Theory for supersolid $^4$He
  Although both vacancies and interstitial have relatively high activation energies in the normal solid, we propose that a lower energy bound state of a vacancy and an interstitial may facilitate vacancy condensation to give supersolidity in $^{4}$He . We use a phenomenological two-band boson lattice model to demonstrate this new mechanism and discuss the possible relevance to the recently observed superfluid-like, non-classical rotational inertial experiments of Kim and Chan in solid $^{4}$He. Some of our results may also be applicable to trapped bosons in optical lattices. 
Xi Dai, Michael Ma, and Fu-Chun Zhang