Higher-order topological insulators in two-dimensional antiferromagnetic and altermagnetic chromium-based group-IV chalcogenides

Abstract

Based on first-principles calculations combined with theoretical analysis, we identify a family of monolayer chromium-based group-IV chalcogenides as a new class of two-dimensional (2D) magnetic higher-order topological insulators (HOTIs). Specifically, the CrC$X_3$ ($X=$ S, Se, Te) and CrSiS${}_3$ monolayers are found to host conventional antiferromagnetic ground states with $\mathcal{P}\mathcal{T}$ symmetry, whereas the Janus compounds Cr${}_2$C${}_2$S${}_3$Se${}_3$ and Cr${}_2$Si${}_2$S${}_3$Se${}_3$ exhibit altermagnetic ground states. We demonstrate that all these monolayer magnetic materials realize 2D HOTI phases, in which the nontrivial topology is protected by lattice $C_3$ rotational symmetry and manifests as zero-dimensional corner states carrying quantized fractional charges. Moreover, upon inclusion of spin-orbit coupling, these systems remain in the HOTI phase and continue to host robust corner-localized states, confirming the stability of their higher-order topological nature. Our results reveal an intrinsic connection between higher-order topology and magnetic order in 2D antiferromagnetic and altermagnetic systems, identifying chromium-based group-IV chalcogenide monolayers as promising platforms for exploring higher-order topological phases and their potential relevance for future topological and spintronic applications.

8 pages, 5 figures

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