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.