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

TL;DR

This study investigates how interactions in a Bose-Einstein condensate within an optical kagome lattice modify the flat band structure, leading to increased group velocity and dispersion, confirmed by experimental measurements and theoretical modeling.

Contribution

It demonstrates that interactions can significantly alter the flat band in an optical kagome lattice, revealing interaction-enhanced group velocity effects.

Findings

Interactions increase the dispersion of the flat band.

Experimental results match predictions of the lattice Gross-Pitaevskii equation.

Lattice distortions caused by interactions explain the observed band structure changes.

Abstract

Geometric frustration of particle motion in a kagome lattice causes the single-particle band structure to have a flat s-orbital band. We probe this band structure by exciting a Bose-Einstein condensate into excited Bloch states of an optical kagome lattice, and then measuring the group velocity through the atomic momentum distribution. We find that interactions renormalize the band structure of the kagome lattice, greatly increasing the dispersion of the third band that, according to non-interacting band theory, should be nearly non-dispersing. Measurements at various lattice depths and gas densities agree quantitatively with predictions of the lattice Gross-Pitaevskii equation, indicating that the observed distortion of band structure is caused by the disortion of the overall lattice potential away from the kagome geometry by interactions.