title: Excitations of a superfluid in a three-dimensional optical lattice.
abstract: We prepare a Bose-Einstein condensed gas in a three-dimensional optical lattice and study the excitation spectrum of the superfluid phase for different interaction strengths. We probe the response of the system by modulating the depth of the optical lattice along one axis. The interactions can be controlled independently by varying the tunnel coupling along the other two lattice axes. In the weakly interacting regime we observe a small susceptibility of the superfluid to excitations, while for stronger interactions an unexpected resonance appears in the excitation spectrum. In addition we measure the coherent fraction of the atomic gas, which determines the depletion of the condensate.
authors.affiliation: Institute of Quantum Electronics, ETH Zürich Hönggerberg, CH-8093 Zürich, Switzerland. Schori@phys.ethz.ch
authors.forename: Christian
authors.initials: C
authors.lastname: Schori
authors.forename: Thilo
authors.initials: T
authors.lastname: Stöferle
authors.forename: Henning
authors.initials: H
authors.lastname: Moritz
authors.forename: Michael
authors.initials: M
authors.lastname: Köhl
authors.forename: Tilman
authors.initials: T
authors.lastname: Esslinger