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

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 Z2 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 Z2 × 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.

Magnon Nernst Effect in Magnon Spin Hall Systems

“…The first one is the bilayer honeycomb ferromagnets with AFM interlayer coupling, which have been proved to host a Z 2 topological invariant. 30,31 Without the Pauli exclusion, the boson usually can not give a quantized response to the external field, the system would thus give a spin Hall effect. Here we take the monoclinic stacked bilayer CrI 3 as the example material, [32][33][34] illustrated in Fig.…”

“…Beisdes the magnon Hall effect, the antiferromagnetic order endows effective magnon spins, giving rise to magnonic analog of spin Hall effect when the two spin branches host opposite Berry curvature. 27,28,30,31 Same to the electronic systems, the magnon spin Hall systems exhibit zero net magnon current in the transverse direction, thus hold a vanishing thermal Hall effect, making it hard to detect experimentally.…”