Bose-Einstein condensation in solid He-4 with a vacancy

Bose-Einstein condensation (BEC) in solid He-4 and its relationship with vacancies have been considered for a long time without conclusive results. Recently, the microscopic theory has succeeded in giving a realistic description of a vacancy also in a low-density solid He-4 where lattice relaxation and delocalization have an essential role. We have studied now the momentum distribution. At densities close to melting at T = 0 K we find the presence of a finite BEC with 0.22 +/- 0.02 atoms in the condensate for each vacancy and the change of the momentum distribution induced by the vacancy is highly non Gaussian. At higher density the condensate becomes vanishingly small. (C) 2000 Elsevier Science B.V. All rights reserved.