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

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Створення порталу інформаційно-довідкових ресурсів міністерства освіти і науки україни

The development of two-dimensional (2D) materials has been experiencing a renaissance since the adventure of graphene. Layered transition metal dichalcogenides (TMDs) are now playing increasingly important roles in both fundamental studies and technological applications due to their wide range of material properties from semiconductors, metals to superconductors. However, a material with fixed properties may not exhibit versatile applications. Due to the unique crystal structures, the physical and chemical properties of 2D TMDs can be effectively tuned through different strategies such as reducing dimensions, intercalation, heterostructure, alloying, and gating. With the flexible tuning of properties 2D TMDs become attractive candidates for a variety of applications including electronics, optoelectronics, catalysis, and energy.

Physical and chemical tuning of two-dimensional transition metal dichalcogenides

Iron pnictide superconductors represent a suggestive alternative to cuprate superconductors for achieving high transition temperatures. Using in situ angle-resolved photoemission spectroscopy, the electronic properties of FeSe are examined as a function of film thickness, providing valuable insights into the mechanism driving the superconductivity in this material.

Interface-induced superconductivity and strain-dependent spin density waves in FeSe/SrTiO3 thin films.

Superconductivity develops in metals upon the formation of a coherent macroscopic quantum state of electron pairs. Iron pnictides and chalcogenides are materials that have high superconducting transition temperatures. In this Review, we describe the advances in the field that have led to higher superconducting transition temperatures in iron-based superconductors and the wide range of materials that are used to form these superconductors. We summarize the essential aspects of the normal state and the mechanism for superconductivity. We emphasize the degree of electron–electron correlations and their manifestation in properties of the normal state. We examine the nature of magnetism, analyse its role in driving the electronic nematicity and discuss quantum criticality at the border of magnetism in the phase diagram. Finally, we review the amplitude and structure of the superconducting pairing, and survey the potential material settings for optimizing superconductivity. Iron-based superconductors display high transition temperatures. The physics behind the unconventional superconductivity of these systems can be investigated by taking into consideration the observed strong electronic correlations and bad-metal behaviour, the nature of their magnetic properties, and the presence of electronic nematicity and of quantum criticalities.

High-temperature superconductivity in iron pnictides and chalcogenides

The recent discovery of high temperature superconductivity in a layered iron arsenide has led to an intensive search to optimize the superconducting properties of iron-based superconductors by changing the chemical composition of the spacer layer that is inserted between adjacent anionic iron arsenide layers. Until now, superconductivity has only been found in compounds with a cationic spacer layer consisting of metal ions: Li+, Na+, K+, Ba2+ or a PbO-type or perovskite-type oxide layer. Electronic doping is usually necessary to control the fine balance between antiferromagnetism and superconductivity. Superconductivity has also been reported in FeSe, which contains neutral layers similar in structure to those found in the iron arsenides but without the spacer layer. Here we demonstrate the synthesis of Lix(NH2)y(NH3)1-yFe2Se2 (x ~0.6 ; y ~ 0.2), with lithium ions, lithium amide and ammonia acting as the spacer layer, which exhibits superconductivity at 43(1) K, higher than in any FeSe-derived compound reported so far and four times higher at ambient pressure than the transition temperature, Tc, of the parent Fe1.01Se. We have determined the crystal structure using neutron powder diffraction and used magnetometry and muon-spin rotation data to determine the superconducting properties. This new synthetic route opens up the possibility of further exploitation of related molecular intercalations in this and other systems in order to greatly optimize the superconducting properties in this family.

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

This paper describes the low-temperature nuclear and magnetic structures of ${\text{La}}_{2}{\text{O}}_{2}{\text{Fe}}_{2}{\text{OSe}}_{2}$ by analysis of x-ray and neutron diffraction data. The material has been demonstrated to order antiferromagnetically at low temperatures, with ${T}_{\text{N}}=\ensuremath{\sim}90\text{ }\text{K}$ a propagation vector of $\mathbit{k}=(\frac{1}{2}0\frac{1}{2})$, resulting in a spin arrangement similar to that in FeTe, despite there being no apparent lowering in symmetry of the nuclear structure.

Low-temperature nuclear and magnetic structures of La 2 O 2 Fe 2 O Se 2 from x-ray and neutron diffraction measurements

A new iron oxyselenide Ce2O2FeSe2: synthesis and characterisation

We report the synthesis of a series of manganese-containing oxychalcogenides with general formula A2O2Mn2OSe2 (A = La, Ce, Pr) as well several new A2O2Fe2OSe2 materials (A = La−Sm). We report the structural, magnetic, and conduction properties of the manganese-containing materials: La2O2Mn2OSe2, Ce2O2Mn2OSe2, and Pr2O2Mn2OSe2. These materials are isostructural with La2O2Fe2OSe2, and are shown to undergo a phase transition on cooling, apparently related to displacement of oxide ions from the [Mn2O]2+ plane. Peak splitting has been observed in the Pr3+ containing material at low temperatures, consistent with a reduction to orthorhombic symmetry. All of the manganese containing materials order antiferromagnetically on cooling with k = (0, 0, 0), TN = 164−184 K, and have a 12 K Mn2+ moment of around 4 μB. The magnetic structure of La2O2Co2OSe2 is also reported. Conductivity studies have shown that La2O2Mn2OSe2 and La2O2Co2OSe2 are both semiconducting, with activation energies of ∼0.24 and 0.35 eV.

Synthesis, Structure and Properties of Several New Oxychalcogenide Materials with the General Formula A2O2M2OSe2 (A = La−Sm, M = Fe, Mn)

Two new oxyselenide materials have been synthesized with composition La2O2MSe2 (M = Mn, Fe). They adopt a new structure type, the β structure, which has been solved and refined from powder X-ray and neutron diffraction data. The structure is described by Ama2 symmetry with unit cell ∼17.5 A × 16.6 A × 4.0 A and consists of sheets of MSen polyhedra separated by La2O2Se blocks. A structural phase transition occurs on cooling involving ordering of M cations and a symmetry reduction to a primitive structure of Pna21 symmetry. Both manganese and iron analogues are antiferromagnetic at low temperature. β-La2O2FeSe2 is a semiconductor in the temperature of 150−300 K with a band gap of approximately 0.7 eV while β-La2O2MnSe2 is insulating at room temperature with band gap of 1.6 eV.

Preparation, Characterization, and Structural Phase Transitions in a New Family of Semiconducting Transition Metal Oxychalcogenides β-La2O2MSe2 (M = Mn, Fe)

David G. Free

Papers

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

Low-temperature nuclear and magnetic structures of La 2 O 2 Fe 2 O Se 2 from x-ray and neutron diffraction measurements

A new iron oxyselenide Ce2O2FeSe2: synthesis and characterisation

Synthesis, Structure and Properties of Several New Oxychalcogenide Materials with the General Formula A2O2M2OSe2 (A = La−Sm, M = Fe, Mn)

Preparation, Characterization, and Structural Phase Transitions in a New Family of Semiconducting Transition Metal Oxychalcogenides β-La2O2MSe2 (M = Mn, Fe)