Strain-Switchable Local Valley Splitting and Carrier-Mediated Piezomagnetism in an Altermagnetic Mn2Se2S Monolayer

Two-dimensional altermagnetic semiconductors are promising for low-stray-field spintronic and valleytronic applications, yet Mn-based systems with strain-tunable local valley splitting and piezomagnetism are rarely studied. Using first-principles calculations, we predict a dynamically stable Mn2Se2S monolayer with an altermagnetic ground state and out-of-plane spin orientation, which remains semiconducting over the Hubbard U = 3.0–4.0 eV range. Uniaxial in-plane strain breaks the diagonal mirror symmetry, yielding a sign-reversible local piezovalley response with a local valence-band splitting varying over a range of 246.13 meV. Carrier doping induces a strain-dependent imbalance between spin-channel occupations, enabling a net magnetization and a doping-tunable piezomagnetic effect. Strain defines −1, 0, and +1 local valley-ordering states, for which the valley-contrasting Berry curvature suggests a possible electrical readout through a transverse response. This work expands Mn-based altermagnetic semiconductors and identifies Mn2Se2S as a candidate for strain-driven valleytronic and piezomagnetic devices.