Can solid helium support superflow? This 40 year old question seemed to have been answered positively in 2004 when Kim and Chan [1] discovered that some of the mass of solid helium inside a torsional oscillator decoupled from the walls.  This effect showed up at temperatures below 0.2K. We performed similar experiments [2] on solid helium at temperatures an order of magnitude higher. We have also observed mass decoupling of around 30%. However, it seems that the phenomenon appears only after disorder is introduced into the crystal. Experiments done with ⁴He containing 100ppm of ³He support the observation that only non faceted helium crystals show large mass decoupling. The growth dynamics of the crystal controls the type of grain boundaries and dislocations. If these are responsible for the mass decoupling, then some change is expected around the roughening temperature. Our results show almost an order of magnitude difference in mass decoupling above and below the crystal's c facet's roughening temperature. Therefore, all our results could be explained in terms of mobile grain boundaries.  It seems that superflow in solids is still out there to be found. 

[1] E. Kim and M. H. W. Chan, Nature 427, 225 (2004)

[2] A. Eyal, O. Pelleg, L. Embon and E. Polturak, Phys. Rev. Lett. 105, 025301 (2010)