The changes that vacancies produce in the properties of hcp solid 4He are studied by means of quantum Monte Carlo methods. Our results show that the introduction of vacancies produces significant changes in the behavior of solid 4He, even when the vacancy concentration is very small. We show that there is an onset temperature where the properties of incommensurate 4He change significantly. Below this temperature, we observe the emergence of off-diagonal long range order and a complete spatial delocalization of the vacancies. This temperature is quite close to the temperature where non-classical rotational inertia has been experimentally observed. Finally, we report results on the influence of vacancies in the elastic properties of hcp 4He at zero temperature.
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