Quantum Matter Seminar

Ultrafast Optical Excitation of Antiferromagnetic or Inertial Magnons in 2D Magnets

For applications in magnonics, there is a considerable experimental effort to excite coherent magnons by ultrafast light, whose frequencies are as high as possible and wavelengths as short as possible. In particular, the advent of two-dimensional (2D) magnetic materials—such as 2D antiferromagnetic semiconductors CrSBr, NiPS3, MnPS3, CrI3 and their van der Waals heterostructures—has made possible recent experiments observing how femtosecond laser pulse excites magnons via exciton mediation. Our theory explains [1] such excitation as the consequence of a novel spin-transfer torque mechanism mediated by excitons, which is also supported by very recent experiments on CrSBr [2]. We additionally predict novel charge pumping [1,3] signatures of exciting magnons. For metallic 2D magnets, we explain [4] how the bath of light-driven electrons can dramatically modify Landau-Lifshitz dynamics to introduce in it either a non-Markovian kernel (with fractal properties in double time plane) or magnetic inertia, depending on the intensity of light. Our extended Landau-Lifshitz equation [4], microscopically derived from Schwinger-Keldysh field theory [5], predicts the possibility of excitation of coherent inertial magnons at frequencies above the standard band of thermal magnons. Thus, both examples offer realizations of high-frequency and all-electrical control of magnetic order in quantum materials where initially electrons interact with the fast-changing electric field of light, which then leads to their photocurrent driving the dynamics of local magnetization.

References

[1] J. Varela-Manjarres, Y. Ren, B. K. Nikolić, Ultrafast optical excitation of magnons in two-dimensional antiferromagnetic semiconductors via spin torque mediated by unbound electron-hole pairs and excitons: Signatures in magnonic charge pumping, Phys. Rev. B 113, 134403 (2026).

[2] N. J. Brennan et al., Excitonic spin torque in a magnetic semiconductor, Nat. Mater. https://doi.org/10.1038/s41563-026-02643-1 (2026).

[3] A. Suresh, U. Bajpai, and B. K. Nikolić, Magnon-driven chiral charge and spin pumping and electron-magnon scattering from time-dependent quantum transport combined with classical atomistic spin dynamics, Phys. Rev. B 101, 214412 (2020).

[4] F. Reyes-Osorio and B. K. Nikolić, Optically induced magnetic inertia and magnons from non-Markovian extension of the Landau-Lifshitz-Gilbert equation, Phys. Rev. Lett. 135, 246701 (2025).

[5] F. Reyes-Osorio and B. K. Nikolić, Gilbert damping in metallic ferromagnets from Schwinger-Keldysh field theory: Intrinsically nonlocal, nonuniform, and made anisotropic by spin-orbit coupling, Phys. Rev. B 109, 024413 (2024).