Creep in solid 4He at temperatures below 1 K

Creep in solid He-4 at temperatures of similar to 100-1000 mK is studied experimentally by detecting the flow of helium through a frozen porous membrane under a constant external force. Creep curves are measured for different temperatures and mechanical stresses. This method has made it possible to detect low creep rates in helium down to the lowest temperatures in these experiments. It is found that throughout this temperature range, creep is thermally activated and the activation energy decreases with falling temperature and increasing mechanical stress. An analysis shows that for temperatures above approximate to 500 mK, Nabarro-Herring diffusive creep takes place in solid helium with mass transfer by self diffusion of atoms and a counterflow of vacancies. The experimental data have been used to obtain the self-diffusion coefficient as a function of temperature for different stresses. At temperatures below approximate to 500 mK creep takes place at a very low flow rate (similar to 10(-13) cm/s) and a very low activation energy (similar to 0.5-0.7 K), while the creep mechanism remains unclear. (C) 2015 AIP Publishing LLC.