Exploring novel magnetic phases in a programmable Fermi-Hubbard simulator

In quantum materials, function follows form: the collective behavior of a large ensemble of electrons crucially depends on the structure of the ionic crystal they inhabit. Ultracold fermionic atoms in optical lattices are a unique platform to understand such emergent phenomena by providing a very clean realization of the Hubbard model, one of the most fundamental models describing strongly correlated quantum matter. Yet, realizing and probing structures inspired by solid-state materials is a challenge beyond simple square geometries.

In this talk, I will report on novel magnetic states realized in a cold-atom Hubbard simulator with programmable lattice geometry. First, we demonstrate the existence of Nagaoka polarons in a particle-doped Mott insulator, imaged as extended ferromagnetic bubbles around single particle dopants that are stabilized by quantum path interference. Key to these observations is a triangular lattice, where kinetic magnetism is strongly enhanced due to frustration and the existence of short-length loops. Second, we report on a ferrimagnetic state realized in a Lieb lattice at half-filling, a paradigmatic example of a flat-band system whose large state degeneracy is lifted by weak interactions. This ferrimagnetic state is characterized by antialigned magnetic moments concomitant with a finite spin polarization, and is shown to be robust over a wide range of interactions. Our work augurs the realization of new cooling schemes based on an adiabatic transformation of the lattice geometry, which would further give access to exotic low-temperature phases of the Hubbard model.