Frustration- and doping-induced magnetism in a Fermi-Hubbard simulator

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(1-3). Promising candidate materials for such phases(4-6) can be described by the Hubbard model on an anisotropic triangular lattice, a paradigmatic model capturing the interplay between strong correlations and magnetic frustration(7-11). 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(12). Here we investigate 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 approximate to 9, we observe at the single-site level how frustration reduces the range of magnetic correlations and drives a transition from a collinear Neel antiferromagnet to a short-range correlated 120 degrees spiral phase. Away from half-filling, the triangular limit shows enhanced antiferromagnetic correlations on the hole-doped side and a reversal to ferromagnetic correlations at particle dopings above 20%, hinting at the role of kinetic magnetism in frustrated systems. This work paves the way towards exploring possible chiral ordered or superconducting phases in triangular lattices(8,13) and realizing t-t' square lattice Hubbard models that may be essential to describe superconductivity in cuprate materials(14).

Automated summary

Geometrical frustration in strongly correlated systems can lead to the emergence of novel ordered states and magnetic phases, such as quantum spin liquids. This study investigates the effects of frustration and doping on the local spin order in a controllable Hubbard model. The results show that frustration reduces the range of magnetic correlations and induces a transition from a collinear Neel antiferromagnet to a short-range correlated 120 degrees spiral phase. The triangular lattice exhibits enhanced antiferromagnetic correlations on the hole-doped side and a reversal to ferromagnetic correlations at high particle dopings.