Abstract
The shear modulus of hcp \(^4\)He decreases significantly above \(\sim \)200 mK, as \(^3\)He impurities unbind from dislocations, unpinning them, and softening the crystal. Here we report shear modulus measurements on a fermi quantum solid: bcc \(^3\)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 \(^4\)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 \(^3\)He crystal, we find an activation energy of 0.27 K, smaller than the 0.7 K binding energy for \(^3\)He impurities in hcp \(^4\)He.
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