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arxiv: 2602.03600 · v2 · submitted 2026-02-03 · ❄️ cond-mat.str-el · cond-mat.mtrl-sci

Evidence for Many-Body States in NiPS₃ Revealed by Angle-Resolved Photoelectron Spectroscopy

Mi{\l}osz Rybak , Benjamin Pestka , Biplab Bhattacharyya , Jeff Strasdas , Adam K. Budniak , Adi Harchol , Vitaliy Feyer , Iulia Cojocariu
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Daniel Baranowski Yaron Amouyal Efrat Lifshitz Markus Morgenstern Magdalena Birowska Krzysztof Wohlfeld
This is my paper

Pith reviewed 2026-05-16 07:25 UTC · model grok-4.3

classification ❄️ cond-mat.str-el cond-mat.mtrl-sci
keywords NiPS3ARPESmany-body statesMott insulatormultipletsvan der Waals antiferromagnetphotoemission
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The pith

ARPES measurements on NiPS3 reveal a weakly dispersive feature from many-body Ni-S multiplet states.

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

The paper measures angle-resolved photoelectron spectra of the van der Waals antiferromagnet NiPS3. It identifies a feature at the valence band edge that does not appear in standard density functional theory plus Hubbard U calculations and does not change when the material orders magnetically. Exact diagonalization of a nickel-sulfur cluster reproduces the energy of this feature through mixed multiplet configurations in the final state. This shows that photoemission can directly probe local many-body physics in such materials, where strong correlations and ligand bonding create spectral structures that mean-field approaches miss.

Core claim

The central claim is that the weakly dispersive feature observed in μ-ARPES at the valence-band edge of NiPS3 arises from low-energy final-state configurations of mixed multiplet d^7 and d^8 L character, as obtained from exact diagonalization of a NiS6 cluster. This feature is absent in DFT+U calculations and unchanged across the Néel transition, implying that ARPES accesses the local Ni-S multiplet physics and reveals a many-body electronic structure beyond mean-field theory in this Mott-insulating van der Waals material.

What carries the argument

Exact diagonalization of the NiS6 cluster model, which generates the mixed d7 and d8L multiplet final states whose energy differences match the position of the additional ARPES feature.

If this is right

  • ARPES can probe local multiplet physics in NiPS3 directly.
  • Mean-field theories like DFT+U fail to capture the valence band edge structure in NiPS3.
  • NiPS3 is an excellent model platform for studying the interplay of strong correlations, reduced dimensionality, and covalent metal-ligand bonding.
  • Both two- and single-particle spectroscopies in NiPS3 require a genuinely quantum many-body description.

Where Pith is reading between the lines

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

  • Similar local many-body features may be observable in ARPES spectra of other van der Waals magnets with covalent bonding.
  • Advanced modeling that combines cluster diagonalization with periodic boundary conditions could better predict spectral features in two-dimensional correlated materials.
  • This result suggests ARPES could be used to study how external tuning parameters affect the local multiplet energies in such systems.

Load-bearing premise

The exact diagonalization of the NiS6 cluster accurately captures the local electronic configurations responsible for the observed spectral feature without significant lattice or nonlocal effects.

What would settle it

If further experiments or larger-cluster calculations show that the feature disperses strongly with momentum or can be reproduced without invoking multiplet states, the assignment to local Ni-S many-body configurations would be falsified.

Figures

Figures reproduced from arXiv: 2602.03600 by Adam K. Budniak, Adi Harchol, Benjamin Pestka, Biplab Bhattacharyya, Daniel Baranowski, Efrat Lifshitz, Iulia Cojocariu, Jeff Strasdas, Krzysztof Wohlfeld, Magdalena Birowska, Markus Morgenstern, Mi{\l}osz Rybak, Vitaliy Feyer, Yaron Amouyal.

Figure 1
Figure 1. Figure 1: MPX3 crystal structure and ARPES results: (a) Atomic structure of MPS3 compounds, illustrating the honeycomb sublattice of MS6 octahedra (top and side views). The yellow-shaded area marks one representative MS6 cluster. (b) Comparison of experimental ARPES curvature plots for MnPS3 [13], FePS3 [14], CoPS3[15]a , and NiPS3 – along the M-Γ-M direction. The dashed boxes refer to the orbital contribution depic… view at source ↗
Figure 2
Figure 2. Figure 2: (a) Schematic illustration of the Brillouin zone of [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: (a)–(e) ARPES curvature plots along M − Γ − M for several photon energies hν as marked on top, T = 40 K. (f)–(j) ARPES intensity as function of energy for k∥ = 0 and the hν marked at the curvature plots above. The experimental data (blue lines) are fitted by two Voigt peaks (green and violet lines) and a Shirley background (orange line) leading to the red line (see text). (k) Peak positions of the two Voig… view at source ↗
Figure 4
Figure 4. Figure 4: (a) Effective band structure of NiPS3 obtained from DFT+U calculations and unfolded onto the primitive (non￾magnetic) Brillouin zone to enable direct comparison with the experimental spectra. The details of the unfolding proce￾dure and its justification are provided in [17]. EV BM denotes the valence band maximum. (b) Atom-projected density of states (pDOS) for Ni, P, and S, showing the dominant Ni–S hybri… view at source ↗
Figure 5
Figure 5. Figure 5: Evolution of the spin-dependent DOS with an increasing value of Hubbard [PITH_FULL_IMAGE:figures/full_fig_p007_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Spin-dependent DOS in the DFT+U approach: (a) nickel (black) and sulfur (lilac grey) orbitals [equivalent to the panel at UH = 1.6 eV in [PITH_FULL_IMAGE:figures/full_fig_p008_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Evolution of the spin-dependent DOS with an increasing value of Hund’s exchange [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
Figure 5
Figure 5. Figure 5: In addition, the correspondingly projected density [PITH_FULL_IMAGE:figures/full_fig_p009_5.png] view at source ↗
Figure 7
Figure 7. Figure 7: The parameter JH quantifies the intra-atomic ex￾change interaction favoring parallel spin alignment across different orbitals of the same atomic shell and thus con￾trols the strength of the effective Hund-driven exchange field acting between the eg and t2g manifolds (see Ap￾pendix B). While the positions of the eg features are found to be almost insensitive to variations of JH, the spin splitting of the fu… view at source ↗
Figure 8
Figure 8. Figure 8: Cartoon explaining two approaches to the calculation of the theoretical ARPES spectrum: (a) Using the DFT [PITH_FULL_IMAGE:figures/full_fig_p011_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: (a) Results of the ED calculations for Ni(3 [PITH_FULL_IMAGE:figures/full_fig_p013_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Evolution of the spin-dependent DOS with an increasing value of Hubbard [PITH_FULL_IMAGE:figures/full_fig_p020_10.png] view at source ↗
Figure 12
Figure 12. Figure 12: Comparison of the transition–metal 3d projected DOS across the MPS3 series (M = Mn, Fe, Co, Ni). (a–d) Spin-resolved 3d projected densities of states for MnPS3, FePS3, CoPS3 and NiPS3, projected onto the t2g (black) and eg (red) orbital manifolds. The shaded background shows the total ligand DOS . For MnPS3 and NiPS3 the antibonding fea￾ture near the top of the valence band is predominantly of eg characte… view at source ↗
Figure 13
Figure 13. Figure 13: Robustness of the Ni 3d electronic structure of NiPS3 against different magnetic orders. (a–d) Spin-resolved Ni 3d projected DOS calculated for four competing magnetic configurations: AFM-zigzag (ground state), AFM-stripy, AFM-Néel, and ferromagnetic (FM), all at U = 2.0 eV. Red and black curves denote the eg and t2g contributions, respectively, while the shaded background shows the total ligand DOS. The … view at source ↗
read the original abstract

We present $\mu$-ARPES spectra of the Mott-insulating van der Waals antiferromagnet NiPS$_3$. Signatures of strong correlations -- such as the onset of atomic or atomic-ligand multiplets and spin-orbit-entangled exciton have been observed in this material by various two-particle spectroscopies, but not previously in photoemission. Our measurements reveal a weakly dispersive feature at the valence-band edge that is absent in DFT+$U$ calculations and remains unchanged across the N\'eel transition. After critically examining and ruling out alternative interpretations, we show that an exact diagonalization of a NiS$_6$ cluster yields low-energy final-state configurations of mixed multiplet $d^7$ and $d^8\underline{L}$ character, whose energy differences are consistent with the observed additional feature. This implies that ARPES directly accesses local Ni-S multiplet physics in NiPS$_3$, revealing a many-body structure beyond mean-field theory. Our results confirm that NiPS$_3$ is an excellent model platform in which strong correlations, reduced dimensionality, and covalent metal-ligand bonding jointly shape both two- and single-particle spectroscopies, underscoring the need for a genuinely quantum many-body description of two-dimensional quantum materials.

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 μ-ARPES spectra on the Mott-insulating van der Waals antiferromagnet NiPS₃. It identifies a weakly dispersive feature at the valence-band edge that is absent from DFT+U calculations and persists across the Néel transition. After examining alternatives, the authors perform exact diagonalization of a NiS₆ cluster and assign the feature to low-energy final states of mixed d⁷ and d⁸L multiplet character whose energy differences match the observed position. This is taken to show that ARPES directly accesses local Ni-S many-body multiplet physics beyond mean-field theory.

Significance. If the assignment is robust, the result establishes that single-particle ARPES can resolve local multiplet structure in a correlated 2D Mott insulator, providing a concrete example where covalency and reduced dimensionality produce observable many-body final states. It strengthens NiPS₃ as a model platform for testing quantum many-body descriptions that unify one- and two-particle spectroscopies.

major comments (2)
  1. [§4] §4 (NiS₆ cluster exact diagonalization): The manuscript states that the calculated energy differences are 'consistent with' the observed feature, but does not tabulate or justify the values of U, charge-transfer energy Δ, and Slater integrals. If these parameters were refined against the ARPES feature position rather than fixed by independent constraints (optical gaps, XAS, or ab-initio estimates), the agreement is non-predictive and weakens the claim that ARPES directly accesses the local multiplet physics.
  2. [§3.3] §3.3 (ruling out alternatives): The abstract asserts that alternatives were 'critically examined and ruled out,' yet the text provides no quantitative metrics (e.g., expected ARPES intensity, dispersion width, or temperature dependence) that would allow an independent reader to verify exclusion of surface reconstruction, minor structural domains, or extrinsic scattering channels.
minor comments (2)
  1. [Figure 3] Figure 3: The overlay of experimental and calculated spectra would be clearer if the cluster final-state energies were marked with vertical lines and the experimental energy resolution was indicated.
  2. Notation: The underline notation for ligand holes (L) is used inconsistently between text and figure captions; a single definition in the methods would improve readability.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thorough review and constructive feedback on our manuscript. We address each major comment point by point below, providing clarifications and indicating where revisions will be made to strengthen the paper.

read point-by-point responses

  1. Referee: [§4] §4 (NiS₆ cluster exact diagonalization): The manuscript states that the calculated energy differences are 'consistent with' the observed feature, but does not tabulate or justify the values of U, charge-transfer energy Δ, and Slater integrals. If these parameters were refined against the ARPES feature position rather than fixed by independent constraints (optical gaps, XAS, or ab-initio estimates), the agreement is non-predictive and weakens the claim that ARPES directly accesses the local multiplet physics.

    Authors: We thank the referee for highlighting this important point on parameter transparency. The parameters in our NiS₆ cluster calculation were not refined to fit the ARPES data. Instead, U was fixed based on optical gap measurements and XAS studies reported in the literature for NiPS₃, the charge-transfer energy Δ was determined from ab-initio estimates and comparisons with similar nickel compounds, and the Slater integrals were taken from atomic calculations with standard reduction factors for solid-state screening. To make this explicit and allow independent verification, we will include a dedicated table in the revised §4 listing all parameter values along with their sources and justifications. This will demonstrate that the agreement with the observed feature is predictive and supports our interpretation of local multiplet physics. revision: yes

  2. Referee: [§3.3] §3.3 (ruling out alternatives): The abstract asserts that alternatives were 'critically examined and ruled out,' yet the text provides no quantitative metrics (e.g., expected ARPES intensity, dispersion width, or temperature dependence) that would allow an independent reader to verify exclusion of surface reconstruction, minor structural domains, or extrinsic scattering channels.

    Authors: We agree that providing quantitative metrics would enhance the rigor of our exclusion of alternative interpretations. In the original manuscript, we examined these alternatives through qualitative arguments based on photon-energy dependence, lack of dispersion, and temperature independence across the Néel transition. For the revised version, we will expand §3.3 to include quantitative estimates: for surface reconstruction, we will add expected intensity ratios from matrix element calculations; for structural domains, we will quantify the dispersion broadening from domain averaging; and for extrinsic scattering, we will compare with temperature-dependent data. These additions will allow readers to better assess the exclusions. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation remains independent of target data

full rationale

The paper reports independent experimental ARPES spectra and compares the observed weakly dispersive valence-band-edge feature to energy differences obtained from exact diagonalization of a NiS6 cluster. The abstract states that these differences are 'consistent with' the feature after alternatives are ruled out, without any quoted equation or section showing that cluster parameters (U, Δ, Slater integrals) were fitted or refined to the ARPES position itself. No self-definitional step, fitted-input prediction, or load-bearing self-citation chain is exhibited that reduces the central assignment to the input data by construction. The experimental data and the cluster calculation are presented as separate inputs whose agreement is interpretive rather than tautological.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the domain assumption that the NiS6 cluster diagonalization captures the relevant local physics and that the observed feature originates from mixed multiplet configurations rather than extrinsic effects.

axioms (1)
  • domain assumption Exact diagonalization of the NiS6 cluster yields low-energy final-state configurations of mixed multiplet d7 and d8L character whose energy differences match the observed feature
    Invoked to link the experimental ARPES feature to many-body states.

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Reference graph

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