Stacking‐Engineered Tunable Altermagnetism in Topological FeSe Bilayer Systems

Altermagnetism and topological insulators represent two of the most transformative frontiers in modern condensed matter physics, spintronics, and quantum information science. Bringing these two paradigms together opens a largely unexplored route toward fundamentally new quantum phenomena. Here, we predict a topological altermagnetic phase in bilayer tetragonal Fe‐based superconductors and reveal it as a highly tunable platform for valley‐polarized anomalous Hall and quantum anomalous Hall physics. Based on first‐principles calculations, we show that the characteristic spin‐splitting and valley polarization can be effectively tuned via applied strain. Moreover, increasing the in‐plane uniaxial strain can induce a topological phase transition from topological insulator phase to quantum anomalous Hall insulator phase. These findings reveal a powerful route for controlling altermagnetism and topology while identifying a realistic material platform for its experimental realization and technological exploitation.