Noncoplanar antiferromagnetism and quantum topological Hall effect in quarter-electron-filled triangular-lattice K0.5RuO2

Transition-metal compounds with triangular lattices exhibit a rich interplay among electron filling, geometric frustration, and electronic correlations, making them an ideal platform for realizing exotic quantum states. Here, using first-principles calculations, we demonstrate that the layered triangular-lattice compound K0.5RuO2 stabilizes a noncoplanar antiferromagnetic state that was previously predicted in a quarter-electron-filled Kondo-lattice model on a triangular lattice. This unconventional chiral magnetic order gives rise to a quantum topological Hall effect (QTHE), namely a quantum anomalous Hall effect (QAHE) driven by noncoplanar spin textures. The resulting topological phase is characterized by a Chern number C = 2, which originates from the two RuO2 layers in the unit cell. The QTHE phase in K0.5RuO2 emerges at 1/4 filling, distinct from the 3/4-filled case where the chiral magnetic order arises from perfect Fermi-surface nesting. Furthermore, based on the modern theory of orbital magnetization, we show that the orbital magnetization varies linearly with the Fermi energy within the topological band gap, providing concrete evidence of the chiral edge states associated with the QTHE phase. These findings establish K0.5RuO2 as a promising material platform for realizing chiral magnetic topological states in quarter-electron-filled triangular-lattice systems and offer guidance for future experimental and theoretical studies of topological chiral antiferromagnets.