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arxiv: 2605.11955 · v1 · submitted 2026-05-12 · ❄️ cond-mat.supr-con · cond-mat.str-el

Recognition: 2 theorem links

· Lean Theorem

Nematicity in LaFeAsO single crystals studied by elastoresistance, high-resolution thermal expansion and shear-modulus measurements

A. U. B. Wolter, B. B\"uchner, C. Hess, F. Caglieris, F. Scaravaggi, L. Wang, R. Kappenberger, R. Klingeler, S. Aswartham, S. Sauerland, S. Sykora, S. Wurmehl, X. C. Hong

Pith reviewed 2026-05-13 04:07 UTC · model grok-4.3

classification ❄️ cond-mat.supr-con cond-mat.str-el
keywords nematicityLaFeAsOelastoresistivityshear modulusWeiss temperatureiron pnictidesstructural transitionelectronic nematicity
0
0 comments X

The pith

LaFeAsO shows mismatched Weiss temperatures for nematic susceptibilities from elastoresistivity and shear modulus.

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

The paper examines nematicity in LaFeAsO single crystals using high-resolution thermal expansion, shear-modulus softening, and elastoresistivity. It reports Curie-Weiss-like divergence in both the shear-modulus-derived susceptibility χ^sh and the elastoresistivity-derived susceptibility χ^er, which is taken as evidence for an electronic origin of the nematic order. The characteristic energy scale of lattice-electronic coupling is extracted as approximately 30 K. Direct comparison to BaFe2As2, however, reveals that the Weiss temperatures of χ^er and χ^sh differ markedly in LaFeAsO while they track each other in BaFe2As2, in disagreement with the matching behavior expected from Landau theory.

Core claim

In LaFeAsO the shear modulus C66 softens toward the structural transition at Ts. Both χ^sh and χ^er display similar Curie-Weiss divergence, yet their Weiss temperatures are significantly different. The same measurements on BaFe2As2 yield matching divergences, as required by the underlying Landau theory that links resistivity anisotropy to uncoupled electronic nematicity. The lattice-electronic coupling energy is deduced to be ~30 K.

What carries the argument

The side-by-side comparison of nematic susceptibilities χ^sh (extracted from shear-modulus data) and χ^er (extracted from elastoresistivity data) that tests whether resistivity anisotropy tracks the same electronic nematicity in both compounds.

If this is right

  • The structural transition at Ts is driven by electronic nematic degrees of freedom in LaFeAsO.
  • The energy scale of coupling between the lattice and the electronic nematic order is ~30 K.
  • The direct mapping from resistivity anisotropy to electronic nematic susceptibility assumed in standard theories does not hold for LaFeAsO.
  • The shear-modulus temperature dependence remains similar across the 1111 and 122 families even though the resistivity response differs.

Where Pith is reading between the lines

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

  • The mismatch may reflect compound-specific scattering channels that affect resistivity but not the elastic response.
  • If the same discrepancy appears in other 1111 compounds, models of nematicity will need separate treatments for the two structural families.
  • Doping studies that track both susceptibilities through the superconducting dome could reveal whether the mismatch correlates with Tc.

Load-bearing premise

That elastoresistivity and shear-modulus data both read out the same underlying electronic nematic susceptibility without extra, material-specific contributions from lattice coupling or scattering that are stronger in LaFeAsO than in BaFe2As2.

What would settle it

New measurements that find the Weiss temperature of χ^er to equal that of χ^sh in LaFeAsO would remove the reported mismatch and restore consistency with standard Landau-theory expectations.

Figures

Figures reproduced from arXiv: 2605.11955 by A. U. B. Wolter, B. B\"uchner, C. Hess, F. Caglieris, F. Scaravaggi, L. Wang, R. Kappenberger, R. Klingeler, S. Aswartham, S. Sauerland, S. Sykora, S. Wurmehl, X. C. Hong.

Figure 1
Figure 1. Figure 1: FIG. 1. (a) Relative length changes ∆ [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Temperature dependence of the nematic susceptibility measured by elastroresistance [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Temperature dependence of the inverse purely electronic nematic susceptibilities ( [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
read the original abstract

Nematicity in LaFeAsO single crystals is studied by means of high-resolution thermal expansion, shear modulus, and elastoresistivity measurements. A softening of the shear modulus $C_{\rm 66}$ towards the structural phase transition at $T_{\rm S}$ is observed. In addition, a similar Curie-Weiss-like divergence of the nematic susceptibilities is found in the temperature dependence of both $\chi^{sh}$ and $\chi^{er}$, which are deduced from the shear modulus (sh) and the elastoresistivity (er) studies, respectively. These observations provide evidence for an electronic origin of nematicity in LaFeAsO. The characteristic energy of the coupling between the lattice and the electronic degrees of freedom is deduced to $\sim$30~K. The comparison to corresponding measurements on BaFe$_2$As$_2$ single crystals reveals a very similar temperature dependence of the shear modulus but yields contrasting results for $\chi^{er}$ : In BaFe$_2$As$_2$, $\chi^{er}$ diverges similarly as the uncoupled nematicity deduced from the shear modulus data as it is expected from the underlying Landau theory. In contrast, the Weiss temperatures of $\chi^{er}$ and $\chi^{sh}$ are significantly different in LaFeAsO. This difference is at odds with the commonly anticipated theories of resistivity anisotropy and electronic nematicity in iron pnictides.

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 elastoresistivity, high-resolution thermal expansion, and shear-modulus measurements on LaFeAsO single crystals. It observes softening of C66 toward Ts, Curie-Weiss-like divergences in both the shear-modulus-derived nematic susceptibility χ^sh and the elastoresistivity-derived χ^er, and extracts a lattice-electronic coupling energy scale of ~30 K. The central claim is that χ^er and χ^sh exhibit similar temperature dependence in BaFe2As2 (consistent with Landau theory) but show significantly different Weiss temperatures in LaFeAsO, providing evidence for an electronic origin of nematicity while challenging common theories of resistivity anisotropy in iron pnictides.

Significance. If the reported mismatch in Weiss temperatures is shown to be intrinsic rather than arising from unaccounted material-specific terms, the work would demonstrate that nematic susceptibility is not universal across pnictide families and would supply a quantitative coupling benchmark (~30 K) for theory. The direct comparison to prior BaFe2As2 data strengthens the contrast but requires the assumption that both probes access the same bare electronic susceptibility.

major comments (3)
  1. [Methods and analysis] The headline claim that the Weiss temperatures of χ^er and χ^sh differ significantly (and thereby challenge standard models) rests on the untested assumption that m66 in LaFeAsO receives no additive contributions from anisotropic quasiparticle scattering or orbital-selective effects that are absent or negligible in BaFe2As2. The methods and analysis sections provide no explicit decomposition or control experiment isolating these terms, which directly undermines the interpretation of the mismatch.
  2. [Results and abstract] No error bars, uncertainties on the fitted Weiss temperatures, or quantitative values for those temperatures are reported in the abstract or main text, nor is sample characterization (e.g., residual resistivity ratio, homogeneity) provided. This prevents assessment of whether the claimed difference exceeds the ~30 K coupling scale or experimental resolution.
  3. [Shear-modulus analysis] The deduction of the ~30 K coupling energy from C66 softening is presented without the explicit fitting equation or the functional form used to separate lattice and electronic contributions; this step is load-bearing for the quantitative claim and for the subsequent comparison to Landau theory.
minor comments (1)
  1. [Introduction] The notation χ^sh and χ^er is introduced without an early, self-contained definition of how each is extracted from the raw data (e.g., the precise relation between m66 and χ^er).

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the constructive comments. We address each major point below and indicate where revisions will be made to improve clarity and rigor.

read point-by-point responses

  1. Referee: [Methods and analysis] The headline claim that the Weiss temperatures of χ^er and χ^sh differ significantly (and thereby challenge standard models) rests on the untested assumption that m66 in LaFeAsO receives no additive contributions from anisotropic quasiparticle scattering or orbital-selective effects that are absent or negligible in BaFe2As2. The methods and analysis sections provide no explicit decomposition or control experiment isolating these terms, which directly undermines the interpretation of the mismatch.

    Authors: We agree that the interpretation of the differing Weiss temperatures assumes m66 primarily probes the nematic susceptibility without dominant material-specific additive terms. Our analysis follows the standard procedure established in the BaFe2As2 literature, where χ^er and χ^sh align as expected from Landau theory; the contrast in LaFeAsO is therefore meaningful within that framework. However, we acknowledge the value of an explicit discussion of possible contributions from anisotropic scattering or orbital selectivity. In the revised manuscript we will add a paragraph in the discussion section addressing these effects and arguing, based on the similar C66 softening in both compounds, that they do not account for the observed mismatch. revision: partial

  2. Referee: [Results and abstract] No error bars, uncertainties on the fitted Weiss temperatures, or quantitative values for those temperatures are reported in the abstract or main text, nor is sample characterization (e.g., residual resistivity ratio, homogeneity) provided. This prevents assessment of whether the claimed difference exceeds the ~30 K coupling scale or experimental resolution.

    Authors: We accept that quantitative Weiss temperatures, their uncertainties, error bars on the fits, and basic sample characterization (RRR, homogeneity) are necessary for readers to judge the significance of the difference. These details are present in the supplementary information and raw data but were not highlighted in the main text or abstract. We will revise the abstract and results section to report the fitted values with uncertainties and add a brief sample-characterization paragraph. revision: yes

  3. Referee: [Shear-modulus analysis] The deduction of the ~30 K coupling energy from C66 softening is presented without the explicit fitting equation or the functional form used to separate lattice and electronic contributions; this step is load-bearing for the quantitative claim and for the subsequent comparison to Landau theory.

    Authors: We agree that the fitting procedure should be stated explicitly. The ~30 K scale is obtained by subtracting a temperature-independent lattice background from the measured C66(T) and fitting the residual softening to a Curie-Weiss form multiplied by the electron-lattice coupling constant λ. In the revised manuscript we will insert the explicit functional form and fitting equation in the methods or results section so that the separation of lattice and electronic contributions is fully transparent. revision: yes

Circularity Check

0 steps flagged

No circularity; purely experimental measurements with independent external comparisons

full rationale

This is an experimental study reporting direct measurements of shear modulus softening, thermal expansion, and elastoresistivity on LaFeAsO. The central observations (Curie-Weiss divergences in χ^sh and χ^er, their Weiss temperature mismatch with BaFe2As2 data, and ~30 K coupling scale) are extracted from raw data fits and compared to independent prior measurements on another material plus external Landau theory. No derivation chain exists that reduces any result to a fitted input or self-citation by construction. No self-definitional steps, fitted predictions, or load-bearing self-citations are present. The paper is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The analysis assumes standard Curie-Weiss form for nematic susceptibility and the applicability of Landau theory for the comparison to BaFe2As2; the ~30 K coupling energy is extracted by fitting the observed divergence.

free parameters (1)
  • coupling energy scale = ~30 K
    Deduced from the temperature dependence of the nematic susceptibilities to be ~30 K.
axioms (2)
  • domain assumption Nematic susceptibility follows Curie-Weiss divergence near Ts
    Invoked to interpret the observed softening and divergence in both χ^sh and χ^er.
  • domain assumption Landau theory predicts matching Weiss temperatures for coupled probes
    Used as the benchmark for the expected behavior in BaFe2As2 and the discrepancy in LaFeAsO.

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