The observation of nonclassical rotational inertia (NCRI) by the torsional oscillator in 2004 gave rise to a renaissance in the study of solid helium-4. Recent theoretical and experimental studies found evidence that disorder in the solid plays a key role in enabling superfluidity. A recent experiment found a marked increase in the shear modulus that shares the same temperature and helium-3 impurity concentration dependence as that of NCRI. This correlation indicates that the onset of superfluidity requires the pinning and stiffening of the dislocation network by helium-3.
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