Ultrafast optical excitation of magnons in 2D antiferromagnetic semiconductors 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,

NiPS$_3$, and MnPS$_3$, 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 \mbox{$\sim 1$ 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 {\em 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 {\em 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 {\em pump} time-dependent charge currents into the attached electrodes, or locally within AF semiconductor that will then emit electromagnetic radiation.