FeAsbased

FeAs-based materials are a class of iron-based superconductors centered on layers of iron coordinated with arsenic. They were first discovered to superconduct in 2008 with a LaFeAsO1−xFx system, showing superconductivity at a Tc around 26 K and launching extensive research into high-temperature superconductivity beyond cuprates.

Crystal structure and families: The key feature is the FeAs layer, where iron forms a square lattice coordinated tetrahedrally by arsenic. Most FeAs-based compounds crystallize in the 1111 (ZrCuSiAs-type) or 122 (ThCr2Si2-type) structures, with other related families such as 111 also known. In these materials, superconductivity typically emerges when the antiferromagnetic parent compounds are electron- or hole-doped or subjected to pressure, altering the electronic structure and magnetic order.

Electronic structure and pairing: The electronic structure of FeAs-based superconductors comprises multiple iron 3d-derived Fermi surfaces, including hole pockets near the Brillouin zone center and electron pockets near the zone corners. The pairing mechanism is widely viewed as unconventional, often described as s± symmetry driven by spin fluctuations, though the precise gap structure can vary by family and level of doping.

Superconducting properties: Tc values span a wide range, from about 8–15 K in many members to approximately 55 K in the highest-doped 1111 compounds, with the 122 family commonly reaching up to around 38 K. These materials typically exhibit high upper critical fields and a strong sensitivity to stoichiometry and impurities.

Synthesis and research status: Synthesis usually involves high-temperature solid-state methods under inert conditions, with doping achieved by substituting oxygen with fluorine, rare-earth elements, or by cation or Fe-site substitutions. FeAs-based superconductors remain an active research area due to their unconventional pairing, interplay with magnetism, and potential for higher Tc and novel physics.