On coherence of Bose field

We study a system of interacting bosons at zero temperature in an atomic trap. Using wave function that models the ground state of interacting bosons we examine the concepts of the order parameter, off-diagonal order a nd coherence of the system. We suggest that the coherence length becomes much smaller than the size of the system if the umber of trapped particles exceeds a certain limit. This behavior is related to the unavoidable existence of two different length scales one determined by the external potential and the second one depending on the two-body forces. (C) 20001 of America.