Abstract
We discuss the relationship between phase shift and energy shift in two-body scattering problem. An effective potential taking effective range of actual inter-atomic potential into consideration is introduced. A modified Gross-Pitaevskii equation is derived using the effective potential. We obtain hydrodynamic equation using the modified Gross-Pitaevskii equation. Bogoliubov mode in uniform system is discussed. It is shown that the excitation energy could be complex when q becomes large and the system will become unstable for certain parameter regimes.
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