Diverse edge states of nanoribbons and excitonic insulator states of the monolayer Ta2Ni3Te5

Adrienn Ruzsinszky; Gabor I. Csonka; Hong Tang; Jiang Wei

REVIEW 3 major objections 6 minor 60 references

Diverse edge states of nanoribbons and excitonic insulator states of the monolayer Ta2Ni3Te5

T0 review · 3 major / 6 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read The paper predicts that a monolayer of Ta2Ni3Te5 is an excitonic insulator, with first dark exciton binding energy 285–586 meV exceeding the 95–169 meV band gap, while nanoribbon edge termination tunes metallic/semiconducting and magnetic…

desk verdict Solid nanoribbon study, but the excitonic-insulator claim rests on a circular scissor and should be re-framed as a hypothesis. read the letter →

arxiv 2505.22955 v1 pith:7SKMDNLL submitted 2025-05-29 cond-mat.mtrl-sci cond-mat.mes-hallphysics.comp-ph

Hong Tang , Jiang Wei , Gabor I. Csonka , Adrienn Ruzsinszky This is my paper

classification cond-mat.mtrl-scicond-mat.mes-hallphysics.comp-ph PACS 71.35.-y73.22.-f71.15.Mb71.20.-b

keywords excitonicinsulatorTa2Ni3Te5monolayernanoribbonedgestatesBethe-SalpeterequationGWapproximationmetaGGAtransitionmetalchalcogenidefirst-principlescalculations

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

Using first-principles calculations, this paper predicts that the monolayer form of the layered chalcogenide Ta2Ni3Te5 is an excitonic insulator—a state in which the most tightly bound electron-hole pair costs less energy than the single-particle gap, so excitons form spontaneously rather than being created by light. In the authors' GW/Bethe-Salpeter and metaGGA-based modified BSE calculations, the first dark exciton has a binding energy of 285–586 meV against a band gap of 95–169 meV, and in the scissor-adjusted GW+BSE run the dark exciton sits at −0.195 eV, below the gap. The paper also shows that nanoribbons cut from the same monolayer can be switched between metallic, semimetallic, and semiconducting behavior (gaps 29.7–60.8 meV) and between ferromagnetic and antiferromagnetic edge order by choosing Ni or Ta edges, ribbon width, and H/F saturation. If these predictions hold, Ta2Ni3Te5 becomes a single material platform for studying exciton condensation and second-order topology and for building edge-tunable nanoelectronic and spintronic devices.

What carries the argument

The load-bearing object is the exciton binding energy $E_b = E_g - E_{\mathrm{exc}}$, computed by the Bethe-Salpeter equation (the many-body equation for correlated electron-hole pairs) and by a metaGGA-based modified BSE that replaces full GW with a screened hybrid-style kernel. The excitonic-insulator condition is simply $E_b > E_g$: the lowest exciton is pulled below the conduction band edge, so the ground state should contain a finite density of excitons. The paper's strongest evidence is the first dark exciton at $-0.195$ eV in the scissor-adjusted BSE and binding energies of $0.586$ eV (LAK) and $0.592$ eV (r2SCAN) against gaps of $0.153$ and $0.169$ eV. For the nanoribbons, the operative mechanism is edge chemistry: the Ni $d$-orbital occupation changes with H versus F saturation and ribbon width, which flips the system between semiconducting, metallic, and magnetically ordered edge states.

What would settle it

Measure the monolayer's quasiparticle band gap directly with ARPES or with tunneling spectroscopy configured to resolve single-particle states: a gap near 417 meV would refute the EI claim because the paper's own unscissored BSE calculation then puts the first dark exciton at +0.14 eV, above the gap. A temperature-dependent gap renormalization in STS or optical absorption below a critical temperature would support it.

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Extended reading notes

Core claim

On its own terms, the central discovery is that the Ta2Ni3Te5 monolayer is an excitonic insulator. After the G0W0 band gap is scissor-corrected to 95 meV to match the experimental STS gap, the Bethe-Salpeter calculation places the first dark exciton at −0.195 eV, giving a binding energy of 285 meV that exceeds the gap; the r2SCAN and LAK based modified BSE calculations give binding energies of 490–586 meV against gaps of 153–169 meV. These negative-energy excitons imply that the low-energy excitations are excitonic rather than single-particle, which the authors use to explain why the STS gap (90–100 meV) is far smaller than the G0W0 quasiparticle gap (417 meV). The same study reports that Ni-edged or Ta-edged nanoribbons host conducting, semiconducting, or semimetallic bands with edge magnetism controlled by termination chemistry, with ferromagnetic edge Ni atoms in F-saturated ribbons and antiferromagnetic coupling in the widest ribbon.

Load-bearing premise

The result rests on treating the 90–100 meV STS gap as the quasiparticle gap in the BSE calculation even though the paper also argues that this measured gap is a renormalized excitonic gap; if the true quasiparticle gap is the unadjusted G0W0 value of 417 meV, the first dark exciton sits above the gap and the excitonic insulator claim fails.

Editorial extensions

If this is right

Monolayer Ta2Ni3Te5 should show a low-energy optical response governed by a dark exciton below the gap, with strong in-plane anisotropy; absorption along the chains peaks near 0.2 eV.
The 90–100 meV gap seen by STS should be interpreted as a renormalized excitonic gap, not the quasiparticle gap, so combined GW/BSE calculations are needed to interpret such measurements in this material.
F-saturated Ni-edged nanoribbons are predicted to conduct through edge Ni d-channels with ferromagnetic (narrow) or antiferromagnetic (wide) edge ordering, making them candidate spin-current channels.
H-saturated Ta-edged nanoribbons (4TNTrb-Ta-H, 6TNTrb-Ta-H) are direct-gap semiconductors with quasi-1D conduction and approximately Dirac-like band crossings near Γ, good settings for Luttinger-liquid or correlated-edge physics.
The predicted EI state does not change the Z2 number, so exciton condensation and second-order topological corner states may coexist in the monolayer.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

Editorial inference: if the EI picture is right, temperature-dependent STS or ARPES on monolayer Ta2Ni3Te5 should reveal a gap that closes or renormalizes at a condensation temperature, analogous to observations in Ta2Pd3Te5; the paper does not predict that temperature.
Editorial inference: because the unadjusted G0W0+BSE calculation places the first dark exciton at +0.14 eV (binding 276 meV, below the 417 meV gap), a single experimental determination of the quasiparticle gap would decide the matter without further modeling.
Editorial inference: the ~1.5 nm exciton extent along Ta chains suggests dielectric environment and strain should tune Eb across the EI boundary, which could be tested by placing the monolayer on different substrates or applying uniaxial strain.
Editorial inference: the nanoribbon rule implied by the calculations—fluorine withdraws charge from edge Ni and switches hydrogen-passivated semiconductors to ferromagnetic metals—could be tested by edge-selective STS and transport on patterned ribbons.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Short version: the nanoribbon half of this paper is a solid, systematic computational study. The excitonic-insulator claim for the monolayer does not hold up as presented.

What's new: this is the first nanoribbon work for Ta2Ni3Te5, and the authors do a thorough job — 15 ribbons, variation of edge (Ni vs Ta), width, and H/F passivation, with a clear classification into metallic/semimetallic/semiconducting and FM/AFM/non-magnetic states. That part is plausible and would be useful to people working on this material family or on edge engineering in quasi-1D chalcogenides. The monolayer EI prediction is new for this material, but it follows the same recipe used for Ta2Pd3Te5, so the novelty is in the application, not the framework.

The problem is the central EI claim. The unscissored G0W0+BSE gives a dark exciton at +0.14 eV with a binding energy of 276 meV against a 417 meV quasiparticle gap. That is a normal bound exciton, not an excitonic insulator. The EI conclusion comes only after scissoring the gap down to 95 meV to match the STS gap. But the paper itself says, in Section 3.1, that the STS gap is likely the renormalized excitonic gap rather than the quasiparticle gap. So the scissor assumes what it is trying to prove: if you force the QP gap to be 95 meV, BSE will almost always produce a negative exciton because the binding energy exceeds the gap. The algebra is essentially forced. The metaGGA+mBSE results do give negative excitons without the scissor, but they depend on screening parameters alpha and mu fitted to RPA dielectric functions; mu and the fitting details are not reported, and the method is not benchmarked against a known non-EI monolayer. So those results are suggestive, not confirmatory.

I want to be fair: the inconsistency is visible in the text, so it is not hidden. The authors clearly state their interpretation of the STS gap, and then use that same number as the QP gap. That is an overclaim, but the underlying calculations are reproducible and the nanoribbon part stands on its own.

Who gets value from this? Anyone working on the Ta2M3Te5 family or nanoribbon edge states. The ribbon classification is the reliable contribution. The EI claim should be treated as a hypothesis for experimental testing, not a demonstrated prediction.

Recommendation: this deserves peer review, but the referee should push for a major revision. Present the unscissored G0W0+BSE result as the primary many-body result, move the scissored calculation to a clearly labeled "what if the STS gap were the QP gap" test, and either benchmark mBSE on a non-EI monolayer or report the full screening parameters. With that re-framing, the paper would be honest and still interesting.

Referee Report

3 major / 6 minor

Summary. The manuscript reports two sets of first-principles results for Ta2Ni3Te5. First, it presents DFT (r2SCAN+SOC) calculations for fifteen nanoribbons with Ni or Ta edges and H/F passivation, finding tunable metallic, semimetallic, and semiconducting behaviors with gaps of 29.7–60.8 meV, and ferromagnetic or antiferromagnetic edge magnetism depending on width and saturation. Second, it presents G0W0+BSE and metaGGA-based modified BSE (mBSE) calculations for the monolayer, claiming that the monolayer is an excitonic insulator with the lowest exciton binding energy (285–586 meV) exceeding the band gap (95–169 meV). The excitonic-insulator conclusion is supported in Section 3.4 by a scissored G0W0+BSE calculation in which the quasiparticle gap is artificially lowered from 417 meV to 95 meV, and by mBSE results with screening parameters fitted to an RPA dielectric function.

Significance. If the excitonic-insulator claim were correct, it would be notable because monolayer Ta2Ni3Te5 is also predicted to be a second-order topological insulator, and the coexistence of excitonic and topological order is an active topic. The nanoribbon results provide a systematic catalog of edge chemistry, magnetism, and band-gap tuning that could be useful for nanoelectronics and spintronics. The paper also includes standard many-body calculations and presents machine-readable-like details of the computational setups. However, the central claim of the paper—the excitonic insulator state—rests on a circular scissor operation and on a model method with parameter sensitivity that is not resolved against the unscissored G0W0+BSE result. The nanoribbon part is sound but secondary; the headline conclusion is not established.

major comments (3)

[Section 3.4 and Section 3.1] The central excitonic-insulator claim is based on a circular argument. Section 3.1 states that the experimental STS gap of 90–100 meV 'likely represents a renormalized excitonic gap rather than the quasiparticle band gap.' Section 3.4 then applies a scissor operation to lower the G0W0 quasiparticle gap to 95 meV—the STS value—and reports a first dark exciton at -0.195 eV with binding energy 285 meV, which is presented as 'confirming that the Ta2Ni3Te5 monolayer is an excitonic insulator.' But the BSE Hamiltonian requires the quasiparticle (single-particle addition/removal) gap as input. The authors have already assumed that the 95 meV STS gap is the excitonic gap, so using it as the quasiparticle gap builds the excitonic-insulator conclusion into the calculation. The unscissored G0W0+BSE result shown in Figure 6(d) gives a dark exciton at +0.14 eV with binding energy 276 meV against a 417 meV gap, which is a normal bound exciton, not an excitonic insulator. Under a rigid scissor, the exciton energy shifts approximately linearly with the gap reduction, so any input gap below roughly 276 meV automatically yields a negative exciton energy; the negative sign is forced by the chosen input, not derived from the electron-hole interaction physics.
[Section 3.4 and Table 1] The metaGGA+mBSE results in Table 1 depend sensitively on the fitted screening parameters, and they disagree qualitatively with the unscissored G0W0+BSE. For LAK, the CHI fitting gives alpha = 0.236 and a binding energy of 586 meV, while the LOPTICS fitting gives alpha = 0.108 and a binding energy of 299 meV; for r2SCAN the corresponding values are 0.241/592 meV and 0.088/254 meV. This factor-of-two spread shows that the 'binding energy exceeds the gap' conclusion is not robust to the choice of the screening model. The paper does not report the second screening parameter mu, nor does it justify why the CHI-derived parameter set is preferred over the LOPTICS one. Moreover, these mBSE results give a negative exciton energy even without the scissor, which is inconsistent with the unscissored G0W0+BSE dark exciton at +0.14 eV. The authors should either explain this discrepancy in a quantitative way, for example by analyzing the difference in screening and starting point, or refrain from claiming that two independent methods confirm the excitonic insulator.
[Abstract and Conclusion] The abstract and conclusion state that the lowest exciton's binding energy is '285–586 meV' and that it exceeds 'the calculated band gap (95–169 meV).' The 95 meV lower end of the gap range is not a calculated quasiparticle gap; it is the scissored value imposed to match the experimental STS gap. Including it in the same range as the calculated r2SCAN/LAK gaps (153–169 meV) conflates an experimental input with a first-principles output and makes the two methods appear more consistent than they are. The range of binding energies also mixes the scissored G0W0+BSE value (285 meV) with the mBSE values (586–592 meV) obtained with a different screening parameter. A clearer presentation would list the method, the quasiparticle gap, the exciton energy, and the binding energy separately for each calculation.

minor comments (6)

[Section numbering] The section numbering jumps from 3.2 to 3.4; there is no Section 3.3. This is presumably a typo but should be fixed.
[Figure captions] The captions of Figures 7 and 8 refer to 'Figure 8(d)' and 'Figure 8(d)' respectively when identifying the first dark and bright excitons; the exciton spectrum is actually shown in Figure 6(d).
[Notation] The text uses 'R2SCAN' in several places, for example in Figure 10 and Section 3.2.1, instead of the consistent 'r2SCAN' notation used elsewhere.
[Reference 21] Reference 21 is listed as 'Wang et al. Prepared.' This is not a complete citation. If it contains the experimental synthesis, STS, and characterization data used in Section 3.1, it must be replaced with a citable reference or the data must be described in the text.
[Table 1 and screening parameters] Table 1 lists only the alpha screening parameter and not the mu parameter, although the text says that both alpha and mu are fitted to the RPA dielectric function. Reporting mu would be important for reproducibility of the mBSE calculations.
[Gap notation in conclusion] The conclusion says 'gaps ~20–50 meV' for the semiconducting Ni-edged H-saturated nanoribbons, whereas the abstract and Section 3.2.2 give 29.7–47.8 meV for these ribbons; the ranges should be consistent.

Circularity Check

1 steps flagged · score 7.0 of 10

The G0W0+BSE 'confirmation' of the excitonic insulator is circular: the STS gap is first called an excitonic gap and then scissored into BSE as the quasiparticle gap, algebraically forcing the dark exciton negative.

fitted input called prediction [Section 3.1 and Section 3.4 (G0W0+BSE scissor; Figs. 6(d), 9(a)-9(c))]
"However, this experimental gap likely represents a renormalized excitonic gap rather than the quasiparticle band gap... To align the band gap with the experimental value (90-100 meV), we applied a scissor operation to adjust the G0W0@PBE gap to 95 meV and performed BSE calculations... The first dark exciton, at -0.195 eV , has a binding energy of 285 meV , exceeding the 95 meV gap, confirming that the Ta2Ni3Te5 monolayer is an excitonic insulator."

The BSE Hamiltonian requires the quasiparticle (single-particle) gap as input, but the paper sets it to the measured STS gap after explicitly labeling that STS gap a 'renormalized excitonic gap.' A rigid conduction-band scissor by Delta shifts BSE exciton energies by approximately -Delta while leaving binding energies nearly unchanged. The unscissored G0W0+BSE first dark exciton is +0.14 eV with a 276 meV binding energy against a 417 meV gap; lowering the gap to 95 meV (Delta=322 meV) algebraically gives about -0.18 eV, close to the reported -0.195 eV. The negative exciton and 'binding energy exceeding the gap' are therefore built in by the fitted input gap, not independently predicted, and the same measurement is used both as input and as confirmation.

full rationale

The nanoribbon results (Sec. 3.2) are self-contained DFT calculations and show no circularity. The central excitonic-insulator claim rests on two computational legs. The G0W0+BSE leg is circular: Section 3.1 states that the 90-100 meV STS gap 'likely represents a renormalized excitonic gap rather than the quasiparticle band gap,' and Section 3.4 then uses that same value as the quasiparticle gap via a scissor operation before running BSE. Since a scissor Delta shifts exciton levels almost rigidly, the reported dark exciton at -0.195 eV is essentially the unscissored +0.14 eV exciton shifted by the imposed gap reduction; the negative energy and the 'binding energy exceeding the gap' conclusion are forced by construction. The metaGGA+mBSE leg is not circular in this way: it yields negative first-exciton energies (-0.42 to -0.43 eV) and binding energies of 586-592 meV from metaGGA gaps of 153-169 meV without using the STS gap as input. Those results do depend on a self-cited mBSE method (Ref. [37]) and on screening parameters fitted to RPA response, but the EI character survives across the CHI and LOPTICS parametrizations shown in Table 1. This independent, parameter-dependent support prevents the highest circularity rating, while the scissored 'confirmation' keeps the score elevated. No uniqueness-theorem or ansatz-via-citation pattern is load-bearing.

Assumptions & free parameters 2 free parameters · 5 assumptions · 0 invented entities

The central EI claim rests on two free parameters: the scissor shift that forces the G0W0 gap from 417 meV to 95 meV, and the fitted mBSE screening parameters alpha and mu. The nanoribbon results depend on the choice of DFT functional and passivation, but these are standard modeling choices. No new entities are introduced.

free parameters (2)

Scissor shift (G0W0 gap adjusted to 95 meV) = 95 meV (shift of -322 meV applied to conduction bands)
Adjusted to match experimental STS gap; without this shift the BSE dark exciton is at +0.14 eV, not negative.
mBSE screening parameters alpha and mu = alpha = 0.236 (LAK, CHI), 0.241 (r2SCAN, CHI), 0.108 (LAK, LOPTICS), 0.088 (r2SCAN, LOPTICS); mu not reported
Fitted to wavevector-dependent RPA dielectric function; the large binding energies (586-592 meV) are sensitive to these parameters, though LOPTICS yields smaller but still large binding (254-299 meV).

assumptions (5)

domain assumption G0W0@PBE provides an adequate quasiparticle starting point for BSE in Ta2Ni3Te5.
The G0W0 gap is 417 meV, far from the metaGGA gaps; using it unscissored gives a positive exciton, so the EI conclusion depends on not using this calculation directly.
ad hoc to paper The experimental STS gap (90-100 meV) is the excitonic gap, and it is valid to use this value as the quasiparticle gap in the BSE calculation after scissoring.
This assumption converts the G0W0+BSE result from a positive to a negative exciton; it contradicts the earlier statement that STS measures the excitonic gap.
ad hoc to paper The mBSE screening model with parameters alpha and mu fitted to the RPA dielectric function captures the true electron-hole screening.
The fitted parameters yield binding energies more than double the full G0W0+BSE value (276 meV); no independent benchmark for this material is given.
domain assumption r2SCAN and LAK metaGGA band gaps are accurate enough to use as quasiparticle gaps for the EI determination.
These gaps (153-169 meV) are about a third of the G0W0 value; the EI conclusion is sensitive to this choice.
domain assumption The H/F-passivated nanoribbon models represent the physically relevant edges of Ta2Ni3Te5.
Passivation schemes are standard, but the predicted gaps and magnetic order are strongly dependent on this modeling choice.

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Cite this review

Pith. "Pith review of Diverse edge states of nanoribbons and excitonic insulator states of the monolayer Ta2Ni3Te5." pith.science (2026). https://pith.science/paper/7SKMDNLL

@misc{pith2026250522955,
  author       = {Pith},
  title        = {Pith review of: Diverse edge states of nanoribbons and excitonic insulator states of the monolayer Ta2Ni3Te5},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/7SKMDNLL}},
  note         = {Machine review of arXiv:2505.22955}
}

read the original abstract

Ta2Ni3Te5, a layered transition metal chalcogenide with quasi-one-dimensional electronic states, exhibits rich topological and correlated phenomena. Using first-principles calculations, we explore Ta2Ni3Te5 nanoribbons, demonstrating tunable electronic and magnetic properties-ranging from metallic to semimetallic and semiconducting (band gaps of 29.7-60.8 meV), and from ferromagnetic to antiferromagnetic-controlled by edge (Ni or Ta), ribbon width, and H/F saturation. Additionally, GW and Bethe-Salpeter equation (BSE) calculations, complemented by metaGGA-based modified BSE, reveal that the Ta2Ni3Te5 monolayer is an excitonic insulator, with an exciton binding energy exceeding its band gap. These diverse properties position Ta2Ni3Te5 nanoribbons and monolayers as promising candidates for nanoelectronics, spintronics, and optoelectronics, motivating further experimental exploration.

Figures

Figures reproduced from arXiv: 2505.22955 by the authors.

**Figure 1.** Relaxed atomic structure of the Ta2Ni3Te5 monolayer calculated using PBE. The supercell vectors a, b, and c align with the x, y, and z axes of the Cartesian coordinate system, respectively. Ta atoms are shown in red, Ni in blue, and Te in green. The right inset depicts the 2D Brillouin zone in the xy plane [PITH_FULL_IMAGE:figures/full_fig_p019_1.png] view at source ↗

**Figure 2.** Atomic structures of Ni-edged Ta2Ni3Te5 nanoribbons (TNTrb-Ni) with three widths: 2TNTrb-Ni, 4TNTrb-Ni, and 6TNTrb-Ni, where the numbers (2, 4, or 6) denote the Ta atoms in the unit cell. Ta atoms are shown in red, Ni in blue, and Te in green [PITH_FULL_IMAGE:figures/full_fig_p019_2.png] view at source ↗

**Figure 3.** Atomic structures of Ta-edged Ta2Ni3Te5 nanoribbons (TNTrb-Ta) with two widths: 4TNTrb-Ta and 6TNTrb-Ta where the numbers (4 or 6) denote the Ta atoms in the unit cell. Ta atoms are shown in red, Ni in blue, and Te in green [PITH_FULL_IMAGE:figures/full_fig_p020_3.png] view at source ↗

Figures from the paper (7 more)

**Figure 4.** Figure 4: Fluorine-saturated 4TNTrb-Ni-F nanoribbon. (a) Band structure along the atomic chain [PITH_FULL_IMAGE:figures/full_fig_p021_4.png]

**Figure 5.** Figure 5: Hydrogen-saturated 2TNTrb-Ni-H nanoribbon. (a) Band structure along the atomic [PITH_FULL_IMAGE:figures/full_fig_p022_5.png]

**Figure 6.** Figure 6: The calculated band structures and optical absorption of monolayer Ta [PITH_FULL_IMAGE:figures/full_fig_p023_6.png]

**Figure 7.** Figure 7: The three isosurface views (in the top-left, top-right and bottom-left subplots) of the [PITH_FULL_IMAGE:figures/full_fig_p024_7.png]

**Figure 8.** Figure 8: The plots for the first bright exciton at 0.21 eV (shown in Figure 8(d)). The isosurface [PITH_FULL_IMAGE:figures/full_fig_p025_8.png]

**Figure 9.** Figure 9: The calculated band structures and optical absorption of monolayer Ta [PITH_FULL_IMAGE:figures/full_fig_p026_9.png]

**Figure 10.** Figure 10: The calculated optical absorption spectrum from LAK+mBSE and R2SCAN+mBSE. [PITH_FULL_IMAGE:figures/full_fig_p026_10.png]

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