Abstract
Using two-photon Bragg spectroscopy, we study the energy of particlelike excitations in a strongly interacting homogeneous Bose-Einstein condensate, and observe dramatic deviations from Bogoliubov theory. In particular, at large scattering length the shift of the excitation resonance from the free-particle energy changes sign from positive to negative. For an excitation with wave number , this sign change occurs at , in agreement with the Feynman energy relation and the static structure factor expressed in terms of the two-body contact. For we also see a breakdown of this theory, and better agreement with calculations based on the Wilson operator product expansion. Neither theory explains our observations across all interaction regimes, inviting further theoretical efforts.
- Received 13 February 2017
DOI:https://doi.org/10.1103/PhysRevLett.118.210401
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