Emerging of Anomalous Higher-Order Topological Phases in Altermagnet/Topological Insulator Heterostructure by Floquet...

Altermagnetism, characterized by collinear-compensated magnetic ordering with vanishing net magnetization, has opened new frontiers in topological matter through its distinctive spin-momentum locking mechanism. We demonstrate that the altermagnetism in altermagnet/topological insulator (TI) heterostructure breaks time-reversal symmetry, and in combination with Floquet engineering, gives rise to anomalous higher-order topological insulator (AHOTI) phases with pure/hybridized $\pi$ -corner modes (CMs). Two fundamental breakthroughs by Floquet engineering are revealed. First, three distinct topological phase transitions are established: trivial-to-AHOTI with pure $\pi$ -CMs (AHOTI-I), higher-order TI-to-AHOTI with hybridized $\pi$ -CMs (AHOTI-II), and mutual transitions between AHOTI-I and -II. Second, the real-space locations of topological CMs-not only conventional 0- but also anomalous $\pi$ -CMs-are synchronously manipulated by Néel vector reorientation, establishing an altermagnetism-programmable paradigm for topological state localization. The emergent topological phases are systematically characterized through the nested dynamical Wilson loop formalism, deliberately adapted for Floquet-driven systems, which shows exact correspondence with the observed CMs. This work establishes a universally versatile platform for non-equilibrium dynamical control of topological phase, thereby providing transformative implications for reconfigurable topological electronics and fault-tolerant quantum computing architectures.