Disorder Phenomena in Quantum Solids with Vacancies

We have investigated disorder phenomena in solid 4He near megting in presence of vacancies by means of Quantum Monte Carlo (MC) simulations using the Shadow Wave Function variational technique and a new exact projector MC algorithm, the Shadow Path Integral Ground State. With these methods the Bose symmetry is manifestly maintained so that defects can be delocalized and relaxation around them is allowed. We have obtained the first microscopic ab-initio calculation of the excitation spectrum of a vacancy. We have studied different crystal lattices (hcp, fcc, bcc) and the vacancy effective mass ranges from 0.3 to 0.39 4He masses. The band width is rather large, it ranges from 6 K to 19 K depending on the crystal lattice and on the particular direction in the lattice. We have also computed the excitation spectrum of longitudinal phonons and we find good agreement with experimental data both for hcp and bcc.