Terahertz field-induced giant symmetry modulations in a van der Waals antiferromagnet

Abstract

Strong-field terahertz (THz) excitations enable dynamic control over electronic, lattice and symmetry degrees of freedom in quantum materials. Here, we uncover pronounced terahertz-induced symmetry modulations and coherent phonon dynamics in the van der Waals antiferromagnet MnPS3, in which inversion symmetry is broken by its antiferromagnetic spin configuration. Time-resolved second harmonic generation measurements reveal long-lived giant oscillations in the antiferromagnetic phase, with amplitudes comparable to the equilibrium signal, driven by phonons involving percent-level atomic displacements relative to the equilibrium bond lengths. The temporal evolution of the rotational anisotropy patterns indicate a dynamic breaking of mirror symmetry, modulated by two vibrational modes at 1.7 THz and 4.5 THz, with the former corresponding to a hidden mode not observed in equilibrium spectroscopy. We show that these effects arise in part from a field-induced charge rearrangement mechanism that lowers the local crystal symmetry, and couples to the phonon modes. A long-lived field-driven response was uncovered with a complex THz polarization dependence which, in comparison to theory, indicates evidence for an antiferromagnetic-to-ferrimagnetic transition. Our results establish an effective field-tunable pathway for driving excitations otherwise weak in equilibrium, and for manipulating magnetism in low-dimensional materials via dynamical modulation of symmetry.

Figures (23)

• Figure 1: MnPS$_3$ properies and THz pump-induced giant oscillations. a, Néel-type layered AFM configuration in MnPS$_3$, blue and red spheres denote Mn atoms with opposite spins. b, Temperature-dependent SHG intensity measured along a lobe in the static SHG-RA pattern, exhibiting an onset below $T_N=78$ K. c, An intense terahertz pump pulse excites the sample and the induced changes are encoded in time-resolved (tr-) SHG-RA patterns produced by a delayed 800 nm probe pulse. d, THz-induced tr-SHG signals for temperatures below (9 K) and above (90 K) $T_\mathrm{N}$. The dark blue arrow indicates the pump polarization direction relative to the static SHG-RA pattern, and the red circle marks the angular position of the monitored signal. Yellow shaded region represents TFISH regime, with the THz pump profile temporally overlaid (black curve). The inset highlights the later-time dynamics, revealing giant oscillations at low temperatures with amplitude comparable to the static SHG signal. Horizontal orange dashed line denotes the equilibrium value. e, Amplitude spectrum of the oscillations at $T=9$ K, displaying prominent phonon peaks at 1.7 THz and 4.5 THz. The THz pump spectrum is shown in gray. f, Temperature-dependence of the spectrum. g--h, Both 1.7 THz (g) and 4.5 THz mode (h) amplitudes decrease with increasing temperature in a order parameter-like fashion.
• Figure 1: Schematic of the experimental configuration. a, Experimental setup for strong-field THz pump - SHG probe spectroscopy. MPA: multi-pass amplifier, OPA: optical parametric amplifier, HWP: half-wave plate, PMT: photomultiplier tube. b--e, Electro-optic sampling (EOS) profiles and the corresponding FFT spectra of THz pump pulses generated from DSTMS (b, d) and OH1 (c, e) crystals.
• Figure 1: Infrared and Raman spectra. a, 2D colormap of IR spectra at 10 K as a function of applied in-plane magnetic field, showing a sharp dip at $\sim 4.5$ THz (150 cm$^{-1}$). b, Horizontal linecuts at select magnetic fields, the blue dashed line at 4.5 THz represents a phonon. Vertical offsets are applied for better visibility. c, Magnetic field-dependent spectra (normalized with the spectrum at 0 T) showing negligible field-induced changes. d, Temperature-dependent Raman spectra. The 4.5 THz phonon is marked in brown dashed line. e, Raman spectra at 5 K as a function of in-plane magnetic field shown in a 2D colormap. No spectral change due to magnetic field was recorded.
• Figure 2: Time- and angle-resolved SHG dynamics upon strong-field THz excitation. a, Pump-induced SHG as a function of pump-probe delay time $t$ and polarizer analyzer angle $\varphi$ displayed in a normalized 2D intensity colormap at 9 K. Upon excitation with THz pulse with $\sim$ 500 kV/cm field strength at $t=0$, the TFISH response (0--2 ps) is followed by a long-lived oscillatory signal. A periodicity of $60\degree$ along the angular direction marks the 6 lobes of the SHG-RA pattern. b, SHG-RA patterns at different pump-probe delays, obtained from vertical linecuts in a (dashed white lines). The radial scale of each polar plot is normalized, with the outer circle corresponding to an amplitude of 1. The dark blue arrow indicates the THz pump polarization relative to the static SHG-RA pattern. For details, see the movies attached in the Supplementary Materials. c, Fourier transform spectrum of the pump-probe signal, revealing the coherent modes at 1.7 THz and 4.5 THz, particularly strong at lobe positions. d, Angular dependence of the 4.5 THz mode as a function of THz polarization. The colored arrows denote polarization directions relative to the SHG-RA pattern. e, Mode amplitudes as a function of normalized THz field, showing predominantly linear scaling.
• Figure 2: Time- and angle-resolved SHG dynamics upon strong-field THz excitation (polarization 2). a, Pump-induced SHG as a function of pump-probe delay time $t$ and polarizer analyzer angle $\varphi$ at 9 K. THz pump with field strength of $\sim$ 500 kV/cm is polarized along a node of the static SHG-RA pattern (purple arrow). The later time dynamics is dominated by a long-lived oscillatory signal, suggesting coherent phonons. b, SHG-RA patterns at different pump-probe delays, obtained from vertical linecuts in a (dashed white lines). The radial scale of each polar plot is normalized, with the outer circle corresponding to an amplitude of 1. c, Fourier transform spectrum of the pump-probe signal, revealing the coherent modes at 1.7 THz and 4.5 THz. The 4.5 THz mode shows a $180^\circ$ periodicity d, Horizontal linecuts showing FFT spectra at lobe positions (indicated by colored circles in the SHG-RA pattern). The linecuts are vertically offset for better visibility.