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

Crystalline defects on the nano-scale, which are called artificial pinning centers (APCs), were successfully introduced into high-temperature superconductors (HTS) by nanotechnology, in order to strongly pin the quantized vortices. The critical current densities, Jc, of the HTS films were dramatically improved by APCs. It is possible to form APCs in high-quality epitaxial films, keeping the desired dimensionality, volume fraction, spatial distribution and so on. The in-field Jc of HTS films at 77 K was improved by one order of magnitude compared with previous values using APCs. This technology can be applied to the coated conductor technology in progress, and a high Jc has already been reported. A current outline of the research is described in this review.

https://iopscience.iop.org/article/10.1088/0953-2048/23/1/014001/pdf

Artificial pinning center technology to enhance vortex pinning in YBCO coated conductors

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

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 = 025 by PLD Room temperature deposition and post annealing of the films yield nearly phase pure films with a pronounced c-axis texture and a strong biaxial in-plane orientation Transport measurements show metallic resistance and onset of superconductivity at 11 K Hc2(T) was determined by resistive measurements and yield Hc2 values of 3 T at 36 K for B||c and 6 T at 64 K for B||ab

Epitaxial growth and anisotropy of La(O,F)FeAs thin films deposited by Pulsed Laser Deposition

Chemical solution deposition (CSD) is a promising way to realize REBa2Cu3O7−x (REBCO; RE = rare earth (here Y, Gd))-coated conductors with high performance in applied magnetic fields. However, the preparation process contains numerous parameters which need to be tuned to achieve high-quality films. Therefore, we investigated the growth of REBCO thin films containing nanometre-scale BaHfO3 (BHO) particles as pinning centres for magnetic flux lines, with emphasis on the influence of crystallization temperature and substrate on the microstructure and superconductivity. Conductivity, microscopy and x-ray investigations show an enhanced performance of BHO nano-composites in comparison to pristine REBCO. Further, those measurements reveal the superiority of GdBCO to YBCO—e.g. by inductive critical current densities, J c, at self-field and 77 K. YBCO is outperformed by more than 1 MA cm−2 with J c values of up to 5.0 MA cm−2 for 265 nm thick layers of GdBCO(BHO) on lanthanum aluminate. Transport in-field J c measurements demonstrate high pinning force maxima of around 4 GN m−3 for YBCO(BHO) and GdBCO(BHO). However, the irreversibility fields are appreciably higher for GdBCO. The critical temperature was not significantly reduced upon BHO addition to both YBCO and GdBCO, indicating a low tendency for Hf diffusion into the REBCO matrix. Angular-dependent J c measurements show a reduction of the anisotropy in the same order of magnitude for both REBCO compounds. Theoretical models suggest that more than one sort of pinning centre is active in all CSD films.

https://iopscience.iop.org/article/10.1088/0953-2048/28/11/114002/pdf

BaHfO3 artificial pinning centres in TFA-MOD-derived YBCO and GdBCO thin films

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

Y2O3 nanoparticles prepared by an inert gas phase condensation process were used to introduce artificial pinning centres in YBa2Cu3O7−δ thin films. The areal density of the particles was varied between 120 and 4200 particles µm−2 without changing the mean particle diameter of approximately 9 nm. Y2O3 particles were deposited on TiO2 terminated SrTiO3 (100) single-crystal substrates with areal densities up to 1654 particles µm−2 and subsequently covered with YBa2Cu3O7−δ by off-axis pulsed laser deposition (PLD). The areal density of the room temperature deposited particles is not changed by the substrate heating during PLD although their height on the substrate decreases. An influence on Jc is demonstrated for particle densities above 1588 particles µm−2, indicating that the substrate decoration with Y2O3 nanoparticles from the gas phase affects the formation of artificial pinning centres in the YBa2Cu3O7−δ films and can be applied to further study of the effect of particle size and areal density on defect formation in YBa2Cu3O7−δ.

https://iopscience.iop.org/article/10.1088/0953-2048/20/9/S19/pdf

Artificial pinning centres in YBCO thin films induced by substrate decoration with gas-phase-prepared Y2O3 nanoparticles

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Papers

Epitaxial growth and anisotropy of La(O,F)FeAs thin films deposited by Pulsed Laser Deposition

BaHfO3 artificial pinning centres in TFA-MOD-derived YBCO and GdBCO thin films

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

Artificial pinning centres in YBCO thin films induced by substrate decoration with gas-phase-prepared Y2O3 nanoparticles