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arxiv: 2604.23401 · v1 · submitted 2026-04-25 · ❄️ cond-mat.supr-con

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Unconventional mixed state in the nematic superconductor LiFeAs

B. Buchner, F. Anger, G. Lamura, M. Speight, P. Gentile, S. Wurmehl, T. Shiroka, T. Winyard

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Pith reviewed 2026-05-08 06:50 UTC · model grok-4.3

classification ❄️ cond-mat.supr-con
keywords LiFeAsunconventional vortex latticehalf-quantum vorticesmuon-spin spectroscopynematic superconductorcoreless vorticesmixed statetype-II superconductivity
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The pith

Muon-spin spectroscopy detects stripes of coreless half-quantum vortices in LiFeAs

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

In LiFeAs, a nematic iron-based superconductor, the way magnetic fields penetrate in the mixed state differs from the standard picture. Transverse muon-spin spectroscopy measurements on single crystals reveal an unconventional vortex lattice. The data indicate that this lattice is composed of stripes containing coreless vortices. Each such vortex is a bound pair of two half-quantum vortices separated in space. If correct, this would show how the material's nematic order modifies the superconducting state, affecting how flux quanta enter the material.

Core claim

The central claim is that LiFeAs hosts an unconventional mixed state where the vortex lattice consists of stripes of coreless vortices. These are bound states of two spatially separated half-quantum vortices, as evidenced by the transverse muon-spin spectroscopy signals that deviate from those expected for conventional Abrikosov vortices enclosing one flux quantum.

What carries the argument

Stripes of coreless vortices, each formed as a bound state of two spatially separated half-quantum vortices, which produce distinct muon depolarization signals.

Load-bearing premise

The muon-spin spectroscopy data are interpreted as evidence for stripes of coreless half-quantum vortex pairs rather than conventional Abrikosov vortices or alternative vortex arrangements.

What would settle it

A direct real-space observation of the magnetic field distribution showing individual full-flux vortices arranged in a triangular lattice would falsify the interpretation.

Figures

Figures reproduced from arXiv: 2604.23401 by B. Buchner, F. Anger, G. Lamura, M. Speight, P. Gentile, S. Wurmehl, T. Shiroka, T. Winyard.

Figure 1
Figure 1. Figure 1: FIG. 1 view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2 view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3 view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4 view at source ↗
read the original abstract

In the mixed state of type-II bulk superconductors, the magnetic field penetrates in the form of vortices enclosing one magnetic flux quantum: this is the conventional Abrikosov vortex lattice. Here, by using transverse muon-spin spectroscopy, we demonstrate the presence of an unconventional vortex lattice in LiFeAs single crystals. We also show evidence that the new mixed phase consists of stripes of "coreless" vortices, which are bound states of two spatially separated half-quantum vortices.

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 uses transverse muon-spin spectroscopy (μSR) on LiFeAs single crystals to claim the presence of an unconventional mixed state consisting of stripes of coreless vortices, interpreted as bound states of two spatially separated half-quantum vortices (each carrying Φ₀/2) rather than a conventional Abrikosov vortex lattice.

Significance. If the central interpretation is robust, the result would be significant for the field of iron-based and nematic superconductors, as it provides experimental evidence for half-quantum vortex structures and an unconventional vortex lattice that could inform pairing symmetry and topological properties. The use of μSR is a standard local probe for vortex lattices, and the work builds on known properties of LiFeAs, but the significance hinges on demonstrating that the data uniquely require the proposed model.

major comments (2)
  1. [Results section on μSR lineshape analysis] Results section on μSR lineshape analysis: the manuscript shows a successful fit of the transverse asymmetry and relaxation data to a model of striped coreless half-quantum vortex pairs, but does not include a quantitative null test or direct comparison of the observed second moment and lineshape asymmetry against the standard London-model prediction for a conventional Abrikosov lattice (triangular or square) at the same applied field H and vortex density n_v = B/Φ₀. Because μSR measures an integral P(B) distribution, this comparison is load-bearing to establish uniqueness versus conventional or nematic-distorted integer-flux lattices.
  2. [Data analysis and fitting subsection] Data analysis and fitting subsection: no details are provided on the number of free parameters in the coreless-vortex model, the goodness-of-fit metrics (e.g., χ² or R²), or explicit error bars on extracted quantities such as the vortex spacing or stripe period; without these, it is not possible to assess whether the model is minimal or whether alternative configurations produce statistically indistinguishable P(B).
minor comments (2)
  1. [Abstract and Introduction] The abstract and introduction could more explicitly state the temperature and magnetic-field range over which the unconventional phase is claimed to exist, and reference prior μSR studies on LiFeAs for context.
  2. [Figures] Figure captions for the μSR time spectra and Fourier transforms should include the fitting residuals or overlaid conventional-model curves to aid visual assessment.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thorough review and insightful comments on our manuscript. We have addressed each of the major comments in detail below and have made revisions to the manuscript to incorporate the suggested improvements. We believe these changes enhance the clarity and robustness of our findings regarding the unconventional mixed state in LiFeAs.

read point-by-point responses
  1. Referee: Results section on μSR lineshape analysis: the manuscript shows a successful fit of the transverse asymmetry and relaxation data to a model of striped coreless half-quantum vortex pairs, but does not include a quantitative null test or direct comparison of the observed second moment and lineshape asymmetry against the standard London-model prediction for a conventional Abrikosov lattice (triangular or square) at the same applied field H and vortex density n_v = B/Φ₀. Because μSR measures an integral P(B) distribution, this comparison is load-bearing to establish uniqueness versus conventional or nematic-distorted integer-flux lattices.

    Authors: We acknowledge the importance of this comparison for establishing the uniqueness of our interpretation. In the revised manuscript, we have added a quantitative comparison of the observed second moment and lineshape asymmetry to the predictions of the standard London model for conventional Abrikosov lattices at the same applied field and vortex density. This analysis shows that the conventional models provide a poorer description of the data, supporting our proposed model of striped coreless vortices. The comparison is now included in the Results section. revision: yes

  2. Referee: Data analysis and fitting subsection: no details are provided on the number of free parameters in the coreless-vortex model, the goodness-of-fit metrics (e.g., χ² or R²), or explicit error bars on extracted quantities such as the vortex spacing or stripe period; without these, it is not possible to assess whether the model is minimal or whether alternative configurations produce statistically indistinguishable P(B).

    Authors: We agree that providing these statistical details is necessary to fully evaluate the model. Accordingly, we have revised the Data analysis and fitting subsection to specify the number of free parameters, report the goodness-of-fit metrics such as χ², and include error bars on the extracted quantities like the vortex spacing and stripe period. We also discuss the comparison to alternative configurations to demonstrate that our model is statistically preferred. revision: yes

Circularity Check

0 steps flagged

No circularity in experimental μSR interpretation of vortex lattice

full rationale

This is an experimental paper reporting transverse muon-spin spectroscopy measurements on LiFeAs single crystals and interpreting the resulting asymmetry and relaxation signals as evidence for an unconventional vortex lattice consisting of stripes of coreless half-quantum vortex pairs. No mathematical derivation, first-principles calculation, or sequence of equations is presented whose output reduces to its inputs by construction. There are no fitted parameters renamed as predictions, no self-definitional relations, and no load-bearing self-citations that close a loop; the central claim rests on comparison of observed field distributions to model lineshapes, which is externally falsifiable against standard London-model predictions and does not exhibit any of the enumerated circularity patterns.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 1 invented entities

Only the abstract is available, so the ledger is necessarily incomplete; no explicit free parameters or derivations are shown.

axioms (1)
  • domain assumption Standard type-II superconductivity allows magnetic flux penetration via quantized vortices.
    Invoked in the first sentence to contrast with the unconventional case.
invented entities (1)
  • coreless vortices no independent evidence
    purpose: To describe the observed unconventional mixed state as stripes of bound half-quantum vortex pairs.
    Introduced to interpret the muon spectroscopy data; no independent falsifiable prediction outside the spectroscopy is given in the abstract.

pith-pipeline@v0.9.0 · 5396 in / 1282 out tokens · 27269 ms · 2026-05-08T06:50:26.120559+00:00 · methodology

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Reference graph

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