The recent discovery of superconductivity with relatively high transition temperature (Tc) in the layered iron-based quaternary oxypnictides La[O1−xFx] FeAs by Kamihara et al. [Kamihara Y, Watanabe T, Hirano M, Hosono H (2008) Iron-based layered superconductor La[O1-xFx] FeAs (x = 0.05–0.12) with Tc = 26 K. J Am Chem Soc 130:3296–3297.] was a real surprise and has generated tremendous interest. Although superconductivity exists in alloy that contains the element Fe, LaOMPn (with M = Fe, Ni; and Pn = P and As) is the first system where Fe plays the key role to the occurrence of superconductivity. LaOMPn has a layered crystal structure with an Fe-based plane. It is quite natural to search whether there exists other Fe based planar compounds that exhibit superconductivity. Here, we report the observation of superconductivity with zero-resistance transition temperature at 8 K in the PbO-type α-FeSe compound. A key observation is that the clean superconducting phase exists only in those samples prepared with intentional Se deficiency. FeSe, compared with LaOFeAs, is less toxic and much easier to handle. What is truly striking is that this compound has the same, perhaps simpler, planar crystal sublattice as the layered oxypnictides. Therefore, this result provides an opportunity to better understand the underlying mechanism of superconductivity in this class of unconventional superconductors.

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Superconductivity in the PbO-type structure α-FeSe

The ternary iron arsenide ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have discovered bulk superconductivity at ${T}_{c}=38\text{ }\text{ }\mathrm{K}$ in $({\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}){\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ with $x\ensuremath{\approx}0.4$. The parent compound ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ crystallizes in the tetragonal ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure, which consists of $(\mathrm{FeAs}{)}^{\ensuremath{\delta}\ensuremath{-}}$ iron arsenide layers separated by ${\mathrm{Ba}}^{2+}$ ions. ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ is a poor metal and exhibits a spin density wave anomaly at 140 K. By substituting ${\mathrm{Ba}}^{2+}$ for ${\mathrm{K}}^{+}$ ions we have introduced holes in the $(\mathrm{FeAs}{)}^{\ensuremath{-}}$ layers, which suppress the anomaly and induce superconductivity. The ${T}_{c}$ of 38 K in $({\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}){\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ is the highest in hole doped iron arsenide superconductors so far. Therefore, we were able to expand this class of superconductors by oxygen-free compounds with the ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure.

Superconductivity at 38 K in the iron arsenide (Ba1-xKx)Fe2As2.

The response of the worldwide scientific community to the discovery in 2008 of superconductivity at T c = 26 K in the Fe-based compound LaFeAsO1−x F x has been very enthusiastic. In short order, ot...

The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

Kamihara and coworkers' report of superconductivity at ${T}_{c}=26\text{ }\text{ }\mathrm{K}$ in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled ``conventional'' superconductors. Clearly, superconductivity and magnetism or magnetic fluctuations are intimately related in the FePn/Ch, and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With ${T}_{c}$ values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review provides an overview, using numerous references, with a focus on the materials and their superconductivity.

Superconductivity in iron compounds

The surprising discovery of high-temperature superconductivity in a material containing a strong magnet—iron—has led to thousands of publications. By placing all the data in context, it becomes clear what we know and where we are headed.

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High-temperature superconductivity in iron-based materials

The hunt for new materials exhibiting high-temperature superconductivity is on again. A complex iron-based oxide, containing lanthanum and arsenic, was recently found to exhibit a transition temperature (Tc) of about 26 K when doped with fluoride ions. That's respectable, but far from the heights achieved in copper oxide superconductors. Now Takahashi et al. show that the application of around 40,000 atmospheres of pressure can raise the Tc of this material substantially, to about 43 K. This is the highest tc yet reported for a non-copper-based material. What is more, this record is unlikely to last for long: the complexity of 'iron oxypnictides' of this type offers considerable flexibility for chemical modification, and we can expect to hear of yet higher transition temperatures. This paper — and the prospect of a new wave of superconductor fever — is the subject of an Editorial in the 24 April issue of Nature (452, 914; 2008). The application of pressure can raise the superconducting transition temperature of oxypnictide (a pnicogen being a group V element) substantially, to a maximum value of about 43 K. This is the highest transition temperature yet reported for a non-copper-based material, but this record is unlikely to last for long: the material system offers considerable flexibility for chemical modification, and we can reasonably anticipate that this record will soon be superseded. The iron- and nickel-based layered compounds LaOFeP (refs 1, 2) and LaONiP (ref. 3) have recently been reported to exhibit low-temperature superconducting phases with transition temperatures Tc of 3 and 5 K, respectively. Furthermore, a large increase in the midpoint Tc of up to ∼26 K has been realized4 in the isocrystalline compound LaOFeAs on doping of fluoride ions at the O2- sites (LaO1-xFxFeAs). Experimental observations5,6 and theoretical studies7,8,9 suggest that these transitions are related to a magnetic instability, as is the case for most superconductors based on transition metals. In the copper-based high-temperature superconductors, as well as in LaOFeAs, an increase in Tc is often observed as a result of carrier doping in the two-dimensional electronic structure through ion substitution in the surrounding insulating layers, suggesting that the application of external pressure should further increase Tc by enhancing charge transfer between the insulating and conducting layers. The effects of pressure on these iron oxypnictide superconductors may be more prominent than those in the copper-based systems, because the As ion has a greater electronic polarizability, owing to the covalency of the Fe–As chemical bond, and, thus, is more compressible than the divalent O2- ion. Here we report that increasing the pressure causes a steep increase in the onset Tc of F-doped LaOFeAs, to a maximum of ∼43 K at ∼4 GPa. With the exception of the copper-based high-Tc superconductors, this is the highest Tc reported to date. The present result, together with the great freedom available in selecting the constituents of isocrystalline materials with the general formula LnOTMPn (Ln, Y or rare-earth metal; TM, transition metal; Pn, group-V, ‘pnicogen’, element), indicates that the layered iron oxypnictides are promising as a new material platform for further exploration of high-temperature superconductivity.

Superconductivity at 43 K in an iron-based layered compound LaO 1-x F x FeAs

Superconductivity at 43 K in an Iron‐Based Layered Compound LaO1‐xFxFeAs.

Electrical resistivity and magnetic susceptibility measurements under high pressure were performed on iron-based superconductor LaFePO and LaFeAsO 1- x F x system. A steep increase in superconducting transition temperature ( T c ) was observed for LaFePO and “optimum-doped” and “over-doped” LaFeAsO 1- x F x . Pressure-induced superconductivity was confirmed in undoped LaFeAsO and SrFe 2 As 2 by electrical resistivity measurements under high pressure. X-ray diffraction measurements were also performed under high pressure up to 10 GPa for LaFePO and LaFeAsO 1- x F x system, where the anisotropic decrease of the lattice constants was observed with applying pressure.

High-pressure Studies on Superconducting iron-based LaFeAsO1-xFx , LaFePO and SrFe2As2

Electrical resistivity and magnetic susceptibility measurements under high pressure were performed on an iron-based superconductor LaFePO. A steep increase in superconducting transition temperature ( T c ) of LaFePO with d T c /d P >4 K/GPa below P = 0.8 GPa was observed. These results are similar to isocrystalline LaFeAsO 1- x F x system reported previously. X-ray diffraction measurements were also performed under high pressure up to 10 GPa, where linear compressibility κ a and κ c are presented.

Pressure Study of Superconducting Oxypnictide LaFePO

Electrical resistivity and magnetic susceptibility measurements under high pressure were performed on an iron-based superconductor LaFePO A steep increase in superconducting transition temperature (Tc) of LaFePO with dTc/dP > 4 K/GPa to a maximum of 88 K for P = 08 GPa was observed These results are similar to isocrystalline LaFeAsO1-xFx system reported previously X-ray diffraction measurements were also performed under high pressure up to 10 GPa, where linear compressibility ka and kc are presented

Kazunobu Arii

Papers

Superconductivity at 43 K in an iron-based layered compound LaO 1-x F x FeAs

Superconductivity at 43 K in an Iron‐Based Layered Compound LaO1‐xFxFeAs.

High-pressure Studies on Superconducting iron-based LaFeAsO1-xFx , LaFePO and SrFe2As2

Pressure Study of Superconducting Oxypnictide LaFePO

Pressure Study of Superconducting Oxypnictide LaFePO