Plastic deformation, vacancy diffusion, and vacancy delocalization in bcc ${}_{}{}^3{}_{}{}^{}\mathrm{He}$

The plastic deformation of bcc ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ crystals has been studied near the melting curve, in the temperature range 0.65 K ≤T≤1.17 K, and for strain rates, ε̇, from 2×${10}^{\mathrm{-}6}$ to 2×${10}^{\mathrm{-}4}$ ${\sec }^{\mathrm{-}1}$. The resulting relations between strain rate and stress, at a given temperature, are accounted for in terms of dislocation climb and vacancy diffusion in the solid He. The temperature dependence of the strain rate at a given stress indicates that a nonclassical mechanism underlies the deformation process. The vacancy diffusion coefficient in bcc ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, as a function of temperature, is deduced from these results, and an energy bandwidth for delocalized vacancies of 0.24±0.05 K is obtained through a fit to theoretical predictions.