Negative absolute temperature states in frustrated optical lattices: towards the kagome flat band

We report on an ultracold-atom setup implementing a variety of hexagonal lattices including triangular and kagome lattices. Both display geometric frustration that gives rise to two non-equivalent maxima for the ground band of the triangular lattice, and a flat band in the kagome case.



We will present an overview of the experimental setup and recent activities, including an investigation of the phase diagram of the bosonic triangular lattice at both positive (unfrustrated) and negative absolute temperatures (frustrated). We observe strikingly different critical interaction strengths for the bosonic superfluid to Mott insulator transition in the two cases. In this context we are also studying the emergence of coherence as the MI-SF transition is crossed. We are working towards loading bosons into the flat band of the kagome lattice and implementing a quantum gas microscope (QGM) for single-site-resolved imaging.