Floquet-Engineered Odd-Parity Altermagnetic Higher-Order Topology in a Two-Dimensional Antiferromagnet Cr$_2$CH$_2$

TL;DR

This paper demonstrates that periodic driving with circularly polarized light can induce and control an odd-parity altermagnetic higher-order topological phase in a 2D antiferromagnetic monolayer, maintaining topological corner states despite band renormalization.

Contribution

It reveals how Floquet engineering can realize and manipulate altermagnetic HOTI phases in antiferromagnetic materials, expanding the understanding of dynamic topological states.

Findings

Periodic driving induces an altermagnetic HOTI phase in Cr$_2$CH$_2$.

Corner states remain robust under Floquet driving across various intensities.

Increasing light intensity transitions the system into an altermagnetic semimetal.

Abstract

Periodic driving provides a platform to dynamically tailor quantum states of matter, yet its impact on symmetry-protected topological phases remains incompletely understood. Here, we demonstrate that periodic driving enables the realization of an odd-parity altermagnetic (AM) higher-order topological insulator (HOTI) phase in the Cr$_2$CH$_2$ monolayer. In equilibrium, Cr$_2$CH$_2$ is a 2D antiferromagnetic (AFM) HOTI protected by $\mathcal C_3$ rotational symmetry, characterized by a symmetry indicator $\chi^{(3)}$ = $\{-2,1\}$ and robust corner states. Under circularly polarized light (CPL), the system develops a f-wave altermagnetic state governed by the symmetry $[C_{2}||\overline{3}_{001}]$ with odd-parity spin splitting. Despite substantial Floquet-induced band renormalization, the $\mathcal C_3$-protected corner states remain intact over a broad range of driving strengths, highlighting the altermagnetic higher-order topology under Floquet driving. As the light intensity increases, the system gradually evolves into an altermagnetic semimetallic state. These results establish a direct connection between magnetism and topology in a periodically driven AFM system, offering a route toward the control of coupled spin and topological transport.