More than 25 years after the discovery of HTS materials, still more than 95% of all manufactured superconductor is LTS, and 90% of this is Nb-Ti, a material with Tc of 9 K and the low Hc2(0) of 15 T. Cuprates, MgB2 and now Fe-based superconductors (FBS) all show promise to displace LTS, but so far only occupy market niches. How to escape from the niches into the mainstream will be the subject of my talk. A useful benchmark is MgB2 which is being made in multi-km lengths with low raw material cost. But it cannot generate magnetic fields > 5 T, even at 4.2 K, because of its low Hc2 (in the inferior direction, only about 15-17 T), rather low Jc and low MgB2 fill factor. These drawbacks also constrain its ability to generate useful fields in the 15-30 K range inaccessible to Nb-Ti or Nb3Sn, where only 2-3 T is possible. In principle the cuprate HTS conductors Bi-2223 tape, REBCO coated conductors and now round wire Bi-2212 all offer major advantages over MgB2, since magnetic fields of over 34 T have now been achieved with Bi-2212 and REBCO. Fe-based superconductors present interesting new possibilities that may make them suitable for applications in ways that neither cuprates nor MgB2 can match. Although it does not presently seem that Tc is likely to rise as high as in the cuprates, they have low Hc2 anisotropy (never more than 10, but in some cases ~ 1). Low anisotropy means rigid vortex lines resistant to thermal fluctuations. Anisotropy of 1-2 is significantly better than in MgB2. The low Hc2 properties that so hold back MgB2 are not an issue, since almost all interesting FBS have Hc2(0) exceeding 50 T and many are in 100 T territory. Like Nb-Ti and REBCO, but unlike Bi-2212, Bi-2223 and Nb3Sn, normal or insulating nanoscale pins can be placed into some FBS (e.g. Co-Ba122 films) and Jc values over 106 A/cm2 at 16 K have been attained. Engineering the current density in thin films by introduction of nanorods and ab-plane interlayer pins has been very effective in developing very high pinning forces, as in REBCO coated conductors. But all large-scale magnet applications depend on conductors and the most desirable conductor form is round, multifilament, well bonded to high conductivity normal metal and twisted so as to minimize hysteretic losses. Of all HTS conductors available today, only Bi-2212 and MgB2 fulfill these requirements though there are some hopes that FBS could also do too and all of these are usable only in the <30 K range. Only REBCO coated conductor and Bi-2223 tapes can be used above 30 K, but neither is round or twisted. This is the dilemma of potential users of HTS conductors. I believe that targeted research could open up the market to HTS conductors but it will imply much greater emphasis on conductor rather than superconductor properties and a new approach to avoiding grain boundary obstruction problems as recent work on Bi-2212 and FBS suggest may be possible.

Current transport properties of coated conductors at a couple of tens of Kelvin which can be reached by using cryo-cooler are attractive for their magnet applications. When applying them to magnets of particle accelerators for various uses including carbon cancer therapy, some of their material properties can be key issues. In this presentation, we focus on the following material-related issues of accelerator magnets wound with coated conductors: influence of the large magnetization of coated conductors with tape shape on the field quality of magnets; designing and winding coils with three-dimensional shapes considering the mechanical properties of coated conductors; assembling coated conductors to attain large current carrying capacities; thermal stability of superconductors against beam losses, which are thermal disturbances peculiar in accelerator magnets. We have been carrying out R&D projects on fundamental technologies for accelerator magnets using coated conductors. Results and future plans of experimental and theoretical studies on the issues will be presented.
This work was supported in part by Japan Science and Technology Agency under Strategic Promotion of Innovative Research and Development Program.

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. The disk-shaped MgB2 bulks (10-100 mm in diameter, 10 mm in thickness) were fabricated from Mg and B powders by using the in-situ technique. The mixed powders were pressed into pellet and heated at 850°C for 3 h under Ar atmosphere. Temperature dependence of trapped magnetic field shows high trapped field of over 4.0 Tesla at 11 K for a magnetized pair of two disc-shaped bulks. Trapped field as a function of time at 20 K shows small magnetic creep rate of less than 2% after 1 day. Moreover, 2D spatial distribution of trapped field measured by a scanning Hall sensor shows that equivalent field contour lines show true circle shape. These superior trapped field properties suggest that MgB2 superconducting bulk magnets would be very interesting for novel permanent magnet applications to be operated by compact cryocoolers.
This work was partially supported by Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science under grant Nos. 23246110 and 22860019 and by the Japan Science and Technology Agency, PRESTO.

Recently, our detection of non-bulk superconductivity with unexpectedly high onset-T_cs up to 49 K in rare-earth-doped CaFe₂As₂ (Ca122) single crystals [1] and Xue et al.&’s report of a T_c up to 50s K in unit-cell FeSe epi-films [2], offer an unusual opportunity to seek an answer to the question posed in the title. Through systematic compositional, structural, and magnetic investigations on rare-earth doped Ca122 single crystals, we have observed [3] a doping-level independent T_c, the simultaneous appearance of superparamagnetism and superconductivity, and the existence of mesoscopic-2D structures in these crystals, thus providing the clearest evidence to date for the possible interface-enhanced T_c in naturally occurring Fe-based superconductors. Such mesoscopic-structures may be attributed to defects as reflected by the presence of superparamagnetism [4] detected in these superconducting samples. Similar observations were also made in the FeSe thin epi-films. We have detected in the 3-4 unit-cell FeSe-films Meissner effect below 1 Oe with extensive weak-links up to ~ 20 K, unconnected small superconducting patches up to ~ 40 K, and an unusual dispersion of diamagnetic moment with frequency up to 70 K [5]. The unusual relaxation of the diamagnetic moment observed in the films suggest that glassy states may exist in the FeSe films below 20 K and 40-70 K. The experimental results will be presented and the implications discussed.
References
1. B. Lv et al., PNAS USA 108, 15705 (2011).
2. Q. Y. Wang et al., Chin. Phys. Lett. 29, 037402 (2012).
3. F. Y. Wei et al., arXiv:1309.0034v2 [cond-mat.supr-con].
4. B. Lv et al., arXiv:1308.3129 [cond-mat.supr-con].
5. L. Z. Deng et al., in preparation.

The BCS theory of superconductivity of phonon mediated pairing and its estimation within the standard density functional methods within the Migdal Eliashberg theory is one of the most remarkable intellectual achievements of the twentieth century. It was so successful that by the early 70&’ s superconductivity was considered by many a completely understood subject, with the maximum achievable critical temperatures believed to have been reached in the A15 compounds. The field of superconductivity research, then took a dramatic turn. Completely new classes of materials such as organics, heavy fermions and transition metal oxides were studied and shown not to fit within existent theory and they exhibit remarkably high transition temperatures.
In this lecture, I will present an overview of the theoretical progress in this active field of research building on the most recent advances in first principles electronic structure methods, hybrid density functionals and realistic implementations of the Dynamical Mean Field Theory (DMFT).

Understanding the origin of the material dependence of the superconducting transition temperature (Tc) in high Tc families can provide guiding principles for obtaining higher Tc materials. In the present study, we focus on the iron-pnictide and cuprate superconductors, and theoretically investigate the origin of the material (lattice structure, element) dependence of Tc. In our theoretical approach, we first perform first principles band calculation, and then construct effective tightbinding models exploiting the maximally localized Wannier orbitals. We apply the fluctuation exchange approximation to the effective models, and solve the Eliashberg equation for superconductivity. This enables us to discuss the material dependence of Tc within the spin-fluctuation-mediated pairing scenario.
For the iron-pnictides, much focus has been put on the Fermi surface configuration from the early stage of the study, where the presence of disconnected electron and hole Fermi surfaces are characteristic of this high Tc family. In addition to this, however, we have recently found that the real space motion of electrons (i.e.,hopping integrals between Fe sites), which can be largely different among materials even when the Fermi surface looks similar, is also quite important and strongly affects the spin fluctuations and Tc. This view provides understanding for the occurrence of high Tc superconductivity in materials in which the nesting between electron and hole Fermi surfaces seems to be ill-conditioned.
For the cuprates, usually a single orbital model that considers the Cu 3dx2-y2 orbital (plus the hybridized oxygen orbitals) is adopted. Here we show that in order to understand the material dependence of Tc, we need to consider the effect of the Cu 3d3z2-r2 and Cu 4s orbitals, which hybridize with Cu 3dx2-y2. We show that reducing the hybridization results in a higher Tc.
Based on these studies for the existing high Tc superconductors, we also try to deduce a guiding principle for obtaining higher Tc by optimizing the spin fluctuation mediated pairing.

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) have emerged as the most attractive opportunity to reduce the cost/performance ratio down to the levels required for energy applications. These quasi-epitaxial multilayered films are deposited on flexible metallic substrates in long lengths without the detrimental influence of grain boundaries. Additionally, they can be accurately nanostructured to achieve very high vortex pinning strengths. All these features make coated conductors very appealing for practical power applications.
In this talk I will present the major recent developments of the different coated conductor architectures being investigated in Europe, particularly those investigated in the scope of the European research program EUROTAPES. Emphasis will be made in describing the novelties associated to the diverse nanostructuration opportunities to enhance vortex pinning, as well as on the progress in the development of cost-effective chemical deposition approaches.
* Research funded from EU-FP7 NMP-LA-2012-280432 EUROTAPES project

AMSC produces Second Generation (2G) HTS wire for utility power applications as well as coil, motor and generator solutions. The wire manufacturing process is based on the RABiTS/MOD technology and yields wire in lengths up to 600+ meters with critical currents up to 500 A/cm-w (77K, self-field). AMSC&’s 2G wire is engineered specifically for power cables, fault current limiters and coils for rotating machine applications. This paper will review the architectures, performance and properties of the various wire types. In addition, to describing the higher critical currents available from the recent increase in the thickness of the YBCO layer, we will also describe the performance of AMSC&’s 2G wire at various temperatures and fields and present progress in optimizing the pinning microstructure of the MOD-based YBCO to maximize Ic at temperatures and fields applicable to coil applications.

Deposition of thin film exhibiting electrical connectivity across several hundred meters of conductor length represents a general technological challenge in any field of electrical applications.
The most pronounced problems are observed in multilayers where HTS film is deposited on the top of several buffers layers which serve as a template for HTS texture growth as well as a diffusion barrier. In this case surface defects of substrate or/and discontinuity of the buffer layers may lead to appearance of specific defects in the superconductor film.
Conditions for cumulating of such defects in the buffer layer system and possibility of their "transfer" to the superconducting film are analysed. We considered also alternative physical mechanisms of self-healing of these defects in the substrate-buffer system.
Influence of different substrate-buffer imperfections on the critical current density, Jc, is demonstrated.
Because of current bypassing effects as well as self-field dependence of Jc the influence of considered imperfections reduces with increasing aspect ratio of the coated conductor tape.
Dominating role of substrate-buffer perfection in reaching a sufficient processing yield is confirmed.

High quality long REBCO coated conductors have been successfully fabricated on flexible polycrystalline metal tapes by pulsed laser deposition (PLD) plus magnetron sputter and ion beam assisted deposition (IBAD) processes. To reduce the cost of coated conductor tapes, simplified tape architecture was developed, i.e. PLD-REBCO/Sputter-CeO₂/IBAD-MgO on metal tapes with amorphous oxide barrier-layer. Under optimized conditions, the IBAD-MgO layers showed pure (001) orientation and excellent in-plane texture. The in-plane phi-scan rocking curve is 4-6 degrees. AFM observation showed MgO layer had very smooth surface. The RMS is less 1nm. On the textured MgO layer, sputter deposited single cerium oxide cap-layer showed pure (001) orientation and excellent in-plane texture of 4-6 degrees. Reel-to-reel PLD process with high deposition rate was already scaled up to 100m/h tape speed. One micrometer thick REBCO films had a high critical current density of over 3.0×10⁶A/cm² (at 77 K, in zero magnetic field). The critical current density of two micrometer thick films is still more than 2.5×10⁶A/cm². Several hundreds of meters long coated conductor tapes with over 500A/cm performance can be routinely fabricated now. The process optimization for kilometer long coated conductor tapes is underway.

In order to achieve the required cost-performance metric for employment of coated conductors in applications such as wind generators and superconducting magnetic energy storage (SMES), we have been working on dramatic improvement of critical current at the operating conditions of these applications i.e. in magnetic fields of 2 to 30 T at temperatures of 4.2 K to 50 K. (Gd,Y)Ba2Cu3Ox (GdYBCO) coated conductors made by metal organic chemical vapor deposition (MOCVD) with high levels of Zr addition (> 15 mol.%) are being explored. Pinning force levels over 300 GN/m^3, 450 GN/m^3 and 1700 GN/m^3 have been achieved at 40 K, 30 K and 4.2 K respectively in 15 mol.% added GdYBCO superconductors in the orientation of field perpendicular to the conductor. We have also discovered very interesting correlations between critical currents at 77 K in low magnetic fields (up to 3 T) and critical currents in high magnetic fields at low temperatures.
This project was partly funded by Advanced Research Projects Agency-Energy (ARPA-E) award DE-AR0000196.

The study of the flux pinning has been developed for the high Jc coated conductor using the artificial pinning center technology. In particular, the results of c.c. doped with the BaMnO3 (BMO) nanorods/nanoparticles contribute to the high Jc c.c. at high magnetic field. Recently some groups report the combination technologies BMO nanorods with nanoparticles.
In the view point of the growth temperature, the varieties of BMO nanorods and its amount of additive, the object of this study is the controlling of shape, slop and width of the nanorods in REBCO film. Furthermore, high Jc c.c. was developed by the knowledge in the flux pinning.
The controlling the growth temperature, SmBCO film and GdBCO coated conductor grown with BaHfO3 nanorods in improving its straight shape on the LaAlO3 substrate and IBAD tape has high pinning force (27 and 23GN/m3)and Birr (15.0 and 15.8T at 77K). Using the low temperature growth technique with the seed layer, we can design and control the high density and the fine BMO nanorods grown in high quality and high Jc SmBCO film. Now we will discuss about the relationship of its distance, strain and superconducting properties.

Multilayered films comprised of BZO or BSO containing YBCO layers and pure YBCO layers have been studied to tune the spatial distribution, density, and pinning anisotropy of nanorods by using PLD. This technique allows us to produce “segmented” BSO nanorods keeping the designed matching field, since the nanorods nucleate and grow just above the upper portion of nanorods underneath the pure YBCO spacing layer. The pinning anisotropy and the spacing are easily varied by changing the pulse number of PLD. In present work, the segmented BSO nanorods were introduced to YBCO thin films for anisotropy tuning of columnar pins. We found that Birr was systematically changed by selection of average nanorod length lp and its spacing ls, and that these variations were explained by the “harmonic oscillator” approach based on the Bose glass state. Jc-B characteristics and field angular dependent Jc also were varied by lp and ls. The experimental and theoretical results indicate that segmented nanorods behave as single columnar pin for B || c, in contrast, as nanoparticle pins for B || ab. The tuning of pinning properties can provide us the further opportunities to attain isotropic and high Jc-B performances as well as high Birr in YBCO films.

Nanostructure control successfully improved pinning properties in YBa2Cu3O7 (YBCO) films, and Ba oxide nanorods are one of the most effective pinning centers. As Critical current density(Jc) optimization has been already actively performed, advanced structural control is needed to realize much higher performance. Several length-scale-structures should be discussed to control vortex pinning properties; interface structure (~atomic scale), size of nanorods (~5 - 10 nm), and spacing of nanorods (a few tens nm). Regardless of extensive studies on nano-scale control of pinning centers (~5 - a few tens nm), atomic-scale interface has not been studied in detail previously. However, since interface between YBCO matrix and nanorods determines elementary pinning force, we believe that the interface is future frontier on pinning control. Therefore, influence of strain, interface electric state and compositional variation on superconductivity at the interface should be understood to control the structure. In the present study, YBCO+BaMO3(M=Sn,Zr) films were fabricated using pulsed laser deposition, and X-ray diffraction (XRD) and transmission electron microscopy(TEM) measurements were carried out. To analyze the interface, first principle calculation was performed. TEM revealed that nanorods contained not only BMO but also Y and that lattice parameters were larger than BMO without Y. In addition to this lattice expansion of nanorods resulting from Y incorporation, compressive stress was induced in the nanorods by YBCO matrix, and stain strongly depends on volume fraction of nanorods. Based on these results, influence of the pinning center interface on critical temperature (Tc) and Jc is discussed.

Historically experimental efforts and theoretical guidance in the quest for very high temperature superconductivity have focused on increasing the pairing interaction. Recently, however, insights into physical (mechanism independent) constraints on very high Tc superconductivity have entered the equation. One of these is that necessarily the Cooper pairs will be very small, approaching unit cell dimensions, and therefore the pairing interaction must be local. In this talk we explore the consequences of this reality and the need for physics and chemistry to join hands in the quest for very much higher temperatures superconductors.

Since the discovery of high-Tc cuprates the quest for new superconductors has shifted toward more anisotropic, strongly correlated materials with lower carrier densities and competing antiferromagnetic orders. While these materials features enhance superconducting correlations, they also result in such serious problems for applications, that the critical temperature Tc and the upper critical field Hc2 become secondary. For instance, the performance of cuprates and Fe-based superconductors at high fields and temperatures is mostly limited by thermally-activated hopping of vortices and electromagnetic granularity. I give an overview of what materials properties may be crucial for these issues and discuss the materials restrictions, which have to be satisfied to make superconductors useful for power applications at high temperatures and magnetic fields. These restrictions become essential at higher temperatures and magnetic fields, particularly for the yet-to-be-discovered room temperature superconductors (RTS). In this case the performance of superconductors would be mostly limited by fluctuations for which higher carrier density and weaker anisotropy are more essential than higher pairing temperature. As the operating temperature increases, the materials parameter space where applications are possible shrinks rapidly.

We review the present state of the understanding and application of high temperature superconductor materials ranging from attempts to explore pairing mechanisms on the energy scale of a few milli-electron-volts to their use to embody terra-kwh continental wide deployment within the electricity enterprise. Examples include the use of density functional theory to study the relative roles of spin-fluctuation and/or lattice vibration induced Cooper pairing to modelling the incorporation of long distance HTSC transmission cables within the same natural gas pipeline rights-of-way infrastructure now emerging worldwide.

The interface between LaAlO3 and SrTiO3, two good band insulators, which was found in 2004 to be conducting [1], and, in some doping range, superconducting with a maximum critical temperature of about 200 mK [2] is attracting of lot of attention. The electron gas has a thickness of a few nanometers at low temperatures and a low electronic density, typically 5 1013 electrons/cm2. Being naturally sandwiched between two insulators, it is ideal for performing electric field effect experiments that allow the carrier density to be tuned.
I will discuss in this presentation superconductivity, the phase diagram of the system and the link with doped bulk SrTiO3 [2,3]; spin orbit and recent experiments on nanostructures that reveal a remarkable tuning of the electronic properties and allow weak localization and weak anti-localization as a function of doping and temperature to be followed [4,5]. Finally, I will discuss an approach that should allow superconducting coupling between different gases to be studied.
[1] A. Ohtomo, H. Y. Hwang, Nature 427, 423 (2004).
[2] N. Reyren, S. Thiel, A. D. Caviglia, L. Fitting Kourkoutis, G. Hammerl, C. Richter, C. W. Schneider, T. Kopp, A.-S. Ruetschi, D. Jaccard, M. Gabay, D. A. Muller, J.-M. Triscone and J. Mannhart, Science 317, 1196 (2007).
[3] A. D. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M. Gabay, S. Thiel, G. Hammerl, J. Mannhart, and J.-M. Triscone, Nature 456, 624 (2008).
[4] A. D. Caviglia, M. Gabay, S. Gariglio, N. Reyren, C. Cancellieri, and J.-M. Triscone, Physical Review Letters 104, 126803 (2010).
[5] A. Fecirc;te, S. Gariglio, A. D. Caviglia, M. Gabay, J.-M. Triscone, Physical Review B (RC) 86, 201105 (2012).

The electron liquid at the LaAlO3/SrTiO3 interface is a model system for the study of superconductivity as it provides a two-dimensional superconductor whose properties can be tuned with an electrical gate field. We developed planar tunnel junctions to study the superconductivity spectroscopically. Our tunnel junctions give access to two important physical parameters: the size of the superconducting gap and the electron-phonon spectral function. The gap increases with decreasing charge density in the entire phase diagram and persists up to temperatures far above Tc in the underdoped regime. These results are analogous to the pseudogap behavior of the high-transition-temperature copper oxide superconductors. The electron-phonon spectral function is independent of the carrier density and the likelihood of the conventional electron-phonon coupling mechanism for superconducting pairing will be discussed.

Ionic gated superconductivity has been so far demonstrated on the atomically flat surfaces of several substances including SrTiO3, ZrNCl, La2CuO4, YBa2Cu3Ox, and MoS2 [1-5]. The device structure is a simple metal-oxide-semiconductor field effect transistor (MOS-FET), but for increasing the accumulated carrier density, the oxide dielectrics are replaced with electrolytes or ionic liquids. These materials are electrically insulating but ionically conducting, allowing charge accumulation with more than one order of magnitude larger than that for oxides. In the presentation, discussion is given on the nature of superconductivity, such as the phase diagrams and difference from the bulk superconductors, with particular emphasis on the two dimensionality of superconductivity. Based on the transport measurements and anisotropy of upper critical fields Hc2, we found that superconductivity is highly two-dimensional with a thickness of 1~2 nm, which is much smaller than the in-plane coherence length. These values are close to the Thomas-Fermi screening lengths, indicating that the carriers are accumulated by the electrostatic mechanism. Another important aspect of electric field effect is that the spatial inversion symmetry is inherently broken in the present device, and might produce a serious impact of paring symmetry, particularly in systems with strong spin-orbit interactions. In fact, the in-plane Hc2 was found considerably enhanced by a factor of more than three in comparison to the value of the Pauli limit. This strongly indicates considerable mixture of triplet nature in the paring in electric field induced superconductivity. [1] K. Ueno et al., Nat. Mater. 7 (2008) 855. [2] J. T. Ye et al., Nat. Mater. 9 (2010) 125. [3] K. Ueno et al., Nat. Nanotechnol. 6 (2011) 408. [4] A. T. Bollinger et al., Nature 472 (2011) 458. [5] J. T. Ye et al., Science 338 (2012) 1193.

2014 marks the 50th anniversary of the discovery of high field superconductivity in Nb3Sn and remarkably, along with the even older Nb-Ti (1961), these two venerable superconductors continue to dominate the production of magnetic fields up to ~24 T. The value of applying Nb-Ti to making accelerator magnets was quickly recognized and the recent confirmation of the Higgs boson at CERN would not have been possible without the ~10,000 Nb-Ti wound magnets operating at 1.9 K of the Large Hadron Collider (LHC). Nb-Ti remains the first choice for most applications because of its low-cost and its excellent uniformity and mechanical properties but to go beyond the 9 T of the LHC or the 11.75 T of the Iseult/INUMAC whole-body Magnetic Resonance Imaging (MRI) project at SACLAY, requires application of Nb3Sn (or Nb3Al) or an HTS conductor. Nb3Sn continues to be developed to higher performance and is critical for high-field NMR and very large magnetic confinement fusion projects such as ITER. Both Nb-Ti and Nb3Sn are very fortunate in properties that have been crucial for their success: Nb-Ti alloys with compositions close to optimum Hc2 precipitate a ductile precipitate at ~400°C which co-draws with the wire to almost match the flux-line lattice spacing at 5 T (4.2 K). Nb3Sn is a brittle intermetallic that cannot be drawn down as a wire but when reacted from Nb filaments embedded in a Cu(Sn) matrix it is protected by the compressive jacket of Cu(Sn) which contracts faster than Nb3Sn when the strand is cooled. In this presentation we will consider how much luck we have left.

High critical current density (Jc) YBa2Cu3O7 (YBCO) superconducting films have been grown on cube-textured metal tapes for the purpose of developing second generation superconducting wires for high temperature, high magnetic field applications. In the standard RABiTS approach, a biaxially crystal-aligned YBCO layer is deposited on a Y2O3/YSZ/CeO2 buffered Ni-W tape. Coated conductors become highly resistive when they are quenched, therefore, to manufacture reliable and safe HTS applications, it is necessary to use conducting (metal) layers such as Cu and Ag to attach to the HTS tapes to stabilize and protect the conductors from damage due to quenches. Presently, insulative oxides are used for the buffer layers, thus thick Ag and Cu layers are required to be deposited as the stabilizer layers on the YBCO layer. However, the high material and process costs for the Ag and Cu layers are one of the major obstacles for achieving low-cost coated conductors. Use of a conductive buffer layer instead of the insulative ones can make coated conductors free from the expensive Ag and Cu stabilizer layers.
We prepared Nb-doped SrTiO3 films on a Ni-electroplated cube-textured Cu tape laminated with SUS316 substrate and epitaxially grew YBCO layers on them. Jc of the YBCO layer on the Nb-STO/Ni/Cu/SUS316 was 1.2 MA/cm2 at 77 K in self field. When changing to normal conductivity, new structure has gradual voltage slope compared with conventional structure. We confirmed that the Cu tape worked as stabilizing layer by applying conductive Nb-doped SrTiO3 as a buffer layer.

The stabilization layer of the YBCO thin-film wire is an essential part of the wire as it protects the wire from over-currents and determines the transport-current characteristics. The transport-current characteristics of the stabilization layer are determined by the specific resistance of the material of the stabilization layer.
Therefore, in this study, the YBCO thin-film wire was first fabricated by depositing materials with different thickness values (Ag) on YBCO thin-film wire with no stabilization layer, using the “Thermal Evaporation Method” with nano-range and micro-range thicknesses to form a stabilization layer. Then the fabricated YBCO thin-film wire was subjected to basic characteristics tests and over-current transport-current tests to investigate their phase transition. Finally, the results of the basic characteristics tests and the over-current transport-current tests were analyzed to present the applications of superconducting power application devices of the YBCO thin-film wire according to the thickness of the wire&’s stabilization layer.

REBa₂Cu₃O_y(RE123) melt-solidified bulks have great potential as strong magnets because of their high J_c in magnetic fields. Therefore, many studies have been done for the development of high performance RE123 bulks by enhancement of J_c thus far. However, the optimization of critical current properties has been attempted mainly at liquid nitrogen temperature, 77 K, while most of their strong magnet applications are expected at lower temperatures. To establish strategies achieving high J_c at low temperatures is necessary for extensive applications of RE123 bulks. Based on these backgrounds, we have fabricated Y123 melt-solidified bulks including fine Y₂BaCuO₅(Y211) precipitates using the powder mixture of Y123 and Y211 prepared by solid-state reaction at low temperature. Features of this method are trying to synthesize Y123 and Y211 simultaneously at a low temperature 800°C starting from Y₂O₃, BaO₂ and CuO under flowing Ar/O₂ gas. After addition of 0.5wt% Pt powder, which is a common method for reducing the particle size of Y211, melt-solidification was performed in air by the cold-seeding method using Nd123 single crystals.
Cylindrical Y123 bulks with single domain were successfully synthesized and they showed enhanced J_c under low fields. Microstructure analysis revealed that the particle size of Y211 in Y123 bulks was decreased and more than 40% Y211 precipitates are smaller than 0.6 mu;m. In addition, more excellent J_c-B properties were observed for Y123 bulks without addition of Pt, while the particle size of Y211 was slightly increased. This might suggest that J_c of Y123 bulks at low temperature and in high fields, where flux pinning due to the interface of Y211/Y123 is less effective, were intrinsically suppressed by addition of Pt. Further improvement J_c in high fields is also important to develop RE123 bulks showing high trapped field properties. The dilute Ga-doping [1], which generates point-defect-like pinning in the Y123 crystal, was applied in the synthesis of Y123 bulks. The dilute Ga-doped Y123 bulks grown from the simultaneously synthesized Y123 and Y211 mixtures maintained high J_c > 0.5 MA/cm² up to 10 T at 40 K accompanying the broad second peak effect. This strongly suggests that over 10 T-class small Y123 bulk magnets can be developed by adopting our new synthesis method.
[1] Y. Ishii et al., Appl. Phys. Lett.89 (2006) 202513.

We report an experimental study of the crystallographic lattice, morphologic characteristics and magnetic feature of Y3Ba5Cu8-xFexO18-δ complex perovskite with doping of Iron x = 0.0597, 0.0625, 0.0825, 0.0925, 0.1225. The structural characterization of the compound was performed by X-ray diffraction and Rietveld refinement using a polycrystalline sample of thematerial. Structural ordering corresponds to an orthorhombic structure, from the space group Pmm2. The lattice parameters of the material are reported here. The superconducting nature of the material was determined by the measuring of the magnetization, as a function of its temperature, in the regime of low and mean field. Magnetic measurements reveal the superconducting behavior, the transition temperature decreases as a function of the Fe concentration. DC resistivity measurements show a bulk critical temperature Tc for the analyzed samples The Fe substitution reduces Tc from 93 K to 84 K.

Traditionally, one dimension X-ray diffraction is used to characterize preferred orientation for the large single grain films. In this report, two dimensions X-ray diffraction (2D-XRD) is used to provide more crystal orientation information of samples, such as the orientation distribution function, tilt angle across grain boundaries, as well as their corresponding superconducting properties. Two different kinds of material: FeSe preferred orientation poly-crystals and YBCO single grain bulks were characterized by 2D-XRD. For the FeSe samples, it is interesting to show that at least four preferred orientation with twin structures were observed by mis-orientation calculation. YBCO crystals with {011}/(001) twin structures were also measured by 2D-XRD, which can act as flux pinning centers to enhance the critical current density. A systematical study of twin structure of both FeSe and YBCO systems on both crystalline and superconducting properties will be reported.
This study was supported by the National Science Council, Taiwan, under Contract NSC 102-2811-M-006-043- and NSC 102-2112-M-006-001-MY3. The two dimensions X-ray diffraction is measured in Establishment of the Instrument Development Center of NCKU.

LFMA (microwave absorption in low magnetic fields) is a highly sensitive tool [1] in searching for possible higher T_C superconducting phases in iron selenide telluride (FeSe_xTe_1-x) based films on different substrates. This is especially effective when combined with SQUID magnetometry and resistivity. LFMA uses an external magnetic field to create either vortices and/or weak links, which are non-resonantly excited by microwave radiation. These different excitations can be detected at low magnetic fields via unique perturbations in microwave response. Such excitations have strong angle dependence in 2-D systems and can be used to carefully probe and understand confinement effects in such systems. [2]
The motivation of this study is to understand the 2-D superconducting behavior of thin layers of iron chalcogenides deposited on different substrates, such as glass and carbon nanotubes. When anisotropic high temperature superconductors are confined in lower dimensions, interesting effects have been observed. Superconductivity has been shown to persist even in monolayers of such materials and, even more interestingly, sometimes superconductivity is enhanced. Such enhanced superconductivity is due to interfacial effects. It has been shown that a few atomic layers of FeSe deposited on strontium titanate exhibits T_C as high as 80K. [3]
We have measured FeSe_xTe_1-x thin films on glass with differing Se and Te content. The LFMA spectra generally exhibit three distinct features in different T-regions: a hysteretic LFMA at low temperatures below T_C (indicating vortices), a step function LFMA at higher temperatures below T_C (indicating Josephson junctions) and non-hysteretic narrow LFMA at higher T (possibly indicating spin-waves). We observe that the hysteretic signal is dominant with high Se content, diminishing completely with FeTe samples. In contrast, the step function response strengthens with Te content. The non-hysteretic narrow is observed in all samples and persists up to 100K. Samples of FeSe_xTe_1-x deposited on carbon nanotubes, either on forests or sheets, were also tested and we will describe these results.
[1] Stankowski, J. Microwave absorption (MMMA) - a contactless method to study superconductors and magnetic nanostructures. Metrology and Measurement Systems Vol. 13, 125-135 (2006).
[2] Pozek, M., Dulcic, A. & Rakvin, B. Comparison of direct and modulated microwave magnetoresistance in ceramic and single crystal YBa₂Cu₃O_{7- δ} . Physica C: Superconductivity 197, 175-182 (1992).
[3] Zheng, F., Wang, Z., Kang, W. & Zhang, P. Antiferromagnetic FeSe monolayer on SrTiO₃: The charge doping and electric field effects. Sci. Rep. 3, (2013).

Fe1.125Te alloys have been synthesized by solid state reaction methods. It crystallizes in Cu2Sb-type structure with lattice constants of a=3.8231 Å and c=6.2808 Å (space group: P/4nmm). The composition of the alloys is confirmed by Energy dispersive X-ray spectrometer (EDX). The effects of nitrogen annealing on Fe1.125Te lattice structure and physical properties have been studied. Samples are annealed at different temperatures and various pressures respectively. It is found that the lattice constants a and c increased after annealed at temperature673~873K. When the temperature is above 1123K, the size of the unit cell returns to the similar size of original samples. The magnetic-thermal curves both show a magnetic transition at 128 K after annealing at 873K in N2 atmosphere and vacuum. The step-like magnetic-thermal curves were observed after annealed at 1173K, which is associated with two magnetic transitions. In vacuum, the transition temperatures are 122 K and 128 K, while in the nitrogen, they are 122 K and 138 K. The resistance-temperature curves indicate a semiconductor to metal transition around 69 K for N2 atmosphere annealing at both 823K and 1173K.

RE doped CaFe₂As₂ system has been attracting various interests due to its high T_c reaching 49 K and anomalous superconducting properties, such as weak diamagnetism at high temperatures above ~20 K and very low J_c, implying the absence of bulk superconductivity.
In order to clarify the origin of anomalous superconductivity in this system, Pr doped and Pr,Co co-doped CaFe₂As₂ single crystals were grown using FeAs flux method with nominal compositions of (Ca_1-xPr_x)(Fe_1-yCo_y)₄As₄. Pr,Co co-doped sample showed two-step transitions at T_c1 = 30~36 K, which is lower than that of Pr doped sample, and T_c2 < 16 K. In addition, post-annealing was performed for these crystals in evacuated quartz ampoules at various temperatures. As a result, we found that samples annealed at 400°C have longer c-axis than that of as-grown ones, indicating existence of the lattice strain in as-grown crystals and this strain is released by annealing at 400°C. Along with the release of the strain, superconducting properties changed drastically. Some of the co-doped samples showed large diamagnetic signal below T_c2 after annealing, suggesting appearance of bulk superconductivity. However, the volume fraction above T_c2 was always very low. Further analysis and discussion on post-annealing and co-doping effects will be shown.

Superconductors of the BSCCO system have been studied due to their present high critical temperature and their important technological applications. This compound presents the general formula Bi2Sr2Ca n-1CunOx+d (n=1, 2 and 3) with three principal compositions 2201, 2212 and 2223 with critical temperature around 15 K, 85 K and 110 K, respectively. In this work, we obtained 2201 and 2212 pure phases using a citrate route. The samples were characterized electrically, chemically, structurally and thermochemically. Enthalpies of formation, ΔH0f, from the constituent oxides at 298 K, were determined by high-temperature oxide melt solution calorimetry using an isoperibol twin microcalorimeter of the Tian-Calvet type with molten sodium molybdate 3Na2O.4MoO3 at 977 K. The enthalpies of drop solution of 2201 and 2212 phases were (-140.5644±43.5245) kJ/mol and (-141.98 ± 33.01) kJ/mol, respectively. These results may contribute significantly to the process of synthesizing a pure high Tc superconductor of the BSCCO system.

Ex-situ MgB₂ samples have higher packing factor compared to in-situ samples. However, intergranular coupling for ex-situ samples is much weaker due to limited contact area and presence of impurities at grain boundaries. Indeed, both J_c and connectivity, which indicates the macroscopic degree of intergranular electrical coupling, for ex-situ samples, were reported to be lower than those of in-situ samples. Since connectivity is a function of packing factor and intergranular connection, much higher connectivity and J_c can be expected for dense in-situ samples if a strong intergranular connection can be achieved. In our previous study, ex-situ MgB₂ bulks prepared by heat-treatment at ~900°C for ~100 h under ambient pressure showed evidence of self-sintering and enhancement of intergranular coupling [1]. However, the temperature range suitable for self-sintering of MgB₂ is limited due to its phase stability corresponding to the equilibrium vapor pressure of Mg. In addition, sintering for a long time would cause practical problems, such as a reaction between the sheath materials and MgB₂ core and possible MgB₂ grain growth. Therefore, the development of new approaches to promote self-sintering in a shorter time is necessary to apply this technique to practical wires. In this study, artificial promotion of self-sintering was attempted to obtain ex-situ MgB₂ bulks with high connectivity. To promote self-sintering, laboratory-made high purity MgB₂ powders were fabricated by three processes, i.e., removing surface MgO to obtain fresh MgB₂ surface, substituting carbon in MgB₂ to introduce defects and accelerate mass transfer, and ball-milling to obtain fine MgB₂ grains. Using these powders, we prepared ex-situ MgB₂ polycrystalline bulk samples by a self-sintering process and evaluated the microstructure and intergranular current transport properties of these samples. By reducing the amount of MgO, the contact area among MgB₂ grains was enlarged, and therefore self-sintering was promoted. By introducing defects by the carbon substitution and reducing the amount of MgO, connectivity increased from 8.3% for the conventional ex-situ MgB₂ to 35% for carbon-substituted MgB₂ sintered for 24 h, which is one-fourth of the optimal sintering time for the conventional bulks [2]. Ball-milled powder had small and uniform grain size, resulting in high connectivity. The results suggest that these processes are effective for achieving superior connectivity through the promotion of self-sintering.
[1] H. Tanaka et al., Supercond. Sci. Technol. 25 (2012) 115022
[2] S. Mizutani et al., submitted to Supercond. Sci. Technol. (2013)

Pore morphologies and superconducting properties of in-situ processed polycrystalline MgB₂ bulk using Mg powders with different shapes were investigated. Three kinds of Mg powders (spherical type, particle size 4-6 mu;m and 12-17 mu;m; plate type, particle size about 44 mu;m) and semi-crystalline B powder below 1 mu;m were used. To understand the pore formation mechanism, MgB₂ bulks were heat-treated by a solid-solid reaction of 600 ^oC for 1 h or 40 h, and a liquid-solid reaction of 900 ^oC for 1 h in an Ar atmosphere. The phase formation, microstructure, density change and critical properties of MgB₂ were examined. The samples heat-treated at 600 ^oC for 1 h showed that the MgB₂ phase formation was insufficient with the remnant Mg, and that Mg diffusion and pore evolution were already observed. Meanwhile, the samples heat-treated at 600 ^oC for 40 h or 900 ^oC for 1 h represented a fully converted MgB₂ phase with a larger pore volume when compared to the sample heat-treated at 600 ^oC for 1 h. It was found that the size and shape of the pores were dependent on the starting Mg powder morphology in in-situ processed MgB₂ samples. The formation of pores was caused by a difference in the diffusion rate between Mg and B at below Mg melting point (650 ^oC) and the capillary motion of Mg at above Mg melting point. A pellet expansion owing to pore evolution and mass loss of Mg after heat-treatment occurred in all samples, which decreased the apparent density. The critical current density of the sample prepared using a smaller spherical Mg of 4-6 mu;m showed the highest values over the applied magnetic fields, although the current capacity of in-situ processed MgB₂ bulks varied with the heat-treatment temperature and time.

The implementation of hybrid-electric-vehicle (HEV) or electric-vehicle (EV) propulsion for automobile transportation been established in the last 5-10 years, and achieves very significant increases of energy efficiencies of 3-4x from the use of non-combustion technologies and ‘smart&’ energy management including brake regeneration. The possibility of electric propulsion for aircraft has increasingly been considered in the last 5 years, and has been successfully implemented in 2-4 passenger aircraft. This paper will summarize recent progress in this field for aircraft, and present case studies of how cryogenic electric power systems can positively impact hybrid-electric or all-electric power systems and capabilities, for different size and power level aircraft. Cryogenic systems will be compared to Cu-wire based systems. The components of 0.1-40 MW-class cryogenic and superconducting power systems will be reviewed, including generators and motors, power transmission cables, power storage devices including Li-batteries and superconducting magnetic energy storage (SMES), power electronics including inverters, and cryogenic technologies. Cryogenic power systems have unique properties such as 10-20x lower weights and ultra-high efficiencies > 98%, which are expected to provide new capabilities and potential significant benefits such as fuel burns reduced > 70% for 400 passenger aircraft powered by hybrid-electric distributed propulsion (HEDP).

Acknowledgments: Air Force Office of Scientific Research (AFOSR), and The Air Force Research Laboratory - Aerospace Systems Directorate (AFRL/RQ).

Wide-ranging needs exist for electrical energy storage systems for many commercial and military applications, and the development of storage systems with new capabilities such as longer cycling rates and higher power and energy densities are always of interest and can have tremendous benefits and positive impacts. Superconducting magnetic energy storage (SMES) devices offer attractive and unique features for energy storage including no theoretical limit to specific power, high cycling efficiencies and charge/discharge rates, and virtually no degradation with cycling. While SMES can achieve the highest power density of any known technology, it traditionally is achieved in magnets with low specific energy densities < 10 Wh/kg. This paper considers new routes to achieve SMES units with much energy densities, to hopefully be competitive with state-of-the-art Li-batteries.
This paper studies the limiting factors of SMES energy densities using present-day and future technology advancements, and particularly considers the use of different commercially available long-length conductors including YBCO, 2nd-generation MgB2, and Nb3Sn wires. If critical current did not depend on field, such as for YBCO with H//ab, an optimum solenoid design would have ratios of outer to inner radius α =1.86 and length to inner radius of 0.95. The MSED ε would scale with stored energy E as ε ~ E2/5. With dependence of critical current on field taken into account, the optimum design for YBCO also including H//c is a pancake with the same α and different scaling ε ~ E1/3. Thus, current and magnetics limits achievable ε only at a fixed E. The overall limit on ε is also imposed by the virial theorem. Without additional structural support ε of a YBCO magnet is limited to ~ 30 Wh/kg. However with introduction of light-weight and strong support materials the upper limit MSED of SMES can be substantially increased, and is likely to exceed that of the best batteries ε ~ 150 Wh/kg.
Acknowledgments: Air Force Office of Scientific Research (AFOSR) and The Air Force Research Laboratory - Aerospace Systems Directorate (AFRL/RQ).

Remarkable progress has been made in improving the superconducting properties of doped (AE)Fe₂As₂ (AE = Sr, Ba) over the last couple of years. Critical current densities (J_c) over 10⁴ Acm^-2 beyond 10 T have been achieved in round wires, and now tapes have demonstrated that J_c can be further raised close to 10⁵ Acm^-2 at 10 T by adding texture. In addition, low anisotropy ~2, high upper critical fields close to 100 T, and the low cost of the starting materials make an encouraging case for why this family of superconductors may be the next in line for niche applications at high fields (>10 T) and temperatures achievable by cryocoolers (>10 K). However, we find that even high J_c wires (4.2 K, self-field >0.16 MAcm^-2) show evidence for significant obstacles to current flow. We conclude that although long-range connectivity does exist in our high J_c samples, there is also clear evidence for percolative current flow suggesting further room for improving J_c(H) performance. Recent effort to raise J_c through texturing, impurity doping, and grain boundary engineering will be discussed for bulk materials as well as for single and multi-filament wires.

After 25 years of Bi2Sr2CaCu2Ox (Bi2212) discovery, and despite significant improvements in the performance of Bi2212/Ag multifilamantary round wires, many unanswered questions remain regarding relationships of microstrcuture and transport behavior of Bi2212/Ag wires. In particular understanding the impact of microstructural defects on different lentgh scales on the transport behavior is a significant challenge. In melt processed multifilamentary Bi2212/Ag round wires the primary impurity is Bi2Sr2CuOx (Bi2201) which forms as mesoscopic grains and nanoscopic intergrowths. In this study, the mesoscopic microstructures are analyzed quantitatively using a statistical approach in which filaments are categorized based on the predominant phases observed by cross-sectional scanning electron microscopy (SEM). A Matlab program is created to analyze the SEM micrographs and categorize over 100 filaments within each image. In total, 26 wires, each heat treated differently so as to vary the critical current density (Jc), are studied. The majority of filaments, 78% of all filaments classified, are either "predominantly Bi2212" or "containing-large-Bi2Sr2CuOx grains". It is found that Jc is directly proportional to the percentage of "predominatly Bi2212” filaments, and is inversely proportional to the percentage of filaments "containg-large- Bi2201 grains". Via a lift-out technique using focussed ion beam (FIB), Bi2212 grains are extracted from filaments with and without significant Bi2201 grains and for the first time the nanoscale characteristics of Bi2201 intergrowths within Bi2212 grains are studied with the aim of atomic resolution images, obtained by an aberration corrected scanning transmission electron microscope (STEM). Several aspects of the Bi2201 intergrowths within Bi2212 grains are studied, including density, chemistry, dimension variety, orientation and Bi2212/Bi2201 interface coherency. By correlating nanoscale characteristics of Bi2201 intergrowths to the Bi2212 coherence length (xi;), magnetization behavior and field dependent transport, the roles of Bi2201 intergrowths in a Bi2212/Ag wire transport are discussed through their impact on c-axis transport and flux pinning.

Magnesium diboride has become an important superconducting material for technological applications very shortly after its discovery. It has high critical temperature (39K), high critical current, absence of weak-links, as well as low cost. Superconducting composites of MgB2 and carbon nanotubes (CNTs) showed significantly improved superconducting properties: CNTs can carry high currents and remain stable in composites providing connections between MgB2 grains, improve tensile strength, and can reduce the amount of oxygen contamination. Here we present a new method to produce ultra-long MgB2 nanowires using free-standing carbon nanotubes sheets as templates. In the first step, the nanotubes were coated with a conformal layer of boron using Laser Assisted Chemical Vapor Deposition. The resultant boron-CNT nanowires have thickness of 70-150 nm. By exposing the boron-CNT nanowires to magnesium vapor the MgB2-CNT composite is formed. MgB2-CNT arrays are flexible and can be easily bent and even twisted into yarns. The MgB2-CNT nanocomposite yarns are superconducting with critical temperature up to 37 K, its critical fields are comparable with conventional MgB2 superconducting wires. The morphology is characterized through scanning and transmission electron microscopy, and correlated with electronic transport measurements, low-temperature SQUID magnetometry and magnetically modulated microwave absorption.
This work is supported by US Air Force Office of scientific research, contract FA9550-09-1-0384.

In conventional Type II superconductors such as NbTi alloys, vortex pinning centers typically consist of small normal metal precipitates such as the α phase of Titanium. In this superconductor the coherence length (about 60 Å) is somewhat larger than the size of the normal precipitates. The pair potential is then locally depressed, making it energetically favorable for vortices to have their cores pinned on the precipitates. This vortex pinning strategy is inapplicable to high temperature superconductors where the in-plane coherence length is only a few lattice spacing (15 Å).Two conditions must be met to pin vortices in that case. First of all, the condensation energy per coherence volume must be substantially larger than kBTc, otherwise no pinning is possible at temperatures of the order of Tc - in other terms the superconductor can only be used at low temperatures, although it has a high Tc. The value of the condensation energy can be obtained from measurements of the electronic heat capacity, and it turns out that YBCO is the only cuprate that meets this condition, particularly when it is overdoped by oxygen. It is therefore the only one in which vortices can in principle be pinned in substantial fields up to temperatures of the order of the transition temperature. Ref.1 gives a detailed discussion of the role played by the condensation energy in the vortex pinning problem.
The second condition is that a way must be found to introduce defects that affect locally the Cu-O-Cu bond length. The inclusion of small grains of an insulating compound having a lattice structure similar to that of the host but with a different lattice parameter, such as Barium Zirconate, has been shown to introduce in the YBCO matrix strains and defects such as stacking faults and dislocations that greatly improve vortex pinning (2). Vortex pinning is not due to the insulating grains themselves, but to the strains introduced by the lattice mismatch at the interface. These results are inconsistent with High Tc models based on band Hamiltonians that do not take into account local lattice deformations. On the other hand, they are consistent with models that emphasize the local nature of the pairing potential in the cuprates, the key parameter being here the Cu- O - Cu bond length. For instance, in the Bond Contraction Pairing model (BCP) (3), a strain of the order of 1% such as found around dislocations at interfaces and grain boundaries (4) is sufficient to completely modify the pairing potential. Consequences of this model will be discussed within the general context of high temperature - short coherence length superconductors.
1) G. Deutscher, "New Superconductors: from granular to High Tc", World Scientific 2006, page 151.
2) A. Llordes et al., Nature Materials 11, 329 (2012).
3) G. Deutscher and P. G. de Gennes, C. R. Physique 8, 397 (2007).
4) G. Deutscher, Appl. Phys. Lett. 96

The optimization of high temperature superconductors (HTS) demands a detailed knowledge about the effects of materials&’ manipulations on the subnanometer scale, because the subtle interaction of a variety of nanoscale defect structures that pin the magnetic flux lattice will dictate the performance of these materials. We have recently demonstrated that the impressive properties of solution deposited-YBa2Cu3O7-x (YBCO) nanocomposites arise from the strains associated with the 3D network of YBa2Cu4O8 (Y124) intergrowths emerging from the spontaneously segregated oxide nanoparticles, and proposed a novel pinning mechanism coupling this lattice strain with superconducting pairing [1]. For that reason, determining the atomic structure of defects as well as understanding how they behave and interact is critical to unravel and control their effect on the physical properties of HTS. In this talk I will present a detailed investigation and characterization of the particular defects found within the material responsible for the strong enhancement of the superconducting properties by means of aberration-corrected scanning transmission electron microscopy (STEM). In addition, STEM in combination with electron energy loss spectroscopy (EELS) and density functional theory (DFT) calculations allowed the unveiling of a complex point defect which has not been previously identified, and determination of how it affects the crystal host structure on a single unit-cell level. STEM-EELS images reveal O-decorated Cu vacancies and local lattice distortions, while DFT calculations confirm their structure and chemistry and establish under which conditions they can be stabilized.
*Research sponsored by MICINN (Consolider NANOSELECT, CSD2007-00041, MAT 2011-28874-C02-01), Generalitat de Catalunya (Pla de Recerca 2009-SGR-770 and XaRMAE) and EU (EUROTAPES, FP7/2007-2013); RyC (JG). Research at ORNL U.S. supported by the U.S. DOE-BES, Materials Sciences and Engineering Division and through a user project supported by ORNL&’s Center for Nanophase Materials Sciences (sponsored by DOE-BES). Research at UCM supported by the ERC Starting Investigator Award and Fundacioacute;n Caja Madrid.
[1] A. Llordes et al., Nature Materials 11, 329-336 (2012)

Arrays of nanodefects embedded in superconducting films allow modifying vortex lattice dynamics, (a state of the art could be found in Ref 1). In this work, we have studied how these nanodefects can modify the superconducting vortices in addition to the vortex dynamics. We show that using magnetic and non-magnetic nanodefects the superconducting currents around the vortex core can be fine-tuned. The hybrid samples are Nb films on top of array of Py or Cu nanodots. The combination of micromagnetic simulations and magnetotransport measurements permits studying the pinning mechanisms for vortices trapped in the nanodefects and interstitial vortices placed out of the nanodefects. Interestingly, the behavior of vortex lattices is completely different when the vortex lattice moves on magnetic nanodefects or on non-magnetic nanodefects. An enhancement of the critical current is observed for trapped vortices on magnetic nanodots in comparison with trapped vortices on non-magnetic nanodots. On the converse the critical current behavior is the opposite when interstitial vortices are present, i. e. samples with arrays of non-magnetic nanodots show an enhancement of the critical current in comparison with critical current in samples with arrays of magnetic nanodots. These conclusions are supported by analyzing (I, V) curves and the vortex glass-vortex liquid second order transition in films with magnetic and non-magnetic pinning centers.
1.M. Vélez, J.I. Martín, J. E. Villegas, A. Hoffmann, E. M. González, J. L. Vicent and I. K. Schuller, J. Magn. Magn. Mat. 320, 2547 (2008)

The irradiation effects of ions, neutrons and electrons, on critical current properties of superconductors have been extensively studied thus far to achieve ultimately high J_c and to elucidate pinning mechanism and characteristics of the vortex system, which are closely related to the electromagnetic anisotropy and distribution of condensation energy. In the present study, effects of high-energy electron irradiation on the critical current properties of RE123 melt-solidified bulks, Y123 thin films, (Ba,K)Fe₂As₂ single crystals and (Ca,Pr)Fe₂As₂ crystals have been investigated. Electron irradiation was performed under an accelerating voltage of 35 MeV with an estimated fluence up to 9.1 x 10¹⁷ e/cm². Various kind of defects, such as columnar defects with random directions and point defects, are introduced by the high-energy electron irradiation. J_cs of the samples were evaluated by the width of magnetization hysteresis loops measured under H // c using the extended Bean model.
For RE123 melt-solidified bulks, electrons were irradiated for three different samples, Y123, (Dy,Ho)123 and (Dy,Er)123. Improvement of J_c by irradiation was confirmed for all samples, while T_c decreased by ~0.5 K. Among these samples, Y123 bulk exhibited highest J_c (~0.7 MA/cm² at 40 K in 2 T) and largest increase in J_c by irradiation suggesting higher condensation energy. However, the large improvement of Jc was limited under low fields below several tesla and their irreversibility fields, Hirr, was not increased. On the other hand, electron irradiation did not improve J_c of Y123 thin films with a thickness of ~0.3 micron synthesized by the fluorine-free MOD method. Ba_1-xK_xFe₂As₂ single crystals with x ~0.40, ~0.50 and ~0.69 showed largely improved J_c and enhanced Hirr after electron irradiation. Difference in J_c before and after electron irradiation strongly depends on x and it was largest for x ~0.40. In addition, those samples did not show any change in susceptibility curves including superconducting transition up to a fluence of 9.1 x 10¹⁷ e/cm². These mean that effects of electron irradiation are more prominent for (Ba,K)Fe₂As₂ than for Y123. (Ca,Pr)Fe₂As₂ crystals showed almost closed magnetization hysteresis loops suggesting J_c ~0 before irradiation and electron irradiation did not improve it at all. This result implies absence of bulk superconductivity in this system.

The discovery of superconductivity in MgB2 in 2001 sparked a great excitement and enthusiasm in superconductivity research community. The relatively high Tc, weak links free grain boundaries, low anisotropy and large coherence length are the major advantages of MgB2. The Tc of MgB2 is as high as 40 K and many groups reported critical current density, Jc values as high as ~107 A/cm2 in the absence of applied magnetic field. However, one of the concerns of using bulk MgB2 for technological applications is its deteriorating superconducting properties at high-fields due to weak pinning. Fortunately, due to its simple structure and ease of synthesis, it is quite feasible to introduce defects (internal/external to the lattice) of the order of its coherence length, which helps in improving its pinning behavior, consequently improving the Jc(H) behavior of the sample. Also, it is advantageous to understand the pinning mechanism in this material which may promote this material to a better alternative to dissipation-less current transport. In this direction, several C-containing dopants such as nano-diamond, CNTs, SiC, graphene, etc., have been found to be very effective in improving the superconducting properties of MgB2. In a recent study, it has been demonstrated that a small addition of reduced graphene improves the superconducting properties of MgB2 by improving grain connectivity. Also, doping of rare-earth (RE) elements in MgB2 are found to be effective in improving flux pinning by introducing REBx impurity phases which act as pinning centres. In this study, we have studied the combined effect of RE-doping as well as graphene oxide (GO) doping on the superconducting properties of MgB2. For this purpose, we have chosen La2O3 and Ho2O3 for doping, both the elements having different magnetic moments. The samples have been prepared using the solid-state reaction route with the composition MgB2, MgB2 + 1wt% RE and MgB2 + 1 wt% RE + 3wt% GO, (RE = La2O3 and Ho2O3) by sintering at 850°C for 3 h in reducing atmosphere of Ar/H2(90/10). The X-ray diffraction results show a hexagonal (P6/mmm) phase along with MgO and REB4 and REB6 impurities phases in small amounts in all samples. The XRD results also show no significant change in the lattice parameters of the doped samples with respect to pure sample. The pure MgB2 sample show Tc (ρ=0) ~ 38.60 K and no substantial change in Tc has been seen in the doped samples as compared to pure sample. Jc(H) has been calculated from the magnetic hysteresis loops measured at 10 K and 20 K. We observe that the RE-doped samples show a significant improvement in Jc as compared to the pure sample in the entire field range (0 -7 T). The Jc(H) has further improved in case of RE and GO co-doped samples. We have observed improvement in Jc(10K, 5T) as high as ~ 20 times in case of Ho2O3 and GO co-doped samples. The possible flux mechanisms playing role in all the samples will be described and discussed in this paper.

It have been reported that BaHfO₃ (BHO) forms nanorod in REBa₂Cu₃O_y (RE = rare earth; REBCO) films deposited by a vapor phase epitaxy such as pulsed laser deposition (PLD) and chemical vapor deposition (CVD), and the BHO nanorods have strong flux pinning force for magnetic fields applied parallel to the c-axis of the REBCO films. We have also reported that the critical current densities (J_c) in magnetic fields were significantly improved in the BHO-doped SmBa₂Cu₃O_y (SmBCO) films deposited on LaAlO₃ (LAO) single crystalline substrates by PLD method.
Empirically, we recognize that the J_cs in self-field (J_c^selfs) of pure SmBCO films on LAO substrates vary from 1.5 to 4.0 MA/cm² at 77 K, even if they are deposited on the same condition. On the other hand, the J_c^selfs of BHO-doped SmBCO films on the LAO substrates with the low BHO contents are constantly over 4.0 MA/cm² at 77 K. As a reason for the reproducibility of J_c^selfs on the BHO-doped SmBCO films, we anticipated that the BHO introduction might improve the transport characteristics at the grain boundaries.
In this study, we aimed to investigate the influence of BHO introduction for the transport characteristics at the grain boundaries in the SmBCO films. We fabricated the BHO-doped SmBCO film on the 5 degree-tilted bicrystal LSAT substrate by PLD method and compared the superconducting properties with the pure SmBCO film.
From the phi;-scan of XRD measurement, it was observed that the SmBCO matrixes grew with the 5 degree-tilted angle at the artificial grain boundaries both in the pure SmBCO film and BHO-doped SmBCO film. The critical temperatures (T_c) of the pure SmBCO and BHO-doped SmBCO films on the single crystalline substrates were 92.5 and 90.7 K, respectively. The T_cs of the films on the bicrystal substrates were 92.7 and 91.1 K, respectively. The T_cs on each substrate were comparable and the influence of the artificial grain boundary was not observed in the T_cs. A clear difference between the pure SmBCO and BHO-doped SmBCO films was observed in J_c^self. The J_c^selfs at 77 K of pure SmBCO and BHO-doped SmBCO films on the single crystalline substrates were 2.63 and 1.62 MA/cm², respectively. And these of the films on the bicrystal substrates were 0.96 and 1.03 MA/cm². Although the J_c^selfs of the films on bicrystal substrates were almost the same, the decrease rates of J_c^self from the films on the single crystalline substrates were significantly different. Those of the pure SmBCO and BHO-doped SmBCO films were 37% and 64%, respectively. This difference should be caused by the BHO introduction. We will discuss about the more details such as I-V characteristics and microstructures at the presentation.

Combination of superconducting (SC) and ferromagnetic (FM) sheets into layered SC/FM composites allows to obtain metamaterial with unusual magnetic properties: the effective magnetic permeability along the sheets could be much higher than in the perpendicular direction. One can design the magnetic cloak consisting of a shell from SC/FM composite protecting its inner space against the penetration of a static magnetic field. Difference between such a device and a usual SC or FM shield will be in its un-detectability by magnetic sensors checking the field distribution outside the cloak. Then one can consider to use such element for protecting sensitive electronic circuitry in an electric machine where the magnetic field is necessary to execute the operation.
We have prepared a series of SC/FM cloaks using commercial coated conductors as the superconducting elements and various kinds of ferromagnetic sheets as the ferromagnetic elements. Finite element calculation taking into account the non-linear properties of both kinds of materials as well as the magnetic hysteresis of superconductor was utilised to optimize the architecture. Experimental testing of the cloaking ability in static magnetic field was performed by scanning the field distribution in vicinity of the cloak. Detectability in low frequency (~100 Hz) AC magnetic fields was tested in an inductance bridge allowing to see both screening and dissipation signals. Recording of magnetization loops allowed to analyse in detail the dynamics of field interaction with the cloak. Reduction of the detector signal due to the cloak was confirmed, however it was still not complete. Tests of various arrangements of superconducting and ferromagnetic materials allowed to identify the main problems hindering to achieve a perfect cloaking.

Superconducting RF (SRF) cavities are normally fabricated from bulk niobium (Nb), a material which is thoroughly studied and approaching its limits. Magnesium diboride has a higher T_c of 39 K, a lower residual resistivity of ~ 0.1 µ#8486;cm (at 42 K), and a higher thermodynamic critical field H_c value comparing with Nb. These properties imply that a MgB₂-coated SRF cavity would work at higher temperature with lower energy dissipation. However, the lower critical field H_c1 of MgB₂ is low and vortex dissipation above H_c1 can lead to degradation of the quality factor and a low RF breakdown field. In this work, we report an enhancement of H_c1 in both c-axis oriented epitaxial and polycrystalline MgB₂ thin films. The value of H_c1(5 K) was increased from 60 mT in a 300 nm-thick MgB₂ film to ~180 mT when the MgB₂ layer thickness was 100 nm in both cases. The microwave properties of the MgB₂ films were characterized as well by using the dielectric resonator technique

A concept of an electric aircraft based on ultra-light superconductive power propulsion is considered. A model that describes power consumption as well as the aircraft dynamics for both conventional and electric aircrafts is developed and characterized. The model demonstrates clear advantages of using a superconducting electric power system over conventional power systems based on combustion, and over conventional electric systems that use copper wiring and components that operate at ambient temperatures. As is shown, a superconductive drive-train is more than ten times lighter and provides up to 70% longer flight range than its copper-wire analogue, and drastically reduces fuel cost compared to combustion-based propulsion. Performance and technological readiness levels (TRL&’s) of superconductor and cryogenic power components including motors/generators, power transmission cables, and power controllers and inverters, will be described and summarized. The performance of sources of electrical energy will also be reviewed, including Li-batteries, fuel cells, flywheels, and superconducting magnetic energy storage (SMES) devices.

Electric networks will experience deep changes due to the emergence of dispersed generation. Variability in power output is a characteristic of wind energy and increased penetration of wind power will present significant operational challenges in ensuring grid security and power quality. This paper addresses the integration of large wind farms into the grid through the beneficial role of superconducting magnetic energy storage (SMES) systems. Although originally conceived as a load-leveling device for nuclear power plants, today&’s utility-industrial realities emphasize other applications of SMES in the development of wind energy. In the industrial section, concerns about power quality and stability have driven the development of a market for micro-SMES devices for power quality applications. The paper reviews the recent history of SMES, performs analysis in terms of the quantity of superconductor required and cost associated with both toroid and solenoid shaped coil using Bi-2223, YBCO and MgB2. Energy storage is optimized by properly designing the bandwidth of SMES. The ultimate aim of this paper is to influence the optimal design and configuration of SMES for land and offshore wind power generation and to propose a roadmap for the resolution of technical barriers related to the integration of wind energy to the electric grid.

MgB2 specimens, with diameters ~25mm and 51 mm and thickness 3-5mm have been obtained by using Hot pressing method. Mixture of Mg and B powders or tablets, made from this mixture, that is preliminarily pressed in the die under 2-4-ton/cm2 pressure, was put in the die. Graphite die with mixture of Mg-B powders or their tablets was put in hot pressing device. In purpose of easily taking out consolidated MgB2 from the die, SIGRAFLEX plate has been put on in graphite die (thickness 0,5mm), that is coated by BN layer. BN layer is formed by using Boron Nitride Aerozol Lubricoat (Manufactured for: ZYP coatings Inc.). The powders are pressed by using hydraulic system, temperature increases gradually up to 1125K and are kept at this temperature for ~ 5divide;7 min. After taking off pressure and they are cooled in vacuum until 425-475K. Cooling of molds is conducted in Ar atmosphere. Grinding and polishing obtained tablets have been conducted by using dry abrasive papers (SiC) in inert atmosphere, after this BN and SIGRAFLEX remains have moved away surfaces. Obtained MgB2 with diameter 27 mm as pellet are without any cracks, but the cracks were observed in the pellets with diameter 51 mm which are preliminary consolidated. If Mg+B mixture will be pressed in graphite die, than cracks will not be observed. The microstructure and intercristalline cracks of the obtained MgB2 samples have been studied by using Electron Microscopy. Microstructure observation exhibits a homogeneous distribution of small pores . The average grain size has been revealed with size less than 5 mu;. The XRD patterns of the samples has shown refection of magnesium diboride and traces of magnesium oxide. Analysis of samples using AES reveals that the samples content maximum which is characteristic for the carbon impurities.

Superconductivity in novel 112-type (Ca_1-xLa_x)FeAs₂ will be presented [1]. The compound crystallizes in a monoclinic structure (space group P2₁), in which FeAs layers alternate with CaAs spacer layers such that monovalent arsenic forms zigzag chains. Superconductivity at 34 K was observed for the x = 0.1 sample, while trace superconductivity was observed at 45 K for the x = 0.21 sample, demonstrating the potential of the 112-phase for higher transition temperature.
Another way to realize higher T_c is co-doping of lanthanum and phosphorus in 122-type CaFe₂As₂, which results in superconductivity at 45 K [2]. This transition temperature is higher than the 38 K in Ba_1-xK_xFe₂As₂, which is the maximum found thus far among the 122 phases. Superconductivity with a substantial shielding volume fraction was observed at 0.12 le; x le; 0.18 and y = 0.06 in Ca_1-xLa_xFe₂(As_1-yP_y)₂. In this doping range, the system exhibits crossover of the lattice collapse transition, which is characterized by the formation of As₂ dimers between the adjacent FeAs layers.
[1] N. Katayama, K. Kudo, S. Onari, T. Mizukami, K. Sugawara, Y. Sugiyama, Y. Kitahama, K. Iba, K. Fujimura, N. Nishimoto, M. Nohara, and H. Sawa, J. Phys. Soc. Jpn. in press.
[2] K. Kudo, K. Iba, M. Takasuga, Y. Kitahama, J. Matsumura, M. Danura, Y. Nogami, and M. Nohara, Sci. Rep. 3 (2013) 1473.

We report the effect of quasi-hydrostatic and hydrostatic pressure on single crystals of the aliovalent La-doped Ca1-xLaxFe2As2 by measuring transport and neutron scattering properties. In contrast to transition metal-doped 122 Fe-based superconductors where superconductivity and antiferromagnetism coexist, these two phases appear to be mutually exclusive in the rare-earth doped series in a manner similar to the 1111-type Fe-based superconductors, suggesting that this high-Tc phase is intrinsic. The unusual dichotomy between lower-Tc systems that happily coexist with AFM and the tendency for the highest-Tc systems to show phase separation provides an important clue to the pairing mechanism in iron-based superconductors.

In the frame of our studies new family of high T_c pnictides superconductors has been discovered. Single crystals and polycrystalline samples of new superconductors Ln₄Fe₂As₂Te_1-xO₄ (Ln=Pr, Sm, Gd) have been grown under high pressure of 3 GPa at 1400-1500°C. In as grown undoped crystals T_c = 25 K has been determined by magnetic and resistivity measurements. As result of fluorine doping T_cof crystals or polycrystalline samples increased up to 45 K for Gd₄Fe₂As₂Te_1-xO_4-yF_y and 40 K for Sm₄Fe₂As₂Te_1-xO₄ . The structure consists of the alternating Fe₂As₂ and Pr₂O₂ layers in the c direction separated by tellurium atoms. It has close resemblance to the LnFeAsO structure. Lattice constants a and b are slightly larger (by = 0.04 Å) while c is considerably larger (by = 21.3 Å). For Ln=Gd pnictogen height and As-Fe-As angle which are highly correlating with T_c, are 1.366(3) Å and 110.47(7)°, respectively. Te site reveals about 10 % of vacancies, which can be source of doping in as grown crystals without intentional doping. The anisotropy γ = 31 of this compound is higher than γ = 7 of Ln1111.
S. Katrych et. al., Physical Review B 87, 180508 ® 2013.

The diversity of coordination chemistry of 5d transition-element platinum allows us to synthesize various superconducting arsenides that include SrPt₂As₂ with PtAs₄ tetrahedra [1], SrPtAs with PtAs₃ triangles [2], and Ca₁₀(Pt₄As₈)(Fe₂As₂)₅ with PtAs₄ planar squares [3]. In contrast, 5d transition-element iridium shows limited coordination geometries; only octahedral and tetrahedral coordination are known in arsenides. In this paper, we report the unprecedented square-planar coordination of iridium in the novel iron iridium arsenide Ca₁₀(Ir₄As₈)(Fe₂As₂)₅ [4]. This compound exhibits superconductivity at 16 K. X-ray photoemission spectroscopy and first-principles band calculation suggest Ir(II) oxidation state, which yields electrically conductive Ir₄As₈ layers. Such metallic spacer layers are thought to enhance the interlayer coupling of Fe₂As₂, in which superconductivity emerges, thus offering a way to control the superconducting transition temperature.
[1] K. Kudo, Y. Nishikubo, and M. Nohara, J. Phys. Soc. Jpn. 79, 123710 (2010).
[2] Y. Nishikubo, K. Kudo, and M. Nohara, J. Phys. Soc. Jpn. 80, 055002 (2011).
[3] S. Kakiya, K. Kudo, Y. Nishikubo, K. Oku, E. Nishibori, H. Sawa, T. Yamamoto, T. Nozaka, and M. Nohara, J. Phys. Soc. Jpn. 80, 093704 (2011).
[4] K. Kudo, D. Mitsuoka, M. Takasuga, Y. Sugiyama, K. Sugawara, N. Katayama, H. Sawa, H. S. Kubo, K. Takamori, M. Ichioka, T. Fujii, T. Mizokawa, and M. Nohara, Sci. Rep. 3, 3101 (2013).

The motivation of this study is to investigate the properties of a unique interfacial superconducting phase in electron-doped Tr_xCa_1-xFe₂As₂ (x asymp; 0.12, Tr = La/Ce/Pr/Nd) pnictides [1], by the Low Field Microwave Absorption (LFMA) technique. Samples are exposed to microwave radiation with frequency nu;_mw ~ 10GHz (and with field intensity on the order of 0.1G), and also to a low strength (order of 1G) magnetic field which modulates at nu;_m = 100kHz. Additionally, a DC field (centered at 0 G) is applied which varies by ±50G. The microwave field causes vortices/fluxons, induced by the modulation and DC fields, to oscillate about their pinning centers, causing a unique microwave absorption spectrum [2,3]. Experiments show a low temperature (T < T_c1 = 22K) LFMA with conventional broadly hysteretic spectrum of a bulk superconductor, which exhibits a transition into a narrow, non-hysteretic LFMA until a higher T_c2, indicative of another superconducting phase with weak linked Josephson Junction superconductivity. The phases may overlap, depending on type of dopant.
Additionally, due to their single crystalline nature, the pnictides can be oriented relative to the microwave polarization and magnetic field direction. The resultant anisotropy in microwave absorption (with extremely small effective critical field H_c1 < 1G for H #8741; c axis) reveals a two dimensional (planar) interfacial superconducting phase above T_c1. Possible microscopic interpretations of the results will be discussed, including filament-like micro-interfaces between highly doped regions and poorly doped regions [1], and infrequent high-T_c atomic layers, with superconductivity mediated by superparamagnetism in adjacent arsenic deficient layers [4].
1. Lv, B. et al. Proceedings of the National Academy of Sciences. 108, 15705-15709 (2011).
2. Shaposhnikova, T. et al. Physica C. 451, 90-97 (2007).
3. Nebendahl, B., et al. Physica C. 209, 362-368 (1993).
4. Lv, B., et al. arXiv:1308.3129 [cond-mat.supr-con] (2013).
This work is supported by the US Air Force Office of Scientific Research, contract FA9550-09-1-0384 "Strengthening superconductivity in nanostructures."

We report on compositional tuning to create excellent field-performance of J_c in ‘self-pinned&’, GdBa₂Cu₃O_7-y (GdBCO) coated conductors (CCs) grown by ultrafast reactive co-evaporation. Angular J_c measurements, micro-Raman scattering spectroscopy, and low temperature laser scanning microscopy (LTLSM) analyses on short samples cut from long km lengths showed the compositional tuning to give J_c's of over 1MA/cm^-2 at 1T, 77K, coinciding with the presence of Gd₂O₃ nanoparticles, self assembled along ‘c&’ and optimum self-field J_c&’s over 3.2 MA/cm² which coincided with the presence of additional liquid presence in the growth process. This liquid is not predicted from equilibrium phase diagram studies, but is believed to arise from quasi-equilibrium growth under the low pO₂ used. Low temperature laser scanning microscopy showed the conductors to be very uniform. With precise compositional control, the liquid assisted reactive co-evaporation process is a very high performance method holding the greatest promise for low cost, high performance coated conductor fabrication.

The development of long-length, high current density Bi2Sr2CaCu2Ox (Bi2212) and MgB2 wires and (RE)Ba2Cu3O7-x (REBCO) coated conductors has now advanced such that superconducting magnets for energy applications and high field applications are progressing rapidly. High current density in long-length conductors is only a necessary-but-not-sufficient condition for magnet success, however, and a number of other issues must be addressed self-consistently for successful magnets and applications to emerge. High current density magnets also tend to have large stored energy and large Lorentz forces, so understanding and preventing conductor degradation and failure, whether triggered by quenching or mechanical challenges, is essential for long-term acceptance of superconducting technologies. In this talk, degradation and failure issues in Bi2212, REBCO and MgB2 conductors are explored. Quenching and mechanical testing are used to identify experimentally the conductor limits under mechanical and thermal loadings. A peridynamic modeling and a COMSOL model are developed and applied to provide computational insight into the intricate micromechanical behaviors. The interplay between experiment and computation are also discussed.

For power applications high current carrying coated conductors are needed. However, their high aspect ratio causes too high magnetization losses. One way to reduce the AC loss is to striate the tapes into filaments. For the structuring of (RE)BCO coated conductors we used a Pico-second laser burning grooves of 18 µm to 21 µm in width (Ag-cap) and 0.5 µm to more than 100 µm in depth. Patterns with up to 120 parallel filaments in 12 mm wide conductors were made. The critical current and the total AC-magnetization loss were measured as a function of the filament count. With increasing number of filaments Ic decreases. This reduction is caused by two contributions, the removed HTS material and current inhomogeneities within the superconductor. The hysteresis loss reduction of 120 filament Ag-cap tapes is nearly two orders of magnitude, as expected. Some remaining filament coupling was investigated.

Superconductors may be grouped into two major classes. The first is conventional metallic, whose pairing mechanism is explained by the BCS theory and electron-phonon coupling. The pairing mechanism of the second class, driven by electron correlations, is still to be worked out. These superconductors have electronic properties that are highly tunable, either by doping or pressure, from a non-superconducting ground state to a superconducting one, thus defining a superconducting ‘dome&’ in the phase diagram. More than 40 families of such superconductors, including high-temperature cuprate and iron-based, heavy fermion, organic, and transition-metal di-chalcogenide superconductors exhibit this ubiquitous phase diagram. All of these materials show intriguing correlated electron states above the dome, and researches agree that the understanding of this electron matter holds the key to the pairing mechanism, and ultimately predictive design of new superconductors, which hold great promise of revolutionary applications, including energy, information technology, and medicine. I will share our guidelines for predictive design, and will also show how our point contact spectroscopy measurements aid in identifying promising candidates.

It has been recently demonstrated that the iron chalcogenide superconductors have a superior high-field performance over low temperature superconductors at 4.2 K.[Nature Communications (2013); Rep. Prog. Phys. (2011)] Although their superconducting transition temperatures (Tc) are typically lower than those of iron pnictides, iron chalcogenides exhibit low critical current anisotropies with very high upper critical field slopes near Tc. In this presentation, I will discuss recent progress in the superconducting films and coated conductors of iron chalcogenides. With a CeO2 buffer, critical current densities (Jc) over 7 MA/cm2 were observed in FeSe0.5Te0.5 films grown on single-crystalline and coated conductor substrates. Furthermore, these films have significantly higher Tc (>20K) as compared to bulk samples (bulk Tc ~15 K) for the entire doping regime of FeSe1-xTex. Structural analysis revealed that these films generally have significantly smaller c-axis and a-axis lattice constant than the bulk value, suggesting that the crystal structure changes have a dominating impact on the superconducting transition in iron-based superconductors. High Jc, low magnetic field anisotropies and relatively strong grain coupling make ironchalcogenide coated conductors particularly attractive for high-field applications at liquid helium temperatures.

Vortex matter in iron-arsenide superconductors exhibits a rich phenomenology that is still largely unexplored, with many shared characteristics with high Tc cuprates. One of them is that vortices are affected by strong thermal fluctuations due to the small coherence length and relatively high critical temperature and anisotropy. This in turn has important consequences in determining how vortices are trapped by different pinning potentials. Angular dependent critical current measurements are extremely useful to determine the nature of the effective pinning centers composed of correlated, planar and randomly distributed point-like pinning centers that comprise the pinning landscape.
Recent studies of the critical current BaFe2As2 thin films with different dopants have shown diversity of pinning landscapes; from clear evidence of strong c-axis correlated pinning with very similar angular dependence to that found in YBa2Cu3O7 (YBCO) with self-assembled columns, as well as samples that show the same phenomenology than YBCO films with nanoparticle despite the very different microstructural origin and temperature range. Comparisons performed at different temperature and field regimes allowed us to draw important conclusions with respect of the strength and efficiency of columnar and nanoparticle vortex pinning and dynamics in cuprates and iron-pnictides.
Another common feature among iron and cuprate high temperature superconductors is the layered structure; consisting of intercalated conducting and insulating planes. This intrinsic layering gives rise to the electronic mass anisotropy as well as a periodic planar pinning potential. Depending on the insulating layer size the anisotropy of the compound can vary from close to 1 up to hundreds for Bismuth- or Mercury-based superconductors. The effect on vortices, also known as intrinsic-pinning, of these periodic planar potentials should not depend on the specifics of different materials but rather be universal. In this talk I will show measurements of the different angular regimes of the vortex dynamics that confirm the generality of the intrinsic pinning found not only in YBCO films but also in iron based superconductors.
Acknowledgment: Work supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering.

In order to determine the potential of the recently discovered iron-based superconductors we investigated the high field properties of several iron-based superconductors, namely Co-doped BaFe₂As₂ (Co-Ba122), P-doped BaFe₂As₂ (P-Ba122) and SmFeAs(O,F) (Sm1111). Despite having the lowest T_c of these three materials, Co-Ba122 thin film results are extremely interesting from the vortex pinning point of view, since it is able to accept an impressively high density of pinning centers well beyond the limit reachable in YBCO. In order to increase the in-field J_c performance and to decrease the effective J_c anisotropy, we studied both single and multilayer films on different substrates with different types and densities of defects. We discovered that Co-Ba122 can accept up to 15-20%vol. of pins, making it effective over a wide temperature and field range without compromising the matrix properties. In the single and multilayer films deposited on SrTiO₃-templated (La,Sr)(Al,Ta)O₃ both self-assembled c-axis aligned nanorods and flat/round-shaped precipitates can be introduced obtaining a large J_c exceeding 10⁵A/cm² 20T and 4.2K with a weak angular dependence and a pinning force density F_p(4.2K) of 45-50 GN/m³ at 15-20T (close to H_Irr/2). In the case of Co-Ba122 films deposited on CaF₂, where an enhanced T_c is induced by strain, a further improvement of the pinning properties has been found beyond that expected from the simple T_c increase. In fact the pinning centers are more than 40% more effective that in films grown on SrTiO₃-templated (La,Sr)(Al,Ta)O₃ with F_p(4.2K) reaching 74 GN/m³ at 22.5 T.
Because of their higher T_c, P-Ba122 and Sm1111 in principle possess even greater potential for applications. We studied clean thin films in high field in order to determine their baseline properties. P-Ba122 achieves a pinning force maximum of 33 and 124 GN/m³ at 10-15 T in the two principal field orientations. In the case of Sm1111 F_p(4.2K) reached 60 GN/m³ at 27.5 T for H//c whereas the F_p maximum occurs above 45T and reaches more than 300 GN/m³ for H//ab. Remarkably J_c in Sm1111 exceeds 0.1 MA/cm² at 45T, even in the weaker orientation for H//c. These baseline properties are very promising for applications and suggest the value of investigating the introduction of pinning centers for raising J_c(H) even more and for anisotropy reduction, especially if any of these properties could be translated into tape or round wire forms.

The relatively large critical temperature Tc around 25 K, high upper critical fields Hc2 up to 50 T for H||c, the low Ginzburg number Gi ~ 10-5, and the low Hc2 anisotropy at 4 K, γH ~ 1.3, render BaFe2As2 (Ba-122) a very interesting candidate for low-temperature high-field applications, such as in high magnetic field coils.
Here, we investigate a Co-doped Ba-122 thin film with a considerable density of ab-planar defects (stacking faults or extra Fe layers) in static magnetic fields up to 35 T. The film was grown by pulsed laser deposition on an Fe-buffered MgO substrate. Whereas the pinning force scales for H||c in the whole temperature range, there is a clear change observed at around 10 K for H||ab. This behaviour is related to a change from plastic pinning at high T to elastic pinning at low T, most likely related to the observed ab-planar defects. The irreversibility field Hirr in the plastic regions is in close vicinity to a vortex glass-liquid transition, investigated by derivatives of R(T) measurements as well as scaling of R(T) and R(H). The angular dependence of Jc shows clear anisotropy due to c-axis correlated defects (dislocation networks), ab-planar defects, and small as well as extended random defects.

Electron and hole doped BaFe₂As₂ (Ba-122) compounds show high upper critical fields with low anisotropies at low temperatures. These properties are favorable for high-field applications and, indeed, high performance K-doped Ba-122 wires have been fabricated by a powder in tube technique [1]. Unlike the powder-in-tube process, in-situ K-doped Ba-122 thin films are hard to prepare due to the high vapor pressure of K. On the other hand, isovalently P-doped Ba-122 thin films with a superconducting transition temperature (T _c) of around 30 K, which is the second highest T_c among the Ba-122 families, have been fabricated readily by both pulsed laser deposition and molecular beam epitaxy (MBE) [2-4]. Additionally, the recent bicrystal experiments showed a high critical current density (J_c) of around 1 MA/cm² even at a large grain boundary angle of 24° and 4 K, far beyond the superconducting properties of Co-doped Ba-122 [3]. Hence, P-doped Ba-122 may be a good candidate for Ba-122 coated conductor application. However, only a few reports on transport properties for P-doped Ba-122 have been published to date. Here we present high-fields (dc up to 35 T) transport properties of P-doped Ba-122 epitaxial thin films prepared by MBE. In-field resistivity measurements showed a clear shift of T_c for both principal crystallographic directions but with a small transition width, similarly to the other Ba-122 systems. A high J_c of over 10⁴ A/cm² is recorded even for H || c at 35 T and 4.2 K. The mass anisotropy γ evaluated from the Blatter scaling is close to 1.2 at 4.2 K. These properties are clearly advantageous to high-field applications.
The research leading to these results has received funding from European Union&’s Seventh Framework Programme (FP7/2007-2013) under grant agreement number 283141 (IRON-SEA). A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement No. DMR-1157490, the State of Florida, and the U.S. Department of Energy. This research has also been supported by Strategic International Collaborative Research Program (SICORP), Japan Science and Technology Agency.
References
1. J. Weiss et al, Nat. Mat. 11, 682 (2012).
2. S. Adachi et al, Supercond. Sci. Technol. 25, 105015 (2012).
3. A. Sakagami et al, Physica C 494, 181 (2013).
4. M. Miura et al, Nat. Commun. 4, 2499 (2013).

The uniaxial strain and hydrostatic pressure effects on YBa2Cu4O8 were investigated through magnetic measurements under strains of up to 25 GPa. The out-of-plane (along the c-axis), in-plane (along the c-plane), and hydrostatic contractions all bring about an increase in the superconducting transition temperature (Tc). For P < 2 GPs, the in-plane contraction caused the largest increase in Tc, and the out-of-plane contraction exhibited the smallest one. This behavior was understood by considering the changes in the symmetry of CuO5 pyramid and carrier of the CuO2 plane. Among the above three types, the optimal Tc in the out-of-plane contraction was realized at the smallest strain value, and for the in-plane contraction, the highest Tc of above 100 K was observed. In the case of the out-of-plane and in-plane contractions, the Meissner signal disappears at a stress corresponding to about 10 GPa. However, under the hydrostatic contraction, the maximum value of Tc was about 100 K at 9-12 GPa, and the Meissner effect was observed at pressures of at least 16 GPa. Thus, we conclude that the disappearance of the Meissner signal is controlled by the selective application of strain.

The tetragonal-to-orthorhombic structural phase transition at T$_S$, coincident or preceding the onset of an antiferromagnetic ground state at T$_N$, in the underdoped regime of quite all families of iron-pnictide and chalcogenide superconductors breaks the four-fold rotational symmetry of the tetragonal phase, implying the onset of a nematic phase. The relevance of nematicity, either electronic in nature or spin-induced, in shaping their phase diagram is certainly one of the most debated issue nowadays. We report on an optical reflectivity study of Ba(Fe$_{1minus;x}$Co$_x$)$_2$As$_2$ with x = 0 and 2.5%, detwinned by uniaxial and in-situ tunable pressure acting as an external symmetry-breaking field. We discover a remarkable optical anisotropy as a function of the applied pressure, very much reminiscent of a hysteretic-like behavior. Its temperature dependence supports the analogy between pressure and external magnetic field with respect to the electronic anisotropy in iron-pnictides and magnetization in ferromagnets, respectively. We estimate the nematic susceptibility, which is Curie-like at temperatures close to and above T$_S$ and which may hint to a ferro-orbital ordering as driving mechanism for both structural and magnetic transitions.

Transparent conductive indium tin oxide (ITO) thin films, electrochemically intercalated with alkali (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺), alkali earth (Mg⁺², Ca⁺²), or complex NH₄⁺ ions show tunable superconducting transition with dome shape behavior of T_c versus electron density around the maximum at 5 K. The zero resistance transition in superconducting state is accompanied with paramagnetic Meissner response to the applied external magnetic field, i.e. the increase of magnetization in field cooling regime. We provide extensive evidences of flax trapping using dc SQUID and mutual inductance measurements. This flux trapped state is metastable and can be tuned by external fields or boundary conditions.

FeSe has the simplest crystal structure among iron-based superconductors. On the other hand, FeTe which has an analogous structure exhibits antiferromagnetic ordering associated with a lattice distortion at 70 K. Partial S or Se substitution suppresses the low temperature magnetic/structural phase transition, and thereby induces superconductivity. At low substitution rates, only filamentary superconductivity is observed although the antiferromagnetic ordering seems to be suppressed. This is because FeTe possesses excess Fe (~9 %) at the interlayer site which is inhomogeneously distributed. As previous reports indicate, excess Fe supplies a substantial amount of electrons to the iron layer, and thus superconductivity is suppressed. Therefore, to achieve bulk superconductivity in Fe-chalcogenide compounds, control of the excess Fe is required.
So far, we have reported that the bulk superconductivity in the Fe-chalcogenides, FeTe1-xSex and FeTe1-xSx, is achieved by oxygen annealing, sulfur annealing and immersion in alcoholic beverages [1-3]. It was proposed that intercalated oxygen compensates the electron carrier of the excess Fe. Furthermore, it is revealed that hot commercial alcoholic beverages have the ability to deintercalate the excess Fe in the sample, and hence bulk superconductivity is achieved [4-6]. In this presentation, we reports details of our research on how to improve superconductivity in Fe-chalcogenides.
[1] Y. Mizuguchi et al., Phys. Rev. B 81 (2010) 214510,
[2] Y. Mizuguchi et al., Europhys. Lett. 90 (2010) 57002.
[3] K. Deguchi et al., Supercond. Sci. Technol. 24 (2011) 055008.
[4] Y. Kawasaki et al., Solid State Commun. 152 (2012) 1135.
[5] K. Deguchi et al., Supercond. Sci. Technol. 25 (2012) 084025.
[6] K. Deguchi et al., Sci. Technol. Adv. Mater. 13 (2012) 054303.

Iron chalcogenide superconductors (11 system) have the simplest crystal structure among iron-based superconductors, as they are composed of only superconducting layers. It is well known that excess-Fe with a valence value of 2+ exists between the layers, and suppresses superconductivity via charge doping [1]. We have succeeded in deintercalating the excess-Fe by the use of alcoholic beverage annealing [2]. This is a result of the chelate effect, which originates in the organic acid which forms part of the alcoholic beverages. In regards to alcoholic beverage annealing, we expected to be able to deintercalate excess-Fe in a shorter time by using an electrochemical reaction with a solution of organic acid.
Polycrystalline samples of FeTe0.8S0.2 were synthesized by a solid-state reaction using powders of Fe, Te, and S. The samples were sealed in an evacuated quartz tube, and sintered at 600 °C for 10 h. Obtained samples were ground, pelletized, and sintered again in an evacuated quartz tube under the same conditions. For the electrochemical reaction, a solution of dissolved 6 g/L of citric acid with ultra pure water was prepared. To characterize the crystal structure of samples, X-ray diffraction (XRD) measurements were performed. The magnetic susceptibility for each sample was also measured by SQUID magnetometer.
The as-grown sample does not show superconductivity. The shielding volume fraction of the samples treated electrochemically for 1 hour increases with increasing applied voltage from 0 V to 1.5 V. Above 1.7 V, it suddenly decreases. Therefore, we have demonstrated that an electrochemical method using citric acid as solution is more effective to deintercalate a large amount of excess-Fe than the alcoholic beverage annealing. By using this method, it is possible to induce superconductivity in other layered compounds which have excess-Fe.

The nature of the superconductive behavior on Fe-superconductors and particularly the interplay between magnetic and superconductive behavior remains a topic of active scientific interest. Here we examine the local superconductive behavior in the Se-doped anti-ferromagnetic parent compound, FeTe, affected by local composition and structural variations on the nanometer level and below. In order to probe the interaction between magnetism and superconductive behavior, we create local magnetic centers by introducing an additional five percent of excess Fe. The compound was further modified by replacing 25% of Te with Se, below the optimal doping of x=0.45. We performed a series of differential conductance spectra over a 40x40 grid on a 10x10 nm area. After we fit the STS curves to that of an s-wave superconductor, we examine the spatial variability of the resulting superconducting gap value. We find that a domain of suppressed SC is found with a magnetic impurity at the center. We model the spatial phase transition of non-SC to SC using Landau-Ginzburg theory and obtain an angle-resolved value for coherence length, as well as phase shift. To understand the experimental results, we create a model system and calculate the local electronic structure using a microscopic Hamiltonian model and Lanczos diagonlization. Subsequently, we introduce a local magnetic impurity and explore the emergence of in-gap (zero bias) states at the defect. Thus, wavefunctions of the system can be found and used to calculate theoretical STM spectra. This approach helps to explain the behavior of this coupled magnetic and superconducting system.

The layered dichalcogenide TiSe₂ exhibits a commensurate charge density wave (CDW) phase that is continuously suppressed upon Cu intercalation (Cu_xTiSe₂), while a superconducting (SC) phase emerges for x>0.04, with a maximum T_C of 4.15K at the Cu doping of x=0.08 [1]. The two condensates, CDW and SC, coexist in a small region of the phase diagram thus making the Cu intercalated TiSe₂ an ideal system to study the interplay between the two collective phenomena. Despite the large research activity, the origin of the CDW phase and its relevance to the occurrence of the superconducting state, remain matter of debates and subject to experimental scrutiny.
We investigated the spatial and electronic structure of the CDW phase in Cu_xTiSe₂ in function of Cu concentration (0le;xle;0.066) by means of low temperature scanning tunneling microscopy and spectroscopy in UHV. We found that the CDW phase is still visible up to the higher Cu concentration investigated (x=0.066) at 4.2K. We also present STM micrographs revealing different CDW features as a function of Cu concentration.
[1] E. Morosan et al., Nature Physics 2, 544 (2006)

We report the superconductivity with onset critical temperatures of approximately 3.1 K and 5.8 K in topological insulator 200-nm-thick Bi2Te3 films with Bi-clusters on the surface. The Bi-clusters were formed naturally during the growth using pulsed laser deposition. Additionally, the magnetoresistance (MR) at temperatures below the superconducting transitions showed the anomalous “bell” and “well” shapes, which originated from the superconducting transitions of Bi2Te3 films and Bi-clusters, respectively. The characteristics of “bell” and “well” MR in Bi-clusters/Bi2Te3 films have been discussed in details.

As compared to the cuprates, iron-based superconductors show large pressure and magnetoelastic effects. The existence of a correlation between spin and lattice degrees of freedom has been speculated since early investigations. It has also been suggested that the structural transition originates from a nematic phase due to spin fluctuations above TN and its coupling to the lattice. We demonstrate that the well-studied “tetragonal” to orthorhombic transition occurring below 140 K in Ba(Fe1-xCox)2As2 for x < 0.055, is actually a transition from one orthorhombic phase (I2mm) to another (Fmmm): The breaking of C4 symmetry occurs above room temperature in unstrained crystals and all of the crystals are twinned with nano-sized domains. The variation of the domain size with x appears to track with Tc and the magnitude of the dynamic Fe moment as gleaned from EELS data. Taken together, these results point to a remarkable direct observation of the effects of magneto elastic coupling and magnetic fluctuations for an improved microscopic description of Fe-based compounds, including the appearance of superconductivity. Compositional nanoscale inhomogeneities present in pnictides and their effect on the superconducting gap will also be discussed. Research supported by the Materials Sciences and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy.

We have used resonant ultrasound spectroscopy (RUS) to measure a thermodynamic signature of broken symmetry at the pseudogap transition in the cuprate YBa2Cu3O6.58. As RUS is a relatively unknown technique, I will give a thorough introduction to the technique and, using the exotic superconductor PuCoGa5 as an example, I will illustrate how the symmetry of elastic moduli can reveal new physics. I will then discuss our new results on YBa2Cu3O6+x quantum oscillations in pulsed fields which reveal the evolution of the Fermi surface toward optimal doping, and discuss how this result ties into the bigger picture of high-Tc.

The asymmetric appearance of superconductivity in copper oxides is associated to the asymmetry of copper coordination with respect to hole- and electron doping. While hole doping into five- and six-fold coordinated CuO₂ planes rapidly induces metallic behavior as well as superconductivity, electron doping into CuO₂ planes with square-planar coordinated copper ions alone is insufficient for the induction of superconductivity. Instead, it is necessary to treat the as-grown RE_2-xCe_xCuO₄ material in reducing environments in order to remove excess oxygen. The subtle change in oxygen content can be traced by measurement of the c-axis length of the crystal. We further show that the Hall coefficient of such materials is positive, irrespective of the doping level, and well coincides with earlier angle-resolved photoemission spectroscopy data.