Stability of a molecular Bose-Einstein condensate in atomic Bose gases*

Recently a molecular BEC has been created out of a bosonic-atom condensate through application of a Feshbach resonance [1]. This observation makes it finally possible to probe a 

bosonic analogue of the widely studied BCS-BEC crossover seen in fermionic systems and allows access to a rich variety of phenomena. In this talk, using a two-channel Feshbach Hamiltonian, we theoretically characterize the associated stability. We do so here in the context of a many-body variational ground state which contains both condensates as well

as depletion and Cooper-pair contributions. We address both wide and narrow Feshbach resonances and show that for a sufficiently narrow resonance a molecular superfluid is stable except for a small region of magnetic fields in the immediate vicinity of the Feshbach resonance [2]; this applies to the resonance of ¹³³Cs atoms at B=19.849 G. By contrast

for more typical broad resonances such as those in ⁸⁵Rb, ⁷Li and ³⁹K the region of instability is substantial and there is little access to the purported quantum critical point phenomena.