Doping a frustrated Fermi-Hubbard magnet

Geometrical frustration in strongly correlated systems can give rise to a plethora of novel ordered states and intriguing magnetic phases such as quantum spin liquids. Promising candidate materials for such phases can be described by the Hubbard model on an anisotropic triangular lattice, a paradigmatic model capturing the interplay between strong correlations and magnetic frustration. However, the fate of frustrated magnetism in the presence of itinerant dopants remains unclear, as well as its connection to the doped phases of the square Hubbard model. Here, we probe the local spin order of a Hubbard model with controllable frustration and doping, using ultracold fermions in anisotropic optical lattices continuously tunable from a square to a triangular geometry. At half-filling and strong interactions $U/t \sim 9$, we observe at the single-site level how frustration reduces the range of magnetic correlations and drives a transition from a collinear N\'eel antiferromagnet to a short-range correlated 120$^{\circ}$ spiral phase. Away from half-filling, magnetic correlations show a pronounced asymmetry between particle and hole doping close to triangular geometries and hint at a transition to ferromagnetic order at a particle doping above $20\%$. This work paves the way towards exploring possible chiral ordered or superconducting phases in triangular lattices, and realizing t-tprime square lattice Hubbard models that may be essential to describe superconductivity in cuprate materials.