Flat bands and ferrimagnetism in a multi-orbital Fermi-Hubbard simulator

Quantum materials usually feature a complex electronic band structure, which is essential for accurately capturing their many-body properties, from cuprates to twisted bilayer graphene. In such multi-band systems, quantum interference can give rise to flat bands with high degeneracy, enabling itinerant magnetism even under weak interactions.

In this talk, we show signatures of a ferrimagnetic state realized with ultracold fermionic atoms in an optical Lieb lattice. These signatures are characterized by antialigned magnetic moments and antiferromagnetic correlations in the presence of a finite spin polarization at half-filling. Our observations remain robust as repulsive interactions increase from the non-interacting limit to the Heisenberg regime, reminescent of a seminal theorem by Lieb in bipartite lattices. Our work opens new avenues for realizing quantum spin liquids at finite particle doping, as well as exotic phases in related multi-orbital systems including kagome lattices.