| iron-based superconductor | |
|---|---|
| Name | Iron-based superconductor |
| Caption | Representative layered structure of an iron pnictide superconductor |
| Discovered | 2008 |
| Discoverer | H. Hosono and collaborators |
| Material class | Iron pnictides and iron chalcogenides |
| Critical temperature | up to ~55 K (bulk) |
| Notable examples | LaFeAsO, BaFe2As2, FeSe |
iron-based superconductor
Iron-based superconductors are a class of high-temperature unconventional superconductors whose conduction bands are dominated by iron 3d electrons in layered transition-metal pnictide or chalcogenide lattices. They matter in Quantum physics because they provide a platform to study strong electronic correlations, multi-band superconductivity, and intertwined orders (superconductivity, magnetism, and nematicity), challenging conventional BCS descriptions and informing theories of correlated electron systems.
Iron-based superconductors were first reported in 2008 with superconductivity in LaFeAsO doped with fluorine, sparking intensive research at institutions such as Tokyo Institute of Technology and laboratories including RIKEN. They exhibit superconducting transition temperatures (Tc) up to about 55 K in bulk materials and higher under pressure or in thin films, situating them alongside cuprate superconductors as high-Tc families. Their multi-orbital character, proximity of antiferromagnetism, and sensitivity to structural tuning make them central to studies of quantum criticality, unconventional pairing, and emergent phenomena in condensed matter physics. Research on these materials has implications for quantum materials design, theoretical methods (e.g., DMFT, DFT), and potential quantum technologies.
Iron-based superconductors broadly divide into families: the 1111-type (e.g., LaFeAsO), 122-type (e.g., BaFe2As2), 11-type (e.g., FeSe), 111-type (e.g., LiFeAs), and various intercalated or doped derivatives. Common features include layers of edge-sharing FeX4 tetrahedra (X = pnictogen or chalcogen) and quasi-two-dimensional electronic structures. Angle-resolved photoemission spectroscopy (ARPES) and quantum oscillation measurements reveal multiple Fermi surface sheets derived from Fe 3d orbitals (d_xy, d_xz, d_yz), leading to multi-band transport and anisotropic effective masses. Structural parameters such as the pnictogen height and Fe–X–Fe bond angles strongly correlate with Tc and magnetic tendencies, motivating crystallographic studies at facilities like Diamond Light Source and Advanced Photon Source.
The pairing mechanism remains debated: leading proposals emphasize spin-fluctuation mediated pairing producing sign-changing s± order parameter, while orbital fluctuations or electron-phonon contributions are considered for specific compounds. Measurements from inelastic neutron scattering showing resonance modes near antiferromagnetic wavevectors, and phase-sensitive probes such as quasiparticle interference and Josephson junctions, support sign-changing gaps in many iron pnictides. Gap structures vary across families and doping: isotropic gaps are observed in some 122 compounds, whereas nodal or strongly anisotropic gaps appear in certain 11 and heavily doped systems. The multi-orbital nature requires theoretical frameworks combining spin-fluctuation theory, multi-band Eliashberg calculations, and numerical methods like quantum Monte Carlo and DMFT.
Phase diagrams of iron-based superconductors display intertwined antiferromagnetic, structural (tetragonal-to-orthorhombic), nematic, and superconducting phases as functions of doping, pressure, or strain. The parent compounds are typically bad metals with stripe-like antiferromagnetic order, and superconductivity emerges upon suppression of magnetism via chemical substitution (e.g., Co, K, P doping) or hydrostatic pressure. Nematic order—electronic anisotropy breaking rotational symmetry without long-range magnetic order—has been identified through transport anisotropy, elastoresistivity, and scanning probe techniques, implicating orbital and spin degrees of freedom. Strong correlations and Hund’s coupling produce orbital-selective correlations, which lead to differentiated quasiparticle coherence among d orbitals and influence normal-state transport and superconducting pairing.
Key experimental methods include ARPES, inelastic neutron scattering at facilities like Institut Laue–Langevin, nuclear magnetic resonance (NMR), muon spin rotation (μSR), scanning tunneling microscopy/spectroscopy (STM/STS), high-pressure diamond anvil cell studies, and thin-film synthesis examined at groups such as University of Tokyo and Stanford University. Landmark discoveries include: the original fluorine-doped LaFeAsO superconductor; tunable Tc via chemical substitution in BaFe2As2; the dramatic Tc enhancement in single-layer FeSe on SrTiO3; observation of neutron resonance modes consistent with sign-changing gaps; and nematicity revealed by elastoresistivity experiments. These results have been published across journals and presented at conferences like the APS March Meeting and International Conference on Strongly Correlated Electron Systems.
While practical applications remain limited compared to conventional superconductors due to fabrication and vortex-pinning challenges, research into wire fabrication, thin-film heterostructures, and high-field magnets continues, with efforts from companies and national labs exploring potential for power transmission and magnet technology. Synthesis techniques include solid-state reactions, flux growth, molecular beam epitaxy (MBE) for films, and chemical intercalation. From a social-justice perspective, the global iron-based superconductivity effort highlights inequities in research funding and infrastructure: promoting open collaboration, capacity-building in under-resourced institutions, and equitable access to synchrotron and neutron facilities can broaden participation and accelerate materials discovery. Continued investment in sustainable and inclusive science is crucial to translate advances in quantum materials into societal benefits.
Category:Superconductivity Category:Quantum materials Category:Iron compounds