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This paper reports the nanoscale texture and microstructure of a high-performance NdFeAs(O,F) superconducting thin film grown by molecular beam epitaxy on a textured MgO/Y$_2$O$_3$/Hastelloy substrate. The NdFeAs(O,F) film forms a highly textured columnar grain structure by epitaxial growth on the MgO template. Although the film contains stacking faults along the $ab$-plane as well as grain boundaries perpendicular to the $ab$-plane, good superconducting properties are measured: a critical temperature, $T _{\rm c}$, of 46 K and a self-field critical current density, $J_{\rm c}$, of $2 \times 10^6 \,{\rm A/cm^2}$ at 4.2 K. Automated crystal orientation mapping by scanning precession electron diffraction in transmission electron microscopy is employed to analyze the misorientation angles between adjacent grains in a large ensemble (247 grains). 99% of the grain boundaries show in-plane misorientation angles ($\Delta \gamma$) less than the critical angle $\theta_{\rm c}$, which satisfies one of the necessary conditions for the high $J_{\rm c}$. Comparing the columnar grain size distribution with the mean distance of the flux line lattice, the triple junctions of low-angle grain boundaries are found to be effective pinning centers, even at high temperatures ($\ge$35 K) and/or low magnetic fields., Comment: Main text with Supplement Material, 32 pages, 16 figures

Resistant hypertension is an important cardiovascular risk factor. This analysis of the JAMP study (Japan Ambulatory Blood Pressure Monitoring Prospective) data investigated the effects of uncontrolled resistant hypertension diagnosed using ambulatory blood pressure (BP) monitoring on the risk of heart failure (HF) and overall cardiovascular events. The JAMP study patients with hypertension and no HF history were included. They had true resistant hypertension (24-hour BP ≥130/80 mm Hg), pseudoresistant hypertension (24-hour BP P =0.012 and 2.24 [95% CI, 1.17–4.30]; P =0.015, respectively) and versus uncontrolled nonresistant hypertension (1.51 [1.03–2.20]; P =0.034 and 3.03 [1.58–5.83]; P Registration: URL: https://www.umin.ac.jp/ctr ; Unique identifier: UMIN000020377.

The most effective way to enhance the dissipation-free supercurrent in presence of magnetic field for type II superconductors is the introduction of defects that acts as artificial pinning centres (APCs) for the vortices. For instance, the in-field critical current density of doped BaFe2As2 (Ba122), one of the most technologically important Fe-based superconductors, has been improved over the last decade by APCs created by ion-irradiation. The technique of ion-irradiation has been commonly implemented to determine the ultimate superconducting properties. However, this method is rather complicated and expensive. Here, we report on a surprisingly high critical current density and strong pinning efficiency close to the crystallographic c-axis for a K-doped Ba122 epitaxial thin film without APCs, achieving performance comparable to ion-irradiated K-doped Ba122 single crystals. Microstructural analysis reveals that the film is composed of columnar grains having width around 30-60 nm. The grains are rotated around the b- (or a-) axis by 1.5 degree and around the c-axis by -1 degree, resulting in the formation of low-angle grain boundary networks. This study demonstrates that the upper limit of in-field properties reached in ion-irradiated K-doped Ba122 is achievable by grain boundary engineering, which is a simple and industrially scalable manner., Main text with Supplement Material

Molecular beam epitaxy of K-doped Ba122 (Ba$_{1-x}$K$_x$Fe$_\text{2}$As$_\text{2}$) superconductor was realized on a MgO substrate. Microstructural observation revealed that the undoped Ba122 served as a perfect buffer layer for epitaxial growth of the K-doped Ba122. The film exhibited a high critical temperature of 39.8 K and a high critical current density of 3.9 MA/cm$^\text{2}$ at 4 K. The successful growth of epitaxial thin film will enable artificial single grain boundary on oxide bicrystal substrates and reveal the grain boundary transport nature of K-doped Ba122., 5 pages, 4 figures, accepted manuscript Supercond. Sci. Technol 2022

In order to improve the electrical transport properties of REBa2Cu3O7−δ nanocomposite films, SmBa2Cu3O7−δ films with and without BaHfO3 nanoparticles were grown by chemical solution deposition, and their microstructural and transport properties were investigated in a detailed study using transmission electron microscopy and transport measurements in magnetic fields up to 24 T. The optimization process of the crystallization step (temperature and oxygen partial pressure) as well as an aging effect, which is due to the release of trapped fluorine, are described. Critical temperature and critical current densities surprisingly improve initially during the aging. Due to the complex microstructure, the additional BaHfO3 nanoparticles have only a positive effect at low magnetic fields for our samples.

The recent realisations of hydrogen doped $Ln$FeAsO ($Ln$=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 $J_{\rm c}$. The superconducting transition temperature $T_{\rm c}$ is comparable to NdFeAs(O,F). Transmission electron microscopy investigation supported that hydrogen is homogenously substituted for oxygen. A high self-field $J_{\rm c}$ of over 10 MA/cm$^2$ 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 $J_{\rm c}$ yielded temperature-dependent scaling parameters $\gamma_{\rm J}$ 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, $\gamma_{\rm J}$ 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 $Ln$FeAsO is highly beneficial for achieving high $J_{\rm c}$ with low anisotropy without compromising $T_{\rm c}$, which is favourable for high-field magnet applications., Comment: 17 pages, 6 figures with Supplementary Information, accepted in Scientific Reports

The iron-based superconductor ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ is emerging as a key material for high magnetic field applications owing to the recent developments in superconducting wires and bulk permanent magnets. Epitaxial thin films play important roles in investigating and artificially tuning physical properties; nevertheless, the synthesis of ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ epitaxial thin films remained challenging because of the high volatility of K. Herein, we report the successful growth of epitaxial ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ thin films by molecular-beam epitaxy with employing a combination of fluoride substrates $({\mathrm{CaF}}_{2},$ ${\mathrm{SrF}}_{2}, \mathrm{and} {\mathrm{BaF}}_{2})$ and a low growth temperature (350--420 \ifmmode^\circ\else\textdegree\fi{}C). Our epitaxial thin film grown on ${\mathrm{CaF}}_{2}$ showed sharp superconducting transition at an onset critical temperature of 36 K, slightly lower than bulk crystals by \ensuremath{\sim}2 K due presumably to the strain effect arising from the lattice and thermal expansion mismatch. Critical current density $({J}_{\mathrm{c}})$ determined by the magnetization hysteresis loop is as high as $2.2\phantom{\rule{0.28em}{0ex}}\mathrm{MA}/\mathrm{c}{\mathrm{m}}^{2}$ at 4 K under self-field. In-field ${J}_{\mathrm{c}}$ characteristics of the film are superior to the bulk crystals. The realization of epitaxial thin films opens opportunities for tuning superconducting properties by epitaxial strain and revealing intrinsic grain boundary transport of ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$.

The electric transport properties of a GdBa2Cu3O 7 − δ thin film containing 12 mol% nano-sized BaHfO3 (BHO) particles grown by chemical solution deposition were investigated in a wide range of temperatures (4.2 ≤T ≤ 77 K) and magnetic fields up to μ 0 H = 19 T. The exponent n of the electric field–current density characteristics ( E ∝ J n ) depends on critical current density J c as (n–1) ∝ J c α (α ∼ 0.45) irrespective of measurement temperature for H ∥ c. On the other hand, this relation does not hold for H ∥ ab. The angular dependence of J c is almost similar to that of n except for the angle close to the ab-plane. A dip of n around this angle regime was observed below 77 K, whereas J c exhibited a maximum. At T ≤ 50 K a tiny peak in the dip was observed that increases with decreasing temperatures. These results suggest that the pinning mechanism changes with temperature for H ∥ ab.

We have grown thin films of CaAgAs by molecular beam epitaxy, which was theoretically proposed to be a topological insulator. The temperature dependence of resistivity and the carrier concentration at 4 K were similar to the reported results of bulk samples. However, the magnetoresistance exhibited a steep increase at low magnetic fields, a behavior not observed for bulk samples. This steep increase of resistivity is ascribable to the weak antilocalization effect and provides clues to the nature of the topological surface state of CaAgAs.

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.

NdFeAs(O,F) thin films having different fluorine contents were grown on 5 deg. or 10 deg. vicinal cut MgO and CaF2 single crystalline substrates by molecular beam epitaxy. Structural characterisations by 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 were derived from the resistivity measurements in the longitudinal and transversal directions. The c-axis resistivity was always higher than the ab-plane resistivity, resulting from the anisotropic electronic structure. The resistivity anisotropy at 300 K was almost constant in the range of 50-90 irrespective of the F content. On the other hand, the resistivity anisotropy at 56 K showed a strong fluorine dependence: the resistivity anisotropy was over 200 for the films with optimum F contents (superconducting transition temperature Tc around 50 K), whereas the resistivity anisotropy was around 70 for the films in the under-doped regime (Tc between 35 and 45 K). The mass anisotropy 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 the ab-plane resistivity was in the range from 2 to 5. On the assumption that the square of the mass anisotropy is equal to the resistivity anisotropy, those values are small compared to the normal state anisotropy.

Grain boundaries (GBs) as two-dimensional defects strongly influence and determine the current carrying capabilities of superconductors, may it be by providing efficient pinning centres or by constituting weak links. In this chapter, GB issues of polycrystalline bulks and wires as well as polycrystalline and bicrystal thin films of Fe-based superconductors are discussed for the most-studied, application-relevant systems: LnFeAs(O,F) (Ln: lanthanoid), P-, Co-, and Ni-containing AeFe2As2, (Ae: alkali earth), and Fe(Se,Te) in comparison to the cuprate high-Tc superconductors.

Superconductivity (SC) in the Ba-122 family of iron-based compounds can be controlled by aliovalent or isovalent substitutions, applied external pressure, and strain, the combined effects of which are sometimes studied within the same sample. Most often, the result is limited to a shift of the SC dome to different doping values. In a few cases, the maximum SC transition at optimal doping can also be enhanced. In this work, we study the combination of charge doping together with isovalent P substitution and strain by performing ionic gating experiments on ${\mathrm{BaFe}}_{2}$(${\mathrm{As}}_{0.8}{\mathrm{P}}_{0.2}$)${}_{2}$ ultrathin films. We show that the polarization of the ionic gate induces modulations to the normal-state transport properties that can be mainly ascribed to surface charge doping. We demonstrate that ionic gating can only shift the system away from the optimal conditions, as the SC transition temperature is suppressed by both electron and hole doping. We also observe a broadening of the resistive transition, which suggests that the SC order parameter is modulated nonhomogeneously across the film thickness, in contrast with earlier reports on charge-doped standard BCS superconductors and cuprates.

An epitaxial NdFeAs(O,F) thin film of 90 nm thickness grown by molecular beam epitaxy on MgO single crystal with Tc = 44.2 K has been investigated regarding a possible vortex glass-liquid tran-sition. The voltage-current characteristics show excellent scalability according to the vortex-glass model with a static critical exponent {\nu} of around 1.35 and a temperature-dependent dynamic exponent z increasing from 7.8 to 9.0 for the investigated temperature range. The large and non-constant z values are discussed in the frame of 3D vortex glass, thermally activated flux motion, and inhomogeneity broadening., Comment: 20 pages, 4 figures

Fe-based superconductors present a large variety of compounds whose physical properties strongly depend on the crystal structure and chemical composition. Among them, the so-called 1111 compounds show the highest critical temperature Tc in the bulk form. Here we demonstrate the realization of excellent superconducting properties in NdFeAs(O1−xFx). We systematically investigated the correlation between the microstructure at the nanoscale and superconductivity in an epitaxial 22 nm NdFeAs(O1−xFx) thin film on a MgO single crystalline substrate (Tc = 44.7 K). Atomic resolution analysis of the microstructure by transmission electron microscopy and atom probe tomography identified several defects and other inhomogeneities at the nanoscale that can act as extrinsic pinning centers. X-Ray diffraction and transmission electron microscopy displayed a broad variation of the a-axis lattice parameter either due to a partially strained layer at the interface to the substrate, high local strain at dislocation arrays, mosaicity, or due to composition variation within the film. The electrical transport properties are substantially affected by intrinsic pinning and a matching field corresponding to the film thickness and associated with the Bean–Livingston surface barrier of the surfaces. The thin film showed a self-field critical current density Jc(4.2 K) of ∼7.6 MA cm−2 and a record pinning force density of Fp ≈ 1 TN m−3 near 35 T for H‖ab at 4.2 K. These investigations highlight the role of the microstructure in fine-tuning and possibly functionalizing the superconductivity of Fe-based superconductors.

The possibility of p-wave pairing in superconductors has been proposed more than five decades ago, but has not yet been convincingly demonstrated. One difficulty is that some p-wave states are thermodynamically indistinguishable from s-wave, while others are very similar to d-wave states. Here we studied the self-field critical current of NdFeAs(O,F) thin films in order to extract absolute values of the London penetration depth, the superconducting energy gap, and the relative jump in specific heat at the superconducting transition temperature, and find that all the deduced physical parameters strongly indicate that NdFeAs(O,F) is a bulk p-wave superconductor. Further investigation revealed that single atomic layer FeSe also shows p-wave pairing. In an attempt to generalize these findings, we re-examined the whole inventory of superfluid density measurements in iron-based superconductors show quite generally that most of the iron-based superconductors are p-wave superconductors., Comment: 35 pages, 15 figures

We report on the growth of NdFeAs(O,F) superconducting thin films by molecular beam epitaxy without having a NdOF secondary layer that was necessary for fluorine doping in our previous studies. The key to realizing the direct growth of a superconducting film was the enhancement of migration of the raw materials on the substrate, which was accomplished by two steps. Firstly, we increased the growth temperature that improved the crystalline quality of parent NdFeAsO thin films. Secondly, the atmosphere in the chamber during the growth was improved by changing the crucible material of the Fe source cell. Highly textured NdFeAs(O,F) thin films with critical temperatures up to 50 K were obtained, and terraces were observed by atomic force microscope, indicating a two-dimensional growth. However, precipitates were also found on the surface, which suggests that enhancing further the migration is necessary for obtaining a NdFeAs(O,F) thin film with a better quality.

We report on the growth of NdFeAs(O,H) epitaxial thin films on MgO (001) substrates and their electrical transport properties. The maximum carrier density of the NdFeAs(O,H) films was more than twice as large as that of our NdFeAs(O,F) films. This enabled us to investigate the physical properties of heavily electron doped NdFeAsO, which could not be achieved with NdFeAs(O,F). The irreversibility field Hirr of NdFeAs(O,H) was larger than that of NdFeAs(O,F) for H || c due to the decrease in the anisotropy of the upper critical field. A very high critical current density of 17 MA cm^−2 was recorded at 4 K, which corresponds to about 13% of the depairing current density of NdFeAs(O,F)., ファイル公開：2021-09-01

Submicron sized strip lines of NdFeAs(O,F) were fabricated and the transport properties were studied. By using laser lithography and dry etching, strip lines with a width of down to 0.84 μm were fabricated. The superconducting critical temperature (T c) was not seriously affected by the fabrication process, but a significant decrease of the critical current density (J c) was observed for strip lines narrower than 2 μm when the sample was patterned by isotropic etching. By employing an anisotropic etching process, on the other hand, the deterioration of J c was largely prevented, and the yield of obtaining a strip line with J c higher than 5 MA/cm2 rose considerably.

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., Invited review

We found a large and unique magnetoresistance (MR) effect for ${\mathrm{CaMn}}_{2}{\mathrm{Bi}}_{2}$. When the magnetic field was applied along the crystallographic $c$ axis at low temperatures, the resistivity increased with the magnetic field and the MR ratio reached several hundred percent, but then it decreased with further increasing the applied field. In addition, the angle dependence measurement revealed a strong anisotropy. This compound is an antiferromagnetic semiconductor with a narrow band gap, and Mn atoms form a corrugated honeycomb lattice. Therefore, a frustration among the magnetic moments is expected, and we propose that our observations can be understood by a nonmonotonic modulation of magnetic fluctuation under the magnetic field.

The discovery of Fe-based superconductors (FBS) as the second class of high-temperature superconducting transition (high-Tc) materials after the cuprates generated a significant impact on the community of fundamental and applied superconductivity research. Whenever a new class of high-Tc superconducting materials is discovered, a lot of effort is devoted to growing single crystals and epitaxial thin films for exploring basic physical quantities. Although several properties of FBS are similar to the cuprates (large upper critical fields, resulting from a short coherence lengths, and small carrier density), others are distinctly different. For instance, in FBS the symmetry of the superconducting order parameter is most likely not a d-wave but an s+-wave, depending on the stoichiometry, crystallographic system, and doping level. Additionally, the critical current densities of FBS are less sensitive to the presence of grain boundaries (GBs) than those of the high-Tc cuprates. These features are highly beneficial for the realization of cheaper conductors for high-field magnets at low temperatures. Indeed, several groups have demonstrated FBS thin films on technical metallic substrates and powder-in-tube processed FBS wires as proof-of-principle studies for conductor applications. FBS on technical substrates also give many opportunities for studying how GB networks affect the critical current and how uniaxial strain impacts the superconducting properties. In this article, we review FBS thin films, especially on technical metallic substrates, and focus on application-relevant properties like pinning improvement by natural and artificial defects as well as the transparency of GBs and GB networks. The recent development of FBS thin films on technical substrates and their superconducting properties are presented and the performance gap with respect to films on single crystals is discussed., Comment: Figures and some of the tables are placed at the end of document

Nanowires of Co-doped BaFe 2 As 2 have been realized and characterized. Thin films having thickness of 20 nm were grown on CaF 2 (100) substrates by pulsed laser deposition. The films were then passivated in situ with a thin layer of MgAl 2 O 4 . Using electron beam lithography, several submicron nanowire structures were patterned and electrically contacted to gold pads. The nanostructures have a superconducting critical temperature T of 16 K, for a 500-nm-wide nanowire, and properties, in terms of normal state resistivity (0.3 mΩ·cm) and critical current (1 MA/cm 2 at 3 K), comparable with those of NbN nanowires. Moreover, they show localization effects in the normal state, while the current-voltage characteristics are dominated by flux creep.

The physical properties of $\mathrm{M}{\mathrm{n}}_{3}\mathrm{CuN}$ 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 (${T}_{C}$) and a sign change of the Hall coefficient at ${T}_{C}$. The analysis of Hall coefficient data revealed a sizable decrease of hole concentration and a large increase of electron mobility below ${T}_{C}$, which is discussed in relation to the electronic structure of this material.

Superconducting FeSe films were electrochemically deposited on rolling-assisted biaxially textured substrate (RABiTS) tape. We observed zero resistivity in the as-electrodeposited FeSe film prepared on the RABiTS when the appropriate voltage was applied while it was dipped into the solution. When the RABiTS tape was dipped in the solution without applying voltage, a thin Se film was deposited on the substrate. The compositional ratio of the FeSe film got closer to the stoichiometric ratio with decreasing the dipping time before applying voltage.

We prepared planar hybrid Josephson junctions using Co-doped BaFeAs thin films and polished single crystals as base electrode, sputtered, and subsequently oxidized titanium layers as barrier and a counter electrode from evaporated lead. On all junctions, RCSJ-like current–voltage characteristics could be observed. The obtained products at 4.2 K range up to 95V for thin film junctions and up to 1.15 mV for junctions on single crystals. For most of the junctions, an excess current with variable contribution to the total current could be observed. Unusual temperature dependencies are discussed for the critical and the excess currents, respectively. Investigations of the junctions under microwave irradiation were performed, therein Shapiro steps up to high orders and very clear modulations of the steps could be observed. Additionally, the origin of the excess current is discussed with respect to the microwave dependence. Furthermore, instead of a single junction, a network-like modulation behavior of the critical current in magnetic fields was observed.

$\hbox{La}_{{\kern-.5pt}1{\kern-.5pt}.{\kern-.5pt}85}{\kern-.5pt}\hbox{Sr}_{{\kern-.5pt} 0{\kern-.5pt}.{\kern-.5pt}15}{\kern-.5pt}\hbox{CuO}_{{\kern-.5pt}4}$ , $\hbox{BaFe}_{{\kern-.5pt}1{\kern-.5pt}.{\kern-.5pt}8}{\kern-.5pt}\hbox{Co}_{{\kern-.5pt} 0{\kern-.5pt}.{\kern-.5pt}2}{\kern-.5pt}\hbox{As}_{{\kern-.5pt}2}$ and $\hbox{FeSe}_{{\kern-.5pt}0{\kern-.5pt}.{\kern-.5pt}5}{\kern-.5pt}\hbox{Te}_{{\kern-.5pt}0.5}$ thin films have been prepared on piezoelectric (001) $\hbox{Pb}(\hbox{Mg}_{1/3}\hbox{Nb}_{2/3})_{0.72}\hbox{Ti}_{0.28}\hbox{O}_{3}$ substrates. By the use of the inverse piezoelectric effect, biaxial strain was induced into the films by applying an electric field to the sample. The strain sensitivity of the upper critical field and of the irreversibility line was determined by the analysis of the resistive transition. A biaxial compression in the order of 0.02% results in a shift of these fields. A consistent description of the field dependence is possible taking into account a strain dependent transition temperature. No influence of the strain state on the anisotropy of the upper critical field was found in first approximation.

YBa 2 Cu 3 O 7-δ (YBCO) films with a thickness of up to 2 μm containing nano-sized BaHfO 3 (BHO) have been grown on biaxially textured metal tapes by pulsed laser deposition. Transmission electron microscopy revealed the inclusion of Y 2 O 3 platelets and rod-like BHO structures. A c-axis oriented growth of the YBCO layer and biaxial oriented incorporation of BHO is confirmed by X-Ray diffraction. BHO expands the c-axis of YBCO, flattens the surface of YBCO and densifies the films. Y 2 O 3 inclusions get smaller and finer distributed, possibly with a positive influence on pinning effects. A superconducting transition temperature T c of about 89 K was determined, decreasing slightly with increasing BHO content. Transport current measurements in the maximum Lorentz force configuration showed an increased irreversibility field H irr and higher pinning force density F p with increasing BHO content in high magnetic fields.

The behavior of submicrometer-sized thin-film YBa 2 Cu 3 O 7-x (YBCO) detectors under illumination with picosecond terahertz (THz) pulses was investigated. Real-time measurements with a temporal resolution of 15 ps full width at half maximum were performed at ANKA, the synchrotron facility of Karlsruhe Institute of Technology, and the UVSOR-III facility at the Institute for Molecular Science in Okazaki, Japan. The capability of YBCO detectors to reproduce the shape of a several picosecond long THz pulse was demonstrated. Single-shot measurements adhering to a reversal of the direction of the electrical field of the THz radiation were carried out. They provided evidence for the electrical field sensitivity of the YBCO detector. Exploiting the electrical field sensitivity of the YBCO detector, the effect of microbunching was observed at UVSOR-III.

The effect of 200-keV He+ ion irradiation on the transport properties of films of the iron-based Ba(Fe1 − x Co x As)2 superconductor has been studied. Contributions to the resistivity and magnetoresistance of irradiated samples from scattering by magnetic and nonmagnetic defects have been separated. It has been shown that mainly nonmagnetic defects are generated in the sample under the corresponding irradiation conditions. This result is important in view of the use of the radiation technique for the study of the effect of defects on the properties of iron-based superconductors.

The temperature dependence of the resistivity of epitaxial Ba(Fe1−xCox)2As2 thin films (with nominal doping x = 0.08, 0.1 and 0.15) has been analysed and compared with analogous measurements on single crystals taken from literature. The ρ(T ) of thin films looks different from that of single crystals, even when the cobalt content is the same. All ρ(T ) curves can be fitted by considering an effective two-band model (with holes and electrons bands) in which the electrons are more strongly coupled with the bosons (spin fluctuations) than holes, while the effect of impurities is mainly concentrated in the hole band. Within this model the mediating boson has the same characteristic energy in single crystals and thin films, but the shape of the transport spectral function at low energy has to be very different, leading to a "hardening" of the electron-boson spectral function in thin films, associated with the strain induced by the substrate

The terahertz and infrared spectra of the complex dynamic conductivity, as well as the temperature dependences of the density of a superconducting condensate and the electronic specific heat of superconducting Ba(Fe1 − x Co x )2As2 compounds, have been analyzed within a Bardeen-Cooper-Schrieffer-like model of a multiband superconductor with strong coupling. It has been shown that the superconducting state of these compounds is determined by three (one electronic and two hole) weakly interacting condensates. The order parameters of the condensates are: Δ1 ≈ 15 cm−1, Δ2 ≈ 21 cm−1, and Δ3 ≈ 30–35 cm−1. The results significantly refine the existing notions on the structure of the superconducting state of Co-doped BaFe2As2 multiband compounds.

We have realized nanowires using Co-doped BaFe2As2 thin films with 20 nm thickness deposited on CaF2 (100) substrates by pulsed laser deposition. The films were passivated in-situ with a thin layer of MgAl2O4. Subsequently, several submicron nanowire structures were patterned using electron beam lithography, and electrically contacted to gold pads. Electrical characterizations of the realized nanostructures show good properties in terms of material resistivity and critical current, with a Tc of 16 K for a 500 nm wide nanowire.

High temperature (high-Tc) 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-Tc Nb3Sn due probably to cost and processing issues. The recent discovery of a second class of high-Tc 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, Hc2, moderate Hc2 anisotropy, and intermediate Tc. Here we report on in-field transport properties of P-doped BaFe2As2 (Ba-122) thin films grown on technical substrates (i.e., biaxially textured oxides templates on metal tapes) by pulsed laser deposition. The P-doped Ba-122 coated conductor sample exceeds a transport Jc of 10^5 A/cm^2 at 15 T for both major crystallographic directions of the applied magnetic field, which is favourable for practical applications. Our P-doped Ba-122 coated conductors show a superior in-field Jc over MgB2 and NbTi, and a comparable level to Nb3Sn above 20 T. By analysing the E-J curves for determining Jc, 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., Comment: Final accepted version can be found in Sci. Rep. 7, 39951 (2017)

We report on the growth of NdFeAs(O,F) thin films on [001]-tilt MgO bicrystal substrates with misorientation angle theta_GB=6{\deg}, 12{\deg}, 24{\deg} and 45{\deg}, and their inter- and intra-grain transport properties. X-ray diffraction study confirmed that all our NdFeAs(O,F) films are epitaxially grown on the MgO bicrystals. The theta_GB dependence of the inter-grain critical current density Jc shows that, unlike Co-doped BaFe2As2 and Fe(Se,Te), its decay with theta_GB is rather significant. As a possible reason of this result, fluorine may have diffused preferentially to the grain boundary region and eroded the crystal structure., Comment: Proceedings of the 30th International Symposium on Superconductivity, 7 figures. Accepted for publication

We report on electrical transport properties of epitaxial Ba(Fe1-xNix)2As2 thin films grown by pulsed laser deposition in static magnetic fields up to 35 T. The thin film shows a critical temperature of 17.2 K and a critical current density of 5.7x10^5 A/cm^2 in self field at 4.2 K while the pinning is dominated by elastic pinning at two-dimensional nonmagnetic defects. Compared to single-crystal data, we find a higher slope of the upper critical field for the thin film at a similar doping level and a small anisotropy. Also an unusual small vortex liquid phase was observed at low temperatures, which is a striking difference to Co-doped BaFe2As2 thin films., Comment: Accepted for publication in Applied Physics Letters, 5 figures