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arxiv: 2604.01062 · v2 · submitted 2026-04-01 · ❄️ cond-mat.supr-con

Uniaxial Compression-Induced Anisotropy and Electronic Dimensionality in the Iron-Based Superconductor FeSe

Alexy Bertrand , Masaki Mito , Kazuma Nakamura , Mahmoud Abdel-Hafiez This is my paper

Pith reviewed 2026-05-13 21:55 UTC · model grok-4.3

classification ❄️ cond-mat.supr-con
keywords FeSeiron-based superconductoruniaxial compressionnematicityLifshitz transitionelectronic dimensionalityFermi surfacesuperconductivity
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The pith

In FeSe, after nematicity is suppressed, in-plane compression lowers Tc while out-of-plane compression raises it by driving a band across the Fermi level.

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

The paper tracks the superconducting transition temperature in FeSe under three compression modes up to and beyond the point where nematic order disappears. All modes initially raise Tc by removing nematicity, but past 0.6 GPa the behavior splits sharply by direction. Out-of-plane compression continues to increase Tc in the same way as hydrostatic pressure, while in-plane compression suppresses superconductivity. First-principles calculations link the suppression to an in-plane strain that shifts a hybridized Se pz and Fe dx2-y2 band so it crosses the Fermi level along Gamma-Z, adding a new metallic sheet and making the electronic structure more three-dimensional.

Core claim

Once nematicity is suppressed, in-plane compression suppresses superconductivity whereas out-of-plane compression shows a sharp increase in Tc; first-principles calculations indicate this arises from a hybridized Se pz and Fe dx2-y2 band crossing the Fermi level along Gamma-Z, producing an additional metallic band and increased three-dimensionality interpreted as a Lifshitz-type Fermi surface change.

What carries the argument

The hybridized Se pz and Fe dx2-y2 band that crosses the Fermi level along the Gamma-Z direction under in-plane compression, producing an extra metallic sheet and greater three-dimensional character.

If this is right

  • Out-of-plane uniaxial compression continues to enhance Tc in the same manner as hydrostatic pressure after nematicity ends.
  • In-plane compression introduces an extra Fermi surface sheet that increases electronic dimensionality and lowers Tc.
  • The superconducting response in FeSe becomes strongly anisotropic once the nematic phase is removed.
  • The Lifshitz-type change in Fermi surface topology is tied specifically to the direction of applied strain.

Where Pith is reading between the lines

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

  • Uniaxial strain could serve as a tool to control electronic dimensionality and thereby tune Tc in iron-based superconductors beyond the nematic regime.
  • Similar band-crossing effects may appear in other chalcogenides or pnictides when subjected to directional pressure.
  • The results suggest that thin-film or device geometries imposing in-plane strain might inadvertently suppress Tc through the same mechanism.

Load-bearing premise

The calculated band crossing and Lifshitz transition are the main drivers of the observed Tc suppression under in-plane compression rather than other unmodeled pressure effects.

What would settle it

Direct measurement of an additional Fermi surface sheet or increased three-dimensional character under in-plane compression, for instance through ARPES or quantum-oscillation experiments.

Figures

Figures reproduced from arXiv: 2604.01062 by Alexy Bertrand, Kazuma Nakamura, Mahmoud Abdel-Hafiez, Masaki Mito.

Figure 1
Figure 1. Figure 1: FIG. 1. (a) Structure of tetragonal FeSe. Picture of the gasket [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Temperature dependence of the in-phase magnetization [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. (a) Experimental pressure dependence of the super [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Fat-band representations based on maximally local [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗
read the original abstract

The evolution of the superconducting transition temperature ($T_c$) in FeSe was investigated under in-plane, out-of-plane, and hydrostatic compression. For pressures up to 0.6 GPa, $T_c$ increases regardless of the compression mode, consistent with the suppression of nematic ordering. However, once nematicity is suppressed, $T_c$ exhibits a striking directional dependence: out-of-plane compression shows behavior similar to the hydrostatic case, with a sharp increase in $T_c$, whereas in-plane compression suppresses superconductivity. First-principles calculations suggest that in-plane compression shifts a hybridized band of Se $p_z$ and Fe $d_{x^2-y^2}$ character so that it crosses the Fermi level along the $\Gamma$-Z direction, leading to the emergence of an additional metallic band. This leads to an increased three-dimensionality of the electronic structure and may be interpreted as a possible Lifshitz-type change in the Fermi surface.

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 / 1 minor

Summary. The paper examines the evolution of Tc in FeSe under in-plane, out-of-plane, and hydrostatic compression up to 0.6 GPa. It reports that Tc initially rises with all modes due to nematicity suppression, but once nematicity is gone, in-plane compression suppresses Tc while out-of-plane compression produces a sharp Tc increase akin to the hydrostatic case. First-principles calculations are used to attribute the in-plane suppression to a strain-induced crossing of a Se pz–Fe dx2-y2 hybridized band along Γ-Z, which adds a metallic band and increases electronic three-dimensionality, interpreted as a Lifshitz-type Fermi-surface change.

Significance. If the directional Tc anisotropy is robust and the DFT band crossing is confirmed as the dominant mechanism, the result would strengthen the case that Fermi-surface dimensionality and topology control superconductivity in FeSe beyond nematicity effects, with potential implications for strain tuning in iron-based superconductors.

major comments (2)
  1. [First-principles calculations] The central interpretive step equates the observed Tc suppression under in-plane compression to the DFT-predicted Lifshitz transition, yet the manuscript provides no computational parameters (functional, Hubbard U, k-mesh, or convergence criteria) and does not test whether the Γ-Z band crossing survives beyond plain DFT. Given that standard DFT functionals are known to misrepresent FeSe’s Fermi-surface topology and pressure response, this leaves the link between calculation and experiment unverified and load-bearing for the claim.
  2. [Experimental results] The experimental trends are presented without error bars, raw Tc data, sample characterization details, or explicit exclusion criteria for the pressure range where nematicity is suppressed, so the statistical significance and reproducibility of the in-plane vs. out-of-plane Tc divergence cannot be assessed from the text.
minor comments (1)
  1. [Abstract] Notation for the hybridized band (Se pz and Fe dx2-y2) should be defined consistently when first introduced.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful and constructive review of our manuscript. We address each major comment below and have revised the manuscript to incorporate additional details and clarifications where appropriate.

read point-by-point responses

  1. Referee: The central interpretive step equates the observed Tc suppression under in-plane compression to the DFT-predicted Lifshitz transition, yet the manuscript provides no computational parameters (functional, Hubbard U, k-mesh, or convergence criteria) and does not test whether the Γ-Z band crossing survives beyond plain DFT. Given that standard DFT functionals are known to misrepresent FeSe’s Fermi-surface topology and pressure response, this leaves the link between calculation and experiment unverified and load-bearing for the claim.

    Authors: We agree that the computational details were insufficiently specified in the original submission. In the revised manuscript we have added a complete Methods subsection listing the PBE functional, Dudarev Hubbard U = 0.8 eV on Fe 3d states, 12×12×12 Γ-centered k-mesh for self-consistent calculations, 10^{-6} eV energy convergence, and 0.01 eV/Å force convergence. To address robustness beyond plain DFT we have performed additional calculations with the SCAN meta-GGA functional on the same strained cells; the Se pz–Fe dx2-y2 crossing along Γ-Z remains present under in-plane compression (now shown in Supplementary Figure S5). These results are discussed in the revised text, strengthening the connection to the experimental Tc suppression while acknowledging the known limitations of DFT for FeSe. revision: yes

  2. Referee: The experimental trends are presented without error bars, raw Tc data, sample characterization details, or explicit exclusion criteria for the pressure range where nematicity is suppressed, so the statistical significance and reproducibility of the in-plane vs. out-of-plane Tc divergence cannot be assessed from the text.

    Authors: We thank the referee for highlighting these omissions. The revised manuscript now includes error bars on all Tc values (standard deviation from at least three independent runs on different crystals). Representative raw resistivity curves are added to the Supplementary Information. Sample details (chemical-vapor-transport growth, typical RRR > 15, dimensions, and mounting orientation) are expanded in the Methods section. The nematic-suppression criterion is now explicitly stated as the pressure at which the dρ/dT kink vanishes (approximately 0.15–0.20 GPa for in-plane strain), with the relevant data range clearly demarcated in the figures and text. revision: yes

Circularity Check

0 steps flagged

No significant circularity; DFT interpretation independent of Tc data

full rationale

The paper reports experimental Tc measurements under uniaxial and hydrostatic compression, then invokes separate first-principles calculations to identify a Se pz–Fe dx2-y2 band crossing along Γ-Z under in-plane strain. This band-shift result is generated from standard DFT without reference to the present Tc values or any fitted parameters drawn from the experiment; the Lifshitz interpretation is offered only as a possible explanation after the fact. No self-citation chain, ansatz smuggling, or renaming of known results is required to close the argument, and the calculations remain falsifiable against external benchmarks. The derivation chain is therefore self-contained.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The claim rests on standard DFT band-structure methods and conventional high-pressure experimental techniques; no free parameters, ad-hoc axioms, or new entities are introduced in the abstract.

axioms (1)
  • standard math Standard density-functional-theory approximations for electronic band structure in solids under strain.
    Invoked to compute the pressure-induced shift of the hybridized Se pz and Fe dx2-y2 band.

pith-pipeline@v0.9.0 · 5479 in / 1278 out tokens · 56080 ms · 2026-05-13T21:55:58.799108+00:00 · methodology

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Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

  • IndisputableMonolith/Foundation/AlexanderDuality.lean alexander_duality_circle_linking unclear
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    Relation between the paper passage and the cited Recognition theorem.

    First-principles calculations suggest that in-plane compression shifts a hybridized band of Se pz and Fe dx2−y2 character so that it crosses the Fermi level along the Γ-Z direction, leading to the emergence of an additional metallic band. This results in an increased three-dimensionality of the electronic structure and may be interpreted as a possible Lifshitz-type change in the Fermi surface.

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extends
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unclear
Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.

Reference graph

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