The interaction that binds electrons into Cooper pairs in iron-based superconductors — the 'pairing glue' — has not been identified. The question was posed in this form by an infrared study of BaFe1.9Pt0.1As2 (Tc = 23 K), which resolved a sharp ~5-meV bosonic mode in the superconducting-state Eliashberg function, absent from the normal state and too low in energy to be purely phononic, and conjectured it to be the neutron spin resonance while noting that no neutron data existed for the compound. Spin-fluctuation-mediated, sign-changing s± pairing remains the leading framework and has gained compound-specific support (superconducting-state kinks in angle-resolved photoemission spectra matching neutron spin resonances in CaKFe4As4), but kink-like features can also arise from purely electronic correlations, nematic-fluctuation and interfacial-phonon channels remain active, and the most recent comprehensive review (2026) concludes that no single microscopic framework accounts for all families. The problem remains open: the microscopic identity of the superconducting-state-only bosonic mode has not been established — in particular, the predicted inelastic-neutron-scattering test of the ~5-meV mode in BaFe1.9Pt0.1As2 has not been performed — and, at the field level, no single microscopic framework identifies the pairing mediator across the iron-based families.

Background

Iron-based superconductors (FeSCs) are layered compounds in which superconductivity emerges in FeAs or FeSe planes upon chemical substitution, pressure, or dimensionality engineering; transition temperatures reach about 55 K in bulk pnictides and exceed 100 K in single-unit-cell FeSe films grown on SrTiO3. Their Fermi surfaces are multiband — cylindrical hole pockets around the Brillouin-zone center and electron pockets around the zone corners, built from several Fe 3d orbitals. In a conventional strong-coupling superconductor such as lead, the bosonic excitations that bind electrons into Cooper pairs (the 'pairing glue', in the standard usage that traces back to Eliashberg theory) are directly visible as features of the electron-boson spectral density, the Eliashberg function α2F(Ω)\alpha^2F(\Omega), which weights each boson energy Ω\Omega by the strength of its coupling to the electrons, in infrared absorption and tunneling spectra. For FeSCs, the analogous question — which bosonic channel provides the glue — has been open since the discovery of the family in 2008. Three candidate channels dominate the debate. First, antiferromagnetic spin fluctuations: the repulsive interband interaction they mediate naturally produces a sign-changing s±s_\pm order parameter, in which the superconducting gap reverses sign between the hole and electron Fermi pockets, as first argued for LaFeAsO1-xFx by Mazin, Singh, Johannes, and Du. Second, orbital fluctuations, which can be amplified by coupling to Fe-sublattice phonons and favor a sign-preserving s++s_{++} state, as developed by Kontani and Onari. Third, phonons themselves, whose calculated electron-phonon coupling in most bulk FeSCs is too weak for the observed transition temperatures, although interfacial phonons of the SrTiO3 substrate are considered a key contributor in monolayer FeSe films. The source of the present candidate is an infrared spectroscopy study of the electron-doped 122-pnictide superconductor BaFe1.9Pt0.1As2 (Tc=23T_c = 23 K; the family label '122' denotes the Ba:Fe:As stoichiometric ratio of the parent compound BaFe2As2) by Xing, Saha, Paglione, and Qazilbash. Comparing the superconducting-state absorption with the normal-state absorption, they observed strong-coupling features analogous to those of lead, and by solving the full isotropic Eliashberg equations they extracted an Eliashberg function containing a large sharp mode at 5.1±0.65.1 \pm 0.6 meV with total coupling constant λ≈3.5\lambda \approx 3.5–4.34.3, together with a weak broad 15-meV mode. The sharp mode is required only when modeling the superconducting state and lies below the energy of the lowest peak of the phonon density of states, so it cannot be due to phonons alone; the authors conjecture that it represents the neutron spin resonance — a collective magnetic excitation that appears only in the superconducting state at the antiferromagnetic ordering wave vector and is generally regarded as a feedback effect of a sign-changing gap rather than the cause of pairing — and they note that inelastic neutron scattering data on BaFe1.9Pt0.1As2 did not exist, explicitly predicting a spin resonance near 5 meV in future neutron experiments. The broad 15-meV mode, present in both states, is attributed either to electron-phonon coupling or to phonon-induced orbital fluctuations, and the authors additionally note the tension that the survival of superconductivity against strong impurity scattering is more consistent with a sign-preserving state. The question is therefore left open at two levels: the compound level (is the superconducting-state-only mode the spin resonance, directly testable by neutron scattering?) and the field level (which channel — spin, orbital, or lattice — mediates pairing across the iron-based families?). The difficulty is intrinsic to the problem: the resonance appears only below TcT_c, so coupling to it cannot by itself establish that it caused pairing; low-energy spectral kinks and optical strong-coupling features can also be generated by purely electronic correlation effects rather than any boson; and the relative strength of the spin, orbital, nematic (rotational-symmetry-breaking), and lattice channels varies from family to family.

Problem Statement

Identify the microscopic origin of the low-energy bosonic modes that spectroscopy resolves only in the superconducting state of the source compound BaFe1.9Pt0.1As2 — in particular the sharp 5.1±0.65.1 \pm 0.6 meV Eliashberg-function mode conjectured, but not confirmed, to be the neutron spin resonance — and thereby the bosonic channel, or combination of channels, that mediates Cooper pairing in this compound. The source study leaves open whether the mode originates from antiferromagnetic spin fluctuations, from orbital fluctuations, or from another mechanism, and explicitly predicts that inelastic neutron scattering on BaFe1.9Pt0.1As2 would find a spin resonance near 5 meV if the mode is due to spin fluctuations; this test has not been performed. The identification must be reconciled with the established constraint set: the absence of the sharp mode from normal-state spectra, the measured mode energies and coupling strengths, the compound's nodeless gap structure, and the tension noted by the source between strong impurity scattering and the survival of superconductivity. The broader field-level question — whether one channel (antiferromagnetic spin fluctuations, orbital fluctuations, phonons including interfacial phonons, nematic fluctuations, or a specified combination) mediates pairing uniformly across the material families (the family labels 1111, 122, 111, 11, and 1144 denote the stoichiometric ratios of the parent compounds, e.g. LaFeAsO1-xFx, BaFe2As2, FeSe, and CaKFe4As4), or whether the mediator is family-dependent — remains open as the context in which the compound-level identification matters, but it is not the question posed by the source.

References

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