title: Excitation spectrum of a Bose-Einstein condensate.
abstract: We report a measurement of the excitation spectrum omega(k) and the static structure factor S(k) of a Bose-Einstein condensate. The excitation spectrum displays a linear phonon regime, as well as a parabolic single-particle regime. The linear regime provides an upper limit for the superfluid critical velocity, by the Landau criterion. The excitation spectrum agrees well with the Bogoliubov spectrum in the local density approximation, even close to the long-wavelength limit of the region of applicability. Feynman's relation between omega(k) and S(k) is verified, within an overall constant.
authors.affiliation: Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 76100, Israel.
authors.forename: J
authors.initials: J
authors.lastname: Steinhauer
authors.forename: R
authors.initials: R
authors.lastname: Ozeri
authors.forename: N
authors.initials: N
authors.lastname: Katz
authors.forename: N
authors.initials: N
authors.lastname: Davidson