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Defects are believed to play a fundamental role in the supersolid state of4He. We report on studies by exact Quantum Monte Carlo (QMC) simulations atzero temperature of the properties of solid 4He in presence of many vacancies,up to 30 in two dimensions (2D). In all studied cases the crystalline order isstable at least as long as the concentration of vacancies is below 2.5%. In the2D system for a small number, n_v, of vacancies such defects can be identifiedin the crystalline lattice and are strongly correlated with an attractiveinteraction. On the contrary when n_v~10 vacancies in the relaxed systemdisappear and in their place one finds dislocations and a revival of theBose-Einstein condensation. Thus, should zero-point motion defects be presentin solid 4He, such defects would be dislocations and not vacancies, at least in2D. In order to avoid using periodic boundary conditions we have studied theexact ground state of solid 4He confined in a circular region by an externalpotential. We find that defects tend to be localized in an interfacial regionof width of about 15 A. Our computation allows to put as upper bound limit tozero--point defects the concentration 0.003 in the 2D system close to meltingdensity.

Study of solid 4He in two dimensions. The issue of zero-point defects and study of confined crystal