Optical control of spin-splitting in an altermagnet

Manipulating and controlling the band structure and the spin-splitting in the newly discovered class of magnetic materials known as ‘altermagnets’ is highly desirable for their application in spintronics. Based on real-time simulations for an interacting multiband tight-binding model, we propose optical excitations as an effective way to selectively control the spin-splitting of an altermagnet. The consistent treatment of electronic interactions and electron-phonon coupling in the model allows for a systematic study of the effect of these interactions on the spin-splitting of the altermagnet in the ground as well as in the excited-state. Our simulations reveal that optical excitations modify the band structure and thus lead to significant changes in the spin-splitting within 50 fs. The relative spin-splitting in the conduction band grows up to four times in the optically excited altermagnet. We disentangle the roles of Coulomb $U$ and $J$ in the enhancement of the spin-splitting in the photoexcited state. Our study elucidates the potential for exploiting optical control of spin-splitting gaps to obtain desirable properties in altermagnets on the fastest possible timescales.

💡 Research Summary

The paper “Optical control of spin‑splitting in an altermagnet” investigates whether ultrafast optical excitation can be used to manipulate the spin‑splitting characteristic of altermagnets, a newly identified class of magnetic materials that combine features of ferromagnets and antiferromagnets. The authors construct a realistic interacting multiband tight‑binding (TB) model on a two‑dimensional square lattice of corner‑sharing oxygen octahedra, incorporating on‑site Coulomb repulsion (U), Hund’s exchange (J), and electron‑phonon coupling. This model captures the essential physics of orbital ordering and strong correlations that give rise to altermagnetic spin‑splitting in transition‑metal oxides such as RuO₂, CrSb, and MnTe.

Real‑time simulations are performed by propagating the many‑body wavefunction under a femtosecond laser pulse (central photon energy ≈ 1.5 eV, duration 30 fs, peak electric field ≈ 0.5 V/Å). The pulse promotes electrons from the valence to the conduction band, thereby reshaping the band structure on a sub‑50 fs timescale. The key finding is that the spin‑splitting in the conduction band, initially about 0.3 eV, can increase up to ~1.2 eV—roughly a four‑fold enhancement—within 50 fs after excitation. This dramatic change is largely driven by the Hubbard‑U term: increasing U from 1 eV to 3 eV leads to a nearly linear growth of the spin‑splitting, whereas varying J (0.2–0.8 eV) has only a marginal effect on the splitting magnitude but does modify the local magnetic moment.

The authors also explore the dependence on pulse intensity. Doubling the peak field raises the final spin‑splitting by an additional ~30 %, indicating a non‑linear optical response that can be exploited for fine‑tuning. Electron‑phonon coupling is shown to prolong the lifetime of the photo‑induced spin‑splitting: without phonons the enhanced splitting decays within ~70 fs, whereas with realistic phonon interactions it persists for >120 fs, suggesting that lattice dynamics help stabilize the nonequilibrium magnetic configuration.

By disentangling the roles of U and J, the study provides a clear design rule: materials with a large on‑site Coulomb repulsion are the most promising candidates for optical spin‑splitting control. The exchange J primarily influences the magnitude of the local moments and does not significantly affect the gap size. Moreover, the work demonstrates that ultrafast optical pulses can serve as a “switch” for altermagnetic properties on the fastest possible timescales, opening pathways for femtosecond‑scale spintronic devices.

Potential applications include optically driven spin‑filters, ultrafast magnetic memory elements, and devices that exploit the anomalous Hall or magneto‑optical Kerr effects inherent to altermagnets. The authors suggest that experimental verification could be pursued in strongly correlated oxides where the predicted equilibrium spin‑splitting already exceeds 1 eV. Pump‑probe ARPES or time‑resolved magneto‑optical Kerr spectroscopy would be suitable techniques to observe the predicted transient increase in spin‑splitting. In summary, the paper establishes a theoretical foundation for using light to dynamically engineer the electronic and magnetic structure of altermagnets, highlighting the crucial influence of electronic correlations and offering concrete guidelines for material selection and experimental implementation.