moiré superlattice

Moiré superlattices formed in two-dimensional (2D) atomic crystals present a powerful platform to study interacting quantum particles in a lattice. Here, we present optical reflection spectroscopy studies on angle-aligned WSe₂/WS₂ bilayers, which form moiré superlattices because of the difference in lattice constant between the two materials. We measure the dependence of the lowest-energy moiré exciton on magnetic field and doping density. At half filling of the first hole moiré band, we observe a Mott insulating state with antiferromagnetic Curie-Weiss behavior. Past half filling, our experiment suggests an antiferromagnetic to weak ferromagnetic quantum phase transition near 0.6 filling. These results can be understood based on a triangular lattice Hubbard model in the strong interaction regime.