Chiral split magnons in metallic g-wave altermagnets: insights from many-body perturbation theory

npj | quantum materials Article Published in partnership with Nanjing University https://doi.org/10.1038/s41535-025-00818-8 Chiral split magnons in metallic g-wave altermagnets: insights from many-body perturbation theory Check for updates 1,2 1234567890():,; 1234567890():,; Wejdan Beida Stefan Blügel1,4 , Ersoy Şaşıoğlu 2 1 1 1,3 , Christoph Friedrich , Gustav Bihlmayer , Yuriy Mokrousov & Altermagnets are a novel class of magnetic materials that bridge the gap between ferromagnets (FMs) and antiferromagnets (AFMs). A key feature is the non-degeneracy of magnon modes where spin splitting occurs, leading to chirality and direction-dependent magnon dispersions governed by symmetry. We explore this in metallic g-wave altermagnets (TPn, where T = V, Cr; Pn = As, Sb, Bi) using density functional and many-body perturbation theories. We analyze the influence of pnictogen substitution on spin splitting and magnon behavior. We uncover anisotropic magnon band splitting aligned with electronic structure, and wavevector- and chirality-dependent damping due to Stoner excitations. We identify regions in the Brillouin zone where the chiral magnon splitting overcomes the damping. These findings suggest altermagnets are promising for spintronic and magnonic technologies, where direction-dependent magnon lifetimes and nonreciprocal magno transport may enable chiral magnon propagation, while wavevector-selective damping could be harnessed for fast and controllable magnetization switching. Altermagnets represent a newly identified class of magnets that uniquely combine characteristics of ferromagnets (FMs) and conventional collinear antiferromagnets (AFMs). For example, consistent with AFMs, altermagnets feature a compensated magnetic structure with zero net magnetic moment. Similar to FMs, they exhibit spin splitting in their electronic bands along specific crystallographic directions1–4. Unlike conventional AFM, this spin splitting does not arise from spin-orbit coupling (SOC), instead, it emerges from the interplay between magnetic exchange interactions and crystal symmetry. This interplay leads to anisotropic spin polarization within the Brillouin zone (BZ), breaking time-reversal symmetry while preserving crystal symmetries such as rotations, i.e., E↑(k) ≠ E↓(− k) without SOC4. The collinear nature of altermagnets implies that the spin remains a good quantum number in the absence of SOC and in difference to noncollinear antiferromagnets, the spin-momentum locking features a common k-independent quantization axis across the Brillouin zone denoting the spin-splitting just as a spin-up, -down splitting. The quantization axis can be changed by rotating the antiferrmagnetic Néel vector relative to the cyrstal lattice with implications on the anomalous Hall effect (AHE), the Dzyaloshinskii-Moriya interaction (DMI) and the current-induced exchange-coupling torques. These characteristics suggest that altermagnets represent a distinct category of materials that deviate from conventional classifications of magnetic order and exhibit unconventional physical properties. A key aspect of understanding the distinctive behavior of altermagnets lies in the study of their spin excitations, which determine their dynamic and transport properties. Spin excitations in magnetic systems encompass both collective magnon (spin-wave) modes and single-particle spin-flip Stoner excitations5,6. In FMs, magnons exhibit a quadratic dispersion near the Brillouin zone center, resulting in low-energy excitations. The ferromagnetic resonance frequency, corresponding to zero-momentum spin excitations in an external magnetic field, typically lies in the gigahertz (GHz) range, depending on material parameters and the applied field7. In contrast, AFMs feature magnons with linear dispersion and significantly higher antiferromagnetic resonance frequencies, which are in the terahertz (THz) regime, due to the additional contribution of the strong exchange interactions8. Altermagnets bridge these two regimes, combining FM-like spin splitting with AFM-like symmetry and frequency response, while also exhibiting unconventional magnonic properties, including the emergence of chiral magnons, which distinguish them from conventional magnetic systems in the absence of SOC9,10. The term chiral magnons has recently gained attention11,12, yet its precise meaning and implications often remain unclear. The concept of 1 Peter Grünberg Institut, Forschungszentrum Jülich and JARA, Jülich, Germany. 2Institute of Physics, Martin Luther University Halle-Wittenberg, Halle (Saale), Germany. 3Institute of Physics, Johannes Gutenberg University Mainz, Mainz, Germany. 4Present address: Physics Department, RWTH-Aachen Unie-mail: ; versity, Aachen, Germany. npj Quantum Materials | (2025)10:97 1 https://doi.org/10.1038/s41535-025-00818-8 magnon polarization has been known for decades, and describes the handedness of spin-wave (magnon) precession in magnetic systems. According to classical (the Landau-Lifshitz equation) and quantum mechanics, a magnetic moment arizing from electrons precesses counterclockwise around an applied magnetic field, which is conventionally defined as having positive polarization. In simple FMs, all magnons share uniquely this positive polarization13, whereas in collinear AFMs, two magnon branches with opposite polarization exist but remain degenerate unless easy-axis or easy-plane anisotropies or a large magnetic field lift the degeneracy8,14. Despite these differences, direct experimental verification of opposite magnon polarization remains challenging, often requiring polarized neutron scattering techniques15–17. In altermagnets, the chiral magnon degeneracy of antiferromagnets is lifted along certain wave-vector directions and chiral magnons emerge as a consequence of exchange interactions and crystal symmetry, rather than from spin-orbit coupling. Unlike chiral magnets, where chirality originates from the Dzyaloshinskii-Moriya interaction in noncentrosymmetric systems, altermagnetic chiral magnons arise from the momentum-dependent spin splitting enforced by symmetry, even in centrosymmetric materials11. This leads to nonreciprocal magnon dispersions, breaking time-reversal symmetry in a distinct way compared to conventional magnetic systems. The resulting asymmetric magnon propagation in altermagnets bears similarities to chiral magnets but stems from a fundamentally different microscopic origin9. Experimentally, the detection of chiral magnons in altermagnets has been demonstrated using inelastic neutron scattering, which has successfully been employed to observe altermagnetic magnon splitting in materials such as MnTe18. Beyond their fundamental significance, chiral magnons in metallic altermagnets exhibit additional complexities due to their coupling with Stoner excitations, which significantly affect both their dispersion and damping. This coupling leads to wavevector-dependent magnon broadening, distinguishi (...truncated)