Advantageous grain boundaries in iron pnictide superconductors

Synthesis and Characterization of [Fe3 (As3 )3 (As4 )]3- , a Binary Fe/As Zintl Cluster With an Fe3 Core

We report here the synthesis and structural characterization of the first binary iron arsenide cluster anion, [Fe₃ (As₃ )₃ (As₄ )]^3- , present in both [K([2.2.2]crypt)]₃ [Fe₃ (As₃ )₃ (As₄ )] (1) and [K(18-crown-6)]₃ [Fe₃ (As₃ )₃ (As₄ )]⋅en (2). The cluster contains an Fe₃ triangle with three short Fe-Fe bond lengths (2.494(1) Å, 2.459(1) Å and 2.668(2) Å for 1, 2.471(1) Å, 2.473(1) Å and 2.660(1) Å for 2), bridged by a 2-butene-like As₄ unit. An analysis of the electronic structure using DFT reveals a triplet ground state with direct Fe-Fe bonds stabilizing the Fe₃ core.

Second magnetization peak, anomalous field penetration, and Josephson vortices in KCa[Formula: see text]Fe[Formula: see text]As[Formula: see text]F[Formula: see text] bilayer pnictide superconductor

We performed magnetization measurements in a single crystal of the anisotropic bilayer pnictide superconductor KCa[Formula: see text]Fe[Formula: see text]As[Formula: see text]F[Formula: see text], with [Formula: see text] [Formula: see text] 34 K, for [Formula: see text] [Formula: see text] [Formula: see text]-axis and [Formula: see text] [Formula: see text] [Formula: see text]-planes. A second magnetization peak (SMP) was observed in the isothermal M(H) curves measured below 16 K for [Formula: see text] [Formula: see text] [Formula: see text]-planes. A peak in the temperature variation of the critical current density, [Formula: see text](T), at 16 K, strongly suggests the emergence of Josephson vortices at lower temperatures, which leads to the SMP in the sample. In addition, it is noticed that the appearance of Josephson vortices below 16 K renders easy magnetic flux penetration. A detailed vortex dynamics study suggests that the SMP can be explained in terms of elastic pinning to plastic pinning crossover. Furthermore, contrary to the common understanding, the temperature variation of the first peak field, [Formula: see text], below and above 16 K, behaves non-monotonically. A highly disordered vortex phase, governed by plastic pinning, has been observed between 17 and 23 K, within a field region around an extremely large first peak field. Pinning force scaling suggests that the point defects are the dominant source of pinning for H [Formula: see text] [Formula: see text]-planes, whereas, for H [Formula: see text] [Formula: see text]-axis, point defects in addition to surface defects are at play. Such disorder contributes to the pinning due to the variation in charge carrier mean free path, [Formula: see text] -pinning. Moreover, the large [Formula: see text] observed in our study is consistent with the literature, which advocates this material for high magnetic field applications.

Enhanced critical current density in K-doped Ba122 polycrystalline bulk superconductors via fast densification

Iron-based superconductors are expected to be used in strong magnet applications owing to their excellent superconducting properties. The process of sintering a mechanically alloyed precursor powder is effective in achieving a high upper critical field and critical current density in BaFe₂As₂ (Ba122) polycrystalline bulk materials. However, when this process is applied to K-doped Ba122, which shows the highest critical temperature in the Ba122 family, suppressing the vaporization of potassium is challenging. In this study, spark plasma sintering (SPS) method was applied to K-doped Ba122 to achieve fast densification. In contrast to the conventional synthesis method, which requires several tens of hours, optimally K-doped bulks with near theoretical density were obtained after only 5 min of SPS, and the magnetic critical current density reached 10⁵ A/cm² at 5 K. The demonstrated superconducting properties suggest that this fast densification technique is a useful tool for applying K-doped Ba122 to bulk trapped field magnets.

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K-doped Ba122 polycrystalline bulks were synthesized via spark plasma sintering

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Nearly single-phase and highly dense bulks were obtained in short period (5 min)

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The magnetic critical current density exceeded 10⁵ A/cm² at 5 K

Critical Current and Pinning Features of a CaKFe4As4 Polycrystalline Sample

We analyze the magnetic behavior of a CaKFe4As4 polycrystalline sample fabricated by a mechanochemically assisted synthesis route. By means of DC magnetization (M) measurements as a function of the temperature (T) and DC magnetic field (H) we study its critical parameters and pinning features. The critical temperature Tc has been evaluated by M(T) curves performed in Zero Field Cooling-Field Cooling conditions. These curves show the presence of a little magnetic background for temperatures above Tc, as also confirmed by the hysteresis loops M(H). Starting from the M(H) curves, the critical current density Jc of the sample has been calculated as a function of the field at different temperatures in the framework of the Bean critical state model. The Jc(H) values are in line with the ones reported in the literature for this typology of samples. By analyzing the temperature dependence of the critical current density Jc(T) at different magnetic fields, it has been found that the sample is characterized by a strong type pinning regime. This sample peculiarity can open perspectives for future improvement in the fabrication of this material.

High Pinning Force Values of a Fe(Se, Te) Single Crystal Presenting a Second Magnetization Peak Phenomenon

The magnetization M of an Fe(Se, Te) single crystal has been measured as a function of temperature T and dc magnetic field H. The sample properties have been analyzed in the case of a magnetic field parallel to its largest face H||ab. From the M(T) measurement, the T_c of the sample and a magnetic background have been revealed. The superconducting hysteresis loops M(H) were between 2.5 K and 15 K showing a tilt due to the presence of a magnetic signal measured at T > T_c. From the M(H) curves, the critical current density J_c(H) has been extracted at different temperatures showing the presence of a second magnetization peak phenomenon. By extracting and fitting the J_c(T) curves at different fields, a pinning regime crossover has been identified and shown to be responsible for the origin of the second magnetization peak phenomenon. Then, the different kinds of pinning centers of the sample were investigated by means of Dew-Hughes analysis, showing that the pinning mechanism in the sample can be described in the framework of the collective pinning theory. Finally, the values of the pinning force density have been calculated at different temperatures and compared with the literature in order to understand if the sample is promising for high-current and high-power applications.

Effect of 1.5 MeV Proton Irradiation on Superconductivity in FeSe0.5Te0.5 Thin Films

Raising the critical current density Jc in magnetic fields is crucial to applications such as rotation machines, generators for wind turbines and magnet use in medical imaging machines. The increase in Jc has been achieved by introducing structural defects including precipitates and vacancies. Recently, a low-energy ion irradiation has been revisited as a practically feasible approach to create nanoscale defects, resulting in an increase in Jc in magnetic fields. In this paper, we report the effect of proton irradiation with 1.5 MeV on superconducting properties of iron–chalcogenide FeSe0.5Te0.5 films through the transport and magnetization measurements. The 1.5 MeV proton irradiation with 1 × 1016 p/cm2 yields the highest Jc increase, approximately 30% at 5–10 K and below 1 T without any reduction in Tc. These results indicate that 1.5 MeV proton irradiations could be a practical tool to enhance the performance of iron-based superconducting tapes under magnetic fields.

Superconducting materials: Challenges and opportunities for large-scale applications

Superconducting materials hold great potential to bring radical changes for electric power and high-field magnet technology, enabling high-efficiency electric power generation, high-capacity loss-less electric power transmission, small lightweight electrical equipment, high-speed maglev transportation, ultra-strong magnetic field generation for high-resolution magnetic resonance imaging (MRI) systems, nuclear magnetic resonance (NMR) systems, future advanced high energy particle accelerators, nuclear fusion reactors, and so on. The performance, economy, and operating parameters (temperatures and magnetic fields) of these applications strongly depend on the electromagnetic and mechanical properties, as well as the manufacturing and material cost of superconductors. This perspective examines the basic properties relevant to practical applications and key issues of wire fabrication for practical superconducting materials, and describes their challenges and current state in practical applications. Finally, future perspectives for their opportunities and development in the applications of superconducting power and magnetic technologies are considered.

Effect of heat treatments on superconducting properties and connectivity in K-doped BaFe2As2

Fe-based superconductors and in particular K-doped BaFe₂As₂ (K-Ba122) are materials of interest for possible future high-field applications. However the critical current density (J_c) in polycrystalline Ba122 is still quite low and connectivity issues are suspected to be responsible. In this work we investigated the properties of high-purity, carefully processed, K-Ba122 samples synthesized with two separate heat treatments at various temperatures between 600 and 825 °C. We performed specific heat characterization and T_c-distribution analysis up to 16 T and we compared them with magnetic T_c and J_c characterizations, and transmission-electron-microscopy (TEM) microstructures. We found no direct correlation between the magnetic T_c and J_c, whereas the specific heat T_c-distributions did provide valuable insights. In fact the best J_c-performing sample, heat treated first at 750 °C and then at 600 °C, has the peak of the T_c-distributions at the highest temperatures and the least field sensitivity, thus maximizing H_c2. We also observed that the magnetic T_c onset was always significantly lower than the specific heat T_c: although we partially ascribe the lower magnetization T_c to the small grain size (< λ, the penetration depth) of the K-Ba122 phase, this behaviour also implies the presence of some grain-boundary barriers to current flow. Comparing the T_c-distribution with J_c, our systematic synthesis study reveals that increasing the first heat treatment above 750 °C or the second one above 600 °C significantly compromises the connectivity and suppresses the vortex pinning properties. We conclude that high-purity precursors and clean processing are not yet enough to overcome all J_c limitations. However, our study suggests that a higher temperature T_c-distribution, a larger H_c2 and a better connectivity could be achieved by lowering the second heat treatment temperature below 600 °C thus enhancing, as a consequence, J_c.

Excess Conductivity Analysis of Polycrystalline FeSe Samples with the Addition of Ag

Bulk FeSe superconductors of the iron-based (IBS) “11” family containing various additions of silver were thoroughly investigated concerning the microstructure using optical microscopy and electron microscopy (TEM and SEM). The measurements of electrical resistivity were performed through the four-point technique in the temperature interval T= 2–150 K. The Aslamazov–Larkin model was employed to analyze the fluctuation-induced conductivity (FIC) in all acquired measurements. In all studied products, we found that the FIC curves consist of five different regimes of fluctuation, viz. critical region (CR), three-dimensional (3D), two-dimensional (2D), one-dimensional (1D), and shortwave fluctuation (SWF) regimes. The critical current density (Jc), the lower and upper critical magnetic fields (Bc1 and Bc2), the coherence length along the c-axis at zero-temperature (ξc(0)), and further parameters were assessed with regards to the silver amount within the products. The analyses discloses a diminution in the resistivity and a great reduction in ξc(0) with Ag addition. The optimal silver doping amount is achieved for 7 wt.%, which yields the best superconducting transition and the greatest Jc value.

Microstructure and Flux Pinning of Reacted-and-Pressed, Polycrystalline Ba0.6K0.4Fe2As2 Powders

The flux pinning properties of reacted-and-pressed Ba_0.6K_0.4Fe₂As₂ powder were measured using magnetic hysteresis loops in the temperature range 20 K ≤ T ≤ 35 K. The scaling analysis of the flux pinning forces ( F p = j c × B , with j c denoting the critical current density) following the Dew-Hughes model reveals a dominant flux pinning provided by normal-conducting point defects ( δ l -pinning) with only small irreversibility fields, H irr , ranging between 0.5 T (35 K) and 16 T (20 K). Kramer plots demonstrate a linear behavior above an applied field of 0.6 T. The samples were further characterized by electron backscatter diffraction (EBSD) analysis to elucidate the origin of the flux pinning. We compare our data with results of Weiss et al. (bulks) and Yao et al. (tapes), revealing that the dominant flux pinning in the samples for applications is provided mainly by grain boundary pinning, created by the densification procedures and the mechanical deformation applied.

Pinning energy and anisotropy properties of a Fe(Se, Te) iron based superconductor

The measurements of DC magnetization M as a function of magnetic field (H) and time (t) have been performed in order to study the superconducting and pinning properties of a Fe(Se, Te) iron based superconductor fabricated by means of the Bridgman technique. By performing the superconducting hysteresis loops M(H) at different temperatures in the case of perpendicular and parallel field, the critical current density J_c (H) has been extracted in the framework of the Bean critical state model for both configurations. The J_c (H) curves have shown the presence of the second magnetization peak effect that causes an anomalous increase in the field dependence of the critical current density. In order to obtain the J_c anisotropy of the sample, we have performed the ratio between perpendicular and parallel critical current density values [Formula: see text] and compared its values with the literature ones. The information regarding the pinning energy U have been extracted by means of the relaxation of the irreversible magnetization M(t) in the case H∣∣c. In particular, performing relaxation measurements at different temperatures and magnetic fields, the temperature dependence of the pinning energy U(T) at different magnetic fields has been obtained showing an anomalous temperature scaling of the curves. The presence of a maximum in the U(T) curves suggests a pinning crossover at a given field and temperature H _cr(T). The H _cr(T) values have been fitted with the equation H _cr(T) = H _cr(0) (1 - T/T*) ⁿ whose results confirm the correlation between the elastic/plastic crossover and the end of the peak effect phenomenon.

Surface reconstruction and charge modulation in BaFe2As2 superconducting film

Whether or not epitaxially grown superconducting films have the same bulk-like superconducting properties is an important concern. We report the structure and the electronic properties of epitaxially grown Ba(Fe_1-x Co _x )₂As₂ films using scanning tunneling microscopy and scanning tunneling spectroscopy (STS). This film showed a different surface structure, [Formula: see text]R45° reconstruction, from those of as-cleaved surfaces from bulk crystals. The electronic structure of the grown film is different from that in bulk, and it is notable that the film exhibits the same superconducting transport properties. We found that the superconducting gap at the surface is screened at the Ba layer surface in STS measurements, and the charge density wave was observed at the surface in sample in the superconducting state.

High-field transport properties of a P-doped BaFe2As2 film on technical substrate

High temperature (high-T_c) superconductors like cuprates have superior critical current properties in magnetic fields over other superconductors. However, superconducting wires for high-field-magnet applications are still dominated by low-T_c Nb₃Sn due probably to cost and processing issues. The recent discovery of a second class of high-T_c materials, Fe-based superconductors, may provide another option for high-field-magnet wires. In particular, AEFe₂As₂ (AE: Alkali earth elements, AE-122) is one of the best candidates for high-field-magnet applications because of its high upper critical field, H_c2, moderate H_c2 anisotropy, and intermediate T_c. Here we report on in-field transport properties of P-doped BaFe₂As₂ (Ba-122) thin films grown on technical substrates by pulsed laser deposition. The P-doped Ba-122 coated conductor exceeds a transport J_c of 10⁵ A/cm² at 15 T for main crystallographic directions of the applied field, which is favourable for practical applications. Our P-doped Ba-122 coated conductors show a superior in-field J_c over MgB₂ and NbTi, and a comparable level to Nb₃Sn above 20 T. By analysing the E − J curves for determining J_c, a non-Ohmic linear differential signature is observed at low field due to flux flow along the grain boundaries. However, grain boundaries work as flux pinning centres as demonstrated by the pinning force analysis.

Enhanced critical-current in P-doped BaFe2As2 thin films on metal substrates arising from poorly aligned grain boundaries

Thin films of the iron-based superconductor BaFe2(As1-xPx)2 (Ba122:P) were fabricated on polycrystalline metal-tape substrates with two kinds of in-plane grain boundary alignments (well aligned (4°) and poorly aligned (8°)) by pulsed laser deposition. The poorly aligned substrate is not applicable to cuprate-coated conductors because the in-plane alignment >4° results in exponential decay of the critical current density (Jc). The Ba122:P film exhibited higher Jc at 4 K when grown on the poorly aligned substrate than on the well-aligned substrate even though the crystallinity was poorer. It was revealed that the misorientation angles of the poorly aligned samples were less than 6°, which are less than the critical angle of an iron-based superconductor, cobalt-doped BaFe2As2 (~9°), and the observed strong pinning in the Ba122:P is attributed to the high-density grain boundaries with the misorientation angles smaller than the critical angle. This result reveals a distinct advantage over cuprate-coated conductors because well-aligned metal-tape substrates are not necessary for practical applications of the iron-based superconductors.

Distinct doping dependence of critical temperature and critical current density in Ba1-xKxFe2As2 superconductor

Since the high transition temperature (High-Tc) superconductivity was discovered in the series of materials containing iron (Fe), their potential for the applications has been extensively scrutinized. In particular, a lot of effort has been made in achieving the high current-carrying ability by revealing the vortex pinning behavior. Here, we report on the critical current density (Jc) for the pristine Ba1-xKxFe2As2 single crystals with various K concentrations (0.25 ≤ x ≤ 0.52) determined by the magnetization hysteresis loop measurements. The x-dependence of Jc is characterized by a spike-like peak at x ~ 0.30, which corresponds to the under-doped region. This behavior is distinct from a moderate Tc dome with a broad maximum spanning from x ~ 0.3 to 0.5. For the under-doped samples, with increasing magnetic field (H), a second magnetization peak in Jc is observed, whereas for the optimally- and over-doped samples, Jc monotonically decreases with H. This result emphasizes that fine tuning of doping composition is important to obtain strong flux pinning. The origin of the characteristic doping dependence of Jc is discussed in connection with the orthorhombic phase domain boundary, as well as the chemical inhomogeneity introduced by the dopant substitutions.

High field superconducting properties of Ba(Fe1-xCox)2As2 thin films

In general, the critical current density, J_c, of type II superconductors and its anisotropy with respect to magnetic field orientation is determined by intrinsic and extrinsic properties. The Fe-based superconductors of the ‘122’ family with their moderate electronic anisotropies and high yet accessible critical fields (H_c2 and H_irr) are a good model system to study this interplay. In this paper, we explore the vortex matter of optimally Co-doped BaFe₂As₂ thin films with extended planar and c-axis correlated defects. The temperature and angular dependence of the upper critical field is well explained by a two-band model in the clean limit. The dirty band scenario, however, cannot be ruled out completely. Above the irreversibility field, the flux motion is thermally activated, where the activation energy U₀ is going to zero at the extrapolated zero-kelvin H_irr value. The anisotropy of the critical current density J_c is both influenced by the H_c2 anisotropy (and therefore by multi-band effects) as well as the extended planar and columnar defects present in the sample.

Large transport J(c) in Cu-sheathed Sr(0.6)K(0.4)Fe2As2 superconducting tape conductors

Copper sheath is the first choice for manufacturing high-T_c superconducting wires and tapes because of its high electrical and thermal conductivities, low-cost and good mechanical properties. However, Cu can easily react with superconducting cores, such as BSCCO, MgB₂ and pnictides, and therefore drastically decrease the transport J_c. Here, we report the fabrication of Cu-sheathed Sr_1−xK_xFe₂As₂ tapes with superior J_c performance using a simple hot pressing method that is capable of eliminating the lengthy high-temperature sintering. We obtained high-quality Sr_1−xK_xFe₂As₂ tapes with processing at 800 ^oC for 30 minutes and measured high T_c and sharp transition. By this rapid fabrication, Cu sheath does not give rise to apparent reaction layer, and only slightly diffuses into Sr-122 core. As a consequence, we achieved high transport J_c of 3.1 × 10⁴ A/cm² in 10 T and 2.7 × 10⁴ A/cm² in 14 T at 4.2 K. The in-field J_c performance is by far the highest reported for Cu-sheathed high-T_c conductors. More importantly, Cu-sheathed Sr-122 tapes also showed a high J_e value of 1.0 × 10⁴ A/cm² in 10 T at 4.2 K, which has reached the widely accepted practical level for applications. These results demonstrate that Cu is a very promising sheath for the practical application of pnictide conductors.

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

This review shows the highlights of a 4-year-long research project supported by the Japanese Government to explore new superconducting materials and relevant functional materials. The project found several tens of new superconductors by examining ∼1000 materials, each of which was chosen by Japanese experts with a background in solid state chemistry. This review summarizes the major achievements of the project in newly found superconducting materials, and the fabrication wires and tapes of iron-based superconductors; it incorporates a list of ∼700 unsuccessful materials examined for superconductivity in the project. In addition, described are new functional materials and functionalities discovered during the project.

Hot pressing to enhance the transport Jc of Sr0.6K0.4Fe2As2 superconducting tapes

High-performance Sr_0.6K_0.4Fe₂As₂ (Sr-122) tapes have been successfully fabricated using hot pressing (HP) process. The effect of HP temperatures (850–925°C) on the c-axis texture, resistivity, Vickers micro-hardness, microstructure and critical current properties has been systematically studied. Taking advantage of high degree of c-axis texture, well grain connectivity and large concentration of strong-pinning defects, we are able to obtain an excellent J_c of 1.2 × 10⁵ A/cm² at 4.2 K and 10 T for Sr-122 tapes. More importantly, the field dependence of J_c turns out to be very weak, such that in 14 T the J_c still remains ~ 1.0 × 10⁵ A/cm². These J_c values are the highest ever reported so far for iron-pnictide wires and tapes, achieving the level desired for practical applications. Our results clearly strengthen the position of iron-pnictide conductors as a competitor to the conventional and MgB₂ superconductors for high field applications.

Growth of c-axis-oriented superconducting KFe₂As₂ thin films

KFe2As2, an iron-based superconductor, is expected to exhibit large spin Hall conductivity, and fabrication of high-quality thin films is requisite for evaluation of this effect and application to spintronics devices. Thin-film growth of KFe2As2 is difficult because of two intrinsic properties; its extremely hygroscopic nature and the high vapor pressure of potassium. We solved these issues by combining room-temperature pulsed laser deposition using K-rich KFe2As2 targets with thermal crystallization in KFe2As2 powder after encapsulation in an evacuated silica-glass tube with all of the processes conducted in a vacuum chamber and a dry Ar atmosphere in a glovebox. The optimized KFe2As2 films on (La,Sr)(Al,Ta)O3 single-crystal substrates were obtained by crystallization at 700 °C, and they were strongly c-axis oriented. The electrical measurements were performed with thin films protected by grease passivation to block reaction with the atmosphere. The KFe2As2 films exhibited a superconductivity transition at 3.7 K.

Huge critical current density and tailored superconducting anisotropy in SmFeAsO₀.₈F₀.₁₅ by low-density columnar-defect incorporation

Iron-based superconductors could be useful for electricity distribution and superconducting magnet applications because of their relatively high critical current densities and upper critical fields. SmFeAsO₀.₈F₀.₁₅ is of particular interest as it has the highest transition temperature among these materials. Here we show that by introducing a low density of correlated nano-scale defects into this material by heavy-ion irradiation, we can increase its critical current density to up to 2 × 10⁷ A cm⁻² at 5 K--the highest ever reported for an iron-based superconductor--without reducing its critical temperature of 50 K. We also observe a notable reduction in the thermodynamic superconducting anisotropy, from 8 to 4 upon irradiation. We develop a model based on anisotropic electron scattering that predicts that the superconducting anisotropy can be tailored via correlated defects in semimetallic, fully gapped type II superconductors.

Thin film growth of Fe-based superconductors: from fundamental properties to functional devices. A comparative review

Fe-based superconductors bridge a gap between MgB2 and the cuprate high temperature superconductors as they exhibit multiband character and transition temperatures up to around 55 K. Investigating Fe-based superconductors thus promises answers to fundamental questions concerning the Cooper pairing mechanism, competition between magnetic and superconducting phases, and a wide variety of electronic correlation effects. The question addressed in this review is, however, is this new class of superconductors also a promising candidate for technical applications? Superconducting film-based technologies range from high-current and high-field applications for energy production and storage to sensor development for communication and security issues and have to meet relevant needs of today’s society and that of the future. In this review we will highlight and discuss selected key issues for Fe-based superconducting thin film applications. We initially focus our discussion on the understanding of physical properties and actual problems in film fabrication based on a comparison of different observations made in the last few years. Subsequently we address the potential for technological applications according to the current situation.

Achievement of practical level critical current densities in Ba₁-xKxFe₂As₂/Ag tapes by conventional cold mechanical deformation

The recently discovered iron-based superconductors are potential candidates for high-field magnet applications. However, the critical current densities (J(c)) of iron-based superconducting wires remain far below the level needed for practical applications. Here, we show that the transport Jc of Ba₁-xKxFe₂As₂/Ag tapes is significantly enhanced by the combination process of cold flat rolling and uniaxial pressing. At 4.2 K, Jc exceeds the practical level of 10(5) A/cm(2) in magnetic fields up to 6 T. The J(c)-H curve shows extremely small magnetic field dependence and maintains a high value of 8.6 × 10(4) A/cm(2) in 10 T. These are the highest values reported so far for iron-based superconducting wires. Hardness measurements and microstructure investigations reveal that the superior J(c) in our samples is due to the high core density, more textured grains, and a change in the microcrack structure. These results indicate that iron-based superconductors are very promising for high magnetic field applications.

Oxypnictide SmFeAs(O,F) superconductor: a candidate for high-field magnet applications

The recently discovered oxypnictide superconductor SmFeAs(O,F) is the most attractive material among the Fe-based superconductors due to its highest transition temperature of 56 K and potential for high-field performance. In order to exploit this new material for superconducting applications, the knowledge and understanding of its electro-magnetic properties are needed. Recent success in fabricating epitaxial SmFeAs(O,F) thin films opens a great opportunity to explore their transport properties. Here we report on a high critical current density of over 10⁵ A/cm² at 45 T and 4.2 K for both main field orientations, feature favourable for high-field magnet applications. Additionally, by investigating the pinning properties, we observed a dimensional crossover between the superconducting coherence length and the FeAs interlayer distance at 30–40 K, indicative of a possible intrinsic Josephson junction in SmFeAs(O,F) at low temperatures that can be employed in electronics applications such as a terahertz radiation source and a superconducting Qubit.

High current superconductivity in FeSe0.5Te0.5-coated conductors at 30 tesla

Although high-temperature superconductor cuprates have been discovered for more than 25 years, superconductors for high-field application are still based on low-temperature superconductors, such as Nb(3)Sn. The high anisotropies, brittle textures and high manufacturing costs limit the applicability of the cuprates. Here we demonstrate that the iron superconductors, without most of the drawbacks of the cuprates, have a superior high-field performance over low-temperature superconductors at 4.2 K. With a CeO(2) buffer, critical current densities >10(6)  A cm(-2) were observed in iron-chalcogenide FeSe(0.5)Te(0.5) films grown on single-crystalline and coated conductor substrates. These films are capable of carrying critical current densities exceeding 10(5) A cm(-2) under 30 tesla magnetic fields, which are much higher than those of low-temperature superconductors. High critical current densities, low magnetic field anisotropies and relatively strong grain coupling make iron-chalcogenide-coated conductors particularly attractive for high-field applications at liquid helium temperatures.

Artificially engineered superlattices of pnictide superconductors

Significant progress has been achieved in fabricating high-quality bulk and thin-film iron-based superconductors. In particular, artificial layered pnictide superlattices offer the possibility of tailoring the superconducting properties and understanding the mechanism of the superconductivity itself. For high-field applications, large critical current densities (J(c)) and irreversibility fields (H(irr)) are indispensable along all crystal directions. On the other hand, the development of superconducting devices such as tunnel junctions requires multilayered heterostructures. Here we show that artificially engineered undoped Ba-122/Co-doped Ba-122 compositionally modulated superlattices produce ab-aligned nanoparticle arrays. These layer and self-assemble along c-axis-aligned defects, and combine to produce very large J(c) and H(irr) enhancements over a wide angular range. We also demonstrate a structurally modulated SrTiO3(STO)/Co-doped Ba-122 superlattice with sharp interfaces. Success in superlattice fabrication involving pnictides will aid the progress of heterostructured systems exhibiting new interfacial phenomena and device applications.

High critical current density and low anisotropy in textured Sr₁-xKxFe₂As₂ tapes for high field applications

From the application point of view, large critical current densities J(c) (H) for superconducting wires are required, preferably for magnetic fields higher than 5 T. Here we show that strong c-axis textured Sr(1-x)K(x)Fe(2)As(2) tapes with nearly isotropic transport J(c) were fabricated by an ex-situ powder-in-tube (PIT) process. At 4.2 K, the J(c) values show extremely weak magnetic field dependence and reach high values of 1.7 × 10(4) A/cm(2) at 10 T and 1.4 × 10(4) A/cm(2) at 14 T, respectively, these values are by far the highest ever reported for iron based wires and approach the J(c) level desired for practical applications. Transmission electron microscopy investigations revealed that amorphous oxide layers at grain boundaries were significantly reduced by Sn addition which resulted in greatly improved intergranular connectivity. Our results demonstrated the strong potential of using iron based superconductors for high field applications.

Iron chalcogenide superconductors at high magnetic fields

Iron chalcogenide superconductors have become one of the most investigated superconducting materials in recent years due to high upper critical fields, competing interactions and complex electronic and magnetic phase diagrams. The structural complexity, defects and atomic site occupancies significantly affect the normal and superconducting states in these compounds. In this work we review the vortex behavior, critical current density and high magnetic field pair-breaking mechanism in iron chalcogenide superconductors. We also point to relevant structural features and normal-state properties.

High intergrain critical current density in fine-grain (Ba0.6K0.4)Fe2As2 wires and bulks

The K- and Co-doped BaFe(2)As(2) (Ba-122) superconducting compounds are potentially useful for applications because they have upper critical fields (H(c2)) of well over 50 T, H(c2) anisotropy γ < 2and thin-film critical current densities J(c) exceeding 1 MA cm(-2) (refs 1-4) at 4.2 K. However, thin-film bicrystals of Co-doped Ba-122 clearly exhibit weak link behaviour for [001] tilt misorientations of more than about 5°, suggesting that textured substrates would be needed for applications, as in the cuprates. Here we present a contrary and very much more positive result in which untextured polycrystalline (Ba(0.6)K(0.4))Fe(2)As(2) bulks and round wires with high grain boundary density have transport critical current densities well over 0.1 MA cm(-2) (self-field, 4.2 K), more than 10 times higher than that of any other round untextured ferropnictide wire and 4-5 times higher than the best textured flat wire. The enhanced grain connectivity is ascribed to their much improved phase purity and to the enhanced vortex stiffness of this low-anisotropy compound (γ~1-2) when compared with YBa(2)Cu(3)O(7-x) (γ~5).