A Microscopic Description of Vacancies in Solid 4He

The changes that vacancies produce in the properties of hcp solid He-4 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 He-4, even when the vacancy concentration is very small. We show that there is an onset temperature where the properties of incommensurate He-4 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 He-4 at zero temperature.