not spontaneously revert to the condition in which it started. The reason is that bending causes the metal to undergo a plastic deformation: an irreversible change of shape that occurs when an applied load exceeds a certain threshold. In Physical Review Letters, Ariel Haziot at the École Normale Supérieure in Paris, France, and colleagues report that the plastic properties of a “quantum solid,” helium-4, can be strikingly different: in helium-4, within a certain temperature range, large plastic deformations can still be reversible . The findings highlight the unusual mechanical properties of quantum solids, and may serve as a model system for understanding the fundamental mechanisms contributing to plasticity.