Iron Based SuperconductorsEdit

Iron-based superconductors are a family of materials that sparked a major shift in the study of high-temperature superconductivity when they were discovered in 2008. These compounds, built around layers of iron coordinated by pnictogen or chalcogen atoms, exhibit superconductivity when their magnetic order is suppressed by chemical substitution, pressure, or other tuning methods. They offer a contrast to cuprate superconductors in their structure, chemistry, and the way superconductivity emerges from a magnetically ordered parent state. The family spans several structural archetypes, most notably iron pnictides and iron chalcogenides, with superconducting transition temperatures that rival many other unconventional superconductors and reveal rich physics in the interplay between magnetism, structure, and electronic correlations. superconductivity iron-based superconductor FeAs layer FeSe LaFeAsO BaFe2As2

Discovery and historical context - The field began with the report of superconductivity in LaFeAsO1−xFx, achieving Tc up to 26 K upon fluorine doping, which was a striking departure from conventional electron-phonon–driven superconductivity and signaled a new family of unconventional superconductors. This finding led to rapid exploration of related compounds and a burst of activity across multiple chemical families. LaFeAsO LaFeAsO1−xFx - Within a few years, several hundred members were identified, extending into multiple structural families and enabling systematic studies of how doping, pressure, and substitutions affect magnetism and superconductivity. Notable families include the 1111-type, 122-type, 111-type, and 11-type materials, each with characteristic phase diagrams that couple antiferromagnetic order to superconductivity. 1111-type 122-type 111-type 11-type

Crystal structure and chemical families - A unifying motif is the FeX layer, where X is a pnictogen (like As) or a chalcogen (like Se). The iron atoms form a square lattice, with X atoms occupying positions that create a tetrahedral coordination around iron. The electronic states near the Fermi level are dominated by iron 3d orbitals, and the layering yields quasi-two-dimensional electronic structure that supports unconventional superconductivity. FeX layer FeSe FeAs - Major structural archetypes: - 1111-type: REFeAsO (where RE is a rare-earth element such as La, Ce, Nd). These layers alternate with oxide layers, and doping (for example with fluorine) suppresses magnetism and induces superconductivity, producing some of the earliest high-Tc records in the family. REFeAsO LaFeAsO - 122-type: AFe2As2 (A = alkaline earth or alkali metal such as Ba, Sr, Ca, or K for hole doping). These materials commonly show large single crystals and robust phase diagrams with magnetism and superconductivity tuned by chemical substitution or pressure. BaFe2As2 - 111-type: AFeAs (A = Li, Na, or other monovalent metals). These are structurally related to the other families but often host distinct magnetic and superconducting behaviors under doping. 111-type - 11-type: FeSe, FeTe, and related chalcogenides with the simplest chemical formula and layered structure. FeSe, in particular, is notable for superconductivity at modest Tc under ambient conditions and dramatic Tc enhancement under pressure or in monolayer form on certain substrates. FeSe - Doping and tuning: Superconductivity typically emerges when antiferromagnetic order in the parent compounds is suppressed by electron or hole doping, or by applying pressure. The phase diagrams show a delicate balance between magnetism, structural transitions, and superconductivity, highlighting the role of competing interactions in these materials. doping antiferromagnetism

Electronic structure, magnetism, and nematicity - The electronic structure of iron-based superconductors features multiple Fe 3d-derived bands crossing the Fermi level, yielding several Fermi-surface sheets. A common theme is hole pockets around the center of the Brillouin zone and electron pockets near the zone corners, with interband scattering playing a central role in many proposed pairing scenarios. Fermi surface Fe 3d orbitals - Magnetism in the parent compounds is usually stripe-like antiferromagnetic order, intimately connected to a structural transition. Superconductivity emerges as this magnetic order is weakened, suggesting that spin fluctuations are relevant to pairing in many materials. spin fluctuations - In several members, nematicity—an electronic anisotropy that breaks rotational symmetry without a long-range magnetic order—appears to be connected to orbital occupancy and lattice distortions, further enriching the phenomenology of these systems. nematic order orbital ordering - The interplay between magnetism and superconductivity in iron-based superconductors contrasts in important ways with cuprates, offering a broader platform to study how magnetism, structure, and electronic correlations can give rise to high-Tc superconductivity. cuprate

Superconducting pairing mechanisms and debates - A widely discussed scenario is s± pairing, in which the superconducting order parameter changes sign between the hole and electron Fermi-surface sheets, a mechanism compatible with pairing driven by repulsive spin fluctuations that connect these sheets. This view is supported by various experimental probes and is a prevalent framework for understanding many iron-based superconductors. s± pairing spin fluctuations - Alternative views have considered s++ pairing (no sign change) arising from orbital fluctuations or electron-phonon–assisted processes, or even nodal or mixed-gap structures in certain materials. The diversity of materials in the family means that multiple pairing channels could be realized, depending on composition, pressure, and dimensionality. s++ pairing orbital fluctuations gap symmetry - The role of electron correlations in iron-based superconductors is a central question. While electron–phonon coupling in these materials is generally not strong enough on its own to explain high Tc, many researchers emphasize moderate correlations and multi-orbital physics that give rise to enhanced pairing tendencies. This stands in contrast to simpler metallic superconductors and aligns with the broader class of unconventional superconductors. electron correlations - External pressure and interface effects can dramatically alter Tc and magnetic behavior. For FeSe, for example, pressure elevates Tc significantly, and monolayer FeSe on certain substrates exhibits Tc values well above the bulk limit, illustrating how lattice, dimensionality, and substrate coupling can tune superconductivity. FeSe pressure monolayer FeSe - Ongoing experiments continue to refine the understanding of the pairing glue, gap structure, and the detailed orbital contributions to superconductivity across the different iron-based families. These efforts include angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), nuclear magnetic resonance (NMR), and other spectroscopic techniques. ARPES STM NMR

Materials progress, notable results, and variability - Tc values across the iron-based families span a broad range. The early 1111-type compounds reached Tc in the mid- to upper-30s and even into the 50s K range for certain rare-earth substitutions with fluorine doping. The 122-type materials often show robust superconductivity with Tc up to the low 40s K range, while the 111-type and 11-type families offer rich chemistry and tunability, including superconductivity in FeSe with relatively low ambient-Tc that can be enhanced via pressure or interfacial effects. Tc LaFeAsO BaFe2As2 FeSe - FeSe stands out for its simplicity and unusual behavior: a relatively low Tc at ambient pressure that increases dramatically under pressure, and striking Tc enhancement in monolayer form on oxide substrates, which has spurred extensive research into interfacial and tensile-strain effects on pairing. FeSe - The ability to synthesize high-quality single crystals and to apply chemical substitutions has enabled detailed phase diagrams that map how superconductivity coexists with or competes against magnetic order and structural transitions. These phase diagrams are a core resource for testing theoretical models of pairing and correlation effects. phase diagram

Controversies and debates (neutral framing) - A central debate concerns the exact symmetry and structure of the superconducting gap across different iron-based materials. While many compounds show evidence consistent with sign-changing s-wave gaps, others reveal nodal behavior or material-specific gap structures, leading to a nuanced view that multiple pairing channels may be realized in this family. gap symmetry - The relative importance of magnetism, orbital physics, and lattice effects in driving superconductivity is an active area of inquiry. Some frameworks emphasize spin-fluctuation–mediated pairing, whereas others highlight orbital fluctuations or electron-phonon contributions in concert with correlation effects. The diversity of materials means that different mechanisms may dominate in different members of the family. magnetism orbital fluctuations electron-phonon coupling - Dimensionality and interfaces can profoundly influence Tc, as shown by findings in monolayer FeSe and related heterostructures. These results have sparked discussion about the role of substrate interactions, strain, and two-dimensional electronic behavior in enabling higher superconducting temperatures. monolayer FeSe interfacial superconductivity - Sample quality, synthesis methods, and measurement interpretation all shape the experimental landscape. Disparate results in some materials have led to careful cross-checks and reproducibility efforts, illustrating how complex multi-band systems can yield seemingly conflicting signals that still fit within a broader, coherent theoretical picture. experimental methods

Applications, outlook, and broader impact - While the practical deployment of iron-based superconductors faces challenges such as material processing, grain boundaries, and chemical stability, their relatively high Tc values and the rich physics they reveal keep them at the forefront of condensed-matter research. The insights gained from iron-based superconductors inform the broader search for higher-Tc materials and enrich the general understanding of unconventional superconductivity. applications - The ongoing exploration across families, pressures, and interfaces continues to influence materials science, electronic structure theory, and the design of new compounds that might unify high Tc with favorable material properties for real-world use. materials science electronic structure theory

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