We have discovered that a helium-4 crystal with no impurity at all is anomalously soft. Its plasticity is large, due to quantum effects. This is because it contains dislocations, which can move macroscopic distances (a fraction of a millimeter) at high speed (several meters per second). Dislocations are lines running through the crystal, where the stacking of atoms is disordered compared to the rest of the crystal. In classical crystals all atoms are completely frozen at low temperature. But in quantum crystals such as helium-4, quantum fluctuations are large and atoms can jump by quantum tunneling from site to site, especially along dislocations where the packing is not as compact as elsewhere. Quantum tunneling is a well-documented process in which particles go through energy barriers without any dissipation because of their wave-like character. Highly mobile dislocations are able to reduce the stiffness of helium-4 crystals by one order of magnitude.
However, very tiny traces of helium-3 impurities are sufficient to stop the motion of dislocations when they attach to them below temperatures of order 100 millikelvin. Apparently, this is what drives these crystals to a “supersolid state”, a highly debated new state of matter, which may be the consequence of mass flow along the core of dislocations when they stop moving.
S. Balibar, The enigma of supersolidity, Nature 464, 176 (2010).
X. Rojas, A. Haziot, V. Bapst, H.J. Maris, and S. Balibar, Anomalous softening of helium 4 crystals, Phys. Rev. Lett. 105, 145302 (2010).