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High-entropy oxide (HEO) superconductors have been developed since very recently. Different superconductors can be produced in the form of a high-entropy compound, including REBa

A low-cost chemical solution deposition technique was employed to prepare YBa2Cu3O7-delta (YBCO) nanocomposite films starting from a colloidal solution containing preformed ZrO2 nanocrystals. As previous publications revealed, an increase in the amount of nanocrystals results in a progressive deterioration of the film properties. The parameters that control this process and their interplay are still unknown in detail. Using definitive screening design (DSD), a design-of-experiments approach, allowed determining which of the multiple growth parameters play a key role for improving the superconducting properties of YBCO nanocomposite films even with a large concentration of nanocrystals. In order to show the potential of DSD, it has been applied for the optimization of two different properties: the critical temperature T-c and the full width at half-maximum of the (005) YBCO reflection. This work shows that DSD is a powerful and efficient method that allows optimizing certain processes with a minimal number of experiments.

A series of carbon-doped single crystals with nominal composition FeSe0.5Te0.5 were synthesized by a self-flux method. X-ray diffraction indicated that the lattice parameter c increases with increasing carbon content, suggesting that carbon atoms enter the lattice. The critical current densities were measured and the flux pinning mechanism and its change with doping were analyzed. These samples showed a higher J c in high magnetic fields as well as a narrower superconducting transition. We studied the impact of the introduction of carbon into FeSe0.5Te0.5 on the temperature dependence of the irreversibility field H irr(T) and upper critical field (H c2). The pinning mechanism for the system was obtained via analysis of J c.

Y1- x Gd x Ba2Cu3O7-BaHfO3 (YGBCO-BHO) nanocomposite films containing 12 mol% BHO nanoparticles and different amounts of Gd, x , were grown by Chemical Solution Deposition (CSD) on Ni5W substrates in order to investigate the impact of the rare-earth stoichiometry on the structure and superconducting properties of these films. For Gd contents x > 0.5, epitaxial YGBCO-BHO films with an approximate thickness of 270 nm self-field critical current density J c at 77 K ∼ 1,5 MA/cm² were obtained. The field dependence of the critical current density J c( B ) shows a much larger accommodation field and lower exponents α in J c ∼ B −α values compared to pristine YBCO films. This is both due to the high amount of individual nanoparticles in the matrix as observed in TEM images and the higher critical temperatures T c. The results show that the CSD is a potential candidate for the preparation of RE BCO films in long-length coated conductors.

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

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 magneto-transport properties are systematically measured under c-direction fields up to 33 T for a series of single-crystal films of intercalated iron-selenide superconductor (Li,Fe)OHFeSe. The film samples with varying degree of disorder are grown hydrothermally. We observe a magnetic-field-enhanced shoulder-like feature in the mixed state of the high-Tc (Li,Fe)OHFeSe films with weak disorder, while the feature fades away in the films with enhanced disorder. The irreversibility field is significantly suppressed to lower temperatures with the appearance of the shoulder feature. Based on the experiment and model analysis, we establish a new vortex phase diagram for the weakly disordered high-Tc (Li,Fe)OHFeSe, which features an emergent dissipative vortex phase intermediate between the common vortex glass and liquid phases. The reason for the emergence of this intermediate vortex state is further discussed based on related experiments and models., 15 pages, 4 figures

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

A low-cost chemical solution deposition technique was employed to prepare YBa

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.

Thin films of Co-doped BaFe2As2 of similar thickness (~40 nm) were grown with different growth rates (0.4 Å s−1 and 0.9 Å s−1) by pulsed laser deposition on CaF2(001) substrates. Analytical transmission electron microscopy (TEM) was applied to analyze the microstructure and secondary phases. The formation of BaF2 and a high concentration of planar defects (mainly stacking faults) are observed for the sample grown at a low rate. A higher growth rate results in high-quality epitaxial films with only few antiphase boundaries. A higher T c was measured for the sample grown at a low growth rate, which is attributed to the difference in strain state induced by the high concentration of defects. Large crystalline Fe precipitates are observed in both samples. Chemical analysis shows a pronounced O and slight F content at the planar defects which highlights the role of O in defect formation. Electron-beam-induced irradiation damage during TEM measurements is observed and discussed.

We report a successful fabrication of long-length rolling-assisted biaxially-textured substrate-based coated conductors (CCs) via chemical solution deposition starting from colloidal YBa2Cu3O7-delta (YBCO) solutions containing 5 mol-% preformed BaMO3 (M = Hf, Zr) (BMO) nanocrystals. Partial optimization of the existing continuous reel-to-reel YBCO processing via design of experiments approach allowed us to obtain YBCO nanocomposites that retain 80%-90% self-field performance of the pristine CC and show a 10%-40% improvement of in-field critical current, which is only a moderate performance improvement compared to similar films on single-crystal substrates prepared on lab-scale. Based on x-ray diffraction and transmission electron microscopy studies, we attribute this to worsening of the YBCO texture and strong coarsening of BMO nanoparticles, particularly, BaZrO3, during processing. Possible process improvements to overcome these effects are discussed.

The technology of electron beam (e-beam) quantitative evaporation of oxide powders at THEVA is quite unique among the many standard in-situ growth methods of high-temperature superconducting (HTS) films. As well known for YBa2Cu3O7-x (YBCO) film growth the temperature and oxygen pressure should be in proximity of the YBCO stability line in the Bormann diagram. In case of e-beam quantitative evaporation, we observe a shift in the DyBa2Cu3O7-x (DyBCO) stability line towards lower pressures and higher temperatures compared to YBCO. We investigated the temperature and oxygen partial pressure boundaries of epitaxial DyBCO growth on inclined substrate deposited MgO (ISD-MgO) buffered Hastelloy tapes. For completeness, the influence of powder stoichiometry and evaporation rate on HTS epitaxial growth is shown. For the first time high-quality HTS films have been grown by e-beam quantitative evaporation without employing an oxygen shuttle and with oxygen in the chamber background instead.

Pristine and BaHfO3 (BHO) nanocomposite GdBa2Cu3O7 (GdBCO) superconducting thin films were deposited at 800 °C, 0.4 mbar O2 partial pressure using a Nd:YAG laser (λ=355 nm) with 10 Hz frequency on MgO substrate. The influence of the oxygenation process on flux pinning and electrical transport properties was investigated by changing the annealing temperature (T ann) between 450 °C and 780 °C and the holding time (t hold) between 10 and 30 min. The irreversibility field shows the largest shift toward higher fields for T ann = 450 °C and thus the largest in-field J c. Angular dependence of J c for most of the pristine GdBCO films exhibits a large peak for B‖c, however the sample with T ann = 550 °C has a large peak in the 90°-120° region, i.e. near B ‖ ab. For BHO nanocomposites, a decrease in T ann increases in-field J c by a factor of 2.

REBa$_{2}$Cu$_{3}$O$_{7}$−x (REBCO, RE = rare earth) compounds with different single RE elements were grown via TFA-MOD (metal-organic deposition of trifluoroacetates) to clarify their T$_{c}$ values when grown by the same preparation method and their processing windows; here: the crystallisation temperatures at a constant process gas composition (pO$_{2}$, pH$_{2}$O). We focussed on the lanthanides (Ln) Nd, Sm, Gd, Dy, Ho, Er and Yb as substituents for Y in the REBCO phase and investigated their growth behaviour in terms of resulting physical (inductive T$_{c}$ and J$_{c}$(77 K)) and structural properties (determined by XRD, SEM, TEM). All phases were grown as pristine films on LaAlO$_{3}$ and SrTiOx$_{3}$ and compared to their respective nanocomposites with 12 mol% BaHfO$_{3}$ for in-field pinning enhancement. With regard to T$_{c}$ and J$_{c}$(77 K), the optima of both values shift towards higher growth temperatures for increasing and decreasing RE ion size with respect to yttrium. Highest T$_{c}$ values achieved so far do not show a trend that can solely be related to the RE ionic size. On the contrary, T$_{c,90}$ values of the LnBCO compounds from Sm to Er range between 94.0 and 95.3 K and are, therefore, significantly larger than the highest values of the average-size non-lanthanide, Y, with T$_{c,90}$ = 91.5 K. J$_{c,sf}$ values at 77 K seem to plateau between 5 and 6 MA cm$^{-2}$ from Sm to Er and are again clearly above the maximum values we ever achieved for Y with 4.2 MA cm$^{-2}$. REBCO phase formations of the very small Yb and large Nd turned out to be more difficult and require further adjustment of growth parameters. All REBCO phases investigated here show distinct dependences of T$_{c}$ on the lattice parameter c.

The electrical transport properties of (Li,Fe)OHFeSe films have been investigated in detail. The sharply textured films, prepared by matrix-assisted hydrothermal epitaxy (MHE) on LaAlO3, show a zero-resistance critical temperature T c of ∼42 K, J c values well above 1 MA cm−2 at low temperatures, and a maximum pinning force density F P of ∼100 GN m−3 at 4 K. The activation energy U 0 for thermal depinning of flux lines has been resolved for low magnetic fields, it agrees well with literature data. The coherence lengths and penetration depth were estimated via upper critical field B c2 and self-field J c, respectively, to be ξ ab ∼ 2.7 nm, ξ c = 0.24 nm, and λ ab ∼ 160–200 nm. The layered crystal structure leads to highly anisotropic and two-dimensional electrical properties, including trapping and lock-in of vortices.

Chemical solution deposition (CSD) of YBa2Cu3O7−δ (YBCO) nanocomposites from colloidal precursor solutions containing double metal oxide preformed nanocrystals is a promising, cost effective and reproducible approach to produce superconducting films with high critical current density (Jc) and enhanced pinning. Here, the influence of the preformed nanocrystal composition on the microstructure and superconducting properties of the YBCO nanocomposite films is studied, with a focus on establishing a simple and scalable process to grow nanocomposites that can be transferred to grow nano-added coated conductors. Colloidal stable BaZrO3, BaHfO3, BaTiO3 and SrZrO3 nanocrystals (3-6 nm in diameter) were synthesized and added to an environment-friendly low-fluorine YBCO precursor solution. High-quality superconducting layers were grown on LaAlO3 single-crystal substrates from these four nanocomposite precursor solutions in a single deposition process, without the need of a seed layer, yielding Jc of 4-5 MA/cm² at 77 K in self-field. The different YBCO microstructures produced by the four types of nanocrystals and the resulting microstrain of the films are compared and related with the magnetic-field and angular dependence of Jc. We demonstrate the BaHfO3-containing nanocomposite as the best-performing with a homogeneous distribution of nanoparticles with 7 nm in average diameter and a high density of stacking faults, which leads to some of the best superconducting properties ever achieved via low-fluorine CSD. The Jc exhibits a much smoother decay in applied magnetic fields and a much more isotropic behaviour for non-parallel magnetic fields, and the pinning force is increased by a factor of 3.5 at 77 K and 1 T with respect to the pristine film.

Chemical solution deposition (CSD) was used to grow Y1−xGdxBa2Cu3O7−δ-BaHfO3 (YGBCO-BHO) nanocomposite films containing 12 mol% BHO nanoparticles and various amounts of Gd, x, on two kinds of buffered metallic tapes: Ni5W and IBAD. The influence of the rare-earth stoichiometry on structure, morphology and superconducting properties of these films was studied. The growth process was carefully studied in order to find the most appropriate growth conditions for each composition and substrate. This led to a clear improvement in film quality, probably due to the reduction of BaCeO3 formation. In general, the superconducting properties of the films on Ni5W are significantly better. For x > 0.5, epitaxial ~270 nm thick YGBCO-BHO films with Tc > 93 K and self-field Jc at 77 K ~2 MA/cm² were obtained on Ni5W. These results highlight the potential of this approach for the fabrication of high-quality coated conductors.

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.

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

Superconducting GdBa2Cu3O7 films with 2.5 wt% of BaHfO3 were deposited by pulsed laser deposition using an Nd:YAG laser ( λ = 355 nm), and their structural and electrical properties were investigated in dependence of substrate temperature T sub and deposition rate (i.e., laser repetition rate v dep). The irreversibility lines show the strongest shift toward higher magnetic fields at intermediate T sub and lower v dep. The self-field J c decreases by a factor of 2 compared with the pristine sample, but the situation improves in the high field region. The angular dependence of J c at 1, 5, and 9 T shows a large c -axis peak at 180o, which indicates strongly correlated pinning at BHO nanocolumns.

We studied the impact of soft ferromagnetic permalloy (Py) on the shielding currents in a strong-pinning superconductor—YBa2Cu3O7−δ with Ba2Y(Nb/Ta)O6 nano-precipitates—by means of scanning transmission x-ray microscopy. Typically and in particular when in the thin film limit, superconductor/ferromagnet (SC/FM) bilayers exhibit isotropic properties of the flux line ensemble at all temperatures. However, in elements with small aspect ratio a significant anisotropy in flux penetration is observed. We explain this effect by local in-plane fields arising from anisotropic magnetic stray fields originated by the ferromagnet. This leads to direction-dependent motion of magnetic vortices inside the SC/FM bilayer. Our results demonstrate that small variations of the magnetic properties can have huge impact on the superconductor.

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 competition between phase formation of BaF2 and Ba(Fe1−x Co x )2As2 on CaF2 single crystals has been analysed. Ba(Fe0.92Co0.08)2As2 thin films have been deposited by pulsed laser deposition. X-ray diffraction, atomic force microscopy and scanning electron microscopy studies have revealed that the formation of secondary phases and misorientations as well as the growth modes of the Ba(Fe0.92Co0.08)2As2 thin films strongly depend on the growth rate. At high growth rates, formation of BaF2 is suppressed. The dependency of the Ba(Fe0.92Co0.08)2As2 lattice parameters supports the idea of fluorine diffusion into the crystal structure upon suppression of BaF2 formation similar as was proposed for FeSe1−x Te x thin films on CaF2. Furthermore, a growth mode transition from a layer growth mechanism to a three-dimensional growth mode at high supersaturation has been found, suggesting similarities between the growth mechanism of iron-based superconductors and high-T c cuprate thin films.

We investigate the field, angle and temperature dependence of the full-width critical current, I c, of pulsed laser deposition-grown GdBa2Cu3O7 coated conductors with and without additional 6 mol% BaSnO3 (BSO) nanoparticles fabricated by SuperOx. The transport characteristics measured from 7 to 77 K and in applied magnetic fields of up to 6 T are complemented by scanning transmission electron microscopy. This combined approach allows for further insight into the vortex pinning mechanism and helps with understanding the enhancement in I c. An exemplary scaling of the pinning force curves versus field at different temperatures confirms the additional contribution to pinning by the BSO nanoparticles. Through the temperature dependence of I c, the weak and strong pinning contributions are determined: strong pinning dominates over almost the entire temperature range especially near the matching field of 1 T, where the largest enhancement in I c is achieved.

Y

Most commercial high-temperature superconducting coated conductors based on ion beam assisted MgO deposited templates use LaMnO3 (LMO) films as the terminating buffer layer. In contrast, coated conductors based on inclined substrate deposition (ISD)-MgO technology are still produced with homoepitaxial (homoepi)-MgO as the cap layer. In this work we report on the deposition of LMO buffer layers on ISD-MgO/homoepi-MgO by electron beam physical vapor deposition. The growth parameters of textured LMO films were studied systematically and their properties were optimized regarding the critical current density (J c) of the subsequently deposited DyBa2Cu3O7−δ (DyBCO) superconducting films. LMO films without outgrowths at the surface were obtained at growth rates of up to 4 Å s−1. Despite the formation of non-stoichiometric LMO films containing 59% La, single-phase films were obtained at substrate temperatures below 775 °C and at oxygen partial pressures of up to 4 × 10−4 mbar due to a large homogeneity region towards La. The J c values of DyBCO films deposited on LMO were found to be independent of the LMO thickness in a range from 50 nm to 450 nm. DyBCO films on LMO reach J c = 0.83 MA cm−2 at 77 K in zero applied field. This value is up to 30% higher than those of DyBCO films grown directly on homoepi-MgO. The wide range of LMO growth parameters and higher J c values of DyBCO on LMO compared to DyBCO on homoepi-MgO make this material attractive for its use in manufacturing coated conductors based on ISD-MgO technology.

Pure and 6 mol% BaHfO3 (BHO)-doped $\text{YBa}_{2}\text{Cu}_{3}\text{O}_{7-\delta}$ (YBCO) films with a film thickness of around 1 $\mu\text{m}$ were prepared on CeO2-buffered ABAD-YSZ templates by pulsed laser deposition. The self-field $J_{\mathrm{c}}$ at 77 K reaches 1.1 MA/cm2 in the doped sample compared with 2.5 MA/cm2 in pure YBCO. Above a magnetic field of 2.2 T along $B\|c$ , $J_{\mathrm{c}}$ of the BHO-doped sample exceeds the $J_{\mathrm{c}}$ of the undoped film. The maximum pinning force density $(F_{\mathrm{P},\max})$ reaches a value of around 3 GN/cm2 for both samples, but $B(F_{\mathrm{P},\max})$ increases from 1.4 T (pure) to a value of 2.9 T (BHO:YBCO). The $J_{\mathrm{c}}$ anisotropy curves of the doped sample show a large and broad peak at $B\|c$ and a strongly reduced anisotropy at all temperatures and fields compared with the pure sample. A complex defect structure with YBa2Cu4O8 intergrowths, Y2O3 precipitates, and BHO nanocolumns with a fan-shaped structure is observed by transmission electron microscopy investigations, which can explain the measured $J_{\mathrm{c}}(B,\theta)$ behavior.

Quasi-multilayer films with perovskite nano-particles have already been investigated and reported in YBCO compounds. Here, we introduce this technique to iron based superconductors by preparing quasi-multilayer nanocomposite films of Co-doped BaFe2As2 with BaZrO3 and BaHfO3 perovskite nanoparticles by a pulsed laser deposition technique using a frequency-tripled Nd:YAG laser (λ = 355 nm) with separate targets of the constituents. We investigated the enhancement of pinning force density and critical current density in relation to critical temperature reduction. Application-relevant properties are discussed in relation to the deposition conditions and the microstructural properties of the films to better understand growth and behavior of strong pinning centers in these materials.

A pristine YBa2Cu3O6+δ film as well as two Zn-doped YBa2(Cu1−x Zn x )3O6+δ films with doping levels x of 0.2 % and 0.4 %, respectively, grown epitaxially on SrTiO3 single crystalline substrates by pulsed laser deposition, were investigated regarding the effect of the doping on the glass-liquid transition line and the pinning potential distribution. Zn doping did not show any effect on the shape of the transition line but caused a suppression of the transition temperature T GL of about 16 K%−1. Although the doping did not lead to an additional pinning effect, the increase of the critical current density j c towards lower temperatures is mainly driven by the variation of the threshold critical current density, j cm. The width of the critical current density distribution j 0 saturates at low temperatures and only drops above ~ 0.9T GL.

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

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

Recent efforts in the development of YBa2Cu3O7-x (YBCO) coated conductors are devoted to the increase of the critical current I-c in magnetic fields. This is typically realized by growing thicker YBCO layers as well as by the incorporation of artificial pinning centers. We studied the growth of doped YBCO layers with a thickness of up to 7 mu m using pulsed laser deposition with a growth rate of about 1.2 nm s(-1). Industrially fabricated ion-beam textured YSZ templates based on metal tapes were used as substrates for this study. The incorporation of BaHfO3 (BHO) or Ba2Y(Nb0.5Ta0.5)O-6 (BYNTO) secondary phase additions leads to a denser microstructure compared to undoped films. A purely c-axis-oriented YBCO growth is preserved up to a thickness of about 4 mu m, whereas misoriented texture components were observed in thicker films. The critical temperature is slightly reduced compared to undoped films and independent of film thickness. The critical current density J(c) of the BHO- and BYNTO-doped YBCO layers is lower at 77 K and self-field compared to pure YBCO layers; however, I-c increases up to a thickness of 5 mu m. A comparison between films with a thickness of 1.3 mu m revealed that the anisotropy of the critical current density J(c)(theta) strongly depends on the incorporated pinning centers. Whereas BHO nanorods lead to a strong B vertical bar vertical bar c-axis peak, the overall anisotropy is significantly reduced by the incorporation of BYNTO forming a mixture of short c-axis-oriented nanorods and small (a-b)-oriented platelets. As a result, the J(c) values of the doped films outperform the undoped samples at higher fields and lower temperatures for most magnetic field directions.

$\hbox{La}_{{\kern-.5pt}1{\kern-.5pt}.{\kern-.5pt}85}{\kern-.5pt}\hbox{Sr}_{{\kern-.5pt} 0{\kern-.5pt}.{\kern-.5pt}15}{\kern-.5pt}\hbox{CuO}_{{\kern-.5pt}4}$ , $\hbox{BaFe}_{{\kern-.5pt}1{\kern-.5pt}.{\kern-.5pt}8}{\kern-.5pt}\hbox{Co}_{{\kern-.5pt} 0{\kern-.5pt}.{\kern-.5pt}2}{\kern-.5pt}\hbox{As}_{{\kern-.5pt}2}$ 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.

Achieving low cost, safe, reproducible, and high performance superconducting thin films of YBa2Cu3O7-delta is essential to bring this material to the energy market. Here, we report on the chemical solution deposition of YBa2Cu3O7-delta nanocomposites from environmentally benign precursors with a low fluorine content. Preformed ZrO2 nanocrystals (3.5 nm) were stabilized in a methanolic precursor solution via two strategies: charge stabilization and steric stabilization. Counter-intuitively, charge stabilization did not result in high quality superconducting layers, while the steric stabilization resulted in highly reproducible nanocomposite thin films with a self-field J(c) of 4-5 MA cm(-2) (77 K) and a much smaller decay of J(c) with magnetic field compared to YBa2Cu3O7-delta without nanocrystals. In addition, these nanocomposite films show a strong pinning force enhancement and a reduced J(c) anisotropy compared to undoped YBa2Cu3O7-delta films. Given the relationship between the nanocrystal surface chemistry and final nanocomposite performance, we expect these results to be also relevant for other nanocomposite research.