Flat Chern bands and correlated states in spiral magnet

Flat topological bands provide a promising platform for realizing exotic correlated quantum phases such as fractional Chern insulators. Here, we predict the van der Waals monolayer ReAg 2 Cl 6 hosts isolated flat Chern bands at the Fermi level in its 120 ∘ antiferromagnetic ground state. Their flatness and nontrivial topology arise from the interplay between coplanar spin order and strong spin-orbit coupling within Re 5 d orbitals, providing a mechanism distinct from moiré systems. The spiral spin texture naturally enlarges the unit cell, reducing carrier densities while preserving sizable interaction scales. Many-body calculations show that fractional fillings of the flat bands can stabilize correlated phases, including fractional Chern insulators and charge-density-wave states. Moreover, the mechanism is generic to a broad family of Re-based compounds, with both spin configuration and flat band topology electrically tunable. These findings establish Re-based coplanar antiferromagnets as a robust, tunable, and experimentally accessible platform for flat Chern bands and correlated topological phases potentially at elevated temperatures.