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arxiv: 2303.02971 · v1 · submitted 2023-03-06 · ❄️ cond-mat.mtrl-sci · cond-mat.mes-hall

Observation of 2D Weyl Fermion States in Epitaxial Bismuthene

Qiangsheng Lu , P. V. Sreenivasa Reddy , Hoyeon Jeon , Alessandro R. Mazza , Matthew Brahlek , Weikang Wu , Shengyuan A. Yang , Jacob Cook
show 15 more authors
Clayton Conner Xiaoqian Zhang Amarnath Chakraborty Yueh-Ting Yao Hung-Ju Tien Chun-Han Tseng Po-Yuan Yang Shang-Wei Lien Hsin Lin Tai-Chang Chiang Giovanni Vignale An-Ping Li Tay-Rong Chang Rob G. Moore Guang Bian
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classification ❄️ cond-mat.mtrl-sci cond-mat.mes-hall
keywords 2D Weyl semimetalbismuthenetopological edge statesphotoemission spectroscopyepitaxial thin filmsspin polarizationFermi string statesinversion symmetry breaking
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The pith

Monolayer bismuthene on SnS(Se) realizes a 2D Weyl semimetal through substrate symmetry breaking.

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

The paper establishes that epitaxial growth of monolayer bismuthene on a SnS(Se) substrate eliminates the material's intrinsic band gap via perturbations that break space-inversion symmetry. This produces gapless, spin-polarized linear band dispersions identified as 2D Weyl fermion states. Spin- and angle-resolved photoemission directly maps the linear dispersion and spin texture, while scanning tunneling spectroscopy detects enhanced edge density of states consistent with Fermi string boundary modes. A reader would care because the result supplies a concrete experimental system for probing topological boundary states that exist only in two dimensions.

Core claim

Monolayer-thick epitaxial bismuthene grown on SnS(Se) substrate forms a 2D Weyl semimetal. Substrate perturbations break space-inversion symmetry and close the intrinsic gap of bismuthene, yielding a gapless spin-polarized Weyl band dispersion. Spin- and angle-resolved photoemission measurements confirm the linear dispersion and spin polarization of these states. Scanning tunneling microscopy and spectroscopy additionally show pronounced local density of states at the sample edge, indicating the presence of Fermi string edge states.

What carries the argument

Substrate-induced breaking of space-inversion symmetry that closes the bismuthene gap and produces gapless spin-polarized Weyl bands.

If this is right

  • The topology of the 2D Weyl cones requires Fermi string edge states as protected boundary modes.
  • Epitaxial thin-film growth becomes a viable route to realize 2D Weyl semimetals.
  • The linear dispersion and spin texture are directly accessible by spin-resolved photoemission.
  • Edge local density of states enhancements become a signature for the new boundary states.

Where Pith is reading between the lines

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

  • Substrate engineering of inversion symmetry could be applied to other monolayer materials to induce similar topological phases.
  • Confirmed Fermi string states would imply protected one-dimensional conduction channels distinct from conventional edge states in quantum spin Hall systems.
  • Momentum-resolved measurements could locate the precise positions of the Weyl points relative to the substrate Brillouin zone.

Load-bearing premise

That the substrate-induced breaking of space-inversion symmetry is the sole mechanism that closes the bismuthene gap and that the measured linear bands and edge density of states arise from intrinsic 2D Weyl states rather than from substrate-induced bands, defects, or hybridization effects.

What would settle it

Detection of an energy gap at the expected Weyl crossing points or absence of spin polarization in the linear bands in photoemission, or lack of enhanced edge density of states in tunneling spectra, would falsify the identification as intrinsic 2D Weyl states.

Figures

Figures reproduced from arXiv: 2303.02971 by Alessandro R. Mazza, Amarnath Chakraborty, An-Ping Li, Chun-Han Tseng, Clayton Conner, Giovanni Vignale, Guang Bian, Hoyeon Jeon, Hsin Lin, Hung-Ju Tien, Jacob Cook, Matthew Brahlek, Po-Yuan Yang, P. V. Sreenivasa Reddy, Qiangsheng Lu, Rob G. Moore, Shang-Wei Lien, Shengyuan A. Yang, Tai-Chang Chiang, Tay-Rong Chang, Weikang Wu, Xiaoqian Zhang, Yueh-Ting Yao.

Figure 1
Figure 1. Figure 1: FIG. 1 [PITH_FULL_IMAGE:figures/full_fig_p021_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2 [PITH_FULL_IMAGE:figures/full_fig_p022_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3 [PITH_FULL_IMAGE:figures/full_fig_p023_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4 [PITH_FULL_IMAGE:figures/full_fig_p024_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5 [PITH_FULL_IMAGE:figures/full_fig_p025_5.png] view at source ↗
read the original abstract

A two-dimensional (2D) Weyl semimetal featuring a spin-polarized linear band dispersion and a nodal Fermi surface is a new topological phase of matter. It is a solid-state realization of Weyl fermions in an intrinsic 2D system. The nontrivial topology of 2D Weyl cones guarantees the existence of a new form of topologically protected boundary states, Fermi string edge states. In this work, we report the realization of a 2D Weyl semimetal in monolayer-thick epitaxial bismuthene grown on SnS(Se) substrate. The intrinsic band gap of bismuthene is eliminated by the space-inversion-symmetry-breaking substrate perturbations, resulting in a gapless spin-polarized Weyl band dispersion. The linear dispersion and spin polarization of the Weyl fermion states are observed in our spin and angle-resolved photoemission measurements. In addition, the scanning tunneling microscopy/spectroscopy reveals a pronounced local density of states at the edge, suggesting the existence of Fermi string edge states. These results open the door for the experimental exploration of the exotic properties of Weyl fermion states in reduced dimensions.

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

1 major / 1 minor

Summary. The paper reports the realization of a 2D Weyl semimetal in monolayer epitaxial bismuthene grown on SnS(Se) substrates. Substrate-induced breaking of space-inversion symmetry is claimed to close the intrinsic bismuthene gap, producing gapless spin-polarized linear Weyl bands whose dispersion and spin texture are observed by spin-ARPES; STM/STS additionally shows enhanced edge LDOS interpreted as topologically protected Fermi string states.

Significance. If the central attribution holds, the result would constitute the first experimental realization of intrinsic 2D Weyl fermions and their protected edge states, providing a new platform for reduced-dimensional topological physics. The combination of SARPES and STM data is in principle capable of supporting such a claim, but the manuscript's evidential strength rests on the untested assumption that interface hybridization and substrate bands do not contribute to the reported features.

major comments (1)
  1. [Results (SARPES and STM sections)] The weakest assumption identified in the skeptic note is load-bearing: the manuscript provides no thickness-dependent ARPES series, bare-substrate reference spectra, or heterostructure DFT calculations that quantitatively separate bismuthene-intrinsic bands from possible SnS(Se) hybridization or defect states in the energy window of the claimed Weyl cones. Without such controls, the linear spin-polarized dispersion cannot be unambiguously assigned to 2D Weyl states of bismuthene.
minor comments (1)
  1. [Introduction] The abstract and main text use 'Fermi string edge states' without a brief definition or reference to the expected LDOS signature of such states in 2D Weyl systems; a short explanatory sentence would improve accessibility.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful and constructive review. We address the single major comment below and will revise the manuscript accordingly to strengthen the band assignment.

read point-by-point responses

  1. Referee: [Results (SARPES and STM sections)] The weakest assumption identified in the skeptic note is load-bearing: the manuscript provides no thickness-dependent ARPES series, bare-substrate reference spectra, or heterostructure DFT calculations that quantitatively separate bismuthene-intrinsic bands from possible SnS(Se) hybridization or defect states in the energy window of the claimed Weyl cones. Without such controls, the linear spin-polarized dispersion cannot be unambiguously assigned to 2D Weyl states of bismuthene.

    Authors: We agree that additional controls would strengthen the attribution. The manuscript currently relies on the observed spin texture (which is not expected from the substrate) and the appearance of the linear bands only in bismuthene-covered regions. We will add heterostructure DFT calculations in the revised version to quantitatively separate the contributions and confirm the bismuthene origin of the Weyl cones. Bare-substrate reference spectra will also be included where available; thickness-dependent ARPES is limited by the monolayer growth mode but can be discussed. revision: yes

Circularity Check

0 steps flagged

No circularity: experimental observation paper with no derivation chain

full rationale

This is an experimental report of ARPES, spin-ARPES, and STM measurements on epitaxial bismuthene. The abstract and available text contain no equations, fitting procedures, theoretical derivations, or predictions that could reduce to self-referential inputs. Claims rest on direct observation of linear dispersion, spin polarization, and edge LDOS, with interpretive attribution to substrate-induced symmetry breaking; no self-citation load-bearing steps, ansatz smuggling, or fitted-input-as-prediction patterns appear. The derivation chain is therefore self-contained as raw data reporting.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

This is an experimental observation paper. No free parameters, mathematical axioms, or new postulated entities are introduced; the claim rests on sample growth and spectroscopic measurements.

pith-pipeline@v0.9.0 · 5830 in / 1205 out tokens · 31977 ms · 2026-05-24T09:50:49.888442+00:00 · methodology

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Forward citations

Cited by 1 Pith paper

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    cond-mat.mes-hall 2024-06 unverdicted novelty 7.0

    Observation of temperature-independent anomalous Hall effect in 68 nm pure bismuth film from 15 mK to 300 K, proposed as intrinsic due to surface Berry curvature.

Reference graph

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