Plastic flow of solid ³He through a porous elastic film

A study of plastic flow of solid ³He through a metalized porous elastic polymer film frozen into the crystal was carried out in the temperature range of 0.1-1 K. The flow was due to mechanical stresses in the crystal induced by external electrical forces. The plastic flow rate of solid helium was determined by measuring the capacitance changes of a capacitor in which the metalized surface of the film served as a movable electrode. Two different regions could be clearly identified on the temperature dependence of the plastic flow rate V(T). Above ~200 mK, V drops exponentially with decreasing temperature, which corresponds to the thermally activated regime of plastic flow. At lower temperatures, the rate V is temperature independent, indicating quantum plastic flow. A detailed analysis of the experimental data was performed in the thermally activated region. The empirical values of the following parameters were defined: the activation volume and energy and the yield strength, corresponding to the onset of macroscopic plastic flow. It was found that the value of the activation volume is 30-70 fold the atomic volume, indicating that the scale of structural rearrangements in the crystal at elementary acts of plastic flow is significantly greater than the atomic size. At the same time the activation energy is close to the vacancy activation energy. The obtained experimental data were analyzed within the framework of the vacancy diffusion and dislocation models of plastic flow. In the dislocation model we estimated the temperature below which the dislocations overcome the Peierls barriers by quantum tunneling. The Appendix discusses the physical basis of the methodology applied as well as the results of the theory of dislocation motion in the Peierls potential, which were used to analyze the characteristics of the plastic flow of solid ³He.