title: Dynamic Onset of Feynman Relation in the Phonon Regime.
abstract: The Feynman relation, a much celebrated condensed matter physics gemstone for more than 70 years, predicts that the density excitation spectrum and structure factor of a condensed Bosonic system in the phonon regime drops linear and continuously to zero. Until now, this widely accepted monotonic excitation energy drop as the function of reduced quasi-momentum has never been challenged in a spin-preserving process. We show rigorously that in a light-matter wave-mixing process in a Bosonic quantum gas, an optical-dipole potential arising from the internally-generated field can profoundly alter the Feynman relation and result in a new dynamic relation that exhibits an astonishing non-Feynman-like onset and cut-off in the excitation spectrum of the ground state energy of spin-preserving processes. This is the first time that a nonlinear optical process is shown to actively and significantly alter the density excitation response of a quantum gas. Indeed, this dynamic relation with a non-Feynman onset and cut-off has no correspondence in either nonlinear optics of a normal gas or a phonon-based condensed matter Bogoliubov theory.
authors.affiliation: Department of Physics, East China Normal University, Shanghai 200241, China.
authors.forename: Y
authors.initials: Y
authors.lastname: Li
authors.affiliation: School of Physical and Engineering Sciences, Tongji University, Shanghai 200092, China.
authors.forename: C J
authors.initials: CJ
authors.lastname: Zhu
authors.affiliation: National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
authors.forename: E W
authors.initials: EW
authors.lastname: Hagley
authors.affiliation: National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
authors.forename: L
authors.initials: L
authors.lastname: Deng