Topological magnon insulators in two-dimensional van der Waals ferromagnets CrSiTe ₃ and CrGeTe ₃ : Toward intrinsic gap-tunability

Abstract

The bosonic analogs of topological insulators have been proposed in numerous theoretical works, but their experimental realization is still very rare, especially for spin systems. Recently, two-dimensional (2D) honeycomb van der Waals ferromagnets have emerged as a new platform for topological spin excitations. Here, via a comprehensive inelastic neutron scattering study and theoretical analysis of the spin-wave excitations, we report the realization of topological magnon insulators in CrXTe 3 (X = Si, Ge) compounds. The nontrivial nature and intrinsic tunability of the gap opening at the magnon band-crossing Dirac points are confirmed, while the emergence of the corresponding in-gap topological edge states is demonstrated theoretically. The realization of topological magnon insulators with intrinsic gap-unability in this class of remarkable 2D materials will undoubtedly lead to new and fascinating technological applications in the domain of magnonics and topological spintronics.

The Magnetic Genome of Two-Dimensional van der Waals Materials

“… 746 , 747 Indeed, recently neutron scattering measurements suggest that the observed spin Dirac gap in CrI 3 is induced by the next nearest neighbor DMIs. 750 , 751 …”

Anisotropic magnon damping by zero-temperature quantum fluctuations in ferromagnetic CrGeTe3

“…In addition, the in-plane lattice parameter a of CrGeTe 3 displays negative thermal expansion around T C = 65 K 27 , consistent with the calculations showing enhanced FM interaction with the expansion of the lattice. Finally, INS experiments found broadened spin waves in CrGeTe 3 throughout the Brillouin zone, suggestive of a strong SLC 26 . Although these results provided circumstantial evidence for SLC in CrGeTe 3 , there is currently no direct experimental proof and a microscopic understanding of the SLC in CrGeTe 3 is lacking.…”

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

“…The CrI 3 , CrBr 3 and other honeycomb ferromagnets with AFM interlayer coupling are promising candidates, and even the kagome ferromagnets. There are already experimental observations of the topology of magnons in monolayer honeycomb ferromagnets, due to the DM interaction [42][43][44], which is necessary in our proposal. The stacking type is not restricted in the AA-type.…”

“…Since the experimental discovery of two-dimensional (2D) ferromagnetism [39][40][41], the van der Waals magnets provide new and versatile platforms to investigate the fundamental physics and also new opportunities for spintronic device design. Recent experimental progresses reveal topological phases in 2D magnetic materials, such as CrI 3 and MnBi 2 Te 4 , etc [42][43][44][45][46][47][48]. The van der Waals magnets show a store of in-plane and out-of-plane magnetic orders.…”