Ultrafast optical excitation of magnons in 2D antiferromagnets via spin torque exerted by photocurrent of excitons: Signatures in charge pumping and THz emission

Recent experiments observing femtosecond laser pulse (fsLP) exciting magnons in two-dimensional (2D) antiferromagnetic (AF) semiconductors -- such as CrSBr, NiPS₃, and MnPS₃, or their van der Waals heterostructures -- suggest exciton-mediation of such an effect. However, its microscopic details remain obscure, as resonant coupling of magnons, living in the sub-meV energy range, to excitons, living in the 1 eV range, can hardly be operative. Here, we develop a quantum transport theory of this effect, in which time-dependent nonequilibrium Green's functions (TDNEF) for electrons driven by fsLP are coupled self-consistently to the Landau-Lifshitz-Gilbert (LLG) equation describing classical dynamics of localized magnetic moments (LMMs) within 2D AF semiconductors. This theory explains how fsLP, of central frequency above the semiconductor gap, generates a photocurrent that subsequently exerts spin-transfer torque (STT) onto LMMs as a nonequilibrium spintronic mechanism. The collective motion of LMMs analyzed by windowed Fast Fourier transform (FFT) decodes frequencies of excited magnons, as well as their lifetime governed by nonlocal damping with the LLG equation due to, explicitly included via TDNEGF, electronic bath. The TDNEGF part of the loop is also used to include excitons via mean-field treatment, utilizing off-diagonal elements of the density matrix, of Coulomb interaction binding conduction-band electrons and valence-band holes. Finally, our theory predicts how excited magnons will pump time-dependent charge currents into the attached electrodes, or locally within AF semiconductor that will then emit electromagnetic radiation. The windowed FFT of these signals contains imprints of excited magnons, as well as their interaction with excitons, which could be exploited as a novel probe in future experiments.