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arxiv: 1906.11424 · v1 · pith:QOC57N6Knew · submitted 2019-06-27 · ❄️ cond-mat.supr-con

Evidence for singular-phonon-induced nematic superconductivity in a topological superconductor candidate Sr_(0.1)Bi₂Se₃

Jinghui Wang , Kejing Ran , Shichao Li , Zhen Ma , Song Bao , Zhengwei Cai , Youtian Zhang , Kenji Nakajima
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Seiko Ohira-Kawamura P. Cermak A. Schneidewind Sergey Y. Savrasov Xiangang Wan Jinsheng Wen
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Pith reviewed 2026-05-25 14:25 UTC · model grok-4.3

classification ❄️ cond-mat.supr-con
keywords nematic superconductivitytopological superconductorelectron-phonon couplinginelastic neutron scatteringSr0.1Bi2Se3phonon linewidthp-wave pairinganisotropic phonons
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The pith

Acoustic phonon linewidths broaden only along [001] at long wavelengths in Sr0.1Bi2Se3, indicating singular electron-phonon coupling that selects nematic p-wave pairing.

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

Inelastic neutron scattering on a superconducting crystal of Sr0.1Bi2Se3 reveals highly anisotropic acoustic phonons whose linewidths increase substantially at long wavelengths, but only along the [001] direction. This direction-specific broadening signals a large electron-phonon coupling that is singular at small momenta. The authors propose that this coupling drives the exotic p-wave nematic superconducting state found across the MxBi2Se3 family. The finding shows that phonon-mediated interactions can produce non-s-wave pairing symmetry in these doped topological insulators.

Core claim

Inelastic neutron scattering measurements on Sr0.1Bi2Se3 find that acoustic phonon linewidths increase at long wavelengths exclusively along [001]. This anisotropy indicates a large and singular electron-phonon coupling at small momenta. The coupling is proposed to give rise to the nematic p-wave superconducting pairing observed in the MxBi2Se3 (M = Cu, Sr, Nb) family, demonstrating that electron-phonon interactions can induce exotic pairing beyond conventional s-wave symmetry and remain consistent with topological superconductivity.

What carries the argument

Direction-specific increase in acoustic phonon linewidths at long wavelengths, taken as evidence of singular electron-phonon coupling at small momenta.

If this is right

  • Phonon-mediated superconductivity can produce p-wave nematic order in the MxBi2Se3 family without requiring additional interactions.
  • The singular coupling at small momenta selects the observed pairing symmetry in these topological candidates.
  • Long-wavelength acoustic modes along the c-axis dominate the interaction that stabilizes the nematic state.
  • Similar phonon anisotropy should appear in other members of the MxBi2Se3 series if the mechanism is general.

Where Pith is reading between the lines

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

  • Measurements on Cu- or Nb-doped samples could test whether the same phonon feature appears across the family.
  • The mechanism offers a route to engineer pairing symmetry via lattice anisotropy in other doped topological insulators.
  • If confirmed, the result would require revisiting assumptions that phonon pairing always favors isotropic s-wave states.

Load-bearing premise

The observed linewidth broadening along [001] is produced by electron-phonon scattering rather than other damping mechanisms, and this coupling alone is enough to select nematic p-wave pairing.

What would settle it

Observation that the [001]-specific linewidth increase disappears in a non-superconducting analog or in calculations that omit electron-phonon scattering would falsify the proposed link to the pairing symmetry.

Figures

Figures reproduced from arXiv: 1906.11424 by A. Schneidewind, Jinghui Wang, Jinsheng Wen, Kejing Ran, Kenji Nakajima, P. Cermak, Seiko Ohira-Kawamura, Sergey Y. Savrasov, Shichao Li, Song Bao, Xiangang Wan, Youtian Zhang, Zhengwei Cai, Zhen Ma.

Figure 1
Figure 1. Figure 1: FIG. 1 [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2 [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3 [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4 [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗
read the original abstract

Superconductivity mediated by phonons is typically conventional, exhibiting a momentum-independent s-wave pairing function, due to the isotropic interactions between electrons and phonons along different crystalline directions. Here, by performing inelastic neutron scattering measurements on a superconducting single crystal of Sr0.1Bi2Se3, a prime candidate for realizing topological superconductivity by doping the topological insulator Bi2Se3, we find that there exist highly anisotropic phonons, with the linewidths of the acoustic phonons increasing substantially at long wavelengths, but only for those along the [001] direction. This observation indicates a large and singular electron-phonon coupling at small momenta, which we propose to give rise to the exotic p-wave nematic superconducting pairing in the MxBi2Se3 (M = Cu, Sr, Nb) superconductor family. Therefore, we show these superconductors to be example systems where electron-phonon interaction can induce more exotic superconducting pairing than the s-wave, consistent with the topological superconductivity.

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

3 major / 1 minor

Summary. The manuscript reports inelastic neutron scattering on superconducting Sr0.1Bi2Se3 and observes that acoustic-phonon linewidths increase substantially at long wavelengths only along the [001] direction. The authors interpret this anisotropy as direct evidence for a large, singular electron-phonon coupling at small momenta that induces the exotic p-wave nematic superconducting state observed in the MxBi2Se3 family, thereby providing an example of phonon-mediated pairing beyond conventional s-wave symmetry.

Significance. If the proposed link between the measured linewidth anisotropy and a momentum-selective pairing interaction could be made quantitative and shown to select the nematic channel, the result would constitute a rare experimental illustration of phonon-driven exotic superconductivity with topological implications. The work would then be of broad interest to the superconductivity community.

major comments (3)
  1. [Abstract] Abstract: the statement that the [001]-specific linewidth increase 'indicates a large and singular electron-phonon coupling at small momenta, which we propose to give rise to the exotic p-wave nematic superconducting pairing' is presented without any quantitative extraction of the coupling matrix element, any model relating linewidth to Im Σ(q,ω), or any derivation showing how the observed momentum dependence stabilizes nematic p-wave over s-wave or other channels.
  2. [Discussion] Main text (discussion of neutron results): no section supplies an explicit calculation (Eliashberg, RPA, or otherwise) that converts the reported long-wavelength linewidth broadening into a singular small-q interaction strength or demonstrates that this interaction is sufficient to select the observed nematic order parameter without additional fine-tuning or competing interactions.
  3. [Results] No comparison is shown to alternative damping mechanisms (anharmonicity, defects, or magnetic scattering) that could produce direction-specific broadening at small q; the attribution to electron-phonon scattering therefore remains an assumption rather than a demonstrated exclusion.
minor comments (1)
  1. [Abstract] The abstract and introduction would benefit from a brief statement of the instrumental resolution and the precise q-range over which the linewidth increase is claimed.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the constructive comments. We respond to each major comment below.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the statement that the [001]-specific linewidth increase 'indicates a large and singular electron-phonon coupling at small momenta, which we propose to give rise to the exotic p-wave nematic superconducting pairing' is presented without any quantitative extraction of the coupling matrix element, any model relating linewidth to Im Σ(q,ω), or any derivation showing how the observed momentum dependence stabilizes nematic p-wave over s-wave or other channels.

    Authors: The abstract summarizes the experimental observation and our proposed interpretation of its implications for pairing. The manuscript does not contain a quantitative extraction of the coupling or a derivation of pairing symmetry selection; the connection is presented as a proposal. We will revise the abstract to make this distinction explicit and to avoid any implication of a completed quantitative analysis. revision: yes

  2. Referee: [Discussion] Main text (discussion of neutron results): no section supplies an explicit calculation (Eliashberg, RPA, or otherwise) that converts the reported long-wavelength linewidth broadening into a singular small-q interaction strength or demonstrates that this interaction is sufficient to select the observed nematic order parameter without additional fine-tuning or competing interactions.

    Authors: No such explicit calculation is present in the manuscript, which reports the neutron scattering data and offers a qualitative proposal. A full microscopic calculation lies outside the scope of this experimental work. In revision we will expand the discussion to include a qualitative argument on how a singular small-q interaction could favor the nematic channel, together with references to relevant theory, while clearly stating the absence of a quantitative demonstration. revision: partial

  3. Referee: [Results] No comparison is shown to alternative damping mechanisms (anharmonicity, defects, or magnetic scattering) that could produce direction-specific broadening at small q; the attribution to electron-phonon scattering therefore remains an assumption rather than a demonstrated exclusion.

    Authors: The strong directional selectivity at long wavelengths, together with the known electronic structure of the material, is most consistent with electron-phonon scattering. We did not, however, present explicit comparisons or calculations ruling out anharmonicity, defects or magnetic scattering. We will add a dedicated paragraph in the revised manuscript discussing these alternatives and the reasons they are considered less likely given the observed momentum and temperature dependence. revision: yes

Circularity Check

0 steps flagged

No significant circularity in the derivation chain

full rationale

The paper reports inelastic neutron scattering data on Sr0.1Bi2Se3 showing [001]-specific acoustic phonon linewidth increase at long wavelengths, interprets the observation as indicating singular small-q electron-phonon coupling, and proposes (without equations) that this coupling gives rise to nematic p-wave pairing. This is an empirical interpretation and proposal rather than a closed mathematical derivation. No self-definitional steps, fitted inputs renamed as predictions, load-bearing self-citations, uniqueness theorems, or ansatz smuggling appear in the abstract or described chain. The link from data to proposal does not reduce to its own inputs by construction and remains open to external falsification.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Abstract-only review; no explicit free parameters, invented entities, or additional axioms are stated beyond standard condensed-matter assumptions about phonon linewidths.

axioms (1)
  • domain assumption Phonon linewidth broadening at small momentum is dominated by electron-phonon scattering
    Invoked when interpreting the [001]-specific increase as singular coupling.

pith-pipeline@v0.9.0 · 5770 in / 1301 out tokens · 24456 ms · 2026-05-25T14:25:59.415970+00:00 · methodology

discussion (0)

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Reference graph

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