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Abstract Superconducting magnets based on high-temperature superconductors (HTSs) have become critical components in cutting-edge technologies such as advanced medical applications. In HTSs, weak links of superconductivity are inevitable at high-angle grain boundaries (GBs). Thus, two adjacent grains should be crystallographically aligned within the critical angle (θ c), for which the intergrain critical current density (J c) starts to decrease exponentially. The θ c of several iron-based superconductors (IBSs) is larger than that of cuprates. However, the decreases in both θ c and intergrain J c under magnetic fields for IBSs are still substantial, hampering their applications in polycrystalline forms. Here, we report that potassium-doped BaFe2As2 (Ba122:K) exhibits superior GB performance to that of previously reported IBSs. A transport J c of over 0.1 MA/cm2 across [001]-tilt GBs with misorientation angles up to θ GB = 24° was recorded even at 28 K, which is a required level for practical applications. Additionally, even in an applied magnetic field, θ c was unaltered, and the decay of the intergrain J c was small. Our results highlight the exceptional potential of Ba122:K for polycrystalline applications and pave the way for next-generation superconducting magnets.

Understanding the nature of grain boundaries is a prerequisite for fabricating high-performance superconducting bulks and wires. For iron-based superconductors [e.g. Ba(Fe,Co)[Formula: see text]As[Formula: see text], Fe(Se,Te), and NdFeAs(O,F)], the dependence of the critical current density [Formula: see text] on misorientation angle ([Formula: see text]) has been explored on [001]-tilt grain boundaries, but no data for other types of orientations have been reported. Here, we report on the structural and transport properties of Fe(Se,Te) grown on CeO[Formula: see text]-buffered symmetric [010]-tilt roof-type SrTiO[Formula: see text] bicrystal substrates by pulsed laser deposition. X-ray diffraction and transmission electron microscopy revealed that [Formula: see text] of Fe(Se,Te) was smaller whereas [Formula: see text] of CeO[Formula: see text] was larger than that of the substrate. The difference in [Formula: see text] between the CeO[Formula: see text] buffer layer and the substrate is getting larger with increasing [Formula: see text]. For [Formula: see text] of the substrates, [Formula: see text] of Fe(Se,Te) was zero, whereas [Formula: see text] of CeO[Formula: see text] was continuously increasing. The inclined growth of CeO[Formula: see text] can be explained by the geometrical coherency model. The [Formula: see text]-axis growth of Fe(Se,Te) for [Formula: see text] of the substrates is due to the domain matching epitaxy on (221) planes of CeO[Formula: see text]. Electrical transport measurements confirmed no reduction of inter-grain [Formula: see text] for [Formula: see text], indicative of strong coupling between the grains.

Abstract The recent realisations of hydrogen doped LnFeAsO (Ln = Nd and Sm) superconducting epitaxial thin films call for further investigation of their structural and electrical transport properties. Here, we report on the microstructure of a NdFeAs(O,H) epitaxial thin film and its temperature, field, and orientation dependencies of the resistivity and the critical current density J c. The superconducting transition temperature T c is comparable to NdFeAs(O,F). Transmission electron microscopy investigation supported that hydrogen is homogenously substituted for oxygen. A high self-field J c of over 10 MA/cm2 was recorded at 5 K, which is likely to be caused by a short London penetration depth. The anisotropic Ginzburg–Landau scaling for the angle dependence of J c yielded temperature-dependent scaling parameters γJ that decreased from 1.6 at 30 K to 1.3 at 5 K. This is opposite to the behaviour of NdFeAs(O,F). Additionally, γJ of NdFeAs(O,H) is smaller than that of NdFeAs(O,F). Our results indicate that heavily electron doping by means of hydrogen substitution for oxygen in LnFeAsO is highly beneficial for achieving high J c with low anisotropy without compromising T c, which is favourable for high-field magnet applications.

The discovery of iron-based superconductors paved the way for advanced possible applications, mostly in high magnetic fields, but also in electronics. Among superconductive devices, nanowire detectors have raised a large interest in recent years, due to their ability to detect a single photon in the visible and infrared (IR) spectral region. Although not yet optimal for single-photon detection, iron-based superconducting nanowire detectors would bring clear advantages due to their high operating temperature, also possibly profiting of other peculiar material properties. However, there are several challenges yet to be overcome, regarding mainly: fabrication of ultra-thin films, appropriate passivation techniques, optimization of nano-patterning, and high-quality electrical contacts. Test nanowire structures, made by ultra-thin films of Co-doped BaFe2As2, have been fabricated and characterized in their transport and intrinsic noise properties. The results on the realized nanostructures show good properties in terms of material resistivity and critical current. Details on the fabrication and low temperature characterization of the realized nanodevices are presented, together with a study of possible degradation phenomena induced by ageing effects.

Epitaxial Fe(Se,Te) thin films were prepared by pulsed laser deposition on (La0.18Sr0.82)(Al0.59Ta0.41)O3 (LSAT), CaF2-buffered LSAT and bare CaF2 substrates, which exhibit an almost identical in-plane lattice parameter. The composition of all Fe(Se,Te) films were determined to be FeSe0.7Te0.3 by energy dispersive X-ray spectroscopy, irrespective of the substrate. Albeit the lattice parameters of all templates have comparable values, the in-plane lattice parameter of the FeSe0.7Te0.3 films varies significantly. We found that the superconducting transition temperature (Tc) of FeSe0.7Te0.3 thin films is strongly correlated with their a-axis lattice parameter. The highest Tc of over 19 K was observed for the film on bare CaF2 substrate, which is related to unexpectedly large in-plane compressive strain originating mostly from the thermal expansion mismatch between the FeSe0.7Te0.3 film and the substrate.

Revealing the universal behaviors of iron-based superconductors (FBS) is important to elucidate the microscopic theory of superconductivity. In this work, we investigate the effect of in-plane strain on the slope of the upper critical field H _c2 at the superconducting transition temperature T _c (i.e. −d H _c2 /d T ) for FeSe _0.7 Te _0.3 thin films. The in-plane strain tunes T _c in a broad range, while the composition and disorder are almost unchanged. We show that −d H _c2 /d T scales linearly with T _c , indicating that FeSe _0.7 Te _0.3 follows the same universal behavior as observed for pnictide FBS. The observed behavior is consistent with a multiband superconductivity paired by interband interaction such as sign change s _± superconductivity.

Highly textured NdFeAs(O,F) thin films have been grown on ion beam assisted deposition-MgO/Y2O3/Hastelloy substrates by molecular beam epitaxy. The oxypnictide coated conductors showed a superconducting transition temperature (Tc) of 43K with a self-field critical current density (Jc) of 7.0 × 104 A/cm² at 5K, more than 20 times higher than powder-in-tube processed SmFeAs(O,F) wires. Albeit higher Tc as well as better crystalline quality than Co-doped BaFe2As2 coated conductors, in-field Jc of NdFeAs(O,F) was lower than that of Co-doped BaFe2As2. These results suggest that grain boundaries in oxypnictides reduce Jc significantly compared to that in Co-doped BaFe2As2 and, hence biaxial texture is necessary for high Jc. [ABSTRACT FROM AUTHOR]

The microstructures of SmFeAs(O,F), Ba(Fe,Co)2As2, and FeTe0.5Se0.5 prepared on CaF2 substrates were investigated using transmission electron microscopy. The SmFeAs(O,F)/CaF2 interface is steep, without a disordered layer. By contrast, a chemical reaction occurs at the interface in the cases of Ba(Fe,Co)2As2 and FeTe0.5Se0.5. The reaction layers are located on opposite sides of the interface for Ba(Fe,Co)2As2 and FeTe0.5Se0.5. We found that the lattice distortion of the three superconducting films on the CaF2 substrates enhances the TC values compared with films prepared on oxide substrates. The origin of this lattice deformation varies depending on the superconducting material. [ABSTRACT FROM AUTHOR]

Understanding the nature of grain boundary (GB) characteristics in combination with creating low-energy GBs by modifying the processing conditions, so-called GB engineering, is of great importance for controlling and reducing the defect density, leading to improved functionalities of polycrystalline metals and ceramics. For superconductors particularly, including both low- and high-temperature superconductors, GB engineering has been developed to improve especially the critical current densities, Jc, across these GBs. The intrinsic physical properties of a given superconductor such as the coherence length, the order parameter symmetry, and their anisotropies would determine the strategy of GB engineering. In this topical review, we present an overview of the GB characteristics and GB engineering of Fe-based superconductors (FBS) in the form of polycrystalline bulks and wires, and thin films with application potential, e.g. for high-field magnet wires. Prior to the FBS, GB engineering of the cuprates and MgB2 are also briefly covered. [ABSTRACT FROM AUTHOR]

NdFeAs(O,F) thin films with different fluorine contents were grown on 5° or 10° vicinal cut MgO and CaF2 single crystalline substrates by molecular beam epitaxy. Structural characterisations from reflection high-energy electron diffraction and x-ray diffraction confirmed the epitaxial growth of NdFeAs(O,F). The resistivities of the ab-plane and along the c-axis (ρab and ρc) were derived from the resistivity measurements in the longitudinal and transversal directions. The c-axis resistivity was always higher than ρab, resulting from the anisotropic electronic structure. The resistivity anisotropy at 300 K was almost constant in the range of 50 ≤ γρ ≤ 90 irrespective of the fluorine content. On the other hand, at 56 K showed a strong fluorine dependence: was over 200 for the films with optimum fluorine contents (superconducting transition temperature Tc around 50 K), whereas γρ was around 70 for the films in the under-doped regime (Tc between 35 and 45 K). The mass anisotropy ( and are the effective masses along the c-axis and on the ab-plane) close to Tc derived from the anisotropic Ginzburg-Landau approach using the angular-dependency of ρab was in the range from 2–5. On the assumption , those values are small compared to the normal state anisotropy. [ABSTRACT FROM AUTHOR]

We have systematically investigated the grain boundary angle (θGB) dependence of transport properties for NdFeAs(O,F) fabricated on [001]-tilt symmetric MgO bicrystal substrates. In our previous study, NdFeAs(O,F) bicrystal films showed a weak-link behaviour even at a θGB of 6°. However, this was caused by an extrinsic effect originating from the damage to both NdFeAs(O,F) and MgO substrate by excess F-diffusion along the grain boundary. To investigate the intrinsic nature of grain boundaries, we minimized the damage to NdFeAs(O,F) and MgO by reducing the deposition temperature of the NdOF over-layer needed for F-doping. The resultant NdFeAs(O,F) bicrystal films have a critical angle of 8.5°, above which Jc starts to decrease exponentially. This critical angle is almost the same as those of other Fe-based superconductors. [ABSTRACT FROM AUTHOR]

To apply tensile or compressive uniaxial strain to functional thin films, we propose a novel approach in combining a piezoelectric-based device and a technical metallic substrate used widely in second generation coated conductors (i.e. superconducting tapes). A strain-induced shift of the superconducting transition temperature of 0.1 K for Co-doped BaFe2As2 was observed along the [100] direction, corresponding to a uniaxial pressure derivative dTc/dp100 = −4 K/GPa. For Mn3CuN, a uniaxial strain derivative along the [100] direction of the Curie temperature dTC/dϵ100 = 13 K/% was observed. The current approach is applicable to various functional thin films in a wide range of temperatures. [ABSTRACT FROM AUTHOR]

The discovery of high-Tc superconductivity in Fe-based superconductors pioneered by Kamihara et al (2008 J. Am. Chem. Soc.130 3296–7) has triggered worldwide research efforts in both science and technology. High-quality epitaxial thin films of Fe-based superconductors are required to develop Fe-based superconducting devices, such as Josephson junctions and superconducting quantum interference devices, and also to develop Fe-based superconducting coated conductors (tapes) with high Jc under strong magnetic fields. Epitaxial films are also required for basic research to explore the intrinsic properties of Fe-based superconductors, since sizable bulk single crystals are difficult to grow for some of the Fe-based superconductors such as LnFeAs(O,F) (Ln: lanthanoid element). There are several families of Fe-based superconductors: Fe(Se,Te) with Tc ∼ 15 K, LiFeAs with Tc ∼ 18 K, (Ae,K)Fe2As2 (Ae = alkaline earth element) with Tc ∼ 38 K for Ae = Ba, and LnFeAs(O,F) with Tc ∼ 55 K for Ln = Nd or Sm. Increasing the number of elements in compounds, the Tc becomes higher. Simultaneously, thin-film growth becomes more difficult in principle, and it was not imagined at the beginning of the research that thin-film growth of five-element compounds, LnFeAs(O,F), could be realized quickly. At present, Tc as high as 58 K exceeding the highest Tc ever reported for bulk samples and high critical current density (Jc) over 3 MA cm−2 are obtained in SmFeAs(O,F). In this topical review, we present an overview of the progress in thin-film growth of representative Fe-based superconductors over a decade, including a detailed description of the growth recipes for each family using molecular beam epitaxy or pulsed laser deposition. Furthermore, we present recent hot topics, such as monolayer FeSe with Tc much higher than bulk Tc, though how high Tc can be reached has not been established. Other examples making use of the advantages specific to thin films such as stabilization of quasi-equilibrium phases, enhancement of Tc by epitaxial strain, etc will also be described. [ABSTRACT FROM AUTHOR]

We report on the growth of epitaxial Fe1+δSe0.5Te0.5 thin films on 0°, 5°, 10°, 15° and 20° vicinal cut CaF2 single crystals by pulsed laser deposition. In situ electron and ex situ x-ray diffraction studies reveal a tilted growth of the Fe1+δSe0.5Te0.5 films, whereby under optimized deposition conditions the c-axis alignment coincides with the substrate [001] tilted axis up to a vicinal angle of 10°. Atomic force microscopy shows a flat island growth for all films. From resistivity measurements in longitudinal and transversal directions, the ab- and c-axis components of resistivity are derived and the mass anisotropy parameter is determined. Analysis of the critical current density indicates that no effective c-axis correlated defects are generated by vicinal growth, and pinning by normal point core defects dominates. However, for H∣∣ab the effective pinning centers change from surface defects to point core defects near the superconducting transition due to the vicinal cut. Furthermore, we show in angular-dependent critical current density data a shift of the ab-planes maxima position with the magnetic field strength. [ABSTRACT FROM AUTHOR]

FeSe0.5Te0.5 thin films were grown by pulsed laser deposition on CaF2, LaAlO3 and MgO substrates and structurally and electro-magnetically characterized in order to study the influence of the substrate on their transport properties. The in-plane lattice mismatch between FeSe0.5Te0.5 bulk and the substrate shows no influence on the lattice parameters of the films, whereas the type of substrate affects the crystalline quality of the films and, therefore, the superconducting properties. The film on MgO showed an extra peak in the angular dependence of critical current density Jc(θ) at θ = 180° (H||c), which arises from c-axis defects as confirmed by transmission electron microscopy. In contrast, no Jc(θ) peaks for H||c were observed in films on CaF2 and LaAlO3. Jc(θ) can be scaled successfully for both films without c-axis correlated defects by the anisotropic Ginzburg–Landau approach with appropriate anisotropy ratio γJ. The scaling parameter γJ is decreasing with decreasing temperature, which is different from what we observed in FeSe0.5Te0.5 films on Fe-buffered MgO substrates. [ABSTRACT FROM AUTHOR]

We have measured the temperature dependence of the in-plane thermal conductivity, kab(T), for both Er-Ba-Cu-O and Ho-Ba-Cu-O superconducting bulks. The absolute values of kab(T) for both bulks are smaller than that of Y-Ba-Cu-O bulk, but are larger than that of Dy-Ba-Cu-O bulk, which is known to show an anomalous small kab. kab(T) for Ho-Ba-Cu-O shows a positive slope (dkab/dT>0) in the normal state, similarly to that for Dy-Ba-Cu-O. On the other hand, a slightly negative slope (dkab/dT<0) is found for Er-Ba-Cu-O. The out-of-plane thermal conductivity, kc, was also measured for Er-Ba-Cu-O bulk, and the anisotropy in the thermal conductivity, (kab/kc), was estimated to be about 2. [ABSTRACT FROM AUTHOR]

The microstructures of SmFeAs(O,F), Ba(Fe,Co)2As2, and FeTe0.5Se0.5 prepared on CaF2 substrates were investigated using transmission electron microscopy. The SmFeAs(O,F)/CaF2 interface is steep, without a disordered layer. By contrast, a chemical reaction occurs at the interface in the cases of Ba(Fe,Co)2As2 and FeTe0.5Se0.5. The reaction layers are located on opposite sides of the interface for Ba(Fe,Co)2As2 and FeTe0.5Se0.5. We found that the lattice distortion of the three superconducting films on the CaF2 substrates enhances the TC values compared with films prepared on oxide substrates. The origin of this lattice deformation varies depending on the superconducting material. [ABSTRACT FROM AUTHOR]

The physical properties of Mn3CuN were studied using thin films. We found that an annealing process was very effective to improve the film quality, the key of which was the use of Ti that prevented the formation of oxide impurities. Using these high-quality thin films, we found strong strain dependence for the ferromagnetic transition temperature (TC) and a sign change of the Hall coefficient at TC. The analysis of Hall coefficient data revealed a sizable decrease of hole concentration and a large increase of electron mobility below TC, which is discussed in relation to the electronic structure of this material. [ABSTRACT FROM AUTHOR]