Creep of solid ⁴He at temperatures below 1 K

The experimental study of creep of solid⁴ He was carried out by recording helium flow across the frozen porous membrane under a constant external force in the temperature range ∼ 100-1000 mK. The creep curves were measured at different temperatures and mechanical stresses. The method used in this work permitted us to record a small creep rate of helium down to the lowest temperature of the experiment. It is found that the creep in solid helium is a thermally activated process everywhere over the temperature region and the activation energy is reduced with decreasing temperature and increasing mechanical stress. The analysis showed that at temperatures above ≈ 500 mK there occurred a Nabar-ro-Herring type of diffusion creep in solid helium where the mass flow was carried out by the self-diffusion of atoms and vacancies flow in the opposite direction. The experimental data permitted us to obtain the self-diffusion coefficient as a function of temperature at different mechanical stresses. At temperatures below ≈ 500 mK the creep process is realized at a very low flow rate (∼ 10^-13 cm/s) and a very low activation energy (∼ 0.5-0.7 K) while the creep mechanism remains unclear.