Excitation Spectrum in Many-Boson Systems

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Abstract
When only two-body correlations are fully taken into account, there appears an energy gap in the excitation spectrum for many-boson systems as shown by Girardeau and Arnowitt and confirmed by Wentzel. This energy gap is shown to disappear and the spectrum to become phononlike again and proportional to the momentum for small momentum, if we construct the eigenmodes of excitations (collective excitations), taking into consideration appropriate higher-order terms.