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.

/pdf/high-temperature-superconductivity-in-iron-based-materials-t2ydrlz60b.pdf

High-temperature superconductivity in iron-based materials

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

Pressure Study of Superconducting Oxypnictide LaFePO

We have performed electrical resistivity and powder X-ray diffraction measurements under high pressures for nickel-based superconducors LaNiXO (X = P and As). Superconducting transition temperatures T c of X = P and As increase to 3.78 K at 1.67 GPa and 3.14 K at 0.75 GPa, respectively. A compressibility of the lattice parameter c of LaNiPO is 1.5 times larger than that of the lattice parameter a , resulting in a linear decrease of an axial ratio c / a .

Pressure effects on superconducting and structural properties for nickel-based superconductors LaNiXO (X = P and As)

The Ruddlesden-Popper series of compounds Lan+1NinO3n+1 are one of the strongly correlated electron systems. Among these compounds, La2NiO4 (n=1) is a well-known two-dimensional antiferromagnetic insulator. Electrical resistivity under high pressure have been measured on R2-xSrxNi1-yCuyO4+δ (R=La, Nd) up to 2 GPa and La3Ni2O7+δ up to ~20 GPa. Elecrical resistivty of La1.92Sr1.08NiO4 under high pressure reveals an enhancement of the electronic localization under pressure. On the other hand, in case of Nd1.6Sr0.4Ni0.5Cu0.5O4+δ, a metal-insulator transition is observed at ~240 K and a metallic state is favorable under pressure, in contrast to La1.92Sr1.08NiO4. In La3Ni2O7+δ pressure also enhances a metallic state up to 2 GPa.

http://iopscience.iop.org/article/10.1088/1742-6596/121/5/052013/pdf

Pressure studies on the electrical properties in R2-xSrxNi1-yCuyO4+δ (R=La, Nd) and La3Ni2O7+δ

Electrical resistivity measurements under high pressure were performed on iron-based superconductor LaFeAsO1-xFx. A steep increase in superconducting transition temperature (Tc) was observed for optimum-doped LaFeAsO0.89F0.11, whereas under-doped LaFeAsO0.95F0.05 shows a moderate increase in Tc with applying pressure. X-ray diffraction measurements were also performed under high pressure up to 10 GPa. A ratio c/a was observed to decrease with applying pressure, which is similar to the pressure effect on cuprate high-Tc superconductors.

Pressure enhancement of superconductivity in an iron-based layered compound LaFeAsO1-xFx

The general chemical formula of ternary filled skutterudites is MT4X12 (where M, T, and X are rare-earth or alkaline-earth metals, transition metals, and pnictides, respectively). These substance are known to show various physical properties. La0.8Rh4P12 has a superconducting transition temperature (Tc) of 13.6 K and LaRh4As12 is a new filled skutterudite compound. In this study, we performed the electrical resistivity and the x-ray diffraction measurements for La0.8Rh4P12 and LaRh4As12 under high pressures. From these measurements, we discuss the condition to rise the Tc in filled skutterudite compounds.

https://iopscience.iop.org/article/10.1088/1742-6596/150/5/052009/pdf

Kazumi Igawa

Papers

Pressure Study of Superconducting Oxypnictide LaFePO

Pressure effects on superconducting and structural properties for nickel-based superconductors LaNiXO (X = P and As)

Pressure studies on the electrical properties in R2-xSrxNi1-yCuyO4+δ (R=La, Nd) and La3Ni2O7+δ

Pressure enhancement of superconductivity in an iron-based layered compound LaFeAsO1-xFx

Pressure studies in filled skutterudite La 0.8 Rh 4 P 12 and LaRh 4 As 12