A Route to Nonrelativistic Altermagnetic Spin Splitting via Ultrafast Light

Abstract: We identify a nonequilibrium route for generating altermagnetic spin splitting in antiferromagnet by ultrafast light. Unlike existing strategies, this route does not require relativistic angular-momentum transfer, static symmetry breaking, or auxiliary external fields. Using real-time time-dependent density functional theory, we demonstrate in the antiferromagnetic perovskite KNiF3 that linearly polarized light can induce momentum-dependent altermagnetic spin splitting by breaking the effective time-reversal symmetry through photoexcited charge redistribution and the resulting lattice distortion. We provide a general symmetry selection rule for this route. These results establish a mechanism for ultrafast control of altermagnetism and extend its material realization into the nonequilibrium regime.

• The paper introduces a novel, nonrelativistic mechanism where ultrafast, linearly polarized light triggers altermagnetic spin splitting in antiferromagnets without relying on spin-orbit coupling.
• Simulations using rt-TDDFT reveal symmetry-selective octahedral rotations up to 12.3° within 400 fs, leading to measurable anomalous Hall conductivity oscillations reaching +400 S/cm.
• The study establishes symmetry and momentum selection rules, offering a robust framework for ultrafast, reversible control of altermagnetic states in cubic perovskite antiferromagnets.

Ultrafast Light-Induced Nonrelativistic Altermagnetic Spin Splitting in Antiferromagnets

Context and Motivation

Altermagnetism (ALM) is characterized by momentum-dependent spin splitting in materials lacking net magnetization, distinct from both classical ferromagnetism and conventional spin-splitting mechanisms dominated by relativistic spin-orbit coupling (SOC). The origin of spin polarization in altermagnets arises from the breaking of composite symmetries such as space-time reversal (PT) and translational-spin-reversal (t'U), as opposed to the symmetry-breaking underlying Kramers degeneracy or SOC-driven splitting. Despite theoretical advances in identifying ALM in low-Z, centrosymmetric antiferromagnets, experimental realization and manipulation of altermagnetic phases remain limited, typically relying on static symmetry breaking methods or relativistic angular-momentum transfer, restricting applicable material systems and operational principles.

This manuscript delineates a nonequilibrium, SOC-free pathway for generating altermagnetic spin splitting in antiferromagnets using ultrafast, linearly polarized light. By employing real-time time-dependent density functional theory (rt-TDDFT) and symmetry analysis, the authors reveal that photoexcitation induces symmetry-selective lattice distortions leading to k-dependent spin splitting, bypassing the necessity for heavy elements or auxiliary external fields.

Mechanism of Light-Induced Altermagnetism

The study focuses on KNiF$_3$ as a prototypical G-type antiferromagnet. In its ground state, KNiF$_3$ exhibits collinear antiferromagnetic order and absence of spin splitting. Optical excitation near the band gap (using a laser with photon energy just above the DFT+U-computed gap of $4.36\,\text{eV}$) transfers electrons from Ni $t_{2g}$ to antibonding $e_g$ orbitals, driving a localized bond elongation. Due to experimental volume constraints, the lattice cannot expand homogenously. Instead, this manifests as symmetry-breaking octahedral rotations – specifically, out-of-phase rotations along Cartesian axes – which disrupt the combined PT and t'U symmetry, thus enabling ALM spin splitting.

rt-TDDFT simulations reveal that after photoexcitation:

• Octahedral rotation angles reach up to $12.3^\circ$ within $400\,\text{fs}$, breaking key symmetries while preserving rotational-spin-reversal symmetry (RU).
• The momentum-dependent spin splitting observed at the valence band maximum exhibits transitions between different symmetry representations: transient $d$-wave followed by $g$-wave spin-splitting patterns, as confirmed by projections onto symmetry-adapted basis sets.
• These results demonstrate that symmetry-selective lattice distortions – not SOC or angular-momentum transfer – underpin the ultrafast emergence of ALM.

Quantitative Results and Experimental Observables

Strong numerical outcomes are documented:

• The emergent anomalous Hall conductivity (AHC) oscillates between $+400\,\text{S/cm}$ in valence bands after laser excitation, a tangible signature of ALM even though calculation requires SOC; the splitting itself arises from lattice symmetry breaking without SOC.
• The excitation population reaches $_3$0 per formula unit, with substantial Ni-F bond elongation to $_3$1, confirmed via rt-TDDFT in nonequilibrium, compared to negligible fluctuations in equilibrium molecular dynamics.
• The degree of symmetry breaking is quantified by the parameter $_3$2, where $_3$3 signals maximal out-of-phase rotation and ALM; this parameter reaches its peak alongside maximal spin splitting and AHC.

Critically, the paper asserts that the mechanism is robust across cubic G-type AFM perovskites, provided the laser polarization is not parallel to the Néel vector – a criterion formalized in their symmetry selection rule. Only transverse lattice distortion modes, symmetry-allowed by the bilinear channel of the incident laser field, drive the ALM transition.

Theoretical Implications and Extension

The work establishes general selection rules:

• Symmetry Rule: Driven phonon modes must belong to symmetric bilinear channels of the laser field, only activating ALM when laser polarization is not aligned with the Néel vector.
• Momentum Rule: Only zone-center ($_3$4) phonons are optically accessible, explaining the selective dynamics observed in KNiF$_3$5 and extending to other cubic perovskite antiferromagnets.

This nonrelativistic, dynamical route for ALM generation expands the realization of altermagnetic materials into the nonequilibrium regime, independently from static fields or heavy-element content. It provides a pathway for ultrafast, reversible control of ALM states, offering practical prospects for spintronic architectures leveraging light-induced symmetry manipulation.

Implications and Speculation on Future Developments

The practical implication is ultrafast optical control of ALM in a broader class of antiferromagnets, circumventing limitations imposed by material composition (e.g., exclusion of heavy elements) and static field constraints. Theoretical ramifications include re-examining the symmetry grounds of spin-splitting phenomena, broadening the toolkit for manipulating electronic and magnetic orders in quantum materials.

Future research may target:

• Time-resolved verification of emergent AHC via magneto-optical Kerr effect or terahertz emission spectroscopy.
• Extension of symmetry selection principles to low-dimensional and layered perovskite systems.
• Development of ultrafast spintronic devices capitalizing on nonequilibrium control of ALM.
• Exploration of light-induced topological responses, persistent spin textures, and higher-order spin-orbit couplings in the ALM regime.

Conclusion

This work identifies and rigorously characterizes a nonrelativistic, ultrafast route for inducing altermagnetic spin splitting in antiferromagnets via symmetry-selective lattice distortion under photoexcited carrier redistribution. The mechanism is quantitatively verified and theoretically generalized, establishing a framework for ultrafast, reversible control of ALM states and greatly expanding the feasible materials and strategies for advanced spintronics.