Flat Chern bands and correlated states in spiral magnet ReAg$_2$Cl$_6$

Abstract

We predict the van der Waals monolayer ReAg$_2$Cl$_6$ hosts isolated flat Chern bands at the Fermi level in its $120^\circ$ antiferromagnetic ground state. Their flatness and nontrivial topology arise from the cooperative effect of coplanar spin order and strong spin-orbit coupling within Re $5d$ orbitals-a mechanism distinct from moir\'e 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 can support fractional Chern insulator and charge-density wave states. Remarkably, the mechanism is generic to a broad family of Re-based compounds, with both spin configuration and flat band topology tunable by electrical manipulation. Our 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.