Modulating band topology and realizing the quantum anomalous Hall effect by composition engineering in VCl(3-x)Brx monolayers

Abstract Composition engineering modifies local ligand fields, orbital hybridization, and crystal symmetry, providing a route to tailor the band topology of two-dimensional magnetic materials. Here, we apply this approach to the VCl(3-x)Brx monolayer system using first-principles density-functional theory. The pristine end members VCl3 x=0 and VBr3 x=3 preserve lattice inversion symmetry (D3d) and exhibit no gapless edge states, whereas the Janus VCl1.5Br1.5 structure x=1.5 lowers the symmetry to C3v, and the asymmetric Cl/Br ligand field drives band inversion which together with ferromagnetic time-reversal symmetry breaking, gives rise to a quantum anomalous Hall (QAH) with Chern number C=1 and chiral edge states within the valence band. Monte Carlo simulations parameterized by ab initio exchange couplings predict a Curie temperature T_c ≈ 29 K. Crucially, representative hole doping of 0.4 holes per unit cell brings the Fermi level closer to the nontrivial gap. The topological band characteristics remain preserved, suggesting a possible route toward the QAH regime through electrostatic tuning. In contrast, random-like alloy configurations at the same nominal compositions fail to sufficiently break lattice symmetry, yielding only tunable anomalous Hall conductivity without quantization. These results demonstrate an ordering-dependent modulation of band topology in VCl(3-x)Brx monolayers and identify the ordered Janus phase as a metastable theoretical candidate.