Following the discovery of long-range antiferromagnetic order in the parent compounds of high-transition-temperature (high-T(c)) copper oxides, there have been efforts to understand the role of magnetism in the superconductivity that occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds. Superconductivity in the newly discovered rare-earth iron-based oxide systems ROFeAs (R, rare-earth metal) also arises from either electron or hole doping of their non-superconducting parent compounds. The parent material LaOFeAs is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility. Although optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping with electrons to induce superconductivity, there has been no direct evidence of SDW order. Here we report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below 155 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then, at approximately 137 K, develops long-range SDW-type antiferromagnetic order with a small moment but simple magnetic structure. Doping the system with fluorine suppresses both the magnetic order and the structural distortion in favour of superconductivity. Therefore, like high-T(c) copper oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range-ordered antiferromagnetic ground state.

Magnetic order close to superconductivity in the iron-based layered LaO1-xFxFeAs systems

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

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

http://www.iop.cas.cn/xwzx/kydt/201106/P020110621599779785576.pdf

The superconductivity at 18 K in LiFeAs system

The recently discovered layered rare-earth metal oxypnictides have reinvigorated research into high-temperature superconductivity. The first of these, found only a few months ago, had a transition temperature of 26 K. A recent paper in Nature reported an iron–arsenic-based material superconducting at 43 K with the application of pressure. Previously only copper oxides superconductors had beaten the 40 K barrier. Now Chen et al. report bulk superconductivity in the samarium–arsenide oxide SmFeAsO1−xFx with a transition temperature of 43 K without this pressure. A report on the discovery of bulk superconductivity in samarium-arsenide oxides SmFeAsO1−xFx with a transition temperature as high as 43 K. Since the discovery of high-transition-temperature (high-Tc) superconductivity in layered copper oxides, extensive effort has been devoted to exploring the origins of this phenomenon. A Tc higher than 40 K (about the theoretical maximum predicted from Bardeen–Cooper–Schrieffer theory1), however, has been obtained only in the copper oxide superconductors. The highest reported value for non-copper-oxide bulk superconductivity is Tc = 39 K in MgB2 (ref. 2). The layered rare-earth metal oxypnictides LnOFeAs (where Ln is La–Nd, Sm and Gd) are now attracting attention following the discovery of superconductivity at 26 K in the iron-based LaO1-xF x FeAs (ref. 3). Here we report the discovery of bulk superconductivity in the related compound SmFeAsO1-xF x , which has a ZrCuSiAs-type structure. Resistivity and magnetization measurements reveal a transition temperature as high as 43 K. This provides a new material base for studying the origin of high-temperature superconductivity.

Superconductivity at 43 K in SmFeAsO1-xFx.

We report the detailed phase diagram and anomalous transport properties of Fe-based high-${T}_{c}$ superconductors ${\mathrm{SmFeAsO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}$. It is found that superconductivity emerges at $x\ensuremath{\sim}0.07$, and optimal doping takes place in the $x\ensuremath{\sim}0.20$ sample with the highest ${T}_{c}\ensuremath{\sim}54\text{ }\text{ }\mathrm{K}$. ${T}_{c}$ increases monotonically with doping; the anomaly in resistivity from structural phase or spin-density-wave order is rapidly suppressed, suggesting a quantum critical point around $x\ensuremath{\sim}0.14$. As manifestations, a linear temperature dependence of the resistivity shows up at high temperatures in the $xl0.14$ regime but at low temperatures just above ${T}_{c}$ in the $xg0.14$ regime; a drop in carrier density evidenced by a pronounced rise in the Hall coefficient is observed below the temperature of the anomaly peak in resistivity. A scaling behavior is observed between the Hall angle and temperature: $\mathrm{cot}﻿{\ensuremath{\theta}}_{H}\ensuremath{\propto}{T}^{1.5}$ for all samples with different $x$ in ${\mathrm{SmFeAsO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}$ system.

Anomalous transport properties and phase diagram of the FeAs-based SmFeAsO1-xFx superconductors.

We synthesized the samples $Ba_{1-x}M_xFe_2As_2$ (M=La and K) with $ThCr_2Si_2$-type structure. These samples were systematically characterized by resistivity, thermoelectic power (TEP) and Hall coefficient ($R_H$). $BaFe_2As_2$ shows an anomaly in resistivity at about 140 K. Substitution of La for Ba leads to a shift of the anomaly to low temperature, but no superconducting transition is observed. Potassium doping leads to suppression of the anomaly in resistivity and induces superconductivity at 38 K as reported by Rotter et al.\cite{rotter}. The Hall coefficient and TEP measurements indicate that the TEP is negative for $BaFe_2As_2$ and La-doped $BaFe_2As_2$, indicating n-type carrier; while potassium doping leads to change of the sign in $R_H$ and TEP. It definitely indicates p-type carrier in superconducting $Ba_{1-x}K_xFe_2As_2$ with double FeAs layers, being in contrast to the case of $LnO_{1-x}F_xFeAs$ with single FeAs layer. A similar superconductivity is also observed in the sample with nominal composition $Ba_{1-x}K_xOFe_2As_2$.

Transport properties and superconductivity in $Ba_{1-x}M_xFe_2As_2$ (M=La and K) with double FeAs layers

We synthesized single phase ${\text{La}}_{0.85}{\text{Sr}}_{0.15}{\text{FeAsO}}_{1\ensuremath{-}\ensuremath{\delta}}$ samples and systematically studied the effect of oxygen deficiency in this system. It is found that partial substitution of Sr for La induces the hole carrier evidenced by positive thermoelectric power (TEP) but no bulk superconductivity is observed. The superconductivity can be realized by annealing the as-grown sample in vacuum to produce the oxygen deficiency. With increasing oxygen deficiency, the superconducting transition temperature $({T}_{c})$ increases and the maximum ${T}_{c}$ reaches 26 K---the same as that observed in the ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ system. TEP changes the sign from positive for the nonsuperconducting as-grown sample to negative for the superconducting samples with oxygen deficiency, while ${R}_{H}$ keeps negative for all samples. It suggests that the dominated carrier in ${\text{La}}_{0.85}{\text{Sr}}_{0.15}{\text{FeAsO}}_{1\ensuremath{-}\ensuremath{\delta}}$ is electronlike.

Superconductivity induced by oxygen deficiency in La 0.85 Sr 0.15 FeAsO 1 − δ

We have studied anisotropic magnetoresistance (MR) and magnetization with rotating magnetic field (B) within $CuO_2$ plane in lightly doped AF $Nd_{2-x}Ce_xCuO_{4}$. \emph{A giant anisotropy} in MR is observed at low temperature below 5 K. The c-axis resistivity can be tuned about one order of magnitude just by changing B direction within $CuO_2$ plane and a scaling behavior between out-of-plane and in-plane MR is found. A "Spin valve" effect is proposed to understand the giant anisotropy of out-of-plane MR and the evolution of scaling parameters with the external field. It is found that the field-induced spin-flop transition of Nd$^{3+}$ layer under high magnetic field is the key to understand the giant anisotropy. These results suggest that a novel entanglement between charge and spin dominates the underlying physics.

D. F. Fang

Papers

Superconductivity at 43 K in SmFeAsO1-xFx.

Anomalous transport properties and phase diagram of the FeAs-based SmFeAsO1-xFx superconductors.

Transport properties and superconductivity in $Ba_{1-x}M_xFe_2As_2$ (M=La and K) with double FeAs layers

Superconductivity induced by oxygen deficiency in La 0.85 Sr 0.15 FeAsO 1 − δ

Giant Anisotropy of Magnetoresistance and "Spin Valve" effect in Antiferromagnetic $Nd_{2-x}Ce_xCuO_{4}$