Observation of ferrimagnetic correlations in a flat-band lattice with a Fermi-Hubbard quantum simulator

Quantum interference in decorated lattice geometries can give rise to highly degenerate "flat" bands with localized eigenstates such as in the case of the Lieb and Kagome lattices. Such electronic models with flat bands are of much interest in the context of itinerant ferromagnetism. In addition, for the bipartite Lieb lattice with an unequal number of sublattice sites, the ground state is expected to show long range ferrimagnetic order with non-zero total spin and antiparallel magnetic moments. Here we report on an experimental observation of a ferrimagnetic state with ultracold fermions loaded into an optical Lieb lattice. We demonstrate how we can prepare a half-filled flat band with the help of tunable interactions and chemical potential. In the presence of finite magnetization, we observe a staggered magnetization with opposite magnetic susceptibilities for the two sublattices over a wide range of interactions. We further study the spin correlations as we continuously tune the geometry between a square and a Lieb lattice, where a transition from canted AFM order to ferrimagnetic order is expected. Lastly we explore the effects of strong interactions in this multiband model and find preliminary signs of an orbital selective Mott transition.