Spatial fragmentation of a Bose-Einstein condensate in a double-well potential

We present a theoretical study of the ground state of a Bose-Einstein conde nsate with repulsive interparticle interactions in a double-well potential, using a restricted variational principle. Within such an approach, there i s a transition from a single condensate to a fragmented condensate as the s trength of the central barrier of the potential is increased. We determine the nature of this transition through an approximate analytic solution as w ell as a numerical solution of our model, in the regime where the interpart icle interactions can be treated perturbatively. The degree of fragmentatio n of the condensate is characterized by the degrees of first- and second-or der spatial coherence across the barrier. [S1050-2947(99)06605-6].