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

https://cyberleninka.org/article/n/818074.pdf

Recent advances in iron-based superconductors toward applications

Iron with a large magnetic moment was widely believed to be harmful to the emergence of superconductivity because of the competition between the static ordering of electron spins and the dynamic formation of electron pairs (Cooper pairs). Thus, the discovery of a high critical temperature (Tc) iron-based superconductor (IBSC) in 2008 was accepted with surprise in the condensed matter community and rekindled extensive study globally. IBSCs have since grown to become a new class of high-Tc superconductors next to the high-Tc cuprates discovered in 1986. The rapid research progress in the science and technology of IBSCs over the past decade has resulted in the accumulation of a vast amount of knowledge on IBSC materials, mechanisms, properties, and applications with the publication of more than several tens of thousands of papers. This article reviews recent progress in the technical applications (bulk magnets, thin films, and wires) of IBSCs in addition to their fundamental material characteristics. Highlights of their applications include high-field bulk magnets workable at 15-25 K, thin films with high critical current density (Jc) > 1 MA/cm2 at ~10 T and 4 K, and an average Jc of 1.3*104 A/cm2 at 10 T and 4 K achieved for a 100-m-class-length wire. These achievements are based on the intrinsically advantageous properties of IBSCs such as the higher crystallographic symmetry of the superconducting phase, higher critical magnetic field, and larger critical grain boundary angle to maintain high Jc. These properties also make IBSCs promising for applications using high magnetic fields.

Epitaxial FeSe0.5Te0.5 thin films with different thickness were grown by pulsed laser ablation deposition on different substrates. High purity phase and fully epitaxial growth were obtained. By varying the film thickness, superconducting transition temperatures up to 21 K were observed, significantly larger than the bulk value 16.2 K. Structural analyses indicated that the c-axis is smaller than the bulk value but it is almost independent of the film thickness and the a-axis changes significantly with the film thickness and is linearly related to the Tc. The latter result indicates the important role of the compressive strain in enhancing Tc. Tc is also related to both the Fe–(Se,Te) bond length and angle, suggesting the possibility of further enhancement.

Tc=21 K in epitaxial FeSe0.5Te0.5 thin films with biaxial compressive strain

High purity epitaxial FeSe0.5Te0.5 thin films with different thickness were grown by Pulsed Laser Ablation on different substrates. By varying the film thickness, Tc up to 21K were observed, significantly larger than the bulk value. Structural analyses indicated that the a axis changes significantly with the film thickness and is linearly related to the Tc. The latter result indicates the important role of the compressive strain in enhancing Tc. Tc is also related to both the Fe-(Se,Te) bond length and angle, suggesting the possibility of further enhancement.

Tc=21K in epitaxial FeSe0.5Te0.5 thin films with biaxial compressive strain

At higher magnetic fields, the critical current density in coated conductors based on biaxially textured metal substrates is limited by the intragrain pinning properties. Therefore, artificial pinning centers were introduced in Y123 films using on-axis as well as off-axis pulsed laser deposition. The nanosized particles were prepared using a quasi-multilayer approach where Hf metal layers having a thickness less than one unit cell were incorporated into Y123 films. Due to the oxidizing deposition atmosphere and solid state reaction with the Y123 phase, nanometer sized perovskite precipitates, which are biaxially textured with a well-defined relationship towards the Y123 lattice were formed. The pinning properties and the anisotropy were characterized using magneto-transport measurements at various fields and temperatures. A significant improvement of as well as irreversibility field B irr could be observed in the off-axis deposited quasi-multilayers.

Improved Pinning in YBCO Based Quasi-Multilayers Prepared by On- and Off-Axis Pulsed Laser Deposition

The introduction of nano-sized Artificial Pinning Centers (APCs) to YBa2Cu3O7−x (YBCO) coated conductors immobilizes flux lines at higher fields, thus increasing their usefulness and commercial applicability. Moreover, careful nano-engineering of these APCs facilitates the fine-tuning of the superconducting properties of coated conductors such as enhanced pinning along specific crystallographic orientations or an overall reduction in anisotropy [1]. Understanding the self-organizational behavior of these APCs and their effect on the superconducting properties of YBCO thin films is the focus of this work.

Microstructure and self-organization of nano-engineered artificial pinning centers in YBa2Cu3O7-x coated conductors

A mechanical wedge polishing procedure that offers a simple, cost-effective and rapid way to look into the depth of a thin film with different surface-sensitive scanning techniques has been developed. As an example of its wide applicability, this method was utilized for the investigation of two differently prepared superconducting YBa2Cu3O7−δ thin films: an Hf-doped film prepared by chemical solution deposition and an undoped film grown by pulsed laser deposition. Upon polishing, the roughness of the samples was reduced to less than 5 nm (peak-to-valley) without influencing the superconducting properties of the films. Thus, nanoscale polishing opens up a unique possibility for microscopic studies with various surface-sensitive techniques. We demonstrate the successful imaging of flux lines by low temperature magnetic force microscopy after polishing a formerly rough as-prepared film. By applying the wedge polishing procedure to the Hf-doped sample, high resolution electron backscattering diffraction investigations reveal the homogeneous distribution of non-superconducting BaHfO3 nanoparticles in the whole volume of the film.

http://iopscience.iop.org/article/10.1088/0953-2048/21/10/105015/pdf

Nanoscale wedge polishing of superconducting thin films—an easy way to obtain depth dependent information by surface analysis techniques

LaFeAsO1−xFx thin films were deposited successfully on (001)-oriented LaAlO3 and MgO substrates from stoichiometric LaFeAsO1−xFx polycrystalline targets with fluorine concentrations up to x = 0.25 by pulsed laser deposition (PLD). Room temperature deposition and post annealing of the films yield films with a pronounced c-axis texture and a strong biaxial in-plane orientation. Transport measurements show metallic resistance and the onset of superconductivity at 11 K. μ0Hc 2(T) was determined by resistive measurements and yield μ0Hc 2 values of 3 T at 3.6 K for and 6 T at 6.4 K for .

https://iopscience.iop.org/article/10.1088/0953-2048/21/12/122001/pdf

Growth and anisotropy of La(O, F)FeAs thin films deposited by pulsed laser deposition

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Papers

Improved Pinning in YBCO Based Quasi-Multilayers Prepared by On- and Off-Axis Pulsed Laser Deposition

Microstructure and self-organization of nano-engineered artificial pinning centers in YBa2Cu3O7-x coated conductors

Nanoscale wedge polishing of superconducting thin films—an easy way to obtain depth dependent information by surface analysis techniques

Growth and anisotropy of La(O, F)FeAs thin films deposited by pulsed laser deposition