Chapter 17 first considers noninteracting elementary bosons for zero and finite temperatures and then discusses the effects of temperature and space dimension on Bose-Einstein condensation. Condensation of massive bosons in a potential trap is also discussed. Next, the chapter discusses condensate fragmentation, showing that the condensate stability against momentum fragmentation requires the introduction of a boson-boson repulsive interaction: exchange processes between the two parts of the fragmented condensate provide a positive extensive contribution to the N-boson energy, which stabilizes the unfragmented condensate. By contrast, spin fragmentation can occur: the condensate is “spin fragmented,” that is, unpolarized, for same-spin bosons undergoing repulsive interactions. The effect of interaction on the elementary boson ground-state energy is also presented, with emphasis on the physical origin of the singularity of the boson correlation energy, and its similarity with the singularity of the electron correlation energy.
Bose--Einstein condensation for particles with repulsive short-range pair interactions in a Poisson random external potential in Rd
We study Bose gases in d dimensions, d≥2, with short-range repulsive pair interactions, at positive temperature, in the canonical ensemble and in the thermodynamic limit. We assume the presence of hard Poissonian obstacles and focus on the non-percolation regime. For sufficiently strong interparticle interactions, we show that almost surely there cannot be Bose--Einstein condensation into a sufficiently localized, normalized one-particle state. The results apply to the eigenstates of the underlying one-particle Hamiltonian.
We present a theoretical study of the ground state of a Bose-Einstein condensate with repulsive interparticle interactions in a double-well potential, using a restricted variational principle. Within such an approach, there is a transition from a single condensate to a fragmented condensate as the strength of the central barrier of the potential is increased. We determine the nature of this transition through an approximate analytic solution as well as a numerical solution of our model, in the regime where the interparticle interactions can be treated perturbatively. The degree of fragmentation of the condensate is characterized by the degrees of first- and second-order spatial coherence across the barrier.