Unconventional Floquet Quantum Spin Hall Effect in Two-Dimensional Altermagnets.

Altermagnetism has recently emerged as a promising platform for achieving exotic quantum states and dissipationless spin transport. Here, we uncover a previously unexplored Floquet realization of the quantum spin Hall effect in 2D altermagnets under elliptically polarized light, a phenomenon that has remained unattainable in previously studied ferromagnetic and antiferromagnetic systems. Remarkably, accompanying the periodic driving, the system undergoes successive topological phase transitions from a second-order topological insulator to a quantum anomalous Hall insulator, and ultimately to a quantum spin Hall insulator. Moreover, the proposed mechanism is confirmed in the altermagnetic V2SSeO monolayer through analyses of corner states, edge states, and quantized anomalous and spin Hall responses, establishing it as a realistic material platform. Our findings establish a link between altermagnetism, Floquet engineering, and topological phases, opening new possibilities for next-generation optically controlled topological spintronic devices.