Higher-order topological semimetal induced by altermagnet

Altermagnetism, characterized by momentum-dependent spin splitting in the absence of net magnetization, provides a fertile ground for realizing topological states. Here, we show that coupling a first-order topological insulator to a d-wave altermagnet leads to a higher-order topological semimetal phase. This phase simultaneously hosts gap-closing points, robust zero-energy edge states, and corner states. The altermagnetic order parameter governs the motion of the gap-closing points within the Brillouin zone, while the Néel vector determines the spatial localization of the corner states. By mapping out the phase diagram, we identify topological transitions between trivial, higher-order insulating, and higher-order topological semimetal phases. Our findings establish altermagnetic heterostructures as a tunable platform for engineering higher-order topological phases in two dimensions.