Interaction-Enhanced Group Velocity of Bosons in the Flat Band of an Optical Kagome Lattice

In the non-interacting limit the kagome lattice exhibits a nearly dispersionless/flat excited band that predicts vanishing group velocity. We place a Bose-Einstein condensate into this s-orbital band and extract its group velocity by analyzing the atomic diffraction pattern. We observe a significant enhancement of the group velocity in the flat band which can be explained by band structure renormalization due to interaction effects. Incorporating interactions via the Gross-Pitaesvkii equation, our calculations indicate that the band structure renormalization is caused by a distortion of the real space atomic distribution away from the kagome geometry.