Thermally activated relaxation of shear stress in solid4He confined in aerogel

Abstract

Torsional oscillator (TO) experiments on ${}^{4}$He 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 ${}^{3}$He 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 ${}^{4}$He 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 ${}^{4}$He. 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.