Exploring competing density order in the ionic Hubbard model with ultracold fermions

TL;DR

This study uses ultracold fermionic atoms in an optical lattice to explore density-ordered phases in the ionic Hubbard model, revealing charge density waves and Mott insulators through various measurements.

Contribution

It demonstrates the realization and measurement of competing density orders in the ionic Hubbard model with ultracold fermions, including experimental detection of charge density waves and Mott insulating states.

Findings

Identification of charge density wave at weak interactions

Observation of Mott insulating state at strong interactions

Complex excitation spectrum revealed by modulation spectroscopy

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 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.