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 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 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.

Figures (4)

• Figure 1: (a,b) Atomic structure of monolayer ReAg$_2$Cl$_6$ from top and side views. The Wyckoff positions 1$a$ and 2$d$ are displayed (notation adopted from Bilbao Crystallographic Server bilbao2bilbao3elcoro2017). The original primitive cell and the magnetic $\sqrt{3}\times\sqrt{3}\times1$ supercell are represented as dashed and solid lines, respectively. (c) Brillouin zone (BZ) of the primitive cell and the supercell. (d) Schematic illustration of (100) AFM with 120$^{\circ}$ spin spiral structure. (e) Crystal field splitting and schematic diagram of AFM exchange between Re 5$d$ electrons.
• Figure 2: Electronic structure and topological properties of monolayer ReAg$_2$Cl$_6$. (a) Band structure and the topological edge states for 120$^{\circ}$ (100) AFM state. Four isolated flat Chern bands are highlighted. (b,c) The distribution of Berry curvature $\mathcal{B}(\mathbf{k})$ and $\text{Tr}[g(\mathbf{k})]$ in the BZ for three $\mathcal{C}=-1$ Chern bands in (a), respectively. $\mathcal{B}(\mathbf{k})$ remains the same sign throughout the whole BZ for VB and the third CB, while their sign is not always negative for the second CB.
• Figure 3: Low energy many-body spectra from ED and PES for $1/3$-filled (a,b) 1$^{\rm{st}}$ VB; (c,d) 2$^{\rm{nd}}$ CB; (e,f) 3$^{\rm{rd}}$ CB. ED with $N_{\text{uc}}=24$ and $27$. Insets of ED show the corresponding locations of nearly degenerate ground states of two cluster sizes (marked by blue circles and red crosses) in the BZ. PES with $N_{\text{uc}}=24$ and $N_{A} = 3$ for the three degenerate ground states in (a,c,e). Here we only show the lowest energy per momentum sectors in addition to the degenerate ground state.
• Figure 4: SOT switching of topology. (a) Schematic of the experimental setup. A spin current $\boldsymbol{\sigma}$, generated via the spin Hall effect of the applied charge current $\bm{I}$ in the substrate, flows parallel to the monolayer and perpendicular to the magnetic easy plane. The resulting damping-like SOT effective fields $\bm{H}_{\text{DL}}$ act on the sublattice moments within the magnetic easy plane. (b) Illustration of the SOT-driven rotation of sublattice moments in the 120$^{\circ}$$(100)$ spiral AFM. (c,d) Evolution of the Chern number of VB and the first CB during the spin rotation. The arrows correspond to the spin configurations shown in Fig. \ref{['fig1']}(d).