Coherent terahertz control of metastable magnetization in FePS3

Abstract

The crystal lattice governs the emergent electronic, magnetic, and optical properties of quantum materials, making structural tuning through strain, pressure, or chemical substitution a key approach for discovering and controlling novel quantum phases. Beyond static modifications, driving specific lattice modes with ultrafast stimuli offers a dynamic route for tailoring material properties out of equilibrium. However, achieving dynamic coherent control of the nonequilibrium phases via resonant excitation of lattice coherences remains largely unexplored. Such manipulation enables non-volatile, on demand amplification and suppression of order parameters on femtosecond timescales, necessary for next generation optoelectronic ultrafast computation. In this study, we demonstrate coherent phononic control of a newly discovered, light-induced metastable magnetization in the van der Waals antiferromagnet FePS3. By using a sequence of terahertz (THz) pulses, we modulate the magnetization amplitude at the frequencies of phonon coherences, whose infrared-active nature and symmetries are further revealed by polarization- and field-strength-dependent measurements. Furthermore, our two-dimensional THz spectroscopy, in tandem with first-principles numerical simulations, shows that these phonons nonlinearly displace a Raman active phonon, which induces the metastable net magnetization. These findings not only clarify the microscopic mechanism underlying the metastable state in FePS3 but also establish vibrational coherences in solids as a powerful tool for ultrafast quantum phase control, enabling manipulation of material functionalities far from equilibrium.

Figures (4)

• Figure 1: Principle of coherent phonon control of the metastable magnetization.a, Equilibrium crystal and magnetic structure of FePS$_3$, with red and blue arrows indicating Fe spins pointing up and down in the out-of-plane direction, respectively. Right panel demonstrates fully compensated up and down spins. b, Crystal lattice displaced along the $\Omega_\text{R}= 3.27$ THz Raman phonon, showing enhanced exchange interactions within the red zigzag chain (thick bonds) and weakened exchange interactions within the blue zigzag chain (thin bonds). Right panel illustrates uncompensated up and down spins, generating net magnetization, $M_z$. c-d, Schematics of IR phonon amplitude as a function of time in double-pump experiment for $\tau=nT_\mathrm{IR}$c, and $\tau=(n+\frac{1}{2})T_\mathrm{IR}$d, demonstrating coherent enhancement c or suppression d. e-f, Corresponding Raman phonon displacement as a function of time resulting from IR phonon driving. g, Schematics of the THz-induced magnetization as a function of THz pulse separation $\tau$, with red and blue shadings correspond to $\tau=nT_\mathrm{IR}$ and $\tau=(n+\frac{1}{2})T_\mathrm{IR}$, respectively.
• Figure 2: Experimental demonstration of coherent control.a, Schematic of the single THz pump experiment, with the yellow and red pulses representing THz pump and 800 nm probe. b, Ellipticity change time traces as a function of $t$ at different temperatures as measured in a. c, Schematics of the double THz pump experiment, with an additional THz pulse (orange) introduced at time delay $\tau$. d, Probe ellipticity change measured at $t=400\ \mathrm{ps}$ as a function of $\tau$ for various temperatures. e, Fourier transform of $\tau$-dependent trace at 118 K in d, revealing a peak at 4.5 THz. f, The metastable state magnitude in b as a function of normalized THz field strength $E_\mathrm{THz}$. The solid line is quadratic fitting. g, The area of the Fourier spectrum from e as a function of normalized THz field strength of the yellow THz pulse. The solid line is linear fitting.
• Figure 3: Identification of the IR-active phonons mediating the lattice displacement.a, Schematic of the THz polarization dependence experiments, where the THz pulses are co-rotated. $\phi$ denotes the angle between THz polarization and the crystallographic $a$-axis. b, Evolution of the oscillation spectrum in Fig. \ref{['fig:fig2']}e with $\phi$. The blue and red shaded areas are Lorentzian oscillator fits with Eq. S7. c, The amplitude of the two Lorentzian oscillators as a function of $\phi$, with $|\sin\phi|$ (red solid line) and $|\cos\phi|$ (blue solid line) fits demonstrating orthogonal polarizations. Data and fits are vertically offset for clarity. d, Left: 2D THz spectrum near $f_t=0\ \mathrm{THz}$ obtained at $T=10$ K and $\phi=45^\circ$. Right: Linecuts along $f_{\tau} =0$ THz of 2D THz spectra measured at $\phi=0^\circ,\ 45^\circ,\ 90^\circ$. The blue and red shaded regions correspond to the blue and red oscillator frequencies in b and c.
• Figure 4: Possible nonlinear interaction pathways.a, Simulated $\Delta M(\tau)$ when two-photon excitation (red), infrared resonant Raman scattering (beige), and ionic Raman scattering (blue) mechanisms are activated for driving the Raman mode $\Omega_\text{R}$. The experimental result is shown in black. b, Fourier transform of the time traces in a. Inset shows the diagram of the nonlinear interactions, where the straight arrows represent photons and wavy arrows represent phonons. c, Atomistic spin dynamics simulation of $\Delta M(\tau)$ with the ionic Raman scattering mechanism activated. d, Fourier transform of c. e-f,. Schematics of the calculated IR phonon eigenmodes at 4.34 THz and 4.80 THz. The red and blue spheres represent Fe ions with spin pointing along opposite directions. The yellow and white spheres represent phosphorus and sulfur atoms, respectively.