  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.
