Role of Defects in Possible Superfluidity of Spatially Ordered Helium

A path integral Monte Carlo investigation of the role of defects in ordered 4He systems has been conducted. We find that interstitial defects in two- and three-dimensional solid 4He can lead to a small but non-zero superfluid fraction, while still maintaining the original solid ordering. Furthermore, a 3He impurity atom initially placed as an interstitial defect is found to relax onto the solid lattice through the promotion of a 4He atom to the interstitial space. We also study the melting transition of pure 4He in two dimensions, where thermally excited defects give rise to unique phases with quasi-long-range order. Finite-size scaling techniques, initially applied to a classical system of Lennard-Jones particles, are found to be able to distinguish two separate melting temperatures. Additionally, coexistence of super fluid and diagonal order was observed in several of our finite-sized samples, raising the possibility that such real- and momentum-space ordering could be held simultaneously by quantum systems in two dimensions