Weyl Magnons in the Non-Coplanar Antiferromagnet MnTe$_2$

Using a combination of band representation analysis, inelastic neutron scattering (INS), magneto-Raman spectroscopy measurements, and linear spin wave theory, we establish that the non-coplanar antiferromagnet MnTe$_2$ is a tunable Weyl magnon material, hosting symmetry-protected topological nodal lines in its magnon band structure, protected by the the non-coplanar nature of the antiferromagnetic ordering, that transition into Weyl magnons upon the application of symmetry-breaking perturbations using an external magnetic field. By constructing a spin model that reproduces the observed INS magnon spectra and field-dependence of the Raman $Γ$-magnons, we directly probe the topological magnon nodal lines and observe their associated signature of non-trivial topology through the pseudo-spin winding of the scattering intensity in angular scans near the nodal lines. Finally, we discuss how to induce Weyl magnons in the spectrum through an external magnetic field, shedding light on future in-field INS and thermal Hall experiments. This work establishes a clear magnonic analog to Weyl electrons, enabling further exploration of topological behavior in bosonic systems and highlighting the interplay between magnetic order and band topology in non-coplanar antiferromagnets.

💡 Research Summary

The authors present a comprehensive study demonstrating that the non‑coplanar antiferromagnet MnTe₂ hosts symmetry‑protected topological magnon nodal lines which can be transformed into Weyl magnons by applying an external magnetic field. Starting from a symmetry‑based band‑representation analysis, they show that Mn atoms occupying the 4a Wyckoff position in the cubic pyrite structure (space group Pa 3̅) give rise to a three‑fold degenerate magnon mode at Γ (Γ + 4) and doubly degenerate modes at the X, M, and R points. Compatibility relations on high‑symmetry lines predict unavoidable nodal lines on the k_i = 0 planes, protected by the lack of an effective time‑reversal symmetry (¯T) that would otherwise forbid Weyl points in coplanar magnets.

High‑quality single crystals grown by the Bridgman method were characterized by inelastic neutron scattering (INS) at 5 K. The measured magnon dispersions along Γ–X, Γ–M, and Γ–R precisely follow the LSWT‑derived band structure, confirming the predicted degeneracies and revealing a clear crossing between the triply degenerate Γ + 4 band and a doubly degenerate band, forming the nodal line. Angular scans of the INS intensity near the crossing display a characteristic pseudo‑spin winding, a direct experimental signature of non‑trivial magnon topology.

Complementary magneto‑Raman spectroscopy identifies two zone‑center magnon modes (≈2.5 meV and ≈3.5 meV) in the ordered phase. When a magnetic field is applied along a low‑symmetry direction (