Ultrafast optical excitation of magnons in 2D antiferromagnetic semiconductors via spin torque mediated by unbound electron-hole pairs and excitons: Signatures in magnonic charge pumping

TL;DR

The paper develops a time-domain quantum transport framework that couples time-dependent nonequilibrium Green's functions with classical Landau-Lifshitz-Gilbert dynamics and exciton physics (TDNEGF+LLG+EX) to explain ultrafast optical excitation of magnons in 2D antiferromagnetic semiconductors. By driving electrons with a femtosecond laser pulse above the semiconductor gap, the model produces spin-polarized photocurrents that exert spin-transfer torque on localized magnetic moments; exciton binding via a Coulomb interaction $U$ further modulates the magnon dynamics, leading to a long-lived bright magnon at frequency $\omega_b$ and additional spectral features. The framework also predicts magnon-induced pumping of time-dependent charge currents and electromagnetic radiation, with windowed FFT analyses revealing signatures at $oldsymbol{\omega_b}$ and higher harmonics (e.g., $4oldsymbol{\omega_b}$), offering new experimental probes of exciton–magnon coupling in materials like CrSBr. Overall, the work connects ultrafast carrier dynamics to magnonics through a self-consistent spintronic mechanism, providing quantitative predictions and guiding future investigations of exciton–magnon interactions in 2D AF semiconductors.

Abstract

Recent experiments observing how femtosecond laser pulse (fsLP) excites magnons in two-dimensional (2D) antiferromagnetic (AF) semiconductors -- such as CrSBr, NiPS$_3$, and MnPS$_3$, or their van der Waals heterostructures -- suggest an important role played by excitons. However, microscopic details of such an effect 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 function (TDNEF) for electrons driven by fsLP is coupled self-consistently to the Landau-Lifshitz-Gilbert (LLG) equation describing classical dynamics of localized magnetic moments (LMMs) residing on magnetic atoms of 2D AF semiconductors. This theory explains how fsLP, of central frequency {\em above} the semiconductor gap, generates a photocurrent that becomes spin-polarized due to the background of LMMs, which, in turn, exerts spin-transfer torque (STT) onto LMMs as a genuinely 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 electronic bath. Finally, our theory also predicts that excited magnons will {\em pump} time-dependent charge currents into the attached electrodes, or locally within 2D AF semiconductor, thereby emitting electromagnetic radiation. The windowed FFT of these two signals contains imprints of excited magnons, as well as possible presence of excitons, which could be exploited as a novel probe in future experiments.

Figures (6)

• Figure 1: Schematic view of a two-terminal setup for TDNEGF+LLG+EX [Fig. \ref{['fig:fig1']}] calculations of ultrafast dynamics of photoexcited electrons by above CB-VB gap femtosecond laser pulse (fsLP), additionally interacting with local magnetic moments (LMMs). The central active region (CAR) consists of two layers of a 2D AF semicoductor described by the top and bottom TB chains (as inspired by the quasi-1D structure of CrSB Klein2023Klein2024Scheie2022Bianchi2023Esteras2022Cui2025) hosting two orbitals, $c$(onduction) and $v$(alence), per site $i$. Electron spin densities (green arrows), $\langle \hat{\mathbf{s}}_{i_c}(t)\rangle$ and $\langle \hat{\mathbf{s}}_{i_v}(t)\rangle$, interact via Kondo exchange [Eqs. \ref{['eq:hamilc']} and \ref{['eq:hamilq']} in the Supplemental Material (SM)] with LMMs at the same site described by classical vector (red arrow) $\mathbf{M}_i(t)$ obeying the LLG Eq. \ref{['eq:llg']}. Note that LMMs are canted by applying an external magnetic field, as also used experimentally Bae2022Diederich2022Diederich2025, thereby introducing noncolinearity of LMMs between the two layers. Electrons on $c$ and $v$ orbitals at the site $i$ interact via inter-orbital local Coulomb interaction of strength $U$ [Eq. \ref{['eq:coulomb']} in the SM], which, when turned on $U >0$, binds photoexcited electrons and holes into excitons Murakami2020Cistaro2022Perfetto2022Perfetto2023. Both chains are attached to semi-infinite ideal NM leads modeled also by 1D TB chains but without any interactions. Via such leads, any spin or charge current pumped within CAR by fsLP, or by excited magnons Suresh2020Evelt2017Ciccarelli2015Kapelrud2013 persisting after fsLP ceases, is drained [Fig. \ref{['fig:fig4']}] toward macroscopic reservoirs kept at the same Fermi energy $E_F$ (i.e., no bias voltage is applied between the leads). We also compute EM radiation, emitted by pumped local charge currents within the CAR, and analyze its frequency content [Fig. \ref{['fig:fig3']}].
• Figure 2: Flowchart of TDNEGF+LLG+EX self-consistent loop combining TDNEGF Gaury2014Popescu2016stefanucci2025 (green box) computation [Eq. \ref{['eq:rhoneq']}] of time-dependent nonequilibrium density matrix ${\bm \rho}^{\rm neq}(t)$ with LLG equation Evans2014Teuling2025 updating (orange box) LMMs $\mathbf{M}_i(t)$. For this study, to the previously developed Petrovic2018Bajpai2019a TDNEGF+LLG scheme, we add a computation (magenta box) employing Cistaro2022 all off-diagonal elements of ${\bm \rho}^{\rm neq}(t)$ to describe the binding of photoexcited conduction-band electrons and valence-band holes into exctions. The loop employs time step $\delta t=0.1$ fs in both quantum (as required for numerical stability of TDNEGF calculations~Gaury2014Popescu2016Petrovic2018) and classical LLG calculations. After each time step, we obtain time-dependent observables listed in the yellow box. In particular, STT is constructed from the expectation value of electron spin $\langle \hat{\mathbf{s}}_i(t)\rangle$ [Eq. \ref{['eq:spin']}] and $\mathbf{M}_i(t)$ via Eq. \ref{['eq:stt']}.
• Figure 3: Time dependence of the Néel vector, defined [Eq. \ref{['eq:Neel_vec']}] for two monolayers of AF semiconductor CAR [Fig. \ref{['fig:fig0']}] in: (a) the absence of excitons ($U=0$); or (c) their presence induced by on-site Coulomb interaction Murakami2020Cistaro2022$U=0.1\gamma$ [Eq. \ref{['eq:coulomb']} in the SM]. Panels (b) and (d) plot the corresponding power spectrum of windowed FFT Press2007Cohen2014, $|N^z(t_c,\omega)|^2$, revealing frequencies and lifetimes of magnons excited by STT from the photocurrent of unbound electron-hole pairs in (a) or those pairs and excitons acting together in (b), respectively. Here $t_c$ denotes the central time of the Gaussian window Press2007Cohen2014 used in FFT. Note that $\omega_b$ labels the frequency of the bright magnon, which corresponds to the same type of magnon observed in experiments of Refs. Bae2022Diederich2022Diederich2025 on in bilayers of AF semiconductor CrSBr. Gray curves on the bottom of panels (a) and (c) depict the vector potential $A^x(t)$ of fsLP.
• Figure 4: Time-dependence of charge current $I_R(t)$ flowing into the right NM lead in Fig. \ref{['fig:fig0']} due to magnonic charge pumping Suresh2020Ciccarelli2015Evelt2017Kapelrud2013 by excited magnons from Fig. \ref{['fig:fig2']} in: (a) the absence of excitons ($U=0$); or (c) their presence induced by on-site Coulomb interaction $U=0.1\gamma$. Panels (b) and (d) plot the corresponding power spectrum of windowed FFT Press2007Cohen2014, $|I_R(t_c,\omega)|^2$. Gray curves on the bottom of panels (a) and (c) depict the vector potential $A^x(t)$ of fsLP.
• Figure 5: Time-dependence of the $E_\mathrm{FF}^x(t)$ component of the electric field of EM radiation emitted into the FF region by magnon (from Fig. \ref{['fig:fig2']}) generated bond charge currents [Eq. \ref{['eq:bondcharge']} in the SM] within the CAR of Fig. \ref{['fig:fig0']} in: (a) the absence of excitons ($U=0$); or (c) their presence induced by on-site Coulomb interaction $U=0.1\gamma$. Panels (b) and (d) plot the corresponding power spectrum of windowed FFT Press2007Cohen2014, $|E_\mathrm{FF}^x(t_c,\omega)|^2$. Gray curves on the bottom of panels (a) and (c) depict the vector potential $A^x(t)$ of fsLP.