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arxiv: 2504.20540 · v1 · submitted 2025-04-29 · ❄️ cond-mat.mes-hall

Spin Wave Dispersion of the van der Waals Antiferromagnet NiPS₃

Ritesh Das , Rob den Teuling , Artem V. Bondarenko , Elena V. Tartakovskaya , Gerrit E. W. Bauer , Jaime Ferrer , Yaroslav M. Blanter This is my paper

Pith reviewed 2026-05-22 18:48 UTC · model grok-4.3

classification ❄️ cond-mat.mes-hall
keywords magnon dispersionDirac nodal linevan der Waals antiferromagnetNiPS3spin wave spectrumtopological magnonsmagnetic phase transitions
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0 comments X

The pith

NiPS3 magnon spectra feature a topologically protected Dirac nodal line that stays intact under external and anisotropy fields.

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

The paper computes the magnon dispersion relations for the two-dimensional zigzag van der Waals antiferromagnet NiPS3 across monolayer, bilayer, and bulk forms by constructing a spin Hamiltonian whose parameters come from first-principles calculations. This same model reproduces the field-driven change from a collinear antiferromagnetic state to a canted state when the field is perpendicular to the easy axis and the spin-flop transition when the field lies along it. The central result is that the calculated spectrum contains a Dirac nodal line whose topological protection remains intact against both applied magnetic fields and variations in magnetic anisotropy. A reader cares because such protected nodal features could underpin dissipationless magnon transport in atomically thin magnetic materials.

Core claim

Using a first-principles-derived spin model, the magnon dispersion of NiPS3 is shown to host a topologically protected Dirac nodal line that is robust against both external magnetic fields and anisotropy fields; the same model quantitatively accounts for the collinear-to-canted transition under a field normal to the easy axis and the spin-flop transition under a parallel field.

What carries the argument

First-principles spin Hamiltonian whose exchange and anisotropy constants are inserted into the magnon band-structure calculation for mono-, bi-, and three-dimensional NiPS3.

If this is right

  • The nodal line supplies a concrete example of topological magnons in a real van der Waals antiferromagnet.
  • The same Hamiltonian framework can be used to predict magnon spectra in related layered magnets once their first-principles parameters are known.
  • Field robustness of the nodal line implies that topological magnon features survive in devices operating under realistic laboratory fields.
  • The calculated transitions provide a microscopic explanation for the experimentally observed magnetic phase boundaries in NiPS3.

Where Pith is reading between the lines

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

  • Similar Dirac nodal lines may appear in other zigzag antiferromagnets once their interlayer couplings are weak enough to preserve two-dimensional character.
  • Engineering the anisotropy or stacking order could move the nodal line to different points in the Brillouin zone, offering a route to tunable magnonic Dirac cones.
  • Because the protection is topological rather than symmetry-based, the nodal line should survive moderate disorder or edge termination in exfoliated flakes.

Load-bearing premise

The first-principles values for exchange and anisotropy constants are accurate enough to describe the low-energy magnon spectrum without further renormalization.

What would settle it

High-resolution inelastic neutron scattering or Brillouin light scattering on NiPS3 single crystals that either detects or fails to detect a linear crossing of magnon bands at the predicted wave-vector location under applied fields of several tesla.

Figures

Figures reproduced from arXiv: 2504.20540 by Artem V. Bondarenko, Elena V. Tartakovskaya, Gerrit E. W. Bauer, Jaime Ferrer, Ritesh Das, Rob den Teuling, Yaroslav M. Blanter.

Figure 1
Figure 1. Figure 1: FIG. 1: (a) Bilayer NiPS [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2: Exchange interactions in NiPS [PITH_FULL_IMAGE:figures/full_fig_p002_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3: Ground state magnetic configurations of an anti [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5: (a) Spin wave dispersion in a monolayer NiPS [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6: Magnon dispersion in a bilayer NiPS [PITH_FULL_IMAGE:figures/full_fig_p007_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7: AFMR frequency in monolayer NiPS [PITH_FULL_IMAGE:figures/full_fig_p007_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: (a) presents the resonance frequency for bilayer NiPS3 under an external field applied along the easy￾axis. The bands are Zeeman-split by the magnetic field. At B = Bc, the lowest frequency band reaches zero at the spin-flop transition. In [PITH_FULL_IMAGE:figures/full_fig_p008_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9: Magnon dispersion in a monolayer NiPS [PITH_FULL_IMAGE:figures/full_fig_p009_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: FIG. 10: Magnon dispersion in bulk NiPS [PITH_FULL_IMAGE:figures/full_fig_p010_10.png] view at source ↗
Figure 12
Figure 12. Figure 12: FIG. 12: Full AFMR frequency in (a) monolayer NiPS [PITH_FULL_IMAGE:figures/full_fig_p010_12.png] view at source ↗
read the original abstract

We calculate the magnon dispersion spectra of the two-dimensional zigzag van der Waals antiferromagnet NiPS$_3$ for monolayer, bilayer, and bulk systems as a function of an external magnetic field. We calculate the exchange and anisotropy constants in our spin model by first principles. We can accurately explain the transition from a collinear to a canted ground state for a magnetic field applied normal to the (in-plane) easy-axis, and a spin-flop transition when the field is parallel to it. A topologically protected Dirac nodal line is present and robust with respect to both external and anisotropy fields.

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 calculates magnon dispersion spectra for monolayer, bilayer, and bulk NiPS₃ using a Heisenberg spin model whose exchange and anisotropy parameters are taken directly from first-principles (DFT) calculations. It examines the field dependence of the spectra, claims to accurately reproduce the collinear-to-canted transition (field normal to easy axis) and the spin-flop transition (field parallel to easy axis), and reports a topologically protected Dirac nodal line that remains gapless and robust against both external magnetic fields and variations in anisotropy.

Significance. If the DFT-derived parameters are sufficiently accurate for the low-energy magnon spectrum, the work supplies a concrete spin-wave calculation for a van der Waals antiferromagnet that links microscopic constants to observable transitions and to a topological feature (Dirac nodal line). The parameter-free (with respect to magnon data) construction is a positive attribute that reduces circularity relative to purely phenomenological fits.

major comments (2)
  1. [Abstract / Spin model construction] Abstract and the spin-Hamiltonian section: the central claims of an 'accurate explanation' of the two field-induced transitions and of a robust, topologically protected Dirac nodal line rest on the assumption that the raw DFT values of J and anisotropy require no renormalization or higher-order corrections. The manuscript does not report a sensitivity analysis showing that the nodal line remains gapless when the DFT parameters are varied within their typical uncertainty range (∼5–10 %), nor does it overlay calculated critical fields against measured values; this omission is load-bearing because small ratio changes are known to gap or shift the nodal line in monolayer NiPS₃.
  2. [Results on magnon dispersion and topology] Dispersion and topology results (presumably §3–4): the topological protection is asserted but the explicit calculation of the relevant topological invariant (e.g., Berry phase or winding number around the nodal line) is not shown, nor is the effect of the external field on the invariant demonstrated. Without these steps the robustness statement cannot be verified independently of the numerical diagonalization.
minor comments (2)
  1. [Figures] Figure captions and axis labels should explicitly state the high-symmetry points used for the dispersion cuts and the field direction relative to the crystallographic axes.
  2. [Methods / Parameter table] A short table comparing the DFT-derived J and anisotropy values with literature values (both experimental and other DFT studies) would improve traceability.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful and constructive review of our manuscript. We address each major comment in detail below. Our responses focus on clarifying the strengths of the DFT-based approach while agreeing to add explicit checks that will strengthen the presentation of the results.

read point-by-point responses

  1. Referee: [Abstract / Spin model construction] Abstract and the spin-Hamiltonian section: the central claims of an 'accurate explanation' of the two field-induced transitions and of a robust, topologically protected Dirac nodal line rest on the assumption that the raw DFT values of J and anisotropy require no renormalization or higher-order corrections. The manuscript does not report a sensitivity analysis showing that the nodal line remains gapless when the DFT parameters are varied within their typical uncertainty range (∼5–10 %), nor does it overlay calculated critical fields against measured values; this omission is load-bearing because small ratio changes are known to gap or shift the nodal line in monolayer NiPS₃.

    Authors: We agree that a sensitivity analysis would provide additional confidence in the robustness of the Dirac nodal line. Although the manuscript employs raw DFT parameters without renormalization, we will add an explicit sensitivity study in the revised version, varying the key exchange and anisotropy constants by ±5 % and ±10 % and confirming that the nodal line remains gapless. Regarding critical fields, our calculated transition points are consistent with the field values reported in experimental literature for both the collinear-to-canted and spin-flop transitions; we will include a direct comparison (table or overlaid plot) in the revised manuscript to make this quantitative agreement explicit. These additions address the referee’s concern while retaining the parameter-free character of the calculation. revision: yes

  2. Referee: [Results on magnon dispersion and topology] Dispersion and topology results (presumably §3–4): the topological protection is asserted but the explicit calculation of the relevant topological invariant (e.g., Berry phase or winding number around the nodal line) is not shown, nor is the effect of the external field on the invariant demonstrated. Without these steps the robustness statement cannot be verified independently of the numerical diagonalization.

    Authors: We appreciate this observation. The persistence of the nodal line under external fields is a direct consequence of the symmetries preserved in the spin Hamiltonian. To allow independent verification, we will include in the revised manuscript explicit evaluations of the Berry phase (or winding number) encircling the nodal line in the Brillouin zone, computed both at zero field and at finite external magnetic fields. These calculations will be presented alongside the dispersion spectra and will confirm that the topological invariant remains nontrivial, thereby demonstrating robustness beyond the numerical diagonalization results alone. revision: yes

Circularity Check

0 steps flagged

No circularity: first-principles parameters feed independent magnon calculation

full rationale

The derivation begins with exchange and anisotropy constants obtained from first-principles calculations, which serve as external inputs independent of the magnon spectrum. These constants are inserted into the spin Hamiltonian to compute the dispersion, identify the Dirac nodal line, and reproduce the collinear-to-canted and spin-flop transitions. No step reduces a claimed prediction or topological feature to a fitted parameter, self-citation, or ansatz that is defined in terms of the output itself. The chain remains self-contained against external DFT benchmarks without any load-bearing self-reference or renaming of known results.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claims rest on the accuracy of DFT-derived exchange and anisotropy constants and on the validity of the classical or linear spin-wave approximation for the chosen Hamiltonian.

axioms (1)
  • domain assumption The spin Hamiltonian with nearest-neighbor exchange and single-ion anisotropy is sufficient to capture the low-energy magnon spectrum.
    Standard assumption in magnon calculations for antiferromagnets; invoked implicitly when the dispersion is computed from the model.

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Reference graph

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