MgB₂ has several attractive natures as superconducting bulk magnet, such as low cost of materials, light weight, weak-link-free homogeneous current flow on a bulk scale and great flexibility of magnet shape designing. In the present study, we performed detailed investigations on trapped field properties and their relationship with local superconducting properties and microstructure of MgB₂ bulks, namely MgB₂ bulks with controlled grain size were synthesized to improve their field trapping properties. The disk-shaped MgB₂ bulks (10-100 mm in diameter and 10 mm in thickness) were fabricated from magnesium and boron powders by using the in-situ technique [1]. To control the grain size of MgB₂, Mg and B powders with molar ratio of 1:2 were mixed and pulverized by ball-milling under various conditions. The mixed powders were pressed into pellet and heated at 600-900°C for 1-24 h under argon atmosphere. Grain size of MgB₂ estimated by secondary electron images was confirmed to be smaller as the rotation speed increased. Grain size of the sample without ball-milling was several micrometers while those ball-milled were several hundred nanometers. Magnetization of MgB₂ bulks was performed by the field-cooling method under external field generated by a superconducting coil magnet. The trapped field as a function of increasing temperature was measured at the center of the surface of bulk sample by using a Hall sensor. The trapped fields of MgB₂ bulk magnet (30 mm in diameter and 10 mm in thickness) without ball-milling process were 2.84, 2.61, 2.27 and 1.85 T at 5, 10, 15 and 20 K, respectively. The MgB₂ bulk prepared from ball-milled powder showed approximately 30% higher trapped field values, 3.72, 3.40, 2.92 and 2.32 T at 5, 10, 15 and 20 K, respectively. The improvement in trapped field is considered to originate in an increase of grain boundary density of MgB₂.
This work was partially supported by the Japan Science and Technology Agency, PRESTO and by Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science under grants Nos. 22860019 and 23246110.
Reference
[1] A. Yamamoto, A. Ishihara, M. Tomita, K. Kishio: Appl. Phys. Lett. 105, 032601 1-4 (2014)

: Progresses in the preparation of superconducting materials are reported. The pressing device named Spark Plasma Sintering (SPS) was modified with the aim of texturing Bi₂Ca₂Sr₂CuO₈ superconductor ceramics. The new process is referred to as “Spark Plasma Texturing” (SPT). During SPT, the bulk material is free to deform itself. As a result, an inter-grain preferential crystallographic orientation is generated, while materials processed by conventional SPS are usually quasi-isotropic.
In other experiments, superconducting MgB₂ discs and rings were consolidated using the rapid SPS process. MgB₂ has not been yet been seriously considered as a superconductor that could be used for magnetic levitation. Here we present preliminary results that show that it presents interesting characteristics. Field cooled levitation force measurements were carried out in a large range of temperatures with a NdFeB magnet as a function of the superconductor-magnet distance. For a given distance, the levitation forces measured between 17 and 32 K take the same value for a ring as for a disc with the same radius. At the minimum magnet-superconductor separation of 1 3mm, the levitation force was around 13 N, that corresponds to a magnetic pressure near 20000 Pa. These measurements have also allowed to determine that the critical temperature is equal to 38 K, in good agreement with the results of other techniques.
These results suggest that bulk MgB₂ superconductors could be a viable variant for magnetic levitation applications. In addition, the similarity of the results obtained with discs and rings support the calculations made by E.H. Brandt [1] that have shown that the screening current in thin disks flow at first along the outside edge of the samples. For both materials, the investigated superconducting properties will be discussed.
[1] E.H.Brandt, Phys.Rev.58, 6506 (1998).

Superconducting properties of MgB₂ are strongly dependent on the boron powder used to perform its synthesis. Here, we describe a method to synthesize nanosized and doped B precursor into the own laboratory. The B prepared with that procedure has been used to prepare MgB₂-based conductors; the wires were manufactured adopting different techniques: in-situ, ex-situ, ½ in-situ - ½ ex-situ (via MgB₄ precursor) and RLI (IMD). A comparison of the superconducting properties among the different techniques and a characterization of boron powder are given. The results support the suitability of the described process to obtain very fine boron particles, which can be homogeneously doped. Preliminary results showed high T_c value for a wire, T_c onset of 39 K, and interesting J_c value were recorded for ex-situ, RLI as well as the via MgB₄ sample. In particular the T_c transition (ΔTasymp;1 K) of doped samples suggested us that good chemical homogeneity can be reached adopting the new B synthesis technique. Furthermore, its implementation for the large scale production of nanostructured boron has been taken in to account.

In this contribution, two different amorphous Boron (B) precursors, boron 95 (purity 95-97%, < 1.5 micron) and nanoboron (purity > 98.5%, < 250nm), were used for the synthesis of bulk and nanosized MgB₂, respectively.
Scanning electron microscope (SEM) images from mixtures of Mg with nanoboron and boron 95 revealed good wetting ability in nanoboron-contained blend. This future enables to reduce synthesis temperature and reaction rate for nanosized MgB₂ in comparison with bulk MgB₂ which is also confirmed by powder x-ray diffraction (PXRD). In addition, transmission electron microscope (TEM) and SEM pictures of nanosized MgB₂ showed that the particle sizes were surprisingly in the range of nanoscale, even smaller than nanoboron precursor. In fact, SEM and TEM pictures demonstrate that there is a linear size correlation between the particles of the reactant B powders and the final product MgB₂. The quantitative Rietveld analysis showed that the concentration of the by-product MgO is reduced by 25% when nanoboron is employed as precursor.
The origin and the role of intrinsic defect centers were also investigated by electron paramagnetic resonance (EPR) spectrometer and the results proved that at nanoscale MgB₂ material contains Mg vacancies whereas EPR signal in bulk MgB₂ is related to oxygen vacancies. Such vacancies influence the connectivity and the conductivity properties which are crucial for the superconductivity applications.
These results showed that the size of boron is one of the important factors to control the MgB₂ quality.

The structural and electronic properties of C₆₀-based solids have been exhaustively explored for more than 20 years. They were long recognized as archetypal examples of molecular superconductors with the highest superconducting transition temperatures (T_c) among all molecular systems known. Their structural and electronic phenomenology and the dominance of strong electron correlations in defining their behavior have posed significant challenges for our understanding of high-temperature superconductivity in these highly correlated organic metals. Here I will review the structures and properties of intercalated fullerides, paying particular attention to the metal-antiferromagnetic-insulator transition at large interfullerene separations.
An apparent exciting development in molecular superconductivity research occurred recently with the reports that alkali metal intercalation of polyaromatic hydrocarbons can lead to the isolation of superconducting materials of unknown composition and structure with T_cs as high as 33 K. However, despite the flare of experimental and theoretical activity in this field, it has not proven possible as yet for the experimental results to be reproduced, while the identities of the putative superconducting phases, including their composition and structure still remain unknown. Therefore, devising new synthetic routes beyond the standard solid state methodology to afford isolation and structural and electronic characterisation of highly-crystalline phase-pure materials in a reproducible fashion is of paramount importance. Here I will report the first results in our exploration of the alkali-phenanthrene phase field -phenanthrene, C₁₄H₁₀ is the smallest phenacene for which superconductivity was reported at 5 K for its K- and Rb-intercalated salts- that include the first unambiguous structural and electronic characterization of alkali phenanthride salts.

Via thin film deposition techniques, we report for the first time stable crystalline hexagonal WO₃ on substrates. Separately, we have also fabricated potassium doped hexagonal KWO₃ thin films via pulsed laser deposition techniques. The hex-WO₃ structure is potentially promising for numerous technological and energy related applications and is one of the structures found for superconducting alkali-doped WO₃. In order to tune the physical properties of the undoped material, we utilized an ionic liquid gating technique. In hex-WO₃, we observe an insulator-to-metal transition, showing the ionic liquid gate to be a viable technique to alter the electrical transport properties of this material. In the KWO₃ samples, we observe superconductivity, as well as disorder induced enhanced Coulomb interactions in the normal state. By comparing the two systems, we conclude with some remarks regarding how superconductivity arises in the hexagonal WO₃.

Over time, various new superconductors have been postulated and realised, ever deepening understanding of the physics of these materials. The synthesis of new superconducting materials not only opens potential routes to high temperature superconductivity, but may give fresh perspectives regarding the mechanism of superconductivity in general and provide cheaper or more processable alternatives to existing materials.
In this work, theoretical postulates presented by Kresin & Ovchinnikov (2006) on the potential of high temperature superconducting metallic clusters are experimentally explored in well-defined atomic clusters of Silver, Gold and Zinc.
We discuss the synthesis of nanoclusters of Silver, Gold and Zinc that have been arranged into macroscopic crystal structures. The nanoclusters exhibit an electronic shell structure increasing the density of states at the Fermi level. A review of the interesting electronic and magnetic behaviour of these materials is explored across a wide range of temperatures, and the experimental realisation of this theory is considered.

Maximising the charge carrier density at the semiconductor interface in a field effect transistor (FET) has always been restricted by the physical limitations of the dielectric material used by the gate electrode. Using a FET setup it is possible to manipulate and examine the electronic behaviour of a material by accumulating charge carriers at the surface. Recent innovations in the area, involving using an ionic liquid (IL) as an electrolyte, have produced a 2D superconducting phase on SrTiO₃ by increasing charge carrier density above what was previously possible. This transition from an insulator to a superconductor was the first of its kind and was achieved using a standard off-the-shelf IL.
This work attempts to describe the possibility of inducing a 2D superconducting phase on a range of novel organic crystals by tailoring the composition of the IL to allow for greater control of carrier density at the IL-crystal interface.
The organic crystals described are polyaromatic hydrocarbons (PAH), hydrogen-terminated, sp² bonded carbon molecules, as some smaller PAH crystals such as picene have already shown superconductivity when chemically doped. As chemical doping is difficult in these systems, field-effect gating should be more accurate means of inducing superconductivity that would be available to a much wider range of crystals.

REBCO coated conductors (CCs) are now fabricated in kilometer lengths and wide applications seems quite possible in the near future. However, they are still far from perfect. Our goal is to develop fast, reliable, nondestructive and low cost methods to study the nature of I_c variations in the conductors. Continuous transport positional variation of I_c, (I_c(x)), of CCs have been non-destructively measured with B||ab-plane and B||c-axis, B=0-1 T, and with a resolution of 2 cm. Full angular dependent I_c measurements for parts of the CC were also carried out and complementary magnetization measurements were carried out using Hall probe arrays with ~ mm spatial resolution, thus allowing measurement of insulated conductors, as well as study in liquid Helium, not just liquid nitrogen. Over the last 3-4 years, the RMS amplitude of the I_c(x) fluctuations has decreased from 6-7% to 1-2%. However, I_c variations between different tapes are still substantial. Some of the differences can be explained by thickness or width variations, but detailed study of the position-, angular-, field- and temperature-dependence of I_c(x) clearly show that cross-section variations alone cannot explain all the I_c changes. For instance, ~ 10% dropouts of I_c in B||c-axis observed in many tapes are accompanied by I_c maxima in the B||ab orientation. This strongly suggests a lengthwise variation of the pinning mechanism. I_c(x) shows very rich behavior. In some tapes I_c is dominated by periodic variations of 28 cm which matches the circumference of the cutting knife used during fabrication. As observed by magnetization imaging, cutting introduces periodic damage to REBCO thin film while tape-width variation amplitude is too small to explain I_c changes. As Fourier analysis reveals, this periodic component is present to some level in all conductors. But in other tapes, rather random, aperiodic fluctuations dominate. Their amplitude is usually inversely proportional to frequency, like in many other physical random systems. Our study indicates both cross-section and morphology variations are responsible for I_c variations. With both continuous, nondestructive measurements and destructive microscopy etc., we are constructing an atlas of typical fingerprints corresponding to identified geometrical and morphological defects which we intend to allow identification of typical defects in long CCs without expensive, destructive and time consuming methods.

We have successfully demonstrated trapping of a magnetic field of 17.6 T in a bulk superconductor at 26 K. This is about 0.4 T in excess of the previous record field obtained by Tomita et al.. Bulk superconductors offer the prospect of convenient, permanent magnet like, fields but of much larger intensity with the very best rare earth magnets achieving little more than 1 T. The utility of such materials is self-evident in spite of the necessity of cryogenic cooling.
The question that immediately arises is as to why it has taken 10 years to achieve a fairly moderate increase on Tomita et al.&’s result. To understand this it is important to consider that the limiting factor to the high field performance of such bulks is, unusually in superconductor applications, not the critical current density of the superconductor itself. In fact, it is the tensile strength of the superconductor that limits performance. The strains inside a bulk superconductor during charging to 17 T can reach ~100 MPa with the strain scaling as B². As the superconductors in question are brittle ceramics featuring a large number of cracks it is clear that this is a challenging problem. Moreover in materials where fracture toughness is important there is often a wide spread of performance since generally it is the one “worst crack” that determines performance.
To achieve high trapped fields, some sort of mechanical reinforcement of the sample is required. Two classes of techniques have been exploited in the literature, steel banding where due to differential thermal contraction a pre-stress is applied to the bulk, and reinforcement with carbon fibre epoxy. We were interested in a quick and easy method which led to our selecting steel banding. We realised, however, that the pre-stress introduced by differential thermal expansion is relatively modest and looked to improve it. We hit upon the simple, and indeed frequently employed in engineering, solution of “shrink fitting”. We carefully machined our bulks to be perfectly cylindrical and prepared slightly under size stainless steel (304L) rings. When heated, these rings expanded and could be slipped over the bulks, as the rings cooled they contracted applying pre-stress to the bulk superconductor. This pre-stress then further increased as the samples were cooled to measurement temperature. In this way we sought to avoid tensile stress sufficient to break them and were able to achieve a field of 17.6 T.

Understanding the interplay of strains initiated at impurity/matrix film and matrix film/substrate interfaces is the key towards controlling strain-mediated self-assembly of nanostructures and is essential to enhancing properties of functional composite oxide thin films. A theoretical model based on the elastic strain theory was developed to understand the effects of elastic properties and interfacial strain on the morphology, dimension, and crystallographic alignment of the impurity nanostructures, such as BaZrO₃, BaSnO₃, BaHfO₃, and YBa₂NbO₆, CeO₃ and Y₂O₃, in c-oriented epitaxial YBa₂Cu₃O_7-x (YBCO) films. Several “phase diagrams” have been obtained to reveal trends of morphology, dimension, and crystallographic alignment of the impurity nanostructures as functions of the dopant volume density (ρ) confirmed in transmission electron microscopy, the dopant/matrix and film/substrate lattice mismatches and the elastic properties of the materials involved. Comparisons with experiment suggests these phase diagrams may provide design rules to controllable self-assembly of dopant nanostructures in YBCO films with magnetic pinning landscape and J_c (H, theta;) optimal to applications.

For improvement of critical current density(J_c) in YBa₂Cu₃O₇(YBCO) coated conductors, nanorods of Ba-containing perovskite are very effective pinning centers. To realize higher J_c in YBCO films containing nanorods, elementary pinning force should be enhanced. The elementary pinning force is dependent on interface sharpness, and interface degradation near nanorods resulting from strain, lattice defects, and compositional variation significantly decreases J_c. To understand the complicated mechanisms of structure and its influence on superconducting properties, combined study of various experimental and theoretical methods is needed. In the present study, structure and superconducting properties in YBCO+BaSnO₃(BSO) films were evaluated, and oxygen vacancies and strain at the interface were discussed based on theoretical calculation and experimental results. Structure of YBCO+BSO films prepared using pulsed laser deposition (PLD) were evaluated using X-ray diffraction (XRD) and transmission electron microscopy (TEM), and critical temperature (T_c) and J_c were measured in the films. To understand the structural results, density functional theory (DFT) calculation and finite element method (FEM) calculation were performed. FEM calculation clarified elastic strain in YBCO+BSO films and partial strain relaxation. DFT calculation showed that oxygen vacancy formation energy depended on strain (value, direction, sign) and that the strain at the interface strongly affected oxygen vacancy formation. These indicate that nanorod-induced tensile-strain in YBCO matrix degrades T_c by increasing oxygen vacancy density. Influence of strain and oxygen vacancies on T_c and J_c at the interface is analyzed quantitatively, and interface design for large elementary pinning force is discussed based on the results.

Energy dissipation is associated with the motion of vortex lattice in a form of vortex flow or creep in superconductors. This dissipation is reduced by the presence of crystalline defects. An important technological challenge in application of superconductors is increasing Jc under magnetic field. Below Jc the vortices are pinned by crystalline defects and can support supercurrent without dissipation. However, the detailed particular of the depinning of vortices from the pinned state are complex, involving the non-equilibrium dynamics of an elastic lattice through a disordered medium. We employed three dimensional (3D) time dependent Ginzburg-Landau (TDGL) simulations to make qualitative measurements of the dependence of Jc on the density, size, and spatial distribution of several kinds of pinning centers, especially, in anisotropic superconductors such as YBCO. A finite difference algorithm was used for solving TDGL equation coupled to the Maxwell equation. Complex link variables were introduced to preserve the gauge invariant properties of the discretized equations. The time derivatives were discretized using a second order semi implicit scheme which, for intermediate values of GL parameter (= 8 - 40), allowed time steps two orders of magnitude larger than commonly used in explicit schemes. We demonstrated the use of the method by solving a fully three dimensional problem of a current carrying superconductors with columnar defects and nanoparticular defects under the tilting magnetic fields. The complex pinning-depining transition in the vicinity of nanorods will be visualized based on the 3D TDGL calculation.

Bi-2212 is the only cuprate superconductor in which high critical current density (J_C) can be developed in a round wire form, which also allows it to be made in multiple multifilament architectures and in the twisted state that benefits low hysteretic losses, isotropic properties and high magnetic field quality. Our previous work showed that overpressure (OP) processing can reduce gas bubbles in the filaments, limit the wire expansion, achieve full densification, prevent leakage, and dramatically increase the critical current density in Bi-2212 round wires. Engineering critical current density (J_E) of 700 A/mm² at 4.2K and 20 T was achieved in wire processed under 100 bars. We made extensive OP study of several series of Bi-2212 wires procured under the Very High Field Superconducting Magnet Collaboration (VHFSMC) and under the DOE Conductor Development Program (CDP). The key goal was to evaluate a broad range of conductors to understand current limiting mechanisms, the effects of filament configuration such as 27x7, 37x18, 85x7, 85x18 and 121x18, and filament size and coupling, precursor powder, carbon content, and sheath materials. Wires made with the Nexans granulate powder all had a higher J_E than those made with Nexans standard (finer) powder and SCI powder. The reason for granulate having higher J_C than standard powder is not yet well understood, but the higher J_E could be due to the lower carbon content in the granulate powder. Transport J_c of a wire with 27x7 filaments and little filament bridging after heat treatment increased by about 40% with decreasing filament diameter (23 to 16 µm). By contrast, the J_c was independent of the filament diameter in the range 11-24 µm diameter in the standard 85x7 and 121x18 architectures. Details of this extensive study will be presented.
The work at the NHMFL was supported by the US Department of Energy Office of High Energy Physics under DE-SC0010421and by the NHMFL, which is supported by the National Science Foundation under NSF/DMR-1157490, and by the State of Florida.

Thin films offer many possibilities for tuning superconducting properties without external pressure or chemical doping. We have already reported a proof-of-principle of this concept by growing doped or non-doped BaFe2As2 thin films on various substrates, in which in-plane strain can control the superconducting transition temperature, Tc, or even induce superconductivity[1-4]. However, the correlation between in-plane strain and electronic properties of BaFe₂As₂ has not been reported so far. Here we explore the phase diagram of Ba(Fe_1-xCo_x)₂As₂ thin films in different in-plane strain states (i.e., tensile and compressive strains) and discuss the effect of in-plane strain on the electronic structure. Our experimental and theoretical investigation indicate that strain affects the band structure similar to carrier doping, albeit the crystal structure changes differently due to strain and doping. As a result, a tensile strain works similarly as an additional electron doping, whereas a compressive strain reduces the actual electron doping level. The obtained general picture of the effect of structural changes on the phase diagram of AEFe₂As₂ (AE: alkari earth elements) pnictides supports relevance of the Fermi surface topology rather than crystal structure for unconventional superconductivity and magnetism of this family.
The research leading to these results has received funding from European Union&’s Sev- enth Framework Programme (FP7/2007-2013) under grant agreement number 283141 (IRON-SEA).
[1] K. Iida et al, Appl. Phys. Lett. 95, 192501 (2009).
[2] F. Kurth et al, Appl. Phys. Lett. 102, 142601 (2013).
[3] P. Pecchio et al, Phys. Rev B 88, 174506 (2013).
[4] J. Engelmann et al, Nat. Commun. 4, 2877 (2013).

Since the discovery of iron-based superconductors, epitaxial thin films have been successfully grown by many groups and they have significantly advanced potential device applications and the understanding of the fundamental physical properties of these new superconductors. In particular, we recently have grown artificially engineered undoped BaFe₂As₂ / Co-doped BaFe₂As₂ compositionally modulated superlattices structures produces ab-aligned nanoparticle arrays and self-assembled c-axis aligned defects that combine to produce very large J_c and H_irr enhancements.
Recently, it has been reported that BaZrO₃ could exist as the nano particle second phase in BaFe₂As₂ matrix since it is chemically stable. We incorporated various mol % BaZrO₃ in Co doped BaFe₂As₂ ceramic targets and have grown BaZrO₃ incorporated Co doped BaFe₂As₂ / undoped BaFe₂As₂ multilayer epitaxial thin films by using pulsed laser deposition to maximize the pinning effect in BaFe₂As₂ system. We will discuss the structural properties of BaZrO₃ incorporated Co doped BaFe₂As₂ multilayer thin films examined by X-ray diffraction (XRD) and transmission electron spectroscopy (TEM) and the superconducting transition temperature (T_c) and critical current density (J_c).

The iron-based superconductors of K-doped Ba(Sr)Fe₂As₂(Ba(Sr)-122) are most potentially useful for high field applications due to their high critical current temperature (T_c) value of ~39K, high upper critical field (H_c2) of over 50T and relatively small anisotropy. However, enhancements of superconducting properties are still needed to boost the successful use of the iron-based superconductors in such applications. Here we present our recent progresses in which we achieved a large enhancement of transport critical current density (J_c) by the applications of conventional cold mechanical deformations.
Ba-122 tapes were prepared by ex situ PIT method. First we prepared high quality (Ba_0.6K_0.4)Fe₂As₂ precursor powder using Ba filings, K plates, Fe powder and As pieces. The precursor powder was packed into a Ag tube, and the Ag tube was deformed to a tape at room temperature with several intermediate annealings and finally the tape was heat treated at 850^oC for 2-4 hr. We found that the application of uniaxial pressing at the final stage of deformation significantly enhanced J_c values. At 4.2 K, the J_c exceeds the practical level of 10⁵ A/cm² in the magnetic fields up to 6T. The J_c-H curve shows extremely small magnetic field dependence and keeps a high value of 8.6×10⁴ A/cm² in 10 T. The microstructure investigations reveal that the high J_c performance in our tapes is due to the high superconducting core density and more c-axis grain oriented microstructure.
However, the uniaxial pressing is not a practical method for long tape conductor fabrications. Furthermore, the Ag sheath was completely annealed and became very soft after the heat treatment. So, the Ag sheathed Ba-122 tape is not practical from the aspect of mechanical strength. In order to solve these problems we fabricated Ba-122 tape conductors applying a new sheath structure of stainless steel (SS) and Ag double sheath and applying only cold rolling. We found that the rolled tapes show J_c values of 7.7x10⁴A/cm² at 4.2K and 10T with high homogeneity. These J_c are the highest values reported so far for iron-based superconducting wires fabricated by scalable rolling process. Application of uniaxial pressing to the double sheathed tape increased J_c at 4.2K and 10T up to 9.0x10⁴A/cm². The transport J_c-H curves for both rolled and pressed tapes both show extremely small magnetic field dependence and the J_c values exceed 3×10⁴A/cm² in 28T, that are much higher than those of low-temperature commercial superconductors. The microstructure investigations indicate that such high J_cwas achieved by higher density of the core and uniform deformation resulting better grain orientation. These results indicate that the combination of the double sheath and the rolling is very promising for fabricating long Ba-122 tape conductors for a high field magnet which can generates fields higher than 20T.

The LHC upgrade relies on magnets operating at fields beyond the reach of Nb-Ti superconductor. With the LHC Hi-Luminosity upgrade, accelerator-quality Nb₃Sn magnets will be for the first time integrated in a real machine, and hundreds of meters long electrical transfer lines based on HTS and MgB₂ conductor will be used for the powering of these new magnets.
Preliminary design studies of future higher energy colliders are based on Nb₃Sn superconductor with performance further enhanced with respect to what till now achieved, and the quest for fields above 16 T motivates studies aiming at demonstrating the potential of HTS conductor for high field magnets.
The activity on conductor development and procurement for the LHC upgrades is presented with a special attention to achieved performance and requirements for both medium and long term applications.

The Powder-In-Tube (PIT) and Rod-Restack-Process (RRP) techniques for the fabrication of Nb₃Sn wires still require optimization because of the necessity to decrease the sub-element size with minimal loss of high J_c and maintenance of high RRR of the Cu stabilizer. For this purpose wire designs and heat treatments are both being modified, thus changing as a consequence also intrinsic properties like T_c, H_c, H_c2 (all of which are composition dependent), as well as structure-sensitive (extrinsic) properties like the grain boundary density, and the amount of A15 phase that can be made within a given package of Nb, Sn and diffusion barrier (DB). Using our recent very detailed study of the 54/61 RRP conductor as a guide [1], we have been analyzing recent finer filament RRP and PIT conductors fabricated at OST and Bruker, respectively, studying T_c distribution (determined by specific heat (C_p)), transport, magnetic and the microstructural properties. In order to reduce Sn leakage though the DB, RRP® wires (Ti-ternary type, 0.778 mm dia., 108/127 design) have been prepared with reduced Sn content. An important result is that the reduced Sn conductors result in only a minor decrease in the non-Cu J_c performance but show an important improvement of RRR (from <50 to >180). Moreover these new wires show a notable improvement in the T_c-distributions with respect to similarly heat treated Ta-doped wires. In fact, the latter has A15 phase with T_c starting from 6 K, whereas the newer standard and low-Sn conductors show no superconducting A15 with T_c below 12-12.5 K. This trend also suggests that Ti doping produces a more homogeneous A15 phase than Ta doping. In the PIT case, different heat treatments for the 192 filaments, 1-0.85mm in diameter, Ta-doped wires have been similarly analyzed. Combining measurements by specific heat (a bulk technique averaging the whole samples in a manner insensitive to position) and magnetization (a position sensitive probe) we found that the Sn-rich, highest T_c A15 is made of poorly connected large grains that do not contribute to transport for optimized heat treatments. This implies that only ~75% of the total A15 made of the small grains is able to carry significant current. A study of how to improve this fraction is now underway.
[1]. C Tarantini, P J Lee, N Craig, A Ghosh and D C Larbalestier, Examination of the tradeoff between intrinsic and extrinsic properties in the optimization of a modern internal tin Nb#8323;Sn conductor, Supercond. Sci. Technol.27, 065013 (2014)
Acknowledgments
Supported at NHMFL by the US Department of Energy - Office of High Energy Physics under grant No. DE-FG02-07ER41451, by the NHMFL (which is supported by the NSF under NSF/DMR-1157490), and by the State of Florida. Supported at BNL by the US Department of Energy under Contract No. DE-AC02-98CH10886.

The Powder-In-Tube (PIT) process for Nb₃Sn competes with the Rod-Restack-Process (RRP) for both highest J_c and the smallest d_shy;eff for upcoming applications in high energy physics, including the Hi-Lumi upgrade at CERN. While both methods produce a final non-Cu package of 50-60% superconducting A15 phase, PIT conductors have a significant fraction of that A15 (~25%) which has a grain size 10x larger than that of the fine grain A15 needed for high critical current density. Despite the high T_c of these coarse grains as judged by heat capacity, they carry little to no current. Therefore, it is crucial to suppress this coarse grain phase and use all of the available Sn for current-carrying, fine grain A15. In a recent 192 filament Bruker PIT wire it was seen that ~15% of these filaments produce strikingly different ratios of A15 and other residual phases. These atypical filaments show a substantial enhancement in the fine grain A15, a decrease of residual Sn in the core and the appearance of a Cu-rich membrane between the growing A15 and the Sn source. This membrane appears to beneficially favor fine, rather than coarse grain A15 and thus offers a method to recover an appreciable superconducting volume in the Nb₃Sn PIT process. If the reaction path for all filaments can be carefully controlled, we should expect a significant increase in J_c, commensurate with the increase in amount of fine grain A15 produced by optimizing Sn utilization.

Nb₃Sn wires have come a long way from the early bronze-processed strands to state-of-the-art restack rod processed and powder-in-tube wires. The former were limited to non-matrix critical current densities below 10⁹ A/m² at 4.2 K and 12 T, whereas the latter achieve 3 middot; 10⁹ A/m² at the same temperature and field, while having superior high-field properties due to upper critical field optimization by ternary element addition.
The field dependence of the pinning force in Nb₃Sn is usually described using a scaling law in which two parameters determine the shape of the function. Deviations of these parameters from the values expected for pure grain boundary pinning are generally ascribed to inhomogeneities as well as to the effects of impurity doping, and often accepted as wire-specific properties without giving much thought to the underlying physics. We argue that this approach is too superficial if further improvements of Nb₃Sn wires are to be attained, since the feasibility of such improvements will strongly depend on how well flux pinning in this material is understood.
Our experimental data were obtained from samples of five types of state-of-the-art multifilamentary wires which differ in terms of production process and additive element, as well as from high-purity binary Nb₃Sn polycrystals. We performed magnetization and transport measurements on these samples, subjected them to repeated fast neutron irradiation, and assessed the resulting changes in the superconducting parameters within the framework of the most extensive Nb₃Sn neutron irradiation study carried out to date. These data are complemented by magnetization maps obtained from scanning Hall probe microscopy, which allow the assessment of the local current distribution within the sub-elements of multifilamentary wires.
Pinning force analyses of our data show that both the magnitude and the functional dependence of the volume pinning force in Nb₃Sn are strongly affected by the introduction of the point-like defects induced by irradiation. Moreover, we found that even in the unirradiated state the pinning function follows the widely accepted scaling law only in a very limited way. On the one hand these results fuel our hopes that Nb₃Sn wires can be significantly improved by changing their pinning landscape, for instance by introducing small normal-conducting precipitates. On the other hand they demonstrate that the currently used scaling laws need to be adjusted to adequately describe all the facets of flux pinning in Nb₃Sn. We will present our major experimental findings, and discuss possible alternatives to the established scaling laws.

In order to use (Re)BCO conductor in the 32T all superconducting user magnet successfully, a variety of its properties should be controlled within specific tolerances. We have developed specific quality assurance (QA) procedures to assure this performance, in addition to standard QA performed by the manufacturer. We present our QA procedures for the (Re)BCO conductor, which includes magnetic field dependence measurements of critical currents for 4 mm wide tapes at 4.2K up to 13.5T at 18^o off tape axis, considered as critical orientation for the 32T coil design. Other parameters tested are the self-field joint resistance at 77 K, cross-sectional area and residual resistivity ratio of stabilizer, dimensional tolerances, and its uniformity. We investigated short samples from 180 conductor piece lengths 60-110 m long each manufactured by SuperPower Inc. produced on two machines: M3 and M4. Visualizing FIB patterned cross sections with SEM, we observed distinctly different ReBCO layer thicknesses, ~1 mu;m for M3 and 1.5-1.6 mu;m for M4 tapes. Average Ag layer thickness is about 1.3mu;m. For most tapes we observed joint resistance below 100nW*cm² but some had joint resistance above 230 nW*cm². We took RRR samples from 30% of all conductors, and all RRR values were above 50. We found that the ratio I_c(4K, 17T, 18^o) / I_c(4K, 17T, HIIc) is asymp;1.8. I_c(H) follows the power law I_c(H) prop; B^-α very well from 7T up to 31T at 4.2 K. We interpret our measurements as indicating improved compositional uniformity in recent tapes that translates into more uniform pinning mechanisms. I_c(4K, 17T, 18^o) extrapolated from data measured up to 13.5T for samples from all production runs has a mean of asymp;372A and standard deviation σ_4K=27%. M3 tapes had average I_c(4K, 17T, 18^o) =292A, while for M4 tapes it is 443A with standard deviation for both about 18%. I_c(77K, SF) measured on both M3 and M4 tapes has much smaller spread: σ_77K=10.9% (mean of 131 A). As expected, we observe weak correlations between I_c at 77K, SF and 4K, 17T, 18^o for recent production. The average lift factor for M3 tapes is 2.3, which is smaller than the value of 3.3 obtained for M4 tapes. We detected a decrease of J_c(77K, SF) with ReBCO layer thickness, but observed no J_c(4K, 17T, 18^o) reduction with ReBCO thickness. This means that pinning centers inefficient at 77K in self-field conditions in thicker M4 tapes become efficient in-field at 4K. For tapes from both machines, we observe no correlation between α and I_c(4K, 17T, 18^o) and detect <α>=0.83 standard deviation of α values σ_α =4.8%. About 12.8% of tapes show I_c (4K, 17T, 18^o) below the 32T specification minimum of 256A based on the field and field angle at the ends of the coils. Comments on optimization of tapes for top and bottom coil parts based on I_c statistics will be given.
Acknowledgement: this work was supported in part by the U.S. National Science Foundation under Grant No. DMR-1157490, DMR-0923070 and the State of Florida.

For fusion superconductors in high magnet fields with currents in the order of 50 kA, single ReBCO coated conductors must be assembled in a cable geometry where combined torsion, axial and transverse loading states are anticipated in the tapes. Axial and transverse loading, caused by differential thermal contraction and electromagnetic forces, may affect the transport properties. The effect of pre-applied twist on degradation of the current carrying capability as a function of controlled tensile stress was studied at 77 K. Results of the effect of transverse pressure applied to tapes showed an influence of the tape thickness layers. The mechanical behavior was modeled using FEM commercial software and accordingly validated with experiments on tapes. The model is then used as a basis to study the behavior of CORC cables during manufacture and under operation conditions.

Achieving high current superconducting wires for large scale applications has been one of the most challenging objectives during all the HTS era. Extraordinary new ideas and materials developments have been demonstrated and second generation YBa₂Cu₃O₇ conductors (coated conductors, CCs) have emerged as the most attractive opportunity to reduce the cost/performance ratio down to the levels required for energy applications. Chemical solution deposition (CSD) has become a very competitive cost-effective technique to obtain nanostructured films and CCs. The development of high performance superconductors and high production throughputs requires, however, a full understanding of the different steps included in the Trifluoroacetate (TFA) or F-free (FF) processes, particularly the crystalline transformation to the superconducting phase and the oxygenation process.
In this presentation we will report on our recent advances concerning the analysis of advanced CSD processing methodologies of YBCO films based on Ink Jet Printing deposition using YSZ-ABAD metallic substrates. We will show that Ink Jet Printing is a practical approach to achieve controlled film thickness in continuous reel-to-reel solution deposition. We will particularly show that in-situ monitoring systems, such as optical microscopy or electrical resistivity measurements, are extremely useful to investigate the influence of several processing parameters on the film shrinkage process, the crystal nucleation and growth and the oxygenation step. Enhanced production throughputs and superconducting performances can be achieved through the use of these methodologies.

Bi-2212 is the only HTS that despite of a very strong structural and electronic anisotropy can be fabricated in the form of almost isotropic practical conductors such as melt processed Bi-2212 round wires, one of the most promising candidates for high-field magnet applications, and melt cast processed Bi-2212 bulk, a very reliable material for applications in fault current limiters, current leads and magnetic shields. Though the materials aspects and physics of the critical currents in these conductors were extensively studied, basic understanding of how high supercurrents flow in the absence of long-range texture is still lacking.
In our work, we addressed this connectivity issue by studying the effects of oxygen doping state and cation composition on superconducting and normal state properties of polycrystalline Bi-2212 with various textures. In particular, we studied the microstructure development, T_c, J_c and resistivity as function of the oxygen doping state in Bi-2212 bulk rods, flat tapes and round wires of varying cation composition
Bi_{2.00 +z}Sr_{2.85 - x}Ca_xCu_2.00O_{8 + δ} (nominal z = 0, 0.08, 0.15; 0.80 le; x le; 1.22; 0.175 le; δ le; 0.256).
We confirmed that changes in cation composition strongly affect the optimum oxygen doping state δ_o (maximum T_c) that increases with increasing Ca contents. We also found that optimizing J_c needs a temperature dependent overdoping, and J_c is a rather strong function of δ even at low temperatures, which suggests a significant effect of changes in condensation energy on the flux pinning.
The consequences of these observations for optimized conductor processing and next experiments addressing the connectivity issue in polycrystalline Bi-2212 are discussed.
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Work partially supported by EuCARD-2. EuCARD-2 is co-funded by the Partners and the European Commission under Capacities 7th Framework Programme, Grant Agreement 312453.

BaMO₃ (BMO, M=Zr, Sn, Hf etc.) becomes very effective artificial pinning centers due to the self-organization within REBCO films. Especially, the BMO forms “nanorod” in the REBCO films grown by vapor phase epitaxy such as pulsed laser deposition (PLD) method.
We have studied the BHO self-organization in the REBCO (RE=Gd and Sm) films depending on the growth conditions such as the additive amount and the growth temperature. The diameter of the BHO nanorods did not depend on the additive amount, while the number density increased with increasing the additive amount. We also checked an influence of the growth temperature. The elevated growth temperature increased the diameter and decreased the number density.
From the viewpoint of film growth, we carried out 2D Monte Carlo simulations for the initial layer growth of a REBCO film with BMO. Monte Carlo (MC) simulation is very useful for investigations of crystal growth. There are many reports about film growth simulated by the MC. In this study, we developed a simple model of the BHO-doped REBCO film growth. We regarded REBCO and BMO as simple cubes with a side of 0.4 nm and the cubes have no anisotropy of the surface free energy density. The cubes arrived from vapor on a site in substrate surface which is consisted of many cubes of the substrate material. We employ standard Metropolis algorithm as follows. In a randomly chosen point of the substrate sites during each MC step, one of three possible events which were the arrival, migration and desorption of the cubes randomly could occur.
In the 2D MC simulations, the REBCO and BMO nuclei occurred randomly. The REBCO nuclei grew up and coalesced. However, due to small amount of the BMO, it was difficult for the BMO nuclei to get fat and to unite with each other. We changed the molar fraction of the BMO and the substrate temperature. As a result, we were able to reproduce the diameter and the number density of BMO nanorods against the BMO molar fraction and the growth temperature, quantitatively.
This work was partly supported by a Grant-in-Aid for Scientific Research (23226014 and 25289358).

Previously, REBa₂Cu₃O_7-δ (REBCO and RE=Gd, Y) superconductor tapes with Zr content of 15 mol.% have been demonstrated by Metal Organic Chemical Vapor Deposition (MOCVD) with critical current densities above 12 MA/cm² at 30 K, 3 T (B||c) and lift factor in critical current above 4. In this work, REBCO tapes with Zr content of 25 mol.% have been produced by MOCVD using a reel-to reel process. The chemical composition of the thin films, surface morphology and phase structure were investigated by Inductively Coupled Plasma (ICP), Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD), respectively. The orientation of BaZrO₃ nanorods created in the films was studied by Transmission Electron Microscopy (TEM). The MOCVD parameters were modified for achieving high self-field and in-field critical currents. The latest improvements in in-field critical current performance of 25 mol.% Zr-added REBCO tapes will be discussed in this presentation.
Keyword: REBCO, MOCVD, BZO, Critical Current, Thin Films

High-Transition-temperature (high-T_c) superconductivity in iron arsenides is usually induced by chemical substitution, or, doping, into parent compounds. To understand the role of doping, we carried out a comparative resistivity and optical reflectivity measurements on BaFe₂As₂ with various chemical substitutions. The charge dynamics of parent BaFe₂As₂ is dominated by incoherent charge carriers, indicative of substantial electron correlation. Electron (Co) and isovalent (P) doping result in transforming the charge dynamics into highly coherent one. On the other hand, charge dynamics remains incoherent for hole (K) doping. In common with any type of doping, high-T_c superconductivity is realized when (1) the normal-state charge dynamics maintains incoherence and when (2) the resistivity associated with the coherent channel exhibits dominant T-linear dependence.
This study has been carried out in collaboration with M. Nakajima, S. Ishida, K. Kihou, C.H.Lee, S. Uchida, and A. Iyo.

[Introduction] Recently new 112 type iron-based superconductors - Ca_1-xRE_xFeAs₂ have been reported for RE = La[1] and Pr [2]. This structure has a monoclinic space group P2₁/m, and is composed of two Ca(RE) planes, anti-fluorite Fe₂As₂ layers, and As₂ zigzag chain layers. The highest T_c reported for this new phase is 43 K for a Sb-doped Ca_0.85La_0.15FeAs₂ sample [3]. Various studies on these new superconductors are needed in order to investigate their superconducting properties. In this study we report the synthesis and characterization of Ca_1-xRE_xFeAs₂ compounds with RE = Ce, Nd, Sm, Eu and Gd.
[Experimental] The Ca_1-xRE_xFeAs₂ samples were synthesized by the high-pressure method. Phase identification was carried out by XRD. Magnetization properties were examined by a SQUID magnetometer and electrical resistivity was measured by the AC four-point-probe method using Physical Property Measurement System.
[Results] 112 phase was successfully formed in all the samples synthesized under moderately high pressure ~2 GPa, small silvery crystals embedded in a uniform opaque black matrix in the bulk samples. From the XRD analyses the samples are composed of Ca(RE)-112 phase with FeAs and FeAs₂ impurity phases. Nd, Sm, Eu and Gd doped samples exhibited diamagnetism suggesting superconductivity, while the Ce doped sample showed a paramagnetic like behaviour down to 2 K without any traces of superconductive transition. The critical temperatures evaluated form the magnetic susceptibility (T_c-mag) for the La-, Pr-, Nd-, Sm-, Eu-, and Gd-doped samples are 24.5, 13.2, 11.9, 11.6, 9.3, and 12.6K, respectively. T_c-mag progressively decreases with increasing atomic number of the substituted RE; the Gd-doped sample showed a relatively high T_c-mag, probably because of the lower doping level. Those results are in agree with the resistivity measurements too, suggesting a general decreasing of T_c with smaller RE substituted atoms.
Distances between Fe planes, d_Fe-Fe, evaluated from the XRD patterns of single crystals dreseases in agreement with the ionic radii of the substituted RE, suggesting a relation between the decrease of Tc and d_Fe-Fe.
We successfully synthesized a series of RE-doped CaFeAs₂ compounds with RE = La-Gd by applying the HPS method. Polycrystalline samples of Ca_1-xRE_xFeAs₂ with RE = La, Pr, Nd, Sm, Eu, and Gd showed clear superconducting behaviour, in contrast to the Ce-doped one. Relatively low Tc&’s observed in our samples and the actual RE concentration largely differed from the nominal ones, which suggests that the precise control of the RE concentration in the samples is crucial for the optimization of the superconducting properties as well as for understanding the factors determining Tc in this new superconductor system.

We present an experimental and theoretical investigation of the pronounced history effect encountered in the global critical current density of polycrystalline high-temperature superconductors. This history manifests in the form of significantly higher critical current densities in the decreasing field branch than found in increasing field at the same value of the applied field.
Polycrystalline Co- and K-doped Ba-122 polycrystals were characterized by magnetization loops and scanning Hall-probe microscopy in order to identify the underlying mechanism with special emphasis on the influence of the grain size. By visualizing the magnetization profiles, our high-resolution Hall scans offer direct evidence of an enhanced global critical current density in decreasing field for all examined grain sizes, with the highest current density found in the samples with the smallest grains (< 1 µm). In samples with a grain size of approximately 5 µm we observed a break-down of the global critical current density, and the simultaneous emergence of intra-granular currents of the grains when ramping the applied field through zero.
We will show that this behavior can be described by a model which uses basic assumptions for the Josephson current across the grain boundaries and the reversible and irreversible currents inside the grains. This model predicts the experimentally observed history effect between the global critical current density in increasing and in decreasing field, and explains why the global current in fine-grained polycrystals across the weak-linke grain boundaries is less sensitive to the applied magnetic field.

INTRODUCTION
Layer structured iron selenide, FeSe has the simplest crystal structures among iron-based superconductors. It shows superconductivity with transition temperature (T_c) of ~13 K under ambient pressure. The T_c increases up to ~37 K by applying high pressure [1-3]. These facts indicate that the FeSe-layers are favorable structures to show superconductivity. When potassium is doped to the interlayer of FeSe, the resulting compound KFe₂Se₂ shows superconductivity at around 31 K under ambient pressure. This is a potential superconductor with much higher T_c by applying high pressure. However, the superconducting properties have no consensus even in the ambient pressure condition, because of its reproducibility, inhomogeneity, and instability, and so on. It is necessary to obtain the high-quality single crystals to clarify the intrinsic properties.
In this study, we cultivate the preparation method for the single crystalline KFe₂Se₂, and investigate its superconducting properties by means of the magnetic susceptibility, electronic properties, and single crystal structural analysis.
RUSULTS and DISCUSSION
The single crystals of KFe₂Se₂ were prepared by “one-step method” [4]. K₂Se, Fe and Se powders were mixed with a stoichiometry of K_0.8Fe₂Se₂. The mixture sealed into evacuated quarts tube was heated up to 900 °C and slowly cooled down, followed by water quenching at a certain temperature. The obtained single crystals quenched at different temperatures have a well-developed crystal habit with homogeneous surfaces. Although the compositional ratios estimated from EDX were almost the same among these samples, the superconducting properties were strongly affected by the quenching temperature. The samples quenched at 550-700 °C showed a sharp superconducting transition at 31-32 K, and its shielding volume fraction was almost 100 %. On the other hand, the samples quenched at lower temperatures less than 400 °C showed broad superconducting transitions with relatively lower T_c&’s and shielding volume fractions. Especially in a furnace-cooled sample, the superconducting fraction was suppressed down to a few %.
The preparation for the single crystals and its superconducting properties will be discussed in detail together with the result of the single crystal structural analysis.
References
[1] Y. Mizuguchi et al., Appl. Phys. Lett. 93, 152505 (2008).
[2] S. Margadonna et al., Phys. Rev. B 80, 064506 (2009).
[3] S. Masaki et al., J. Phys. Soc. Jpn. 78, 063704 (2009).
[4] T. Ozaki et al., Euro. Phys. Lett., 98, 27002 (2012).

A particular family of the iron chalcogenides superconductors (termed the 11-system), for instance FeSe, Fe(Te,S), and Fe(Te,Se), have the simplest crystal structure among the iron-based superconductors. The iron chalcogenides have only superconducting layers without blocking layers, which indicates that this system is suitable to clarify the mechanism of iron-based superconductivity.
Thus many synthesis processes have been actively studied and reported such as solid state reaction, single crystal growth by a low-temperature vapor-transport technique, and a method by electrochemical deposition and so on [1].
However, several studies have found that a small amount of excess Fe is trapped between the layers during the synthesis. This excess Fe suppresses superconductivity by doping the excess electrons to the superconducting layers. Attempts to suppress the effect of excess Fe, then to induce superconductivity have been done.
Recently, we have succeeded in inducing superconductivity in the11-system compounds, FeTe_1-xS_x and FeSe_1-xTe_x, by annealing in an organic acid solution and sulfur annealing, which are effective to remove excess Fe from the interlayer [2, 3].
Most recently, we have reported that to induce superconductivity excess Fe in FeTe_0.8S_0.2 sample was electrochemically removed, (i.e. de-intercalated), by applying voltage to the citric acid solution. This is the first report of inducing superconductivity using an electrochemical reaction in the iron-based superconductors.
The variety of synthetic methods and inducement of superconductivity in iron chalcogenides are discussed in detail.
[1] S. Demura et al., J. Phys. Soc. Jpn 81 (2012) 043702
[2] A. Yamashita et al., Solid State Communications 200 (2014) 29-31
[3] A. Yamashita et al., arXiv: 1407.4932

In this talk we will provide some new insights into the materials synthesis and the crystal growth of modern superconductors, where high-pressure conditions are often crucial for the success. After a short introduction to the technological aspects and the role of high-pressure, high-temperature techniques in inorganic synthesis, we will focus on some typical examples encompassing our most recent achievements.
First, we will review the high-pressure exploration of the Mg-B-N system. Here we discovered the simultaneous crystal growth of completely different types of materials: a rare two-band superconductor MgB₂ and a wide-band semiconductor hBN. Besides the interesting physics, both these materials hold great potential for practical applications.
Successively, we will highlight the key role of the extreme conditions in the growth of Fe-based superconductors, where a careful control of the composition-structural relations is vital for understanding the material behavior. The availability of high-quality Ln1111 single crystals allowed us to obtain intrinsic and directionally dependent superconducting properties, such as H_c2, J_c, and their anisotropies. In addition, in SmFeAs(O,F) we could observe also a distinct change in the nature of the vortices: from well-pinned slow-moving Abrikosov-like to weakly-pinned fast-flowing Josephson-like. The varying nature of the vortices reflects a delicate balance between some key material properties, such as the coherence length and the interlayer separation.
Finally, we will further emphasize the beneficial role of the high-pressure, high-temperature conditions in exploring the crystal growth of various intermetallic superconductors, such as MgCNi₃, Mo₃Al₂C, and APt₃P (A = Sr, Ca, La). The underlying correlations and the general trends between composition, structure, and superconductivity in these superconducting materials will be discussed as well.

In order to be disclosed to the practical applications (lossless current transportation, winding of magnets and so on), superconducting materials should possess not only T_c, but also J_c (critical current density) and pinning force (F_p) as large as possible to have a wide application range. Introduction of nanosized Artificial Pinning Centers (APCs) was widely used to strongly enhance J_c and F_p of High Temperature Superconductors (HTSC) like YBa₂Cu₃O_x (YBCO, T_c = 92 K) in magnetic field. Furthermore, in the past ten years, the nano-engineering approach to control microstructure, distribution, concentration and dimensionality of APCs has been demonstrated to be a powerful tool to produce thin films with excellent performance.
For example, the ablation of mixed YBCO- 4 wt% BaSnO₃ (BSO) - targets by Pulsed Laser Deposition (PLD) [1] have made possible the fabrication of high quality YBCO thin films incorporating BSO in form of nanorods, which are classified as one-dimensional APCs (1D-APCs). YBCO films added with 4 wt% BSO have J_c = 0.3 MA/cm² and F_p^MAX = 28.3 GN/m³ (77K, 3T, B//c). However, J_c is intrinsically anisotropic with the direction of applied magnetic field (with a maximum for B//c axis) and this is a critical issue for practical applications, since the value of J_c might be constant in all directions of applied magnetic field. Subsequently, such issues have been solved by introducing isotropic (or 3D) pinning (Y₂O₃ nanoislands) to the YBCO films: for example, the 5.44 A% Y₂O₃ added sample presented F_p^MAX =14.3 GN/m³ (77K, 3T, B//c)) [2]. Combining the 1D- and 3D-APCs pinning, with coexistence of BSO nanorods and Y₂O₃ nanoparticles in multilayered films, encouraging results was obtained with F_p^MAX =17.6 GN/m³ (77K, 2.2T, B//c) in [(90 nm YBCO+BSO)/(30 nm YBCO+Y₂O₃)]×3 multilayer [3].
Despite to such encouraging results, the performance of superconducting thin films requires a breakthrough for the practical application. In particular, the irreversibility field (B_irr) needs to be improved. Typical values of B_irr (77K) for YBCO-BSO prepared by PLD is about 11T [1], while recently Xu et al reported B_irr = 14.8T (77K) for MOCVD (Y,Gd)BCO doped with 15% vol Zr, though F_p^MAX = 14 GN/m³ (at 6T) is about half respect to the YBCO-BSO [4]. Further expansion of the F_p and B_irr is necessary to have real applicable thin films.Our ultimate approach to extend the applicability limits of the superconducting films will be discussed at the conference..
[1] P. Mele et al., Supercond. Sci. Technol. 21 (2008) 032002
[2] P. Mele et al., Supercond. Sci. Technol. 2014, in press.
[3] P. Mele et al., in preparation
[4] A. Xu et al., APL Materials 2 (2014) 064111

The solution processing of YBa₂Cu₃O_7-x (YBCO) coated conductors has been extensively studied, however the dependence of J_c on the magnitude and the orientation of the magnetic field still poses problems for practical applications in high magnetic fields. This can be circumvented by the incorporation of inert metal oxide nanoparticles in the YBCO coating. In-situ created nanostructures present in the YBCO layer have proven their effectiveness. In these nanocomposites the nanostructures tend to attain random orientation with respect to the epitaxial YBCO, generating incoherent interfaces within the epitaxial YBCO matrix that causes localized nanostrained regions, that has been correlated with a strong enhancement of vortex pinning.^[1] However, the size, morphology and distribution of in-situ created nanostructures an thus the associated nanostrain cannot be controlled in the layer. The less established ex-situ approach, in which first nanoparticles are synthesized by wet chemical methods and are afterwards added to the YBCO precursor solution, offer the promise of a better control over particle growth, size, size distribution and shape and thus also over the introduced nanostrain.
This research is devoted to the processing of ZrO₂ and HfO₂ nanocrystals (NCs), useful for ex-situ pinning in YBCO. We explored a microwave-assisted solvothermal and a heating-up synthesis method for the preparation of the NCs.^[2] Both synthesis methods give rise to small NCs (3 - 10 nm in diameter) and narrow size distributions, characterized by means of XRD, DLS and TEM analyses. The two methods lead to NCs with different surface chemistry, yet in both cases the NCs are capped with organic ligands to prevent agglomeration, ensuring colloidal stability in apolar solvents.^[3] However, as the YBCO precursor typically provides a more polar environment, e.g. methanol, an important aspect of this research involves finding the appropriate stabilizing molecule and studying the particle stability in the polar YBCO precursor solution.
After ensured particle stability in the YBCO precursor solution, the YBCO-NCs suspension were deposited on a La₂Zr₂O₇ substrate through spin coating to study the influence of the NCs in the YBCO coating. The effect of different concentrations of APCs in YBCO were investigated, bringing us closer to the effective incorporation of high-temperature superconductors in high and alternating magnetic field applications, relevant in alternative energy generation.
[1] X. Obradors, T. Puig, A. Palau, et al. (2011) Comprehensive Nanoscience and Technology 303-349
[2] J. De Roo, K. De Keukeleere, J. Feys et al. (2013) Journal of Nanoparticle Research 15:1778
[3] J. De Roo, F. Van den Broeck, K. De Keukeleere, et al. (2014) JACS 9650-9657

Introduction of artificial pinning centers (APCs) into the REBa₂Cu₃O_y (REBCO: RE = rare earth) coated conductors (CCs) has been a successful approach to achieve the improvement of critical current density (Jshy;_c) performance. Among the APC, BaMO₃s (BMOs: M = Zr, Sn, Hf) are very promising materials, because BMOs become strong pinning centers and morphologies of the BMOs could be easily controlled by deposition conditions.
Recently, we have fabricated SmBCO films with BHO nano-rods on [001]-tilt bicrystal (LaAlO₃)_0.3-(SrAl_0.5Ta_0.5O₃)_0.7 (LSAT) substrates at a high substrate temperature, in order to investigate superconducting properties and structures of BHO nano-rods at grain boundaries (GBs). From TEM observations, we found that the BHO nano-rods grew parallel to the c-axis of the SmBCO and some BHO nano-rods grew along the lines with GBs. Moreover, we could consider that the transport properties at the GBs of the BHO-doped SmBCO films were improved via the flux pinning by the BHO nano-rods for the Abrikosv Josephson votices.
We reported that BMO form radially nano-rods structures: “fireworks structures” in REBCO films fabricated at a low substrate temperature. Experimentally, we found that one firework could be composed of nano-rods grown in one REBCO grain.
In this work, aiming to reveal structures of inclined nano-rods and transport properties at GBs, we fabricated SmBCO films with BHO nano-rods inclined for c-axis of SmBCO on bicrystal substrates. We observed structures of the BHO nano-rods on the GBs by TEM observations. A 3.0 vol.% BHO-doped SmBCO superconducting film was deposited on a 50 [001]-tilt bicrystal LSAT substrate by pulsed laser deposition method adopting a low-temperature growth (LTG) technique. The LTG technique consists of seed layer deposition and subsequent film deposition at a lower temperature. The film shows T_c = 89.3 K and J_c = 0.5 MA/cm² under a self-field. We will discuss growth mechanisms of BHO nano-rods at GBs.

The longitudinal magnetic field (LMF) effects mean various peculiar superconducting electromagnetic phenomena under the force-free state. Experimentally, the enhancement of the critical current density (J_c) under a certain magnetic field from the J_c under self-field (J_c^self) was reported. There are many reports about the J_c enhancement for the intermetallic superconductors such as the Nb-Ti wire. However, there are few reports for REBa₂Cu₃O_y (REBCO, RE = rare earth) high-temperature superconductors. In addition, determining factors of the J_c enhancement peak are unexplained.
So far, we have reported a J_c enhancement by the LMF effect only on a multilayered SmBa₂Cu₃O_y (SmBCO) film with short BaHfO₃ (BHO) nanorods like 3D pinning centers, and we have never observed it on SmBCO films with BHO nanorods. Therefore, we consider that existence of the 3D artificial pinning centers is important for the J_c enhancement under the force-free state because flux-line is in translational motion along the film surface normal and in rotational motion in ab plane by force-free model. And we speculate about a range of the interface density [/mu;m], which means number of interface between pure SmBCO layer and SmBCO layer with BHO nanorods, is exist to lead to the LMF effect. We evaluated the J_c properties under the force-free state as a function of the interface density along the film surface normal.
In this study, we fabricated BHO-doped SmBCO multilayered films on LaAlO₃ (100) (LAO) substrates by pulsed laser deposition (PLD) method. These films consisted of pure SmBCO layers and SmBCO layers including BHO nanorods with same film thickness. We fixed 2 vol.% BHO contents in all BHO-doped SmBCO layers for the constant nanorods density and the diameter. We changed the layer number in order to control the interface density along the film surface normal. The total film thickness was fixed to 450 nm and the interface density along the substrate surface normal was in the range from 18 to 107 /mu;m.
As a result, all of the multilayered films showed c-axis orientation on the LAO substrates from the X-ray diffraction measurements and magnetic field dependence of J_c of the each film in B//c showed similar tendency. The J_c peak by the LMF effect from J_c^self at 77 K was observed in the 53, 71, 107 /mu;m multilayered films. Maximum J_c enhancement was 8% to the J_c^self value and the enhancement was observed at 0.15 T on 71 /mu;m film. The J_c peak of all the films were also around 0.15 T. However, the J_c peak was not observed in the 18, 35 /mu;m film. This result indicates that the interface density which is more than around 50 /mu;m leads to the LMF effect.
This work was supported in part by Giant-in-Aid for Scientific Research (Nos. 23226014 and 25289358) from the Japan Society for the Promotion of Science (JSPS). We would like to thank Prof. T. Matsushita belonging to Kyushu Institute of Technology for fruitful discussions.

The fabrication of nanostructured YBa₂Cu₃O₇ (YBCO) thin films by the incorporation of nanosized oxide secondary phases has been shown to strongly enhance YBCO performance (at high magnetic fields and in a wide temperature range), ensuing great potential for use in a broad number of power applications. We have demonstrated that in chemical solution deposited (CSD) YBCO nanocomposites, vortex pinning seems to be dominated by the nanostrain generated within the YBCO matrix ensuing from partial dislocations associated with intergrowths (extra Cu-O chain layers).[1] The challenging goal is to induce a specific defect structure in the YBCO matrix through the control of the nanoparticle characteristics (composition, size and orientation) and the intergrowth scenario. Here we present an extensive research on CSD-YBCO nanocomposites prepared following two different approaches. First, the CSD in-situ approach where tight tuning of the processing conditions allowed us to obtain homogeneously distributed and randomly oriented second phases with small particle size (<20 nm).[2] Also, we have investigated the influence of multiphase segregation (BZO+Y₂O₃, Ba₂YTaO₆+BZO, Ba₂YTaO₆+Y₂O₃) on the generation of structural defects in the YBCO matrix and how they modify the pinning performance. The second approach named ex-situ is a radically new concept consisting of the incorporation of prefabricated nanoparticles (NP) colloids (CeO₂, ZrO₂, CoFe₂O₄, BZO) in the YBCO precursor solution in order to have much better control of the NP characteristics. We have already identified the critical issues in this new route: YBCO-NP precursor solution stability, presence of aggregates, film homogeneity, reactivity between NP and YBCO, cation incorporation and NP distribution. We will discuss how all these critical parameters affect YBCO microstructure and vortex-pinning landscape. We have used advanced x-ray diffraction characterization, scanning transmission electron microscopy and transport measurements to deeply evaluate all these CSD-YBCO nanocomposites.
[1] A. Llordes, A. Palau, J. Gazquez, M.Coll, et al., Nature Materials, 2012 11, 329-336.
[2] M. Coll, R. Guzman, P. Garces, et al., Supercond. Sci. Technol. 2014, 27, 044008.
The research leading to these results has received funding from EU-FP7 NMP-LA-2012-280432 EUROTAPES and NanoSC COST action MP1201, and MAT2011-28874-C02 Spanish project.

SQUIDs (Superconducting Quantum Interference Device) made of low critical temperature (T_c) superconductors are known as the most sensitive magnetic sensors and have been used in a wide range of applications [1]. Devices made of high critical temperature superconductors (HTS) can further expand the domain of applications, provided they have comparable performances. One way to improve the sensitivity and the dynamic range is to create arrays of SQUIDs with a random distribution of sizes. Such multiple-loop configuration called SQIFs (Superconducting Quantum Interference Filters) display a unique dip in the voltage response function around zero magnetic field [2]. SQIFs therefore allow the determination of the absolute magnetic field value, and their robust design makes these devices more tolerant to the spread in JJs characteristics compared to an array of identical SQUIDs. SQIF devices therefore fulfil key requirements for their use as highly sensitive magnetometers.
One of the most promising and reliable technology to fabricate Josephson Junctions (JJs) with HTS materials is based on the ion irradiation process [3]. The planar geometry of such JJs, offers the great advantage of a high integration level of SQUIDs arrays, with well controlled JJs characteristics. We obtained recently a significant result with an array of 2000 SQUIDS, which displays a sensitivity of 1000 V/T [4].
In this paper, we report on a serial SQIF, composed of N=18 SQUIDs, made by ion irradiation of the prototypical HTS YBa₂Cu₃O₇. Our design gives us the possibility to probe both the individual response of each SQUID and the global SQIF response in magnetic field. Thus we are able to study the role played by each SQUID and JJs parameters on the SQIF output voltage antipeak versus magnetic field. Our study will lead to a better understanding of SQIF characteristics and paves the way toward future SQIF with improved design, control and performances.
[1] The SQUID Handbook, J. Clarke & A. I. Braginski (Eds), Wiley-VCH (2004)
[2] C.Haussler et al., J. Appl. Phys. 89, 1875 (2001)
[3] N. Bergeal et al., J. Appl. Phys. 102, 083903 (2007)
[4] S. Ouanani et al, Journal of Physics: Conference Series, (EUCAS2013), 507, 042008 (2014)

Superconducting nanowire single-photon detectors (SNSPD) based on ultra-thin films have become the preferred technology for applications that require high efficiency single-photon detectors with high speed, high timing resolution, and low dark counts rates. Such applications include quantum information, quantum optics, light detection and ranging (LIDAR), distributed fiber sensing and integrated circuit testing. Since demonstration of the first SNSPD using NbN thin films, an increasingly large number of materials are being explored. At NIST, we have demonstrated the highest system detection efficiency (SDE) to date for SNSPDs at a wavelength of 1550nm of 93%, using amorphous thin films of WSi_x integrated in an optical cavity for enhanced absorption [1]. Since then, we have been investigating a number of amorphous thin film alloys such as MoSi, MoGe, and WRe with the goal of optimizing SNSPDs in terms of higher operation temperature, high efficiency and high speed.
To understand the material impact we have measured superconducting transition temperature (T_c) as a function of film thickness and sheet resistance, as well as critical current densities. We will present experimental data comparing these materials to models of a universal scaling of the properties of thin superconducting films [2], relating superconducting transition temperature, film thickness and sheet resistance. Other material parameters (from measurements or literature), such as superconducting energy gap and coherence lengths will be related to device parameters such as device critical currents, operation temperature and speed.
[1] F. Marsili, et. al., Nature Photonics 7, 210-214 (2013).
[2] Ivry Y. et al., arXiv:1407.5945 v

The fabrication of metamaterials, in which electromagnetic waves are manipulated by varying the effective permittivity and permeability in three dimensions, is a major area of research worldwide. One approach to producing materials with a controllable spatial distribution of electromagnetic properties is the synthesis of polymer matrix composites containing different volume fractions of metallic, dielectric or magnetic filler particles. High temperature superconducting (HTS) filler particles can be used to produce temperature-dependent electromagnetic properties, with a controlled reduction in permeability as the filler material is cooled into the superconducting state. These composite materials are attractive for variety of potential applications such as active switching and shielding devices for electrical machines, as well as potentially being useful as microwave frequency flux guides for MRI applications. In this study, the basic electromagnetic properties of cast HTS composites have been measured with various volume fractions of YBCO powder, and the permittivity (ε_r) and permeability (mu;_r) values measured at room and 77K. The electromagnetic properties of these HTS composites have been correlated with the homogeneity and density of YBCO particle distributions in the polymer matrix measured using scanning electron microscopy. We show that the ε_r values of the HTS composite are enhanced with increasing volume fraction of YBCO powder at both room and low temperature. A large drop of mu;_r values with volume fraction is observed at 77K, owing to the diamagnetic response of the superconducting particles. Thus, the refractive index of the composites can be controlled both by volume fraction and temperature.
In order to achieve spatial variations in electromagnetic properties, we have investigated 3D printing of superconducting composites, a simple and versatile technique for fabricating objects with complex materials properties using polymer-based materials. Our initial experiments have demonstrated that the filament loaded with a chosen volume fraction of YBCO powder can be extruded and we will report results on simple devices fabricated by this technique.

In thin film superconductors placed in a gradually increasing (or decreasing) transverse magnetic field, the smooth propagation of the flux front can be violated by sudden bursts of flux motion. These dramatic events occur due to a thermomagnetic instability causing large amounts of flux to rush in (or out) from seemingly arbitrary positions along the edge. Magneto-optical imaging (MOI) of the flux motion in films of many superconducting materials have shown that it is a generic feature of these avalanches that they form non-repetitive branching structures. The present work presents striking experimental evidence that during their propagation such avalanches possess electromagnetic wave properties.
MOI was used to observe flux avalanches in a 170 nm thick rectangular NbN film, where part of the area was coated with a 0.9 micron thick Cu layer shaped as a long strip parallel to the film edge. It is shown that the individual avalanche branches, when propagating across boundaries between uncoated and coated areas of the superconductor, they are refracted in full quantitative agreement with Snell's law. For the present sample, a refractive index of n = 1.4 was found.
The Cu-strip was also used as an electrode allowing time-resolved measurements of the voltage pulse induced by each avalanche branch. This determines the propagation speed of the branches, and we find for the bare NbN film a speed of v₀ = 11.5 km/s, and for the metal coated part v₁ = 8.2 km/s. The fact that v₀/v₁ = n gives strong additional support to the electromagnetic wave scenario.
The origin of the refraction is believed to be caused by the dendrites propagating of an electromagnetic shock wave, similar to damped modes considered previously for metals [1]. The analogy is justified by the large dissipation taking place during the avalanches, which can raise the local temperature significantly, even far above Tc [2].
[1] E. H. Brandt, Phys. Rev. B 49, 9024 (1994); 50, 4034 (1994).
[2] M. Baziljevich et al., Appl. Phys. Lett., 105, 012602 (2014).

Superconductivity and magnetism are two long range order cooperative effects which compete with each other. Recently, this antagonist behavior has been turned around, since magnetic nanostructures could enhance superconducting properties, among many examples could be the so-called domain wall superconductivity, see for instance the recent work by Iavarone et al (1). In the present work we have followed the opposite approach; we show that superconductivity is an appropriate tool to probe the magnetic behavior of tiny nanomagnets. Superconducting films in direct contact with nanomagnet arrays are fabricated on Si substrates using a combination of different techniques as sputtering, electron beam lithography, and reactive ion etching, hellip;. The hybrid samples are patterned to a bridge which allows magnetotransport measurements with the applied magnetic field applied perpendicular to the films. We show that the superconducting vortices discriminate among different magnetic states including magnetic vortex states. Superconducting vortices are sensitive to the magnetic vortex state cores which show very small stray magnetic fields. The magnetoresistance curves, measured in the mixed state, probe and distinguish the presence of magnetic vortices, in-plane magnetization and perpendicular magnetization in the remanent state of magnetic nanodots. Finally, we show that we can explore in details the magnetization states following the dissipation induced by the motion of superconducting vortices.
(1) M. Iavarone et al. Nature Comm. 5, 4766 (2014)

The manufacturing of MgB₂ superconducting wires through the ex-situ process has been implemented in a new production line with very high throughput for an HTS material. The ex-situ route, although more complex and delicate than the in-situ one, has been selected for the scaling-up to large manufacturing volumes because of the higher homogeneity of the wire performance along its length, as well as the higher n-value and better mechanical properties. The MgB₂ filament density larger than 80% guarantees a more uniform current flow than with the in-situ, in which the filament mass density is typically below 50%. A significant effort has been devoted in strongly reducing microstructural defects and inhomogeneities that may cause local fluctuations of the critical current over very long lengths. New in-line technologies for MgB₂ wires have been implemented in order to increase the reliability of long length conductors, as eddy currents defect inspection.
The good chemical and mechanical compatibility of MgB₂ with a number of pure elements and alloys has allowed us to develop and produce multi-Km long wires with various filament architectures, matching the requirements of very different applications, as MRI magnets, rotating machines, induction heaters, fault current limiters and cables.

Enormous efforts have been done since the discovery of MgB₂ in 2001 toward improving its superconducting parameters to enable it for technical applications especially in MRI magnet. In commercial MRI still we depend on the low T_C superconductor NbTi which requires liquid helium for its operation. The soaring price of liquid helium has forced to look for an alternative. MgB₂ is a good alternative which can be operated at higher temperature around 20K well below T_C (39K) using liquid hydrogen or pulse tube cryo-cooler. The carbon doping in MgB₂ improves the critical current J_C(H) in higher field however degrades it in lower field. The degradation of J_C(H) in the low field region is mainly due to impurity phases and voids which decrease the effective superconducting cross-sectional area. Thus to improve the J_C(H) in the low field around 3T is the need for MRI application. This can be achieved by increasing the connectivity and mass density in the pure MgB₂. Various preparation technique has been adopted such as cold high pressure densification (CHPD), cold and hot iso-static pressure, internal magnesium diffusion (IMD) ex situ in situ combination etc. However all the techniques have its own merits, demerits and difficulties especially in scaling up for commercial production. The combination of in situ and ex situ technique has also been studied but it is inconclusive. This technique is very simple and easy to scale up.
In the present report we adopted the combination of in situ and ex situ technique for the preparation of the un-doped MgB₂. We optimize the ratio of ex situ in the in situ and ex situ technique and found 25% of ex situ MgB₂ gives remarkable enhancement in J_C(H) in the low field region. The values of J_C(H) at 20 K are J_c(0) = 5.2 ' 10⁵ A/cm²and J_c(3 T) = 3.3 ' 10⁴ A/cm² . This is the highest reported in un-doped bulk MgB₂ in the low field region. These enhancements are 1.7 times with respect to the insitu preparation. At 5 K, the J_c(3 T) and J_c(6 T) are 1.9 ' 10⁵ A/cm² and 2 ' 10⁴ A/cm² respectively, which are 2.4 and 4.3 times that of the in situ preparation. These samples were characterized by XRD ,SEM, VSM and resistivity measurement. We concluded that the enhancement in J_C(H) in the samples prepared by combination of in situ and ex situ technique is due to enhancement in grainconnectivity and pinning centers.

High critical current density Nb₃Sn wires fabricated by the Rod Restack Process (RRP®) are to be used in the new dipole and quadrupole magnet upgrades for the Large Hadron Collider. The present prototype conductor design for this application is a 132/169 stack with a wire diameter of 0.85 mm or 0.7 mm, corresponding to subelement diameters of 49 µm and 40 µm for the quadrupole and the dipole magnets respectively. The prototype wires we received from Oxford Superconducting Technology (OST) have an atomic Nb:Sn ratio of 3.6:1 (reduced Sn) to preserve RRR, and it has been observed that the critical current density (J_c) of the larger diameter wires averages 2700 A/mm² at 12 T and 4.2 K, while the smaller wires average only 2400 A/mm² with much larger scatter. This tendency of lower quality in smaller subelement sizes is also observed in J_c#8209;optimized wires (Nb:Sn = 3.4:1) where larger subelements reach up to 3000 A/mm² but smaller subelements are less effective. Our preliminary studies show evidence of higher Nb dissolution and uneven Sn distributions in subelements below 40 µm which may explain the lower performance. These effects have also been observed by Pong et al. and Scheuerlein et al. in recent heat treatment optimization studies where undesirable reaction paths resulted in excessive formation of large grain Nb#8323;Sn of low flux pinning capability. Pong et al. have also noticed that the highest J_c wires with the maximum amount of fine-grain Nb₃Sn (better quality) are the result of a reaction involving a regulating membrane of solid Cu-Sn phase that protects the Nb filament pack from dissolution by the Sn-rich liquid. Our preliminary studies suggest that the component dimensions of the smaller subelements disrupt the integrity of this regulating membrane and facilitate Nb dissolution (accompanied by the formation of large grain Nb₃Sn). We will discuss explicit tests of this conjecture using these state#8209;of#8209;the#8209;art RRP wires aimed at producing the highest possible J_c in the smallest possible subelement size.

Spin-fluctuation model, proposed immediately after the discovery of
Fe-based superconductors, remains the most popular scenario for this
phenomenon. On the other hand, other proposals have been put
forward, such as charge or orbital fluctuations. Criticism of the
spin-fluctuation mechanism is often (nearly always) based upon a
specific, weak coupling interpretation, which implies an intimate
connection between the Fermi surface shape, in particular its nesting
properties, and superconductivity. The critics attack the
spin-fluctuation model based on quantitative disagreement of particular
scenarios based upon the spin-fluctuation theory with selected
experiments, such as detailed topology of the superconducting gap
or specific rate of Tc suppression by impurities. In this
talk I will introduce the notion of qualitative evidence, independent on
particular implementation of the general theory, such as the pi-
junctions in corner Josephson contacts in nigh-Tc cuprates, and of
quantitative evidence, based on the level of numerical agreement of
theoretical predictions with the experiment. I will argue for the
primacy of the former type of evidence, and will show examples of
qualitative evidence favoring the spin-fluctuations mechanism.

Monolayer oxide supported films of FeSe have shown evidence for high temperature superconductivity possibly of a different type than in bulk Fe-based superconductors. Here I report calculations of the electronic structure and magnetism of such films in relation to possible superconducting pairing and observed experimental gap structures. The results indicate the effects of disorder on the electronic structure are important and should be considered when comparing experimental measurements of the Fermi surface with calculations. Scenarios for superconductivity are discussed based on the results, with the conclusion that these monolayers may be very similar to bulk Fe-based superconductors.

With its ability to probe directly the momentum-resolved single-particle electronic structure, angle-resolved photoemission spectroscopy (ARPES) is a very powerful tool to investigate the origin of the electronic properties of strongly correlated electron systems. This is particularly true for the study of multi-band systems with complicated Fermi surfaces, such as the Fe-based superconductors. Here I show evidence that local physics is important in this class of materials, including for the superconducting pairing mechanism, which cannot be satisfactorily described with weak coupling approaches. I also discuss the evolution of the electronic correlations as a function of band filling in the parent compounds of one family of transition metal pnictides, and show that the largest electronic correlations are observed for the Fe material.