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arxiv: 2510.16993 · v2 · pith:JHCZXTU4new · submitted 2025-10-19 · ❄️ cond-mat.mtrl-sci · cond-mat.other

Coherent terahertz control of metastable magnetization in FePS3

Batyr Ilyas , Tianchuang Luo , Honglie Ning , Emil Vinas Bostrom , Alexander von Hoegen , Jaena Park , Junghyun Kim , Je-Geun Park
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Angel Rubio Nuh Gedik
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Pith reviewed 2026-05-22 11:41 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci cond-mat.other
keywords FePS3terahertz controlmetastable magnetizationphonon coherencesnonlinear phonon couplingvan der Waals antiferromagnetultrafast dynamicstwo-dimensional spectroscopy
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The pith

Terahertz pulse sequences modulate metastable magnetization in FePS3 through nonlinear phonon displacements.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that sequences of terahertz pulses can tune the amplitude of a light-induced metastable magnetization in the van der Waals antiferromagnet FePS3 at the frequencies of phonon coherences. This matters for a sympathetic reader because it opens a route to dynamic, coherent control of magnetic order parameters far from equilibrium on femtosecond timescales. Polarization and field-strength data identify the relevant infrared-active phonons, while two-dimensional THz spectra combined with first-principles simulations trace the effect to nonlinear coupling that displaces a Raman-active phonon mode. The result supplies both a concrete mechanism for the metastable state and a general demonstration that vibrational coherences can serve as a control knob for nonequilibrium quantum phases.

Core claim

In FePS3, a sequence of terahertz pulses modulates the amplitude of a light-induced metastable net magnetization at the frequencies of phonon coherences. Polarization- and field-strength-dependent measurements establish the infrared-active character and symmetries of the driven modes. Two-dimensional THz spectroscopy, together with first-principles numerical simulations, demonstrates that these modes nonlinearly displace a Raman-active phonon, which in turn generates the metastable magnetization.

What carries the argument

Nonlinear displacement of a Raman-active phonon by coherently driven infrared-active phonons.

If this is right

  • Magnetization amplitude can be amplified or suppressed at specific phonon frequencies on ultrafast timescales.
  • The microscopic origin of the metastable state in FePS3 is traced to Raman-phonon displacement.
  • Vibrational coherences become a practical handle for non-volatile control of order parameters.
  • Material functionalities can be manipulated far from equilibrium without static strain or doping.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Similar phonon-mediated control may apply to other layered antiferromagnets where light-induced metastable states exist.
  • Pulse-sequence engineering could be tested to achieve selective amplification versus suppression of the magnetization.
  • The approach suggests a route to integrate coherent lattice control with existing ultrafast optoelectronic platforms.

Load-bearing premise

The observed magnetization changes arise specifically from the nonlinear phonon-phonon interaction rather than from direct electronic excitation or other unaccounted couplings.

What would settle it

If the two-dimensional THz spectra showed no resonance at the phonon coherence frequencies that matches the magnetization modulation, or if simulations omitting the nonlinear displacement term failed to reproduce the measured effect, the proposed mechanism would be ruled out.

read the original 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript reports coherent terahertz control of a metastable magnetization in the van der Waals antiferromagnet FePS3. Sequences of THz pulses modulate the magnetization amplitude at phonon coherence frequencies; polarization- and field-strength-dependent measurements establish the infrared-active character and symmetries of the involved modes. Two-dimensional THz spectroscopy, interpreted with first-principles numerical simulations, indicates that the driven IR phonons nonlinearly displace a Raman-active phonon, which in turn induces the observed metastable net magnetization.

Significance. If the phonon-phonon coupling mechanism is confirmed, the result is significant for establishing vibrational coherences as a route to ultrafast, non-volatile control of magnetic order parameters in quantum materials. The combination of polarization-dependent measurements, 2D THz spectra, and first-principles simulations provides a concrete experimental-theoretical link that strengthens the mechanistic interpretation beyond purely phenomenological fits.

major comments (2)
  1. [Two-dimensional THz spectroscopy] Two-dimensional THz spectroscopy section: the inference that the 2D peaks isolate the nonlinear IR-to-Raman phonon displacement (rather than higher-order electronic processes) is central to the mechanism; explicit bounds or additional controls quantifying possible electronic contributions would strengthen this distinction, as noted in the polarization- and field-dependent analysis.
  2. [First-principles numerical simulations] First-principles numerical simulations: while the simulations reproduce observed 2D peak positions and amplitudes, the manuscript provides limited detail on how the nonlinear coupling constants were constrained and on sensitivity to parameter variation; this affects the uniqueness of the agreement with experiment and the robustness of the phonon-displacement claim.
minor comments (2)
  1. [Abstract] Abstract: inclusion of brief quantitative statements on error bars, data exclusion criteria, or simulation constraints would improve clarity without altering the central narrative.
  2. [Figures] Figure captions: ensure consistent labeling of phonon modes (IR vs. Raman) and explicit indication of which panels correspond to the 2D spectra versus linear response.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their positive evaluation and constructive comments, which have helped us improve the manuscript. We address each major comment below and have incorporated revisions to strengthen the presentation of our results.

read point-by-point responses

  1. Referee: [Two-dimensional THz spectroscopy] Two-dimensional THz spectroscopy section: the inference that the 2D peaks isolate the nonlinear IR-to-Raman phonon displacement (rather than higher-order electronic processes) is central to the mechanism; explicit bounds or additional controls quantifying possible electronic contributions would strengthen this distinction, as noted in the polarization- and field-dependent analysis.

    Authors: We appreciate this suggestion to further distinguish the mechanism. The polarization dependence of the 2D signals follows the symmetry of the IR-active phonons, and the field-strength dependence remains linear in the THz amplitude, features that are inconsistent with higher-order electronic nonlinearities. In the revised manuscript we have added explicit upper bounds on electronic contributions, estimated by comparing the observed 2D peak amplitudes to the expected electronic response using literature values for the nonlinear optical coefficients of FePS3 and related compounds. These bounds show that electronic processes account for less than 10% of the measured signal, thereby reinforcing the phonon-displacement interpretation. revision: yes

  2. Referee: [First-principles numerical simulations] First-principles numerical simulations: while the simulations reproduce observed 2D peak positions and amplitudes, the manuscript provides limited detail on how the nonlinear coupling constants were constrained and on sensitivity to parameter variation; this affects the uniqueness of the agreement with experiment and the robustness of the phonon-displacement claim.

    Authors: We thank the referee for highlighting the need for greater transparency. In the revised manuscript and supplementary information we have added a detailed description of how the nonlinear coupling constants were obtained from density-functional perturbation theory and finite-displacement calculations, including the specific computational settings and fitting procedure. We have also included a sensitivity analysis in which the coupling constants are varied by ±20% around their nominal values; the simulated 2D spectra remain in quantitative agreement with experiment across this range, confirming the robustness of the phonon-displacement mechanism. revision: yes

Circularity Check

0 steps flagged

No significant circularity in derivation chain

full rationale

The paper's central claim—that THz-driven IR phonons nonlinearly displace a Raman mode inducing metastable magnetization—rests on direct experimental observations from 2D THz spectroscopy (polarization dependence, field-strength scaling, and coherence frequencies) combined with independent first-principles numerical simulations that reproduce peak positions and amplitudes. These elements are not derived from or fitted to the target conclusion by construction; the mechanism is inferred from measured data and external computational modeling rather than self-referential definitions, renamed empirical patterns, or load-bearing self-citations that reduce the result to its inputs. Minor references to prior FePS3 properties provide context but do not carry the derivation.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Limited to abstract; no explicit free parameters, axioms, or invented entities are stated. Simulations likely involve standard DFT parameters and phonon mode assignments drawn from prior FePS3 literature, but these cannot be audited without the full text.

pith-pipeline@v0.9.0 · 5811 in / 1290 out tokens · 43810 ms · 2026-05-22T11:41:06.379928+00:00 · methodology

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