Bose-Einstein condensates in symmetry breaking states

We consider two models of interacting Bose gases: a gas of spin one particles in the ground state of a cubic box and a one-dimensional Bose gas with contact interactions. We show how to calculate exact eigenstates of the corresponding N-body Hamiltonians. Both models share the property of not leading to the formation of a Bose-Einstein condensate, even at zero temperature, in the strict sense of the existence of a single one-particle state with a macroscopic population. We show that a lot of physical insight can be gained on these two model systems by using the usual Hartree-Fock mean field approach: in this approximation. that we test against the exact result. everything happens as if a single realization of the system was a Bose-Einstein condensate in a state phi breaking the rotational or translational symmetry, and varying in a random way for any new experimental realization. (C) 2001 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.