Thermally activated relaxation of shear stress in solid 4He confined in aerogel

Torsional oscillator (TO) experiments on He-4 show supersolid behavior that appears to be associated with disorder. However, confining helium in the pores of an aerogel does not enhance the supersolid decoupling, even though x-ray measurements confirm that the crystals are highly disordered. Solid helium's shear modulus also shows anomalous behavior below 200 mK, stiffening as mobile dislocations are pinned by He-3 impurities at low temperatures. A highly porous material such as aerogel should also provide effective pinning sites for dislocations. We have made shear modulus measurements on solid He-4 grown in a 95% porosity aerogel. We see large modulus decreases as the samples are warmed but these occur at much higher temperatures and over a broader range than in bulk He-4. The frequency dependence of the modulus and dissipation are consistent with a thermally activated process. The activation energy is approximately 16 K and may be associated with vacancy motion.