Quantum simulation of 3He impurities and of 4He interstitials in solid 4He

We have studied the role of an atomic 3He impurity and an interstitial 4He atom in two- and three-dimensional solid 4He using path integral Monte Carlo (PIMC) simulation. We find that when a substitutional 3He impurity is introduced, the impurity becomes localized and occupies an ideal lattice site. When an interstitial 3He impurity is introduced in the 4He solid, we find that the impurity becomes localized at a substitutional position and, thus, promotes the extra 4He atom to the interstitial space. As a consequence we find that the one-body density matrix (OBDM) and the superfluid fraction, for the case of a 4He solid with an interstitial impurity, are very similar to those calculated for a 4He solid with a 4He interstitial atom. Namely, while the off-diagonal OBDM approaches zero exponentially with increasing particle displacement for the "pure" solid, an interstitial 4He atom or a 3He impurity appear to enhance it at long distances. Finally, the effective mass of the 3He impurity quasiparticle in 2D and 3D crystalline 4He is estimated.