title: Roton-maxon excitation spectrum of Bose condensates in a shaken optical lattice.
abstract: We present experimental evidence showing that an interacting Bose condensate in a shaken optical lattice develops a roton-maxon excitation spectrum, a feature normally associated with superfluid helium. The roton-maxon feature originates from the double-well dispersion in the shaken lattice, and can be controlled by both the atomic interaction and the lattice modulation amplitude. We determine the excitation spectrum using Bragg spectroscopy and measure the critical velocity by dragging a weak speckle potential through the condensate-both techniques are based on a digital micromirror device. Our dispersion measurements are in good agreement with a modified Bogoliubov model.
authors.affiliation: James Franck Institute, Enrico Fermi Institute and Department of Physics, University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: Li-Chung
authors.initials: LC
authors.lastname: Ha
authors.affiliation: James Franck Institute, Enrico Fermi Institute and Department of Physics, University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: Logan W
authors.initials: LW
authors.lastname: Clark
authors.affiliation: James Franck Institute, Enrico Fermi Institute and Department of Physics, University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: Colin V
authors.initials: CV
authors.lastname: Parker
authors.affiliation: James Franck Institute, Enrico Fermi Institute and Department of Physics, University of Chicago, Chicago, Illinois 60637, USA and Joint Quantum Institute, University of Maryland, College Park, Maryland 20742, USA.
authors.forename: Brandon M
authors.initials: BM
authors.lastname: Anderson
authors.affiliation: James Franck Institute, Enrico Fermi Institute and Department of Physics, University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: Cheng
authors.initials: C
authors.lastname: Chin