title: Exploring competing density order in the ionic Hubbard model with ultracold fermions.
abstract: We realize and study the ionic Hubbard model using an interacting two-component gas of fermionic atoms loaded into an optical lattice. The bipartite lattice has a honeycomb geometry with a staggered energy offset that explicitly breaks the inversion symmetry. Distinct density-ordered phases are identified using noise correlation measurements of the atomic momentum distribution. For weak interactions the geometry induces a charge density wave. For strong repulsive interactions we detect a strong suppression of doubly occupied sites, as expected for a Mott insulating state, and the externally broken inversion symmetry is not visible anymore in the density distribution. The local density distributions in different configurations are characterized by measuring the number of doubly occupied lattice sites as a function of interaction and energy offset. We further probe the excitations of the system using direction dependent modulation spectroscopy and discover a complex spectrum, which we compare with a theoretical model.
authors.affiliation: Institute for Quantum Electronics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Michael
authors.initials: M
authors.lastname: Messer
authors.affiliation: Institute for Quantum Electronics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Rémi
authors.initials: R
authors.lastname: Desbuquois
authors.affiliation: Institute for Quantum Electronics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Thomas
authors.initials: T
authors.lastname: Uehlinger
authors.affiliation: Institute for Quantum Electronics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Gregor
authors.initials: G
authors.lastname: Jotzu
authors.affiliation: Institute for Theoretical Physics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Huber
authors.affiliation: Institute for Quantum Electronics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Daniel
authors.initials: D
authors.lastname: Greif
authors.affiliation: Institute for Quantum Electronics, ETH Zurich, 8093 Zurich, Switzerland.
authors.forename: Tilman
authors.initials: T
authors.lastname: Esslinger