Iron-Based Practical Superconductors

The year 2008 turned out to be a landmark year for the superconductivity community when Hosono discovered superconductivity in iron-based compounds. Iron, which has large magnetic moment, was traditionally considered as an opponent of superconductivity because of the contradiction between the static electron spin ordering in Fe and dynamic formation of electron pairs of opposite spins in superconductors. The discovery opened the floodgate to the emergence of a large variety of iron-based superconductors (IBSCs) popularly known through the acronyms 1111, 111, 122, 11 and more. These IBSCs are characterized by very high Hc2 (> 70 T at 20 K), low anisotropy γ (< 2), moderate weak link effect and large critical grain misorientation angle 9° compared to the small value of 3°–5° for YBCO. All IBSCs share in common Fe-pnictogen (P or As) or a Fe-chalcogen (Se or Te). A \(T_{\text{c}}\) of 26 K was reported in La(O0.89F0.11)FeAs, the so-called 1111 compound when F− was substituted for O2−. The substitution leads to a positive charge transfer to insulating La2O2 layer and a negative charge transfer to the Fe2As2 conduction layer. These are called electron-doped superconductors. \(T_{\text{c}}\) as high as 55 K was reached in 1111 SmFeAsO1−xFx and 1111 NdFeAsO1−xFx by replacing La by larger ionic radii rare earth in the parent compound LaO1−xFxFeAs. Similar increase in \(T_{\text{c}}\) can be achieved by applying high pressure. The common feature of all the 1111-type compounds is that superconductivity appears only when antiferromagnetic ordering (SDW) is fully suppressed at a certain level of doping. Coexistence of SDW and superconductivity has, however, been observed in a number of 122 superconductors of the type Sr/BaFe2As2. The compound 122 Ba1−xKxFe2As2 is a hole-doped superconductor as K+ is substituted at the Ba2+ site. In contrast, the Co-doped 122 Ba(Fe2-xCox)2As2 is electron doped. Compound (Ba0.6K0.4)Fe2As2 has a maximum \(T_{\text{c}}\) = 38 K. Iron chalcogenides, FeSe (Te, S) also called the 11 phase compounds have the simplest structure with a low \(T_{\text{c}}\) = 8 K. Pressure studies on IBSCs have shown interesting results. \(T_{\text{c}}\) increases with pressure in underdoped IBSC, \(T_{\text{c}}\) remains nearly unchanged in optimally doped compounds, and \(T_{\text{c}}\) decreases with pressure almost linearly in over-doped compounds. The cost-effective powder-in-tube (PIT) technique has turned out to be the ideal route to wires/tapes. Transport \(J_{\text{c}}\) as high as 105 A/cm2 (4.2 K, 10 T) has already been achieved in 122 superconductors which is more than the threshold set for practical superconductors up to a field of 10 T. Recent production of very uniform 122 tape of more than 100 m single length is a step forward in scaling-up the production to kms long wires/tapes. This augers well for the use of this IBSC conductor for building high-field magnets in the near future.