Plastic Deformation and Creep in Solid Helium

Helium crystals are quantum solids, with unusual mechanical properties. Quantum zero point motion prevents helium from freezing, unless pressure is applied, and helium crystals are extremely compressible, with elastic constants orders of magnitude smaller than those of conventional solids. In such quantum solids, tunneling allows atomic exchange and defects may move easily at low temperatures. The unusual mobility of dislocations and isotopic impurities in ${}^4$He crystals can reduce their shear modulus at small strains by as much as 90%. For large strains, solid helium crystals are extraordinarily soft and ductile near their melting points, flowing under millibar stresses. At low strain rates, this high-temperature creep is thermally activated and involves diffusion of vacancies, which allow dislocations to move via climb. At low temperatures, these processes freeze out and helium crystals are much less ductile. Deformation proceeds via sudden slip events—dislocation avalanches—with a wide range of sizes and timescales. In this paper, we review experiments on plastic deformation and flow in solid ${}^4$He and ${}^3$He during the past 50 years, and discuss the plastic deformation mechanisms in solid helium that have been identified from these experiments.