COMPARATIVE-STUDY OF VACANCIES IN THE BCC AND HCP PHASES OF HE-4 USING SHADOW WAVE-FUNCTIONS

We use the shadow wave function formalism (SWF) to determine the energ y of formation of single and double vacancies in He-4 crystals at T=0 K. Data is presented for both the bce and hcp phases. The activation e nergy for single vacancies in bcc He-4 was found to be 6.7+/-3.9 K, ab out 43% of that in the hcp He-4, 15.6+/-3.9 K. By determining the occu pation of the Voronoi regions of the crystal sites, we determined the location of vacancies in the crystal and studied the relaxation of the neighboring atoms. We also present data on the correlations between v acancies, and between vacancies and He-3 impurities. Following the pos ition of the vacancy through successive configurations we observed the motion of the vacancy as seen in our Monte Carlo simulations. On the shorter Monte Carlo time scales, greater vacancy motion was observed i n the bcc phase than in the hcp phase.