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

Altering the composition of the spacer layers present in iron-based superconductors is one strategy for increasing the temperature below which they superconduct. Now, intercalating FeSe with molecular spacer layers is also shown to enhance the superconducting transition temperature.

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Enhancement of the superconducting transition temperature of FeSe by intercalation of a molecular spacer layer

Exploration of new superconductors and functional materials, and fabrication of superconducting tapes and wires of iron pnictides

Iron selenide superconductors exhibit a number of unique characteristics that are helpful for understanding the mechanism of superconductivity in high-Tc iron-based superconductors more generally. However, in the case of AxFe2Se2 (A = K, Rb, Cs), the presence of an intergrown antiferromagnetic insulating phase makes the study of the underlying physics problematic. Moreover, FeSe-based systems intercalated with alkali metal ions, NH3 molecules or organic molecules are extremely sensitive to air, which prevents the further investigation of their physical properties. It is therefore desirable to find a stable and easily accessible FeSe-based superconductor to study its physical properties in detail. Here, we report the synthesis of an air-stable material, (Li0.8Fe0.2)OHFeSe, which remains superconducting at temperatures up to ~40 K, by means of a novel hydrothermal method. The crystal structure is unambiguously determined by a combination of X-ray and neutron powder diffraction and nuclear magnetic resonance. Moreover, antiferromagnetic order is shown to coexist with superconductivity. This synthetic route opens a path for exploring superconductivity in other related systems, and confirms the appeal of iron selenides as a platform for understanding superconductivity in iron pnictides more broadly.

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Coexistence of superconductivity and antiferromagnetism in (Li0.8Fe0.2)OHFeSe.

The iron-based superconductors that contain FeAs layers as the fundamental building block in the crystal structures have been rationalized in the past using ideas based on the Fermi surface nesting of hole and electron pockets when in the presence of weak Hubbard $U$ interactions This approach seemed appropriate considering the small values of the magnetic moments in the parent compounds and the clear evidence based on photoemission experiments of the required electron and hole pockets However, recent results in the context of alkali metal iron selenides, with generic chemical composition ${A}_{x}{\mathrm{Fe}}_{2\ensuremath{-}y}{\mathrm{Se}}_{2}$ ($A=\mathrm{\text{alkali}}$ metal element), have challenged those previous ideas since at particular compositions $y$ the low-temperature ground states are insulating and display antiferromagnetic order with large iron magnetic moments Moreover, angle-resolved photoemission studies have revealed the absence of hole pockets at the Fermi level in these materials The present status of this exciting area of research, with the potential to alter conceptually our understanding of the iron-based superconductors, is here reviewed, covering both experimental and theoretical investigations Other recent related developments are also briefly reviewed, such as the study of selenide two-leg ladders and the discovery of superconductivity in a single layer of FeSe The conceptual issues considered established for the alkali metal iron selenides, as well as several issues that still require further work, are discussed

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Colloquium: The unexpected properties of alkali metal iron selenide superconductors

Observation of superconductivity at 30 ∼ 46K in A x Fe 2 Se 2 (A = Li, Na, Ba, Sr, Ca, Yb, and Eu)

https://www.cas.cn/ky/kyjz/201206/P020120606643875877834.pdf

Observation of superconductivity at 30∼46K in AxFe2Se2 (A = Li, Na, Ba, Sr, Ca, Yb, and Eu)

New iron selenide superconductors by intercalating smaller-sized alkali metals (Li, Na) and alkaline earths using high-temperature routes have been pursued ever since the discovery of superconductivity at about 30 K in KFe2Se2, but all have failed so far. Here we demonstrate that a series of superconductors with enhanced Tc=30~46 K can be obtained by intercalating metals, Li, Na, Ba, Sr, Ca, Yb, and Eu in between FeSe layers by the ammonothermal method at room temperature. Analysis on their powder X-ray diffraction patterns reveals that all the main phases can be indexed based on body-centered tetragonal lattices with a~3.755-3.831 A while c~15.99-20.54 A. Resistivities show the corresponding sharp transitions at 45 K and 39 K for NaFe2Se2 and Ba0.8Fe2Se2, respectively, confirming their bulk superconductivity. These findings provide a new starting point for studying the properties of these superconductors and an effective synthetic route for the exploration of new superconductors as well.

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Observation of superconductivity at 30 K~46 K in AxFe2Se2 (A = Li, Na, Ba, Sr, Ca, Yb, and Eu)

The ubiquitous coexistence of majority insulating 245 phases and minority superconducting (SC) phases in AxFe2–ySe2 (A = K, Cs, Rb, Tl/Rb, Tl/K) formed by high-temperature routes makes pure SC phases highly desirable for studying the intrinsic properties of this SC family. Here we report that there are at least two pure SC phases, KxFe2Se2(NH3)y (x ≈ 0.3 and 0.6), determined mainly by potassium concentration in the K-intercalated iron selenides formed via the liquid ammonia route. K0.3Fe2Se2(NH3)0.47 corresponds to the 44 K phase with lattice constant c = 15.56(1) A and K0.6Fe2Se2(NH3)0.37 to the 30 K phase with c = 14.84(1) A. With higher potassium doping, the 44 K phase can be converted into the 30 K phase. NH3 has little, if any, effect on superconductivity. Thus, the conclusions should apply to both K0.3Fe2Se2 and K0.6Fe2Se2 SC phases. K0.3Fe2Se2(NH3)0.47 and K0.6Fe2Se2(NH3)0.37 stand out among known superconductors as their structures are stable only at particular potassium doping levels, and hence t...

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Superconducting phases in potassium-intercalated iron selenides.

Two new layered oxyselenides, Ba2MO2Ag2Se2 (M = Co, Mn), have been successfully synthesized via solid-state reaction. It is found that these two compounds, consisting of the infinite MO2 square planes and antifluorite-type Ag2Se2 layers separated by barium, possess new structural features while keeping I4/mmm symmetry. A detailed calculation on the discrete coordination of transition metals by oxygen in the two compounds and Ba2ZnO2Ag2Se2 revealed quite different energy landscapes. The calculated results indicate that the manganese compound favors adoption of the I4/mmm space group, while the cobalt compound could be at the boundary of the transition between the I4/mmm and Cmca phases. In Ba2CoO2Ag2Se2, the coexistence of a large barium ion and a Ag2Se2 layer expands the oxide layer significantly and results in the largest Co–O bond length in the square-planar sheet ever reported. Ba2CoO2Ag2Se2 is near-stoichiometric, whereas Ba2MnO2Ag2Se2 contains 7% silver vacancies, which is explained by the mixed vale...

Structures and Physical Properties of Layered Oxyselenides Ba2MO2Ag2Se2 (M = Co, Mn)

Our recent progress on the preparation of a series of new FeSe-based superconductors and the clarification of SC phases in potassium-intercalated iron selenides are reviewed here. By the liquid ammonia method, metals Li, Na, Ca, Sr, Ba, Eu, and Yb are intercalated in between FeSe layers and form superconductors with transition temperatures of 30 K~46 K, which cannot be obtained by high-temperature routes. In the potassium-intercalated iron selenides, we demonstrate that at least two SC phases exist, KxFe2Se2(NH3)y (x ≈ 0.3 and 0.6), determined mainly by the concentration of potassium. NH3 has little, if any, effect on superconductivity, but plays an important role in stabilizing the structures. All these results provide a new starting point for studying the intrinsic properties of this family of superconductors, especially for their particular electronic structures.

https://iopscience.iop.org/article/10.1088/1674-1056/22/8/087412/pdf

Xiaofang Lai

Papers

Observation of superconductivity at 30∼46K in AxFe2Se2 (A = Li, Na, Ba, Sr, Ca, Yb, and Eu)

Observation of superconductivity at 30 K~46 K in AxFe2Se2 (A = Li, Na, Ba, Sr, Ca, Yb, and Eu)

Superconducting phases in potassium-intercalated iron selenides.

Structures and Physical Properties of Layered Oxyselenides Ba2MO2Ag2Se2 (M = Co, Mn)

Exploring FeSe-based superconductors by liquid ammonia method