Correlated magnetism of moiré exciton-polaritons on a triangular electron-spin lattice

We demonstrate evidence of correlated magnetism for exciton-polaritons in a MoSe_2/WS_2 moiré heterostructure with near-parallel alignment subject to electron doping. In our experiments, interactions between electrons and moiré excitons are controlled electrostatically by field-effect doping, and the polaritonic regime of strong light-matter coupling is established in an open cryogenic microcavity. Remarkably, at filling fractions around one electron per moiré cell, we observe drastic and nonlinear enhancement of the effective polariton Landé factor as a hallmark of correlated magnetism, which is cavity-controlled via resonance tuning of light and matter polariton constituents. Our work establishes moiré van der Waals heterostructures as an outstanding platform for studies of correlated phenomena in the presence of strong light-matter coupling and many-body phases of lattice-ordered excitons, charges and spins.