The role of defects in Supersolid Helium-4

We seek an explanation for the observed non-classical rotational inertia in solid Helium-4. We study the activation energy for a single vacancy in an otherwise perfect crystal, which is huge and leads to the conclusion that there are no thermal vacancies in solid Helium-4 at the experimentally relevant temperatures. We also study grain boundaries and find that they can be mechanically stable when brought into contact with liquid, and can turn superfluid. Contact with recents experiments is made.