# Iron-based superconductors: families and evidence

A multiorbital materials guide that separates parent compounds, doped superconductors, interface spectra and candidate gap structures.

## Scope

Iron-based superconductors contain Fe–pnictogen or Fe–chalcogen layers in several structural families. Multiple Fe orbitals contribute to their low-energy electronic structure, and parent magnetic or structural transitions must be distinguished from superconductivity in a substituted sample. Kamihara and colleagues' fluorine-doped LaOFeAs discovery established a high-temperature 1111 platform. Its original abstract reports a transition near 26 K; the exact operational criterion remains unassigned in this review. [R1](#ref-source-62a72493865c59d7)

A 122 arsenide, a binary 11 chalcogenide and a single-layer interface are different experimental systems. Rotter and colleagues studied K-substituted BaFe2As2, while Hsu and colleagues reported superconductivity in PbO-type FeSe. Neither result establishes that every related composition has the same gap, transition temperature or magnetic ground state. Starting mixture, refined phase composition and nominal formula should remain separately identifiable. [R2](#ref-source-1a3690e5f1d38f3e) [R3](#ref-source-205024adeb431322)

## Mathematical assumptions

The coefficient $y$ in $\mathrm{BaFe}_{2-y}\mathrm{Co}_y\mathrm{As}_2$ and the site fraction $x$ in $\mathrm{Ba}(\mathrm{Fe}_{1-x}\mathrm{Co}_x)_2\mathrm{As}_2$ obey $y=2x$. Eleven exact atom-count checks verify this normalization. Likewise, nominal $\mathrm{FeSe}_{1-x}$ becomes $\mathrm{Fe}_{1+\delta}\mathrm{Se}$ with $\delta=x/(1-x)$ when normalized to one Se. Five checks preserve the elemental ratio. These conversions do not measure crystal occupancy, mobile-carrier density or the location of a defect. [R7](#ref-source-7bc9538ec7783cf2)

Bulk fractions also have a precise but limited meaning. For superconducting and magnetic fractions $f_S$ and $f_M$ of the same volume, their overlap lies between $\max(0,f_S+f_M-1)$ and $\min(f_S,f_M)$. Two explicit arrangements with 95% and 90% fractions give 85% or 90% overlap. This set-theory bound neither establishes atomic-scale coexistence nor removes probe calibration and sample-matching requirements. A microscopic interpretation must use additional spatial, temporal and model evidence. [R8](#ref-source-c817382f57bbb81d)

A multiband gap $\Delta_i(\mathbf{k})$ is defined on specified bands. Mazin and colleagues' LaFeAsO1−xFx model connects hole and electron pockets by repulsive singlet pair transfer; its leading eigenvector has opposite signs. This is a material-specific proposal, not a consequence of merely containing iron. Orbital weights and accessible momentum transfers enter the response, as the separate Maier–Scalapino two-orbital calculation makes explicit.

Opposite signs do not require equal gap magnitudes. In a local near-transition illustration with positive constant one-spin densities of states $N_h,N_e$, a shared cutoff, reciprocal interband interaction $V>0$ and zero intraband terms, use $\lambda_{ij}=-V_{ij}N_j$. The leading mode has $\lambda_+=V\sqrt{N_hN_e}$ and $\Delta_h/\Delta_e=-\sqrt{N_e/N_h}$. In synthetic normalized units, $N_h=1$, $N_e=4$ and $V=0.1$ give $\lambda_+=0.2$ and a gap vector proportional to $(2,-1)$. Six exact rational checks and six attractive-interband controls verify the eigenvectors and density-of-states transformation. This is a linearized model example, not a fitted iron-based gap ratio or a zero-temperature prediction. Band edges inside the pairing window, energy-dependent densities of states and unequal cutoffs require a more general kernel. The [multiband](/scwiki/multiband-pairing) and [s±](/scwiki/s-plus-minus) pages give the broader assumptions and probe limits. [R6](#ref-source-7c2b55d128348e6c) [R11](#ref-source-5717dacf7df85626)

## Experimental criteria

The revised Rotter author manuscript gives a resistive midpoint of 38.1 K for Ba0.6K0.4Fe2As2, a first deviation near 39 K and a reported zero-resistance point of 37.2 K. Figure 4 also labels 37.2 K as its 10% threshold, so the term zero retains an unresolved instrumental-threshold caveat. Its 90–10% width is 1.5 K. Quoting only the rounded 38 K headline loses distinctions that matter in a searchable record. [R2](#ref-source-1a3690e5f1d38f3e)

The same revision measures magnetic cycles at 0.5 and 1 mT, replacing the earlier manuscript's 10 mT example. Diamagnetism begins near 38.3 K; shielding and field-cooled responses differ and depend on density and demagnetization assumptions. The powder contains a minor FeAs impurity. Selected revised-manuscript figures have been checked, but the final published methods and raw data have not been completely compared here. [R2](#ref-source-1a3690e5f1d38f3e)

Hsu's final FeSe report identifies zero resistance near 8 K in a sample labelled by nominal FeSe0.88 composition. Magnetic susceptibility at 30 G and a broad heat-capacity anomaly provide complementary evidence. Its field-dependent transition uses 50% of normal resistance, with applied fields through 9 T; the roughly 16.3 T upper-critical-field value is a fit extrapolation. Nominal selenium deficiency and a fitted critical field must not be promoted to directly measured phase occupancy or field limits. [R3](#ref-source-205024adeb431322)

McQueen and colleagues subsequently found nearly stoichiometric superconducting FeSe and strong preparation sensitivity. Their selected neutron-refinement analysis gives Fe1.01±0.02Se, with limited ability to distinguish tiny deviations. Nominal composition, oxygen contamination and annealing temperature vary across the comparison. A sample labelled Fe1.03Se showed no superconductivity down to 0.6 K; that lower measurement bound is not a superconducting transition. The proposed defect-site explanation remains a hypothesis. [R7](#ref-source-7bc9538ec7783cf2)

## Pressure and magnetism

Medvedev's author manuscript associates a resistive onset of 36.7 K with 8.9 GPa in Fe1.01Se. Broadened transitions and curve offsets require careful interpretation; at 15 GPa the authors report residual resistance associated with a second phase. Their resistance method uses a diamond-anvil cell, platinum contacts, a van der Pauw arrangement and ruby pressure measurements at low and room temperature. The silicone-oil medium specified for the Mössbauer experiment must not automatically be assigned to the separate resistance experiment. [R4](#ref-source-a631bcd83d23567b)

The earlier Mössbauer nondetection of static order is not a universal FeSe phase diagram. Bendele and colleagues later reported pressure-induced static magnetism using zero-field muon-spin rotation and AC susceptibility in nominal FeSe0.94 and FeSe0.98 samples. Their selected revised manuscript specifies Daphne 7373, pressure-cell background treatment and low-temperature pressure calibration. Its Figure 3 supports a pressure-dependent coexistence interpretation near and above 0.8 GPa, with model-dependent spatial conclusions. Different probes, samples and pressure environments must remain explicit. [R4](#ref-source-a631bcd83d23567b) [R8](#ref-source-c817382f57bbb81d)

## Evidence for gap structure

Christianson's neutron experiment concerns K-doped Ba0.6K0.4Fe2As2. Its superconductivity-related resonance and the relevant coherence-factor model constrain candidate signs, without directly determining every iron-based gap. Background, momentum, orbital matrix elements and independently measured gap scales remain necessary parts of that interpretation. A resonance measurement cannot be substituted for a family-wide phase verdict. [R5](#ref-source-716c338ddfa726d3) [R6](#ref-source-7c2b55d128348e6c)

At a FeSe/SrTiO3 interface, Lee's replica bands and mean-field gap-closing scale retain their spectroscopic labels. Faeth's later photon-energy comparison supports an intrinsic electron–phonon interpretation within its stated analysis, but does not determine all contributions to pairing enhancement. A gap-closing temperature is not interchangeable with zero resistance or a magnetic transition. These interface results remain linked to the detailed [multiband](/scwiki/multiband-pairing) discussion rather than silently entering a bulk Tc comparison. [R9](#ref-source-f9fe6e9bc3792dcc) [R10](#ref-source-ac66116dfd6e4887)

## Limits and disputes

Several orbitals, correlations, nematicity and magnetic proximity help organize the evidence without selecting one universal pairing mechanism. A mass renormalization is not automatically a larger pairing eigenvalue. Complete scientific acceptance remains open, including final-version comparisons, full supplementary methods and raw-data analysis. Differences between a source's claim and this article's review scope are preserved in the references. The historical mechanism narrative also needs its published qualification: Mazin and colleagues' note 8 says that the magnetic instability they initially described as ferromagnetic was misidentified. Keep that qualification with the original abstract; it is not a universal statement that doping suppresses ferromagnetism in every iron-based superconductor. [R5](#ref-source-716c338ddfa726d3) [R6](#ref-source-7c2b55d128348e6c) [R7](#ref-source-7bc9538ec7783cf2) [R8](#ref-source-c817382f57bbb81d)

## Discovery implications

Test a specific model with controlled composition, structure, pressure or disorder changes, and identify the confounding changes. Preserve actual measurement criteria and sample provenance when comparing results. For a proposed coexistence regime, report what each probe resolves and how sample fractions were calibrated. For a candidate pairing mediator, explain which alternative survives the same observation. [R2](#ref-source-1a3690e5f1d38f3e) [R4](#ref-source-a631bcd83d23567b) [R8](#ref-source-c817382f57bbb81d)

## Related topics

Continue with [s± pairing](/scwiki/s-plus-minus), [multiband pairing](/scwiki/multiband-pairing), [magnetic substitution](/scwiki/magnetic-substitution-and-superconductivity), [nematic order](/scwiki/nematic-order), and [two-dimensional interfaces](/scwiki/two-dimensional-interfaces).

## Material cases

Values describe individual reports, not a ranking. Unknown means the checked source did not supply the field at this review scope.

| Material and sample | Reported temperature | Criterion | Probe | Pressure | State | Evidence |
|---|---|---|---|---|---|---|
| LaFeAsO1-xFx (aliases: iron-based, iron based superconductors); F-doped 1111 material | 26 K; approximate reported transition | transition; unknown | discovery report | ambient; unknown GPa | equilibrium | experiment; [source](#ref-source-62a72493865c59d7) |
| Ba0.6K0.4Fe2As2 (aliases: iron-based, iron based superconductors); Rotter2008 revised authorv2 K-substituted122 polycrystal | 38.1 K; 38.1K midpoint in authorv2;90–10%width1.5K,firstdeviation~39K. Complete final comparison open. | transition; resistive-midpoint | Four-probe resistance | ambient; unknown GPa | equilibrium | experiment; [source](#ref-source-1a3690e5f1d38f3e) |
| Fe1.01Se (aliases: FeSe, iron selenide, iron-based, iron based superconductors); pressurized beta-Fe1.01Se | 36.7 K; 36.7K onset at8.9GPa in selected authorv1; broad transition,not assigned zeroresistance. | transition; resistive-onset | DAC van der Pauw resistance; ruby pressure at low androomtemperature | high; 8.9 to 8.9 GPa | unknown | experiment; [source](#ref-source-a631bcd83d23567b) |
| Ba0.6K0.4Fe2As2 (aliases: iron-based, iron based superconductors); Rotter2008 revised authorv2 K-substituted122 polycrystal | 37.2 K; Authorv2 prose calls37.2K zero;Figure4also labelsT10=37.2K. Instrumental threshold caveat retained. | transition; reported-zero-resistance | Four-probe resistance | ambient; unknown GPa | equilibrium | experiment; [source](#ref-source-1a3690e5f1d38f3e) |
| FeSe0.88 (aliases: FeSe, iron selenide, Hsu FeSe, iron-based); Hsu2008 nominal starting composition; superconducting phase occupancy not inferred | 8 K; Approximatezeroresistance; nominalformula notactualSevacancy measurement. | transition; zero-resistance | Four-probe resistance, silverpastecontacts | ambient; unknown GPa | unknown | experiment; [source](#ref-source-205024adeb431322) |
| Fe1.03Se (aliases: FeSe, iron selenide, McQueen Fe1.03Se, iron-based); McQueen nominal Fe1.03Se: no superconductivity down to0.6K | 0.6 K; Lower temperature bound of nondetection,notTc. | measurement; superconductivity-nondetection-bound | Low-temperature sample characterization; see selected source scope | ambient; unknown GPa | unknown | experiment; [source](#ref-source-7bc9538ec7783cf2) |
| FeSe/SrTiO3 (aliases: FeSe STO, single-layer FeSe); Lee one-unit-cell film, author v6 Figure 2 mean-field fit | 58 K; 58±7 K ARPES gap-closing fit, not a zero-resistance transition. | spectroscopic-gap; ARPES-mean-field-gap-closure | Temperature-dependent ARPES | ambient; unknown GPa | unknown | experiment; [source](#ref-source-f9fe6e9bc3792dcc) |

## Sources

### Ref source-62a72493865c59d7

[R1] Iron-Based Layered Superconductor La[O1−xFx]FeAs (x = 0.05–0.12) with Tc = 26 K (2008). [DOI 10.1021/ja800073m](https://doi.org/10.1021/ja800073m). Publication: journal-article. Check: publisher-original-abstract-and-date. Checked: 2026-10-19. Publisher original abstract indexed: online23February2008,issue19March; F substitution and approximate26K. Full methods require access; institutional copy timedout25s. No exact operational criterion assigned.
### Ref source-1a3690e5f1d38f3e

[R2] Superconductivity at 38 K in the Iron Arsenide(Ba1−xKx)Fe2As2 (2008). [DOI 10.1103/PhysRevLett.101.107006](https://doi.org/10.1103/PhysRevLett.101.107006). Publication: journal-article. Check: selected-revised-author-transition-and-figure-check. Checked: 2026-10-19. v1 submitted29May2008;v2 revised17July2008; journal5September2008. Selected v2pp2–3/p3Figures3–5 visually checked against v1 text. 38.1K resistive midpoint,37.2K reportedzero,~39K firstdeviation; magnetic data0.5/1mT rather than v1 10mT. Figure4 labelsT10=37.2K,also calledzero in prose: retain threshold caveat. Publisher finalPDF401; no final/raw comparison.
### Ref source-205024adeb431322

[R3] Superconductivity in the PbO-type structure α-FeSe (2008). [DOI 10.1073/pnas.0807325105](https://doi.org/10.1073/pnas.0807325105). Publication: journal-article. Check: selected-final-transport-heat-and-methods-check. Checked: 2026-10-19. Final PNASEarlyEdition3pages,selectedpp1–3 andp2Figures2–4 visually checked. NominalFeSe0.88 sample,8Kzero/magnetic onset,30Gmagnetism,50%Rnfieldcriterion up9T,fittedHc2~16.3T. Nominal starting ratios are not measured siteoccupancy. Full raw fit/phase refinement not reproduced; later stoichiometry paper separately cited.
### Ref source-a631bcd83d23567b

[R4] Electronic and magnetic phase diagram of β-Fe1.01Se with superconductivity at 36.7 K under pressure (2009). [DOI 10.1038/nmat2491](https://doi.org/10.1038/nmat2491). Publication: journal-article. Check: selected-author-pressure-methods-and-figure-check. Checked: 2026-10-19. Authorv1 submitted12March2009,relatednmat2491. Selectedpp3–6results,pp12–13methods,p15Figure2 visually:36.7Konset at8.9GPa,broad transitions,offsetzero lines; resistanceDACvanderPauw/ruby. Silicone oil is specified for Mossbauer,not automatically resistance medium. No full final comparison/raw curves; magnetic nondetection remains source-specific.
### Ref source-716c338ddfa726d3

[R5] Unconventional superconductivity in Ba0.6K0.4Fe2As2 from inelastic neutron scattering (2008). [DOI 10.1038/nature07625](https://doi.org/10.1038/nature07625). Publication: journal-article. Check: selected-final-neutron-methods-and-figures-check. Checked: 2026-10-19. Final printed pp. 930–931 selected methods/results; p. 931 Figures 2–4 visually checked. Polycrystalline K-doped sample, MERLIN energies, vanadium normalization, 7 K/50 K and integration windows distinguished. Resonance excess fades near the transition; finite background remains. Full synthesis supplement, raw detector counts and a unique phase assignment are not independently verified.
### Ref source-7c2b55d128348e6c

[R6] Unconventional Superconductivity with a Sign Reversal in the Order Parameter of LaFeAsO1−xFx (2008). [DOI 10.1103/PhysRevLett.101.057003](https://doi.org/10.1103/PhysRevLett.101.057003). Publication: journal-article. Check: selected-final-pairing-kernel-and-qualification-check. Checked: 2026-10-19. Published29July2008; selected final pp057003-1–4: x=.1 calculated pockets, singlet coupling matrix/eigenvector, susceptibility approximation and note8 qualification of originally misidentified ferromagnetic instability. arXivv4 identification also checked; typeset2018 not publication year. Full DFT/RPA and gap amplitudes not reproduced. Publisher PDF text read; screenshot request failed.
### Ref source-7bc9538ec7783cf2

[R7] Extreme sensitivity of superconductivity to stoichiometry in Fe1+δSe (2009). [DOI 10.1103/PhysRevB.79.014522](https://doi.org/10.1103/PhysRevB.79.014522). Publication: journal-article. Check: selected-author-composition-sensitivity-check. Checked: 2026-10-19. Journal29January2009; authorv1 selectedpp3–5 andp18Figure2 visually. Neutron refinednearFe1.01±0.02Se; preparation/oxygen andanneal temperatures influence transitions. Nominal Fe1.03Se has noSCdown0.6K,notTc=0.6K. No site-specific defect determination or full final/raw refinement.
### Ref source-c817382f57bbb81d

[R8] Pressure Induced Static Magnetic Order in Superconducting FeSe1−x (2010). [DOI 10.1103/PhysRevLett.104.087003](https://doi.org/10.1103/PhysRevLett.104.087003). Publication: journal-article. Check: selected-author-muon-methods-and-phase-diagram-check. Checked: 2026-10-19. Authorv2 revised2December2009,journal25February2010. Selectedpp1–3 andp3Figure3 visually: nominalFeSe0.94/.98,Daphne7373,piston-cylinder,lowTpressurePb/In,AC96Hz/~.1mT,cell-background-subtractedZFmuSR;staticorder near.8GPa. Spatial coexistence inference model-dependent,not settled solely byvolume fractions; no full final/raw fits.
### Ref source-f9fe6e9bc3792dcc

[R9] Interfacial mode coupling as the origin of the enhancement of Tc in FeSe films on SrTiO3 (2014). [DOI 10.1038/nature13894](https://doi.org/10.1038/nature13894). Publication: journal-article. Check: selected-author-results-and-figure-check. Checked: 2026-10-19. Nature publication 13 November 2014; identified author v6 revised 11 May 2015. Selected pp. 3–6, Figure 2 p. 14 visually checked and Figure 4 continuation p. 17 visually checked. Gap closing 58±7 K is a mean-field fit, not zero resistance; predicted enhancement assumes a magnetic baseline. Full supplementary growth/ARPES methods, final-version comparison and raw fitting remain open.
### Ref source-ac66116dfd6e4887

[R10] Interfacial Electron-Phonon Coupling Constants Extracted from Intrinsic Replica Bands in Monolayer FeSe/SrTiO3 (2021). [DOI 10.1103/PhysRevLett.127.016803](https://doi.org/10.1103/PhysRevLett.127.016803). Publication: journal-article. Check: selected-author-methods-and-figures-check. Checked: 2026-10-19. Author v1 submitted 5 February 2021; published 30 June 2021 per indexed final journal first page, not issue date 2 July. Selected pp. 2–4 methods/results; p. 3 Figures 2–3 visually checked. 15 K, photon energies 21–75 eV; photon-energy invariance, replica spacing and a forward-scattering fit. Final PDF download failed; supplements and full raw spectra/refits remain open.
### Ref source-5717dacf7df85626

[R11] Theory of neutron scattering as a probe of the superconducting gap in the iron pnictides (2008). [DOI 10.1103/PhysRevB.78.020514](https://doi.org/10.1103/PhysRevB.78.020514). Publication: journal-article. Check: selected-author-two-orbital-response-check. Checked: 2026-10-19. Journal published 31 July 2008. Author v1 submitted 3 May 2008; PDF typeset 2018 is not publication. Selected pp. 1–3, p. 2 equations visually checked. Orbital matrix elements, approximate velocities, singlet/triplet response distinguished. No full final-version or RPA numerical reproduction. D4 follow-up re-read author pp1–3 orbital matrix elements and RPA construction; no new numerical susceptibility reproduction.

## Claim evidence and conditions

### scwiki-d4-claim-scope-1

Iron-based superconductors contain Fe–pnictogen or Fe–chalcogen layers in several structural families. Multiple Fe orbitals contribute to their low-energy electronic structure, and parent magnetic or structural transitions must be distinguished from superconductivity in a substituted sample. Kamihara and colleagues' fluorine-doped LaOFeAs discovery established a high-temperature 1111 platform. Its original abstract reports a transition near 26 K; the exact operational criterion remains unassigned in this review.

Evidence: hypothesis. Sources: source-62a72493865c59d7.

Conditions: Source version, composition, probe, criterion, pressure environment and sample preparation are material-specific; gaps, phase, carriers and nominal chemistry are distinct.

Verification scope: Named selected primary passages/figures, original abstracts and exact notation/volume bounds; complete scientific acceptance remains open.

### scwiki-d4-claim-scope-2

A 122 arsenide, a binary 11 chalcogenide and a single-layer interface are different experimental systems. Rotter and colleagues studied K-substituted BaFe2As2, while Hsu and colleagues reported superconductivity in PbO-type FeSe. Neither result establishes that every related composition has the same gap, transition temperature or magnetic ground state. Starting mixture, refined phase composition and nominal formula should remain separately identifiable.

Evidence: hypothesis. Sources: source-1a3690e5f1d38f3e, source-205024adeb431322.

Conditions: Source version, composition, probe, criterion, pressure environment and sample preparation are material-specific; gaps, phase, carriers and nominal chemistry are distinct.

Verification scope: Named selected primary passages/figures, original abstracts and exact notation/volume bounds; complete scientific acceptance remains open.

### scwiki-d4-claim-mathematical-assumptions-1

The coefficient $y$ in $\mathrm{BaFe}_{2-y}\mathrm{Co}_y\mathrm{As}_2$ and the site fraction $x$ in $\mathrm{Ba}(\mathrm{Fe}_{1-x}\mathrm{Co}_x)_2\mathrm{As}_2$ obey $y=2x$. Eleven exact atom-count checks verify this normalization. Likewise, nominal $\mathrm{FeSe}_{1-x}$ becomes $\mathrm{Fe}_{1+\delta}\mathrm{Se}$ with $\delta=x/(1-x)$ when normalized to one Se. Five checks preserve the elemental ratio. These conversions do not measure crystal occupancy, mobile-carrier density or the location of a defect.

Evidence: theory. Sources: source-7bc9538ec7783cf2.

Source relationship: context-for-local-derivation. McQueen supplies the stoichiometry/occupancy context. Atom-count normalization identities and checks are local arithmetic, not refined occupancies or mobile-carrier measurements.

Conditions: Source version, composition, probe, criterion, pressure environment and sample preparation are material-specific; gaps, phase, carriers and nominal chemistry are distinct.

Verification scope: Named selected primary passages/figures, original abstracts and exact notation/volume bounds; complete scientific acceptance remains open.

### scwiki-d4-claim-mathematical-assumptions-2

Bulk fractions also have a precise but limited meaning. For superconducting and magnetic fractions $f_S$ and $f_M$ of the same volume, their overlap lies between $\max(0,f_S+f_M-1)$ and $\min(f_S,f_M)$. Two explicit arrangements with 95% and 90% fractions give 85% or 90% overlap. This set-theory bound neither establishes atomic-scale coexistence nor removes probe calibration and sample-matching requirements. A microscopic interpretation must use additional spatial, temporal and model evidence.

Evidence: theory. Sources: source-c817382f57bbb81d.

Source relationship: context-for-local-derivation. Bendele supplies experimental coexistence context. Set-theoretic overlap bounds and explicit volume arrangements are local reasoning, not microscopic coexistence measurements.

Conditions: Source version, composition, probe, criterion, pressure environment and sample preparation are material-specific; gaps, phase, carriers and nominal chemistry are distinct.

Verification scope: Named selected primary passages/figures, original abstracts and exact notation/volume bounds; complete scientific acceptance remains open.

### scwiki-d4-claim-mathematical-assumptions-3

A multiband gap $\Delta_i(\mathbf{k})$ is defined on specified bands. Mazin and colleagues' LaFeAsO1−xFx model connects hole and electron pockets by repulsive singlet pair transfer; its leading eigenvector has opposite signs. This is a material-specific proposal, not a consequence of merely containing iron. Orbital weights and accessible momentum transfers enter the response, as the separate Maier–Scalapino two-orbital calculation makes explicit.

Opposite signs do not require equal gap magnitudes. In a local near-transition illustration with positive constant one-spin densities of states $N_h,N_e$, a shared cutoff, reciprocal interband interaction $V>0$ and zero intraband terms, use $\lambda_{ij}=-V_{ij}N_j$. The leading mode has $\lambda_+=V\sqrt{N_hN_e}$ and $\Delta_h/\Delta_e=-\sqrt{N_e/N_h}$. In synthetic normalized units, $N_h=1$, $N_e=4$ and $V=0.1$ give $\lambda_+=0.2$ and a gap vector proportional to $(2,-1)$. Six exact rational checks and six attractive-interband controls verify the eigenvectors and density-of-states transformation. This is a linearized model example, not a fitted iron-based gap ratio or a zero-temperature prediction. Band edges inside the pairing window, energy-dependent densities of states and unequal cutoffs require a more general kernel. The [multiband](/scwiki/multiband-pairing) and [s±](/scwiki/s-plus-minus) pages give the broader assumptions and probe limits.

Evidence: theory. Sources: source-7c2b55d128348e6c, source-5717dacf7df85626.

Source relationship: mixed-source-and-local-derivation. Mazin supplies the LaFeAsO hole/electron-pocket sign-changing proposal and coupling-eigenvector framework; Maier supplies orbital/scattering context. The pure-interband unequal-DOS limit, chosen1:4DOS example and rational controls are local near-Tc derivations, not source-fitted material parameters.

Conditions: Near-Tc linearized real reciprocal two-band model; zero intraband terms, positive constant one-spin DOS, shared cutoff, interband repulsionV>0. Numerical values synthetic. No zero-temperature gap-ratio prediction.

Verification scope: Mazin final selected pp057003-1–4, including note8; Maier authorv1 selected pp1–3/Eqs13–17; exact local eigenvector/DOS checks. DFT, full RPA and material gap curves not reproduced.

### scwiki-d4-claim-experimental-criteria-1

The revised Rotter author manuscript gives a resistive midpoint of 38.1 K for Ba0.6K0.4Fe2As2, a first deviation near 39 K and a reported zero-resistance point of 37.2 K. Figure 4 also labels 37.2 K as its 10% threshold, so the term zero retains an unresolved instrumental-threshold caveat. Its 90–10% width is 1.5 K. Quoting only the rounded 38 K headline loses distinctions that matter in a searchable record.

Evidence: experiment. Sources: source-1a3690e5f1d38f3e.

Conditions: Source version, composition, probe, criterion, pressure environment and sample preparation are material-specific; gaps, phase, carriers and nominal chemistry are distinct.

Verification scope: Named selected primary passages/figures, original abstracts and exact notation/volume bounds; complete scientific acceptance remains open.

### scwiki-d4-claim-experimental-criteria-2

The same revision measures magnetic cycles at 0.5 and 1 mT, replacing the earlier manuscript's 10 mT example. Diamagnetism begins near 38.3 K; shielding and field-cooled responses differ and depend on density and demagnetization assumptions. The powder contains a minor FeAs impurity. Selected revised-manuscript figures have been checked, but the final published methods and raw data have not been completely compared here.

Evidence: experiment. Sources: source-1a3690e5f1d38f3e.

Conditions: Source version, composition, probe, criterion, pressure environment and sample preparation are material-specific; gaps, phase, carriers and nominal chemistry are distinct.

Verification scope: Named selected primary passages/figures, original abstracts and exact notation/volume bounds; complete scientific acceptance remains open.

### scwiki-d4-claim-experimental-criteria-3

Hsu's final FeSe report identifies zero resistance near 8 K in a sample labelled by nominal FeSe0.88 composition. Magnetic susceptibility at 30 G and a broad heat-capacity anomaly provide complementary evidence. Its field-dependent transition uses 50% of normal resistance, with applied fields through 9 T; the roughly 16.3 T upper-critical-field value is a fit extrapolation. Nominal selenium deficiency and a fitted critical field must not be promoted to directly measured phase occupancy or field limits.

Evidence: experiment. Sources: source-205024adeb431322.

Conditions: Source version, composition, probe, criterion, pressure environment and sample preparation are material-specific; gaps, phase, carriers and nominal chemistry are distinct.

Verification scope: Named selected primary passages/figures, original abstracts and exact notation/volume bounds; complete scientific acceptance remains open.

### scwiki-d4-claim-experimental-criteria-4

McQueen and colleagues subsequently found nearly stoichiometric superconducting FeSe and strong preparation sensitivity. Their selected neutron-refinement analysis gives Fe1.01±0.02Se, with limited ability to distinguish tiny deviations. Nominal composition, oxygen contamination and annealing temperature vary across the comparison. A sample labelled Fe1.03Se showed no superconductivity down to 0.6 K; that lower measurement bound is not a superconducting transition. The proposed defect-site explanation remains a hypothesis.

Evidence: experiment. Sources: source-7bc9538ec7783cf2.

Conditions: Source version, composition, probe, criterion, pressure environment and sample preparation are material-specific; gaps, phase, carriers and nominal chemistry are distinct.

Verification scope: Named selected primary passages/figures, original abstracts and exact notation/volume bounds; complete scientific acceptance remains open.

### scwiki-d4-claim-pressure-and-magnetism-1

Medvedev's author manuscript associates a resistive onset of 36.7 K with 8.9 GPa in Fe1.01Se. Broadened transitions and curve offsets require careful interpretation; at 15 GPa the authors report residual resistance associated with a second phase. Their resistance method uses a diamond-anvil cell, platinum contacts, a van der Pauw arrangement and ruby pressure measurements at low and room temperature. The silicone-oil medium specified for the Mössbauer experiment must not automatically be assigned to the separate resistance experiment.

Evidence: experiment. Sources: source-a631bcd83d23567b.

Conditions: Source version, composition, probe, criterion, pressure environment and sample preparation are material-specific; gaps, phase, carriers and nominal chemistry are distinct.

Verification scope: Named selected primary passages/figures, original abstracts and exact notation/volume bounds; complete scientific acceptance remains open.

### scwiki-d4-claim-pressure-and-magnetism-2

The earlier Mössbauer nondetection of static order is not a universal FeSe phase diagram. Bendele and colleagues later reported pressure-induced static magnetism using zero-field muon-spin rotation and AC susceptibility in nominal FeSe0.94 and FeSe0.98 samples. Their selected revised manuscript specifies Daphne 7373, pressure-cell background treatment and low-temperature pressure calibration. Its Figure 3 supports a pressure-dependent coexistence interpretation near and above 0.8 GPa, with model-dependent spatial conclusions. Different probes, samples and pressure environments must remain explicit.

Evidence: experiment. Sources: source-a631bcd83d23567b, source-c817382f57bbb81d.

Conditions: Source version, composition, probe, criterion, pressure environment and sample preparation are material-specific; gaps, phase, carriers and nominal chemistry are distinct.

Verification scope: Named selected primary passages/figures, original abstracts and exact notation/volume bounds; complete scientific acceptance remains open.

### scwiki-d4-claim-evidence-for-gap-structure-1

Christianson's neutron experiment concerns K-doped Ba0.6K0.4Fe2As2. Its superconductivity-related resonance and the relevant coherence-factor model constrain candidate signs, without directly determining every iron-based gap. Background, momentum, orbital matrix elements and independently measured gap scales remain necessary parts of that interpretation. A resonance measurement cannot be substituted for a family-wide phase verdict.

Evidence: experiment. Sources: source-716c338ddfa726d3, source-7c2b55d128348e6c.

Conditions: Source version, composition, probe, criterion, pressure environment and sample preparation are material-specific; gaps, phase, carriers and nominal chemistry are distinct.

Verification scope: Named selected primary passages/figures, original abstracts and exact notation/volume bounds; complete scientific acceptance remains open.

### scwiki-d4-claim-evidence-for-gap-structure-2

At a FeSe/SrTiO3 interface, Lee's replica bands and mean-field gap-closing scale retain their spectroscopic labels. Faeth's later photon-energy comparison supports an intrinsic electron–phonon interpretation within its stated analysis, but does not determine all contributions to pairing enhancement. A gap-closing temperature is not interchangeable with zero resistance or a magnetic transition. These interface results remain linked to the detailed [multiband](/scwiki/multiband-pairing) discussion rather than silently entering a bulk Tc comparison.

Evidence: experiment. Sources: source-f9fe6e9bc3792dcc, source-ac66116dfd6e4887.

Conditions: Source version, composition, probe, criterion, pressure environment and sample preparation are material-specific; gaps, phase, carriers and nominal chemistry are distinct.

Verification scope: Named selected primary passages/figures, original abstracts and exact notation/volume bounds; complete scientific acceptance remains open.

### scwiki-d4-claim-limits-and-disputes-1

Several orbitals, correlations, nematicity and magnetic proximity help organize the evidence without selecting one universal pairing mechanism. A mass renormalization is not automatically a larger pairing eigenvalue. Complete scientific acceptance remains open, including final-version comparisons, full supplementary methods and raw-data analysis. Differences between a source's claim and this article's review scope are preserved in the references. The historical mechanism narrative also needs its published qualification: Mazin and colleagues' note 8 says that the magnetic instability they initially described as ferromagnetic was misidentified. Keep that qualification with the original abstract; it is not a universal statement that doping suppresses ferromagnetism in every iron-based superconductor.

Evidence: hypothesis. Sources: source-716c338ddfa726d3, source-7c2b55d128348e6c, source-7bc9538ec7783cf2, source-c817382f57bbb81d.

Source relationship: context-for-editorial-analysis. The published Mazin note8 explicitly qualifies its earlier instability assignment. General warnings about inference and remaining review limits are editorial synthesis, not a family-wide mechanism measurement.

Conditions: Source version, composition, probe, criterion, pressure environment and sample preparation are material-specific; gaps, phase, carriers and nominal chemistry are distinct.

Verification scope: Named selected primary passages/figures, original abstracts and exact notation/volume bounds; complete scientific acceptance remains open.

### scwiki-d4-claim-discovery-implications-1

Test a specific model with controlled composition, structure, pressure or disorder changes, and identify the confounding changes. Preserve actual measurement criteria and sample provenance when comparing results. For a proposed coexistence regime, report what each probe resolves and how sample fractions were calibrated. For a candidate pairing mediator, explain which alternative survives the same observation.

Evidence: hypothesis. Sources: source-1a3690e5f1d38f3e, source-a631bcd83d23567b, source-c817382f57bbb81d.

Conditions: Source version, composition, probe, criterion, pressure environment and sample preparation are material-specific; gaps, phase, carriers and nominal chemistry are distinct.

Verification scope: Named selected primary passages/figures, original abstracts and exact notation/volume bounds; complete scientific acceptance remains open.

## Revision and review

Article ID: scwiki-d4. Revision: b7e54e7cd9530cffbf59. Updated: 2026-10-19.

Review scope: Rotter revised authorv2 transition/figures compared with v1; Hsu selected final methods/figures; Medvedev,McQueen andBendele selected author results/figures; inherited C2/B12 scopes; exact composition and overlap checks. Follow-up: Mazin selected final coupling/eigenvector and note8; Maier orbital-response passages; exact local pure-interband DOS/gap-ratio checks.

- Complete scientific acceptance remains open.
- Kamihara original abstract only; full operational criterion unassigned.
- Author manuscripts are versioned; final publication/supplement/raw comparisons remain open.
- Rotter37.2K reportedzero also appears asT10 inFigure4; preserve the threshold caveat.
- Nominal chemistry and bulk fraction bounds do not establish crystal siteoccupancy or atomicscale coexistence.
- The new unequal-DOS example is restricted to the linearized common-cutoff model, not a material fit or a zero-temperature gap ratio; full DFT/RPA remains unreproduced.

Source: [D_material_families/D4_iron_based.md](https://github.com/JackZH26/jzis-superconductivity/blob/0a7c9d6696804ec26aa12f8eed68b55c9b3c0fcf/D_material_families/D4_iron_based.md), commit 0a7c9d6696804ec26aa12f8eed68b55c9b3c0fcf, SHA-256 55f48dddf6d9d0890bd6a4b5de30276884799eedfa9af42578bb4b03f42edbc7.