Emergent altermagnetism and topological response in Janus MnPSX monolayers.

Through a comprehensive computational study, we demonstrate that g-type altermagnetism can be effectively engineered in Janus MnPS₃ monolayer via chalcogen substitution. Replacing one sulfur layer with O, Se, or Te breaks inversion symmetry and induces a charge density asymmetry, lifting Kramers degeneracy and resulting in a momentum-dependent spin splitting characteristic of altermagnetic materials. The magnitude of this splitting is governed by the interplay of electronegativity and atomic radius, with oxygen substitution yielding the largest effect due to its strong Mn-O and P-O bonds driven by high electronegativity and small atomic size. Remarkably, the structural asymmetry and charge redistribution also facilitate a topological response, leading to the emergence of a topological phase with nontrivial quantum spin Hall order. Our findings reveal a new pathway to tailor altermagnetism and topological properties in 2D materials through charge density engineering, opening exciting prospects for spintronic and quantum topological applications in layered magnetic systems.