Floquet Quantum Anomalous Hall Effect with In-Plane Magnetization in Two-Dimensional Altermagnets

Altermagnets and the quantum anomalous Hall effect (QAHE) are intrinsically important for advancing low-dissipation spintronics. However, the emergence of QAHE in altermagnets remains elusive. Here, we realize the Floquet QAHE in a 2D altermagnet with in-plane magnetization and, in particular, put forward that Floquet–Bloch band engineering drives a topological phase transition from the second-order topological insulator (SOTI) to a QAH insulator. Taking the square lattice as an example, the Janus V2XTeO (X = Se and S) monolayers are investigated as the potential systems to access the viability of the proposed scheme. The Janus V2XTeO monolayers are prototypical materials for altermagnets, and without light irradiation, they are SOTIs distinguished by nontrivial corner states. Driven by a time-periodic optical field, engineered light intensity triggers the topological phase transition that gives rise to QAHE, concurrently evidenced by a quantized Chern number of C=1 and a chiral edge state. These findings demonstrate the exotic QAHE in altermagnets, providing a prototype platform for intrinsic topological phenomena that is expected to draw great experimental attention.