Abstract
Based on density functional theory (DFT) calculations, we predict that a monolayer of OsCl₃---a layered material whose interlayer coupling is weaker than in graphite---possesses a quantum anomalous Hall (QAH) insulating phase generated by the combination of honeycomb lattice of osmium atoms, their strong spin-orbit coupling (SOC) and ferromagnetic ground state with {\em in-plane} easy-axis. The band gap opened by SOC is \mbox{E_g ≃ 67 meV} (or \mbox{≃ 191 meV} if the easy-axis can be tilted out of the plane by an external electric field), and the estimated Curie temperature of such {\em anisotropic planar rotator} ferromagnet is TC 350 K. The Chern number C=-1, generated by the manifold of Os t_2g bands crossing the Fermi energy, signifies the presence of a single chiral edge state in nanoribbons of finite width, where we further show that edge states are spatially narrower for zigzag than armchair edges and investigate edge-state transport in the presence of vacancies at Os sites. Since 5d electrons of Os exhibit {\em both} strong SOC and moderate correlation effects, we employ DFT+U calculations to show how increasing on-site Coulomb repulsion U: gradually reduces E_g while maintaining C = -1 for 0 < U < U_c; leads to metallic phase with E_g = 0 at U_c; and opens the gap of topologically trivial Mott insulating phase with C=0 for U > U_c.