Shear Modulus and Dislocations in bcc Solid He

The shear modulus of hcp He decreases significantly above 200 mK, as He impurities unbind from dislocations, unpinning them, and softening the crystal. Here we report shear modulus measurements on a fermi quantum solid: bcc He. In contrast to previous low-frequency measurements, which did not show dislocation softening in this system, we have observed a drop in shear modulus, accompanied by a dissipation peak, which we attribute to the unpinning of dislocations as He impurities unbind. For large stresses, impurities cannot pin the dislocations and the low temperature stiffening is suppressed. At high frequencies, the modulus changes and dissipation peaks shift to higher temperature, indicating that the unbinding is thermally activated. For a 58 bar bcc He crystal, we find an activation energy of 0.27 K, smaller than the 0.7 K binding energy for He impurities in hcp He.