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

Superconductivity was recently observed in the binary iron-based compound, FeSe. It is now shown that under pressure, the transition temperature can rise above 36 K. In addition, no static magnetic ordering is observed for this system, contrary to FeAs superconductors. The discovery of new high-temperature superconductors1 based on FeAs has led to a new ‘gold rush’ in high-TC superconductivity. All of the new superconductors share the same common structural motif of FeAs layers and reach TC values up to 55 K (ref. 2). Recently, superconductivity has been reported in FeSe (ref. 3), which has the same iron pnictide layer structure, but without separating layers. Here, we report the magnetic and electronic phase diagram of β-Fe1.01Se as a function of temperature and pressure. The superconducting transition temperature increases from 8.5 to 36.7 K under an applied pressure of 8.9 GPa. It then decreases at higher pressures. A marked change in volume is observed at the same time as TC rises, owing to a collapse of the separation between the Fe2Se2 layers. No static magnetic ordering is observed for the whole p–T phase diagram. We also report that at higher pressures (starting around 7 GPa and completed at 38 GPa), Fe1.01Se transforms to a hexagonal NiAs-type structure and exhibits non-magnetic behaviour.

Electronic and magnetic phase diagram of β-Fe1.01Se with superconductivity at 36.7 K under pressure

We report the superconductivity in iron-based oxyarsenide Sm[O1-xFx]FeAs, with the onset resistivity transition temperature at 55.0K and Meissner transition at 54.6 K. This compound has the same crystal structure as LaOFeAs with shrunk crystal lattices, and becomes the superconductor with the highest critical temperature among all materials besides copper oxides up to now.

https://iopscience.iop.org/article/10.1088/0256-307X/25/6/080/pdf

Superconductivity at 55 K in Iron-Based F-Doped Layered Quaternary Compound Sm[O1-xFx] FeAs

A series of big single crystals of BaFe2?xNixAs2 have been prepared by the FeAs self-flux method, with nominal nickel doping x = 0?0.12. The dimensions of the cleaved crystals are over 10 mm along ab plane and ~2 mm in maximum along the c direction. The measurements of x-ray diffraction, electrical resistance and magnetic property are carried out on the crystals. For the undoped parent compound BaFe2As2, both resistance and magnetization data display an anomaly associated with spin density wave and/or structural phase transition, with the transition temperatures at ~138 K. For Ni-doped BaFe2?xNixAs2 crystals, the superconducting critical temperature Tc ranges from 4.3 K for x = 0.06 sample to 20 K for the optimally doped x = 0.10 crystal.

Single crystal growth and characterizations of iron arsenide superconductor BaFe 2-x Ni x As 2 (0.0 ≤ x ≤0.12)

Single crystals of undoped and nickel-doped BaFe2-xNixAs2 (x = 0, 0.04) have been grown by FeAs self-flux method. The maximum dimension of the crystal is as large as similar to 1 cm along ab plane. The crystalline topography of a cleaved crystal surface is examined by scanning electron microscope (SEM). By x-ray powder diffraction (XRD) experiments using pure silicon as an internal standard, precise unit cell parameters (tetragonal at room temperature) are determined: a = 3.9606(4) angstrom (1 angstrom=0.1 nm), c = 13.015(2) angstrom for BaFe1.96Ni0.04As2 and a = 3.9590(5) angstrom, c = 13.024(1) angstrom for BaFe2As2. DC magnetization and transport measurements are performed to check superconducting transition (T-c = 15 K for x = 0.04) and other subtle anomalies. For BaFe1.96Ni0.04As2 crystal, the resistance curve at normal state shows two distinct anomalies associated with spin and structure transitions, and its magnetization data above 91 K exhibit a linear temperature dependence clue to spin density wave (SDW) instability.

http://iopscience.iop.org/article/10.1088/1674-1056/19/8/086101/pdf

Structural and physical properties of undoped and nickel-doped BaFe 2-x Ni x As 2 （x = 0,0.04） single crystals

Superconducting La1.937SrO0.063CuO4 crystals grown by the travelling-solvent floating-zone technique were thermally treated under various temperatures and oxygen pressures for moderately adjusting the oxygen content. The response of intrinsic electronic property of the crystals to the change of hole density in La2-xCuO4 in the vicinity of the magic doping of x = 1/16 (= 0.0625) is studied in detail by magnetic measurements under various fields up to 1T. It is found that when the superconducting critical temperature (TC) increases with the oxygen content, there appears also a new subtle electronic state that can be detected from the differential curves of diamagnetic susceptibility dχ/dT of the crystal sample. In contrast with the intrinsic state, the new subtle electronic state is very fragile under the magnetic fields. Our results indicate that a moderate change in oxygen doping does not significantly modify the intrinsic electronic state originally existing at the magic doping level.

Effect of heat treatment on electronic phase in underdoped La 2- x Sr x CuO 4 single crystal

Bulk superconducting La2CuO4+δ single crystals are obtained by using electrochemical intercalation technique from the as-grown insulating samples. Oxidation is carried out by constant current I=10µA at temperature T=70°C and room temperature, respectively. Structure and magnetic properties are studied by low-temperature X-ray diffraction and susceptibility measurements. A superconducting phase with Tc of 19K and δ-0.12 can be attributed to the formation of oxygen clusters. Room temperature oxidation is inhomogeneous: two superconducting phases with Tc1 of 24K and Tc2 of 8K and an antiferromagnetic phase are coexisting in the crystal. It is found that the appearance of Tc in this system has the "step" tendency.

Zhou Fang

Papers

Superconductivity at 55 K in Iron-Based F-Doped Layered Quaternary Compound Sm[O1-xFx] FeAs

Single crystal growth and characterizations of iron arsenide superconductor BaFe 2-x Ni x As 2 (0.0 ≤ x ≤0.12)

Structural and physical properties of undoped and nickel-doped BaFe 2-x Ni x As 2 （x = 0,0.04） single crystals

Effect of heat treatment on electronic phase in underdoped La 2- x Sr x CuO 4 single crystal

Electrochemical oxidation of La2CuO4 single crystals