TUE-CMS

Abstract: Since the original proposal by Chester and Legget, supersolid phenomena like non-classical rotational inertia (NCRI) has been observed, and confirmed in solid $^4$He. The sharp drop in the period of oscillation of a torsional oscillator (TO) filled with solid $^4$He, either in the form of crystallites frozen within porous Vycor glass or as a bulk polycrystal below 200 mK has been attributed to the appearance of a new supersolid phase, the NCRI fraction (NCRIF) within the solid which decouples from the solid $^4$He and the TO. While the possibility of local melting either in the surface or within grain boundaries have been systematically ruled out by careful experiments, there are experiments which suggest the prominent role for crystal defects. NCRI depends crucially on sample preparation, and the NCRIF can reduce drastically due to annealing of the sample which presumably removes defects. By combining recent theories of plasticity in solids coupled to the superfluid order parameter, we show that supersolidity in solid $^4$He is a transient phenomenon which occurs during plastic deformation at dislocation pile-ups and may be removed during annealing. The decrease of the supersolid fraction follows a $\sim t^{-1/2}$ law. Supersolidity is absent if climb motion of dislocations is suppressed.