Shear Modulus and Dislocation Effects in bcc $$^3$$He

The large zero-point motion and rapid atomic exchange in quantum solids like helium make defects extremely mobile. Previous shear modulus measurements have shown that mobile dislocations can soften the shear modulus of high purity hcp $$^4$$ He by as much as 90%, and that isotopic impurities ( $$^3$$ He atoms) pin dislocations and restore the intrinsic shear modulus at low temperatures. In this paper, we report shear modulus and dissipation measurements on solid $$^3$$ He in its body-centred cubic (bcc) phase. The lighter mass makes $$^3$$ He an even more quantum solid, with much higher exchange frequencies, but its spin and different crystal structure can change the nature of dislocations and their behavior. Our measurements show that the shear modulus of bcc $$^3$$ He had many similarities to that of hcp $$^4$$ He, including a thermally activated transition from a stiff state to a soft state as $$^4$$ He impurities unbind from dislocations when the temperature increases. The impurity binding energies are smaller than those in hcp $$^4$$ He and depend more strongly on pressure than a simple elastic binding model would suggest. In contrast to hcp $$^4$$ He, the shear modulus of bcc $$^3$$ He continues to decrease at temperatures up to 1 K, accompanied by a second dissipation peak with a larger activation energy. This behavior suggests that dislocation motion may be limited by intersections in tangled network in bcc $$^3$$ He but can overcome these barriers at high temperatures by thermal activation.