Higher-order Topological Phases of Magnons in van der Waals Honeycomb Ferromagnets

TL;DR

This paper proposes a theoretical model for higher-order topological magnon insulators in van der Waals honeycomb ferromagnets, revealing protected corner and hinge modes with potential applications in magnonic devices.

Contribution

It introduces a second-order topological phase in layered ferromagnets with novel symmetry protections and topological features not previously explored in magnon systems.

Findings

Identification of Z2 topological phase protected by pseudo-time-reversal symmetry.

Discovery of hinge modes propagating along stacking directions in cuboid geometries.

Establishment of a Z2×Z topology with chiral hinge modes robust against disorder.

Abstract

We theoretically propose a second-order topological magnon insulator by stacking the van der Waals honeycomb ferromagnets with antiferromagnetic interlayer coupling. The system exhibits Z$_{2}$ topological phase, protected by pseudo-time-reversal symmetry (PTRS). An easy-plane anisotropy term breaks PTRS and destroys the topological phase. Nevertheless, it respects a magnetic two-fold rotational symmetry which protects a second-order topological phase with corner modes in bilayer and hinge modes along stacking direction. Moreover, an introduced staggered interlayer coupling establishes a Z$_{2}$$\times$Z topology, giving rise to gapped topological surface modes carrying non-zero Chern numbers. Consequently, chiral hinge modes propagate along the horizontal hinges in a cuboid geometry and are robust against disorders. Our work bridges the higher-order topology and magnons in van der Waals platforms, and could be used for constructing topological magnonic devices.