Valley polarization and symmetry-driven spin splitting in altermagnetic Fe2S2O and Janus Fe2SSeO monolayers and bilayers

Abstract

The recent discovery of altermagnetism has established a new paradigm for realizing spin-polarized electronic states in collinear antiferromagnets without relying on spin–orbit coupling or net magnetization. Here, we report a comprehensive first-principles DFT (density functional theory) study aimed at understanding the structural, electronic and magnetic properties of Fe₂S₂O and the newly proposed Janus Fe₂SSeO monolayers and bilayers. We first confirm that the Fe₂S₂O monolayer hosts an altermagnetic ground state, where uniaxial strain and carrier doping provide efficient control of the valley polarization and piezomagnetic response. Extending the study to bilayers reveals that the altermagnetic behavior of Fe₂S₂O strongly depends on interlayer stacking symmetry, magnetic configuration and external perturbations. In contrast, we demonstrate that the novel Janus Fe₂SSeO bilayer constitutes a previously unexplored intrinsically symmetry-broken altermagnetic system. The inequivalent S- and Se-terminated surfaces permanently remove inversion and mirror symmetries, preventing the restoration of spin degeneracy and stabilizing robust momentum-dependent spin splitting in both AA and AA${}^{\prime }$ stacking configurations under different magnetic orders without applying external electric fields. Symmetry analysis further reveals that the interplay between interlayer registry and crystal-spin symmetry governs the spin-layer coupling and electronic reconstruction. Our findings establish Janus bilayer engineering as an effective route toward intrinsic altermagnetism and provide a promising platform for designing two-dimensional spintronic and valleytronic materials based on symmetry-controlled electronic states.