Ultrafast electron dynamics in altermagnetic materials

Altermagnets constitute a new class of magnetic materials that combine properties previously thought to be exclusive to either antiferromagnets or ferromagnets, and have unique properties of their own. In particular, a combination of symmetries connecting magnetic sublattices gives rise to a band spin splitting exhibiting unconventional d, g, or i-wave character. Their unique electronic properties have already led to new spin-dependent transport effects. Here, we consider their spin and charge dynamics on ultrafast timescales. We use a minimal tight binding model that captures the main features of the altermagnetic candidate material KRu$_4$O$_8$. In the framework of this model, we compute the spin-dependent electronic scattering dynamics after ultrashort-pulse excitation and show through these microscopic calculations how electron-electron and electron-phonon scattering processes redistribute optically excited carriers in a 2D slice of the Brillouin zone. We find that the optically excited spin polarization is long lived (~1ps) compared to the electron-electron momentum scattering lifetime of roughly 10fs. This contrasts remarkably with the much shorter spin lifetimes observed in typical ultrafast electronic spin dynamics in conventional ferromagnets and antiferromagnets, making these pulse-driven spin excitation experiments a key probe of altermagnetism.

💡 Research Summary

This paper presents a comprehensive theoretical investigation of ultrafast electron and spin dynamics in altermagnetic materials, focusing on the candidate compound KRu₄O₈. Altermagnets are a newly identified class of magnetic systems that combine features of ferromagnets and antiferromagnets while exhibiting unconventional momentum‑dependent spin splittings (d‑, g‑, or i‑wave symmetry). The authors first obtain the electronic band structure and spin texture of KRu₄O₈ from density‑functional theory (DFT). The DFT results reveal two well‑separated conduction bands near the Fermi level that display a characteristic d‑wave altermagnetic spin texture: the expectation value of the spin along the crystallographic x‑axis alternates sign across the Brillouin‑zone, producing distinct spin‑up and spin‑down “domains” in k‑space.

To enable tractable time‑dependent simulations, the two DFT bands are fitted to a minimal tight‑binding (TB) model defined on a square lattice. The TB Hamiltonian reads
H(k)=C(k)σ₀+Δ_c(k)σ_z+t_{Iz}(k)σ_x,
where C(k) encodes nearest‑neighbor hopping, Δ_c(k) captures the altermagnetic spin splitting (including both cos(k_x)−cos(k_y) and sin(k_x)sin(k_y) terms), and t_{Iz}(k) represents a spin‑orbit coupling term that mixes the σ_x component. The fitted parameters reproduce the DFT band dispersion and spin texture with high fidelity.

The system is then excited by an ultrashort, linearly polarized laser pulse (photon energy ħω_L ≈ 4.15 eV, polarization angle φ = 45° relative to the