Tunable magnon Weyl points in ferromagnetic pyrochlores

TL;DR

This paper demonstrates that magnon dispersion in ferromagnetic pyrochlores hosts tunable Weyl points with topological magnon arcs, controllable via external magnetic fields, revealing a new class of topological magnon phenomena.

Contribution

It introduces the existence of tunable Weyl points in magnon spectra of ferromagnetic pyrochlores and explores their topological surface states and experimental detection methods.

Findings

Weyl points appear in magnon dispersion relations of ferromagnetic pyrochlores.

External magnetic fields can tune the position and orientation of Weyl points and magnon arcs.

Surface magnon arcs connect projections of Weyl points, resembling Fermi arcs in electronic Weyl semimetals.

Abstract

The dispersion relations of magnons in ferromagnetic pyrochlores with Dzyaloshinskii-Moriya interaction is shown to possess Weyl points, i.\,e., pairs of topological nontrivial crossings of two magnon branches with opposite topological charge. As a consequence of their topological nature, their projections onto a surface are connected by magnon arcs, thereby resembling closely Fermi arcs of electronic Weyl semimetals. On top of this, the positions of the Weyl points in reciprocal space can be tuned widely by an external magnetic field: rotated within the surface plane, the Weyl points and magnon arcs are rotated as well; tilting the magnetic field out-of-plane shifts the Weyl points toward the center $\bar{\Gamma}$ of the surface Brillouin zone. The theory is valid for the class of ferromagnetic pyrochlores, i.\,e., three-dimensional extensions of topological magnon insulators on kagome lattices. In this Letter, we focus on the $(111)$ surface, identify candidates of established ferromagnetic pyrochlores which apply to the considered spin model, and suggest experiments for the detection of the topological features.