Superconductivity is induced by doping electrons into the 1111-type iron pnictides.
Indirectly electron doping has been performed by partial replacement of oxygen ions with fluorine ions or with oxygen vacancies. However, generic phase diagrams reported so far are incomplete due to the solubility limit of F- and oxygen vacancy.
We have found new electron doping method to overcome these obstacles and obtained complete phase diagrams covering overdoped regions. The results obtained are summarized as below:
(1) The Tc dome is extended to x~0.5 in REFeAsO1-xHx.
(2) The shape of the dome has a trapezoidal shape.
(3) There are two dome structures in the La-1111 system. The dome in the first is the same as reported so far by F-doping but the optimal Tc is higher in the second dome in 0.25(4) The characteristics of the second dome in the La-1111 are commonly observed for other RE-1111 systems.
The origin of high Tc in the 1111 system is discussed based on inelastic neutron diffraction measurements and theoretical calculations.

FeTe is one of the parent compounds of Fe-based superconductors. It exhibits an antiferromagnetic ordering associated with a lattice distortion at 70 K. The partial S and Se substitutions suppress the low temperature structural/magnetic phase transition, and thereby produce superconductivity. At low substitution rate, only filamentary superconductivity is observed although the antiferromagnetic ordering seems to be suppressed. This is because that FeTe possesses excess Fe (7 - 25 %) at the interlayer site. A previous report indicated that excess Fe supplies a substantial amount of electrons and it has a magnetic moment, and thus the physical properties of FeTe depend on the content of excess Fe. Therefore, to achieve bulk superconductivity in Fe-chalcogenide compounds, reducing 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 air exposure [1], water immersing [2], and oxygen annealing [3, 4]. It was proposed that oxygen suppresses the magnetic moment 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 [5-7]. In this presentation, we reports details of our research on the Fe-chalcogenide superconductors.
[1] K. Deguchi et al., Physica C 470 (2010) S340.
[2] Y. Mizuguchi et al., Phys. Rev. B 81 (2010) 214510.
[3] Y. Mizuguchi et al., Europhys. Lett. 90 (2010) 57002.
[4] Y. Kawasaki et al., Solid State Commun. 152 (2012) 1135.
[5] K. Deguchi et al., Supercond. Sci. Technol. 24 (2011) 055008.
[6] K. Deguchi et al., Supercond. Sci. Technol. 25 (2012) 084025.
[7] K. Deguchi et al., arXiv: 1210.5889.

Iron-based superconductors (IBS) have excellent properties for their potential applications to wires that produce high magnetic field. Namely, in most of IBSs, the upper critical field is over 40 T [1], and the electromagnetic anisotropy is less than three [2], which makes the motion of vortices, leading to energy dissipation, weaker. In most IBSs, J_c at 5 K under self-field ranges between 10⁵ A/cm² and 10⁶ A/cm². In order to enhance J_c further, introduction of defects plays an important role. Chemical substitution is one of the ways. On the other hand, irradiation of swift particles is another way. We have reported that heavy-ion irradiation is very effective in enhancing J_c in Co-doped BaFe₂As₂ [3]. We have also reported the enhancement of J_c by the proton irradiation, which creates point defects, also in Co-doped BaFe₂As₂ crystal [4]. Effect of other swift heavy-ions and protons are summarized in Ref. [5].
In this study, we explore how the enhancement of J_c by irradiation changes from one IBS system to another. We explore the effect of proton irradiation in (Ba,Sr)(Fe,Co) ₂As₂, Ba(Fe,Ru)₂As₂ [6], two kinds of Ca-Fe-Pt-As compounds [7,8], and Fe(Te,Se) [9,10]. We compare the effect of proton irradiation in these IBSs with that in Co-doped BaFe₂As₂, where the doubling of pinning potential without changing the glassy exponent has been found [4].
[1] A. Gurevich, Nature Mater. 10, 255 (2011).
[2] Y. Nakajima, T. Taen, and T. Tamegai, J. Phys Soc. Jpn. 78, 023702 (2009).
[3] Y. Nakajima, Y. Tsuchiya, T. Taen, T. Tamegai, S. Okayasu, and M. Sasase, Phys. Rev. B 80, 012510 (2009).
[4] T. Taen, Y. Nakajima, T. Tamegai, and H. Kitamura, Phys. Rev. B86, 094527 (2012).
[5] T. Tamegai, T. Taen, H. Yagyuda, Y. Tsuchiya, S. Mohan, T. Taniguchi, Y. Nakajima, S. Okayasu, M. Sasase, H. Kitamura, T. Murakami, T. Kambara, and Y. Kanai, Supercond. Sci. Technol. 25, 084008 (2012).
[6] T. Ishibashi, Y. Nakajima, and T. Tamegai, Physics Procedia 27, 104 (2012).
[7] Q. P. Ding, Y. Tsuchiya, S. Mohan, T. Taen, Y. Nakajima, and T. Tamegai, Phys. Rev. B 85, 104512 (2012).
[8] Q. P. Ding, Y. Tsuchiya, Y. Sun, T. Taen, Y. Nakajima, and T. Tamegai, J. Phys. Soc. Jpn. 81, 114723 (2012).
[9] T. Taen, Y. Tsuchiya, Y. Nakajima, and T. Tamegai, Phys. Rev. B 80, 092502 (2009).
[10] Y. Sun, T. Taen, Y. Tsuchiya, Z. X. Shi, and T. Tamegai, submitted.

High quality FeSe_0.5Te_0.5 thin films have been made with various substrates, such as SrTiO₃, LaAlO₃ and YSZ, and buffer layers, such as CeO₂. T_c as high as 20K with superconducting transition width about 1K was obtained. This T_c is much higher than that of the bulk (~15K). These films carry high critical current density J_c above 1MA/cm² at liquid helium temperature. The J_c holds above 1×10⁵A/cm² under magnetic field as high as 30T at 4.2K, with very low J_c anisotropy (< 3 for field parallel and perpendicular to the film surfaces). We also made textured FeSe_0.5Te_0.5 thin films on a buffered metal template with similar results as those films made on single crystalline substrates. This shows that iron chalcogenides have a very promising future for high field application at liquid helium temperature. Pinning force analysis indicates the point defect flux pinning mechanism, suggesting a straightforward approach for the conductor optimization.

Enormous strains can exist in thin films when one material is deposited on another due to differences in crystal lattice parameters and thermal expansion behavior between the film and the underlying substrate or arising from defects formed during film deposition. As a result, the properties of thin films can be dramatically different from the intrinsic properties of the corresponding unstrained bulk materials. While such strain often leads to degraded film properties, the judicious use of substrates and growth parameters offers the opportunity to enhance particular properties of a chosen material in thin film form, namely strain engineering. In particular, strain in iron based superconductor (FeSC) thin films could affect the superconducting transition temperature (Tc) of the films. So far, several investigations on 11 and 1111 FeSC phases showed that the Tc of strained thin films can exceed the maximum Tc of bulk material. However the reported Tc of strained 122 thin films is still lower than bulk. We have studied the strain effect in Co-doped Ba-122 thin film through the thermal expansion mismatch between Co-doped Ba-122 and the substrate. The Tc at zero resistivity (Tc,ρ=0) of strained Co-doped Ba-122 thin film is as high as 26.7K examined by both the electrical transport measurement and the magnetization measurement, which is even higher than Tc of bulk Co-doped Ba-122.

Among a series of high-Tc superconductors (HTS), (Bi, Pb)₂Sr₂Ca₂Cu₃O_y is the most prospective candidate for HTS wires with high critical current capacity, commercial productivity, and homogeneity over 1 km-long. DI-BSCCO (Dynamically-Innovative BSCCO) is the high performance multi-filamentary wires produced with the controlled-overpressure (CT-OP) sintering technique. The recent R&D activities have succeeded in commercialization of the 1 km-long wires with I_c up to 200 A (77 K, self-field), corresponding to 450 A/cm-w. Further, I_c of the R&D wires has topped out at 270 A (77 K, self-field), corresponding to higher than 600 A/cm-w. These performances are attractive for HTS applications and many prototypes have already been evaluated worldwide. To expand use of DI-BSCCO wires in various HTS application fields, we have improved the other required properties such as J_e, in-field critical current capacity at low temperature, mechanical strength and so on. The updated commercial and R&D activities will be shown in the presentation.

(Bi,Pb)₂Sr₂Ca₂Cu₃O_x (Bi-2223) HTS tapes are now commercially produced and can provide the uniform high critical current around 200 A (at 77K, self-field) over 1km. Therefore, various proto-type magnet applications are developed using Bi-2223 tapes. In this presentation, high field insert coils, liquid nitrogen cooled magnet, and conduction-cooled MRI magnet will be introduced.
A layer-wound coil using 540 m (in total) of Bi-2223/Ag/Cu-alloy tapes was fabricated for high field insert magnet. The winding has the dimensions: inner dia. 57.1 mm, outer dia. 88.9 mm, coil height: 153.9 mm, 44 layers (2,355 turns). This coil carried 193.3 A (0.1 mu;V/cm criterion) and generated 3.3 T in external 12 T (15.3 T in total). In addition, current carrying performance of recent Bi-2223 tapes in high fields (up to 17 T) will be reported.
A liquid nitrogen bath magnet was fabricated using eight double-pancake coils of Bi-2223/Ag tapes. This magnet has the dimensions: inner dia. (cold bore) 106 mm, outer dia. 211 mm, winding height: 81 mm, 1,390 turns (total tape length: 640 m). Iron flanges are put on both top and bottom ends of the winding in order to reduce the magnetic field perpendicular to the tape surface in the winding. The magnet carried 65 A (center field: 0.8 T) at 77.3 K (in liquid nitrogen bath with 1 bar vapor) and 102 A (1.25 T) at 67 K (0.2 bar).
A 3T MRI magnet system (500 mm room temperature bore) for human brain was fabricated. The magnet is composed of five layer-wound coils of Bi-2223/Ag/Cu-alloy tapes (total tape length: 44.6 km) and cooled using a G-M cryocooler. The system was magnetized to 1.5 T and the magnetic-field homogeneity of 5 ppm was attained. The first MR image at 1.5 T using HTS magnet was obtained. The system was successfully magnetized to 3 T (stored energy: 2.3 MJ).
This work was partly supported by Japan Science and Technology Agency, JST, under System Developing Program for Advanced Measurement and Analysis (SENTAN), Strategic Promotion of Innovative Research and Development Program (S-innovation), and Strategic International Research Cooperative Program.

Bi2Sr2CaCu2Ox (Bi2212) is the only material among current high temperature superconductors that offers round wire technology. At present, Bi2212/Ag wires are manufactured by filling Ag-tube with oxide powders, deforming into wire, restacking and then heat treating using partial-melt processing. During these processes, significant amount of voids and secondary phases emerge, which greatly undermines magnetic and transport properties. Here, an alternative approach of forming the Bi2212 phase is studied using metallic precursors. This approach involves the synthesis of Bi-Sr-Ca-Cu alloy via mechanical alloying, resulting in much higher density than oxide precursors. The metallic precursors are then oxidized directly into superconducting phase. This approach aims to solve the fundamental problem of Bi2212 superconductors by increasing the Bi2212 density as well as suppressing formation of voids and secondary phases via direct oxidation under solid state conditions and at temperatures for which Bi2212 is a stable phase. Results from DTA/TG analysis, quench studies, microstructure and phase characterization, and SQUID magnetometry are reported.

Bi2223 thin films on SrTiO3(100) fabricated by RF sputtering have been investigated. It is very difficult to get single phase of Bi-2223 thin films. Recently, we fabricated Bi,Pb2223 thin films by the combination of RF sputtering and annealing with Bi,Pb2223 pellets. The high Tc value of 101 K and high Jc values of 0.9 MA/cm2 at 77 K and in self field were obtained in these films. In this study, we tried to fabricate Bi,Pb2223 thin films using another way, in order to study the mechanism of phase transformation from Bi-2212 to Bi-2223.
We fabricated a multi layered films as a precursor of Bi,Pb-2223 by using two kinds of sputtering targets of nominal compositions of Bi,Pb2212 and CaCuPbO. After deposition by sputtering, multi-layered precursor was annealed with Bi,Pb-2223 pellets at 840oC and for 100 hours. From the XRD measurements, the peaks of precursor film were Bi-2212 phase. According to the ICP measurement, composition of multi-layered film was Bi:Pb:Sr:Ca:Cu=1.8:0.4:2.0:1.8:3.0. From the TEM and STEM observation, we confirmed that multi-layer of Bi,Pb2212 layer and amorphous CaCuO layer was formed. After annealing, we checked the Tc by resistivity and magnetization measurements. The Tc values by four probe method was reached Tczero = 101 K. On the other hand, we observed two step transition at 101 K and 60 K, by magnetization measurement. Furthermore, we observed two phases of Bi-2223 and Bi-2212 by XRD measurement. These results suggested that Bi,Pb-2223 phase transformed from multi-layered precursor film of Bi-2212 and CaCuO formed.
This study was partially supported by KAKENHI (22246079) with a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology. This study was partially supported by Nano-Integration Foundry in NIMS.

Past research has shown that not only can ferromagnetic shields have an influence on critical currents of coated conductors but also do they strongly affect ac losses in these sorts of superconducting tapes. Data regarding BSCCO tapes is available while ReBCO tapes have yet to be characterised. We will present measurements on these kinds of tapes together with an insight into preparation of the samples. Our research shows that special care has to be taken when working with unstabilised coated conductors because of the fragility and relative exposure of the superconducting layer. Furthermore, we will show how numerical simulation can be successfully employed to better understand the underlying physical principles and to optimise the coating process.

Coated conductors are attracting broad interests for large scale applications of superconductors such as electrical power machines and magnets for accelerators. Although large current carrying capacities are required in many large scale applications of superconductors, the critical currents of coated conductors are not more than a couple of hundreds amperes. Therefore, a number of coated conductors are assembled to obtain a large current carrying capacity. In these cases, coated conductors interact with each other electromagnetically. Furthermore, in some assembled forms such as Roebel cable, three-dimensional geometries of coated conductors lead to more complicated electromagnetic behavior. We studied electromagnetic behaviors of coated conductors through numerical electromagnetic field analyses, where their three-dimensional geometries as well as assembled structures were considered. The influence of microscopic electromagnetic phenomena on macroscopic electromagnetic quantities such as ac losses and magnetic fields generated in magnets were discussed based on the visualized microscopic electromagnetic phenomena in coated conductors. With respect to ac losses, calculated values were compared with measured values.
This work was supported in part by Japan Science and Technology Agency under Strategic Promotion of Innovative Research and Development Program.

Coated conductors, punched to a meander shape and assembled to the ROEBEL geometry provide high current AC cables which are in particular suited for application in windings like the rotor/stator coils of sc motors/generators, transformers or magnets. The properties of straight samples were investigated so far under manifold boundary conditions and will be reviewed with respect to current capacity in different fields and at different temperatures, the AC losses and the prospects for improvements. The application however will usually be in a coil arrangement which has a large impact on the cable behaviour. We present first measurements and modelling of the cable properties in a pancake coil structure and efforts on realizing a layered winding. Larger operation currents exceeding 20 kA in background fields as necessary in Fusion magnets exceed the potential of a ROEBEL cable and require even more complex cable designs as a Rutherford cable. We report about the status to prepare and test a Rutherford cable sample made from ROEBEL strands. Of most importance is the qualification of bending ability of the strands which have to withstand a complex bending situation around the central former. We report on transport measurements, bending experiments and the situation for AC losses and the upgrade strategy of the cable design.

Developments in 2G HTS conductor performance continue to drive the limits of magnet performance. Recent developments in 2G HTS conductor and coil technology are presented highlighting the ability of 2G HTS wire to function under varying operating conditions without degradation. The challenges of using 2G HTS wire in various coil constructions and applications are discussed. Several applications where the conductor is subjected to demanding operational performance requirements include high field insert coils and rotating machinery. Recent coils have taken advantage of the advances in 2G HTS conductor with advanced pinning that provides for improved in field performance of the conductor. These advancements, as well as the ability of the wire to be tailored (stabilization, insulation, ac losses) to fit various operating parameters will also be discussed. SuperPower is expending considerable effort in 2G HTS coil development to improve coil performance for a broad range of applications and provide technical support in the use of 2G HTS wire in various coil winding configurations.

TBD

Superconductors carrying high density currents with little or no heating offer unique solutions to fundamental grid challenges of the 21st century: high capacity power delivery in urban areas, long distance transmission of renewable electricity from remote sources to population centers, and high capacity, low weight offshore wind turbines. The high density electrical current in superconductors, many times that of conventional copper, is the key functional advantage of superconductors that cannot be matched with conventional technology.
Wide penetration of superconducting power technologies depends on raising the performance and lowering the cost of second-generation high temperature superconducting coated conductors based on YBa₂Cu₃O₇. Both objectives are achievable by breaking the “glass ceiling” that limits the critical current to about 30% of its theoretical expectation. We show that the critical current of commercial coated conductors based on YBa₂Cu₃O₇ can be raised by a factor of two by proton irradiation that creates point defects and clusters of point defects that immobilize vortex motion. The proton irradiation results demonstrate that the “glass ceiling” on critical current is not an intrinsic limitation and can be broken with the proper combination of pinning defects, enabling significant cost reductions in the cost of grid applications of superconductivity.
This work was supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (GWC, WKK, YJ, KK).

Magnetically coupled superconductor-ferromagnet hybrids offer advanced routes for nanoscale control of superconductivity. Scanning tunneling microscopy and scanning magnetic force microscopy coupled to magneto-transport measurements reveal rich vortex phase diagram.
The magnetic stripe domain of the ferromagnet induces periodic local magnetic induction in the superconductor, creating a series of pinning and anti-pinning channels for vortices, that have been observed with scanning tunneling microscopy and magnetic force microscopy at low temperature. Such laterally confined Abrikosov vortices form chains. When the experimental parameters of the superconductor and the ferromagnet are properly tuned spontaneous formation of Vortex-Anti-Vortex (V-AV) pairs is observed in these systems. The key role for formation of V-AV pairs is played by the out-of-plane magnetization of the ferromagnet and by the ratio of the magnetic domain width to the superconducting film thickness.
We have investigated the effect of inhomogeneous stray fields of the ferromagnet on superconductivity in Nb/Py and Pb/[Co/Pd] systems. Low temperature magnetic force microscopy and low temperature scanning tunneling microscopy have been employed for the local characterization of these systems.
Tunneling spectroscopy and conductance maps show clear indications of domain wall and reverse domain superconductivity. Close to the transition temperature (Tc) and in zero applied field, we visualized the emergence of superconductivity in regions above the separation between adjacent magnetic domains on length scales of the order of the coherence length. We also find an increase in Tc for certain values of applied field above magnetic domains of the opposite polarity. At low temperature, deep in the superconducting state, the magnetic stripe domain of the ferromagnet induces spontaneous formation of V-AV pairs.
Such S/F structures are attractive model systems that offer the possibility to control the strength of the superconducting nucleus either by applying an external magnetic field or through rearrangement of the magnetic domain structure and provide a tool for vortex manipulation.

1This work was supported by US DOE Grant No. SC0004556 and UChicago Argonne, LLC, Operator of Argonne National Laboratory. Argonne, a U.S. Department of Energy Office of Science Laboratory, is operated under Contract No. DE-AC02-06CH11357.

The title compound IrTe2 crystallizes in a trigonal CdI2-type structure with regular triangular lattices of iridium. IrTe2 exhibits a first-order phase transition from the trigonal to a monoclinic structure at approximately 250 K. The transition is characterized by a formation of short Ir-Ir bonds along the one side of the triangular lattice, namely b axis. In this talk, we demonstrate that the monoclinic phase can be suppressed by substituting Pt for Ir and superconductivity with a transition temperature up to 3.1 K emerges in the vicinity of the structural phase boundary [1]. Details of the crystal structure, normal-state properties, as well as superconducting properties of Pt-doped IrTe2 will be addressed.
[1] S. Pyon, K. Kudo, and M. Nohara, J. Phys. Soc. Jpn. 81, 053701 (2012).

Chemical solution deposition (CSD) has become a very competitive cost-effective technique to obtain nanostructured films and coated conductors. The development of high performance superconductors and high production throughputs requires, however, a full understanding of the different steps included in the TFA process, particularly the crystalline transformation to the superconducting phase and the oxygenation process. On the other hand, vortex pinning landscape engineering is foreseen as the route to high performance YBCO coated conductors at high fields. In solution-derived nanocomposites this is achieved generating randomly oriented second phase nanoparticles which induce huge isotropic pinning forces.
In this presentation we will report on our recent advances concerning the analysis of advanced processing methodologies of YBCO films grown through the Trifluoroacetate approach on single crystals with different sorts of cap layers and on YSZ-ABAD metallic substrates. We have particularly used in-situ resistance measurements to investigate the influence of processing parameters on nucleation, growth and oxygenation. This knowledge is used to optimize the solution deposition and processing conditions of CSD-grown buffer and YBCO layers in order to obtain high quality coated conductors. Parameters such as water partial pressure, total pressure and growth temperature of films deposited by ink jet printing have been modified to achieve high quality tapes at high growth rates. Highly textured YBCO films are obtained on Ce0.9Zr0.1O2 (CZO) buffered ABADYSZ/SS substrates. A two-step growth process has been implemented to reach critical current densities above 2 MA/cm2 at 77K with high growth rates (G> 1nm/s).
Finally, we will report on YBCO-TFA nanocomposites including BaZrO3, Y2O3, BaCeO3 and Ba2(Ta,Y)O6 second phase nanoparticles. A highly effective novel pinning mechanism, coupling superconducting pairing to lattice strain is proposed. The nanoscale strain is evaluated from X-ray diffraction line broadening while HRTEM and STEM investigation evidences a ramified shape of inhomogeneously distributed nanostrained regions.
References
[1] X. Obradors et al.; Physica C, http://dx.doi.org/10.1016/j.physc.2012.04.020
[2] X. Obradors et al.; Supercond. Sci. and Technol. 25, 123001 (2012)
[3] J. Gutierrez et al.; Nature Mater. 6, 367 (2007).
[4] A. Llordés et al.; Nature Mater. 11, 329 (2012)

THEVA developed a technology for the fabrication of coated conductors based on ISD-MgO buffer layers and an all-PVE approach for buffer layers, HTS, and silver coating. A reel-to-reel process was successfully developed in the past and good performance of the tapes could be demonstrated. In order to further improve the performance of our coated conductors we then have been focusing on two main routes, thick HTS films and Zr-doping. Usually, growth defects, which are hard to avoid in a production environment, can lead to a strong deterioration of the critical current density with thickness. On our MgO buffer layers a special growth mode of the HTS layer was found where defects can be overgrown. As a result, no deterioration of the critical current with thickness is observed and critical currents in excess of 1000 A/cm-width are possible. To improve the magnetic field performance the influence of doping by Zr was analyzed. TEM revealed BaZrO₃ nanoparticles but no nanorods in our HTS films. A decrease of the J_c-anisotropy was measured. Latest results of the two main routes of coated conductor performance improvement at THEVA will be discussed in this presentation.

Thick REBCO coated conductors have been deposited on RABiTS and IBAD-MgO tapes by pulsed laser deposition process. Although thin REBCO films (less than 0.8 micrometer) had a very high critical current density J_c of 4.0x10⁶ A/cm²(at 77 K, self field) on both of RABiTS and IBAD-MgO tapes, thick REBCO films showed quite different microstructure and superconducting properties. When REBCO film thickness increased over 1.0 micrometer, with increasing of REBCO film thickness the Jc values of REBCO films on RABiTS tapes decreased much fast than REBCO films on IBAD-MgO tapes. SEM and AFM observations showed that thick REBCO films on RABiTS tapes had micrometer-scale grains and relatively broad grain boundaries which blocked superconducting current paths. However, thick REBCO films on IBAD-MgO tapes still had sub-micrometer-scale grains and very narrow grain boundaries. In addition, it was found that thick REBCO films on RABiTS tapes had a-axis dominated a-/c-axis mixed orientations, while 2 micrometer thick REBCO films on IBAD-MgO tapes still had nearly pure c-axis orientation with critical current of over 500 A/cm-width (at 77K, self field). Microstructure analysis and superconducting property measurement showed that it is easier to fabricate thick REBCO coated conductors by PLD on IBAD-MgO tapes than on RABiTS tapes.

A fundamental challenge for developing YBa2Cu3O7-δ (YBCO) -based thin films and tapes with enhanced critical current densities for electronic and power applications is to immobilize the magnetic flux lines and thus prevent resistive losses in the presence of large magnetic fields. Vortex pinning is extremely dependent on the microstructure of the YBa2Cu3O7-δ layers, and many thin-film defects have been proposed as flux pinning sites in YBCO. Unfortunately, the naturally occurring defects are not effective enough at large magnetic fields. Yet, vortex pinning landscape engineering is foreseen as the route to high performance YBCO coated conductors at high fields, while solution deposited-YBa2Cu3O7-δ-BaZrO3 and YBa2Cu3O7-δ-Ba2YTaO6 nanocomposites appear as a very robust approach to prepare low cost coated conductors with substantially improved superconducting properties1. The addition of Zr and Ta metalorganic precursors to the initial solution leads to the formation of YBa2Cu3O7-δ(YBCO) epitaxial films with basically non-coherent nanoparticles randomly distributed within the YBCO film. Their presence changes drastically the microstructure of the YBCO matrix, generating incoherent interfaces, which ultimately promote lattice disorder and defects homogeneously distributed all over the film. Using aberration corrected scanning transmission electron microscopy (STEM) we have been able to unambiguously identify the atomic structure of these individual defects, their intrinsic self-assembling behavior as well as their interaction, which is critical to the understanding of the flux pinning efficiency of these HTS superconductors. We will show that the embedded secondary phases act as extra nucleation centers for the formation of twin boundaries, resulting in films with higher density of twin domains and shorter twin spacing. Both nanoparticles and the associated defects distort the twin boundaries affecting both their spacing and coherence across the entire sample thickness. These effects yield changes in dimensionality, size and even orientation of the TBs, which will definitely affect their roles as pinning sites or as flux channels. In particular, it is shown that vortex channeling is strongly precluded in nanocomposites.

Practical applications of superconducting power cables using long length tapes of high-T_c superconductors (HTS), such as Bi(Pb)2223 and RE123, have been started in these several years. In addition, various types of magnets, motors and generators using these HTS tapes have been demonstrated for extensive applications in future. Although national projects have strongly supported most of these developments thus far as a part of energy and environmental policy, further investigations for developing HTS conductors with higher performance are necessary to promote their broad utilizations in various fields.
The engineering critical current density J_e of these commercial HTS tapes is just above usable level, for example, approximately 200 A/mm² at 77 K under low magnetic fields, at the present stage, while intrinsic potentials of these HTS materials are much higher. Among many factors degrading performance of the practical HTS materials, we have been focusing on the nonstoichiometry of both cation and oxygen compositions. Recent our studies revealed that very precise control of the cation composition is crucial for determining intra- and inter-grain critical current properties of HTS compounds. In addition, control of oxygen content is always important to control carrier doping state, which directly affects T_c, electromagnetic anisotropy, pinning strength corresponding to the condensation energy and inter-grain coupling. Moreover, various doping techniques are also effective ways to control critical current characteristics of HTS materials.
Important factors to determine critical current properties of polycrystalline Bi(Pb)2223 and RE123 conductors and effective ways to bring out their intrinsic potentials will be comprehensively discussed from a viewpoint of “chemical control” based on our recent studies. Furthermore, their ultimate and feasible performance for extensive applications will also be mentioned.

This study shows the enhanced superconductivity of single grain Y-Ba-Cu-O (YBCO) bulk superconductors with the addition of nano-scale second phases of Y₂Ba₄CuAgO_y (Y2411(Ag)). The sol-gel process was utilized to fabricate fine and uniform Y2411(Ag) particles at a relative low reaction temperature than the solid state [1]. With the addition of different amounts of Y2411(Ag), at first, the T_c of these composites remained approximately constant at 90 ± 1 K and narrow width of Δ T implied no substitution interactions occur between YBa₂Cu₃O_7minus;δ (Y123) matrix and Y2411(Ag) phase during melting process. On the other hand, the superconducting properties of J_c and trapped field were observed to increase with an increasing Y2411(Ag) content in YBCO samples. With the addition of Y2411(Ag), J_c reached 6 x10⁴ A cm^-2 at 77K. This was almost 3 times higher than that of the un-doped sample (~2 x10⁴ A cm^-2). Notably, the values of the maximum trapped field (B_t,max) increased linearly as the Y2411(Ag) content increased. In the bulk with the addition of 4 mol % Y2411(Ag) ,B_t,max was enhanced to be 2.5 times higher than that of un-doped sample. Microstructure observations showed that Y2411(Ag) particles resulted in nano-scale (10-20nm) inclusions and distributed homogeneously in the Y123 matrix after melt-growth process. The enhancing of superconductivity was attributed to the improvement in microstructure by the addition of Y2411(Ag).
Ref:
[1] P.W. Chen, C.J. Liu, S.Y. Chen, Y.T. Nien, and I.G. Chen*, IEEE Transactions on Applied Superconductivity, 21, 3, 2710-2713 (2011)
This study was supported by the National Science Council, Taiwan, under Contract NSC 100-2627-E-006 -002 and NSC 101-2627-E-006 -001

Recent advances in MgB2 conductors are leading to a new level of conductor performance. Based on the use of an internal Mg diffusion or Mg infiltration starting point, but also includes optimized powders, proper chemistry, and an improved architecture, dense MgB2 structure with not only a high critical current density Jc, but also a high engineering critical current density, Je, can be obtained. Such a conductor could be described as a second generation MgB2 conductor. In this paper, a series of these conductors has been prepared using a 2% C doping level. Scanning electron microscopy and associated energy dispersive X-ray spectroscopy were applied to characterize the microstructure and composition of the wires, and a dense MgB2 layer structure was observed. The best layer Jc for our monofilamentary samples is 1.07x105 A/cm2 at 10 T, 4.2 K. This is about 10 x higher than that seen in the best PIT strands, and consistent with the best IMD strands in the literature. Previous IMD strands had lower Je values because of small reacted IMD layers. However, optimization of the layer design has led to monofilamentary Je values substantially higher than the corresponding PIT strands -- our best Je is seen to be 1.67x104 A/cm2 at 10 T, 4.2 K (a factor of higher than the best PIT). Optimization of the transport properties of these 2G wires is discussed in terms of B-powder choice, area fraction, and the MgB2 layer growth mechanism. Supplementing the studies on monofilaments with 2% C doping, we have also explored 3 and 4% C additions, with further improvements in high field Jc for monofilaments. We also have measured multifilamentary structures with 18 filaments in the form of one meter samples on modified ITER barrels. The 4.2 K, 5 T engineering critical current density, Je is 5.7x104 A/cm2, which is 33% higher than the best Je of PIT strands.

Niobium is the metal of choice for superconducting radio-frequency (SRF) cavities for particle accelerators because it has the highest critical temperature of any element in the periodic table and can be plastically deformed into complex geometries. Differences in sub-surface chemistry from bulk niobium are believed to determine the performance of SRF cavities, such as the frequently observed high-field Q (quality factor) drop. In this study, the subsurface chemistry of niobium has been characterized utilizing ultraviolet laser-assisted local-electrode atom-probe (LEAP) tomography employing ps laser pulsing together with aberration-corrected scanning transmission electron microscopy (STEM). The superior spatial resolution and analytical sensitivity of a LEAP tomograph permits us to determine the subsurface composition on an atom-by-atom and atomic layer-by-layer basis. The three-dimensional reconstructions from LEAP tomographic analyses demonstrate different behaviors for niobium oxides and niobium hydrides in pure niobium, as well as interaction with structural imperfections, such as dislocations and grain boundaries in SRF-grade Nb coupon material.
Atomic-resolution Z-contrast imaging and electron energy-loss spectroscopy combined with in-situ heating and cooling experiments is used to examine the atomic and electronic structures of Nb near the cavity surface. We have developed a methodology to quantify the local Nb valence and employ this method to assess the oxygen diffusion towards the cavity surface during the low temperature bake. In addition, we will demonstrate that hydrogen atoms incorporated into the Nb crystal, forming β-NbH precipitates, can be directly visualized using annular bright field imaging.
By using both atomic-resolution STEM analysis and LEAP tomography sequentially on the same needle-like sample, we find remarkable agreement between the stacking sequence of the 5 nm thick Nb oxide surface layer determined by LEAP tomography and EELS as NbO₂, Nb₂O₅, and NbO on top of metallic Nb. Below the surface of Nb grains, we find an sim;40 nm thick region with a high concentration of H, which in some areas forms an ordered β-NbH phase at room temperature.
Acknowledgment:
This research was supported by the Fermi Research Alliance LLC under contract number DE-AC02-07CH11359 with the US Department of Energy. The atom-probe LEAP tomograph at Northwestern University was purchased and upgraded with funding from NSF-MRI (DMR 0420532) and ONR-DURIP (N00014-0400798, N00014-0610539, N00014-0910781). NUCAPT is supported by the National Science Foundation's MRSEC program (DMR-1121262). The work at UIC is supported by the University Research Associate (URA) Visiting Scholars Program at the Fermi National Accelerator Laboratory. The acquisition of the UIC JEOL JEM-ARM200CF is supported by an MRI-R2 grant from the National Science Foundation (DMR-0959470).

We report the flux pinning characteristics of high-J_c GdBa₂Cu₃O_7-δ (GdBCO) coated conductors (CCs) on LaMnO₃ (LMO)-buffered IBAD MgO template fabricated by the Reactive Co- Evaporation Deposition & Reaction (RCE-DR) process. In this study, we have carefully analyzed the microstructural features of GdBCO CCs by transmission electron microscopy to find flux pinning sites. From these analyses, GdBCO CCs were found to include a lot of elongated round Gd₂O₃ particles and a small amount of Cu-O phase trapped within the GdBCO matrix. Normally, the Gd₂O₃ are relatively large and thus not so effective as to improve the critical current density (J_c) in high fileds. By varying the processing conditions, we could control the average size of Gd₂O₃ particles within the GdBCO matrix. Detailed effects of refined Gd₂O₃ particles on the Jc(B,T,theta;) properties of GdBCO CCs will be presented. In addition, we will also present carefully analyzed crystal defects around Gd₂O₃ particles within the GdBCO matrix responsible for flux pinning in high fields.

We deposited an amorphous Bi film at 4.2K with a thickness between 3 and 6.5 nm on top of previous deposited Co clusters layer having a mean size of ~ 4.5 nm. Total amount of deposited clusters corresponds to a mean layer thickness of 2.3 and 5 nm. In-situ transport measurements were performed between 2 and 100 K. Electrical resistance measurements on as prepared samples show hopping (tunneling) conductivity as as σ = σo exp[-(To/T)^1/2] above the superconducting transition (Tc) and re-entrance behavior with hopping (tunneling) conductivity again below Tc (for 3nm of Bi) and σ = σo exp[-(To/T)^1/3] for ticker Bi (~5 nm) . After heating up to asymp; 60 K the amorphous Bi crystallizes. The crystallized Bi films on top of the Co-clusters layer, on the other hand, show a strong decreasing in conductivity with decreasing temperature with a power-law scaling as σ = σo T^1/2, indicate that these films are close to a quantum critical point (QCP). This can be explained also by an opening of a gap in the density of states (DOS) at the Co/Bi interface layer of the Bi film due to the presence of Co magnetic moments and/or coulomb interaction due to disorder.

Large YBCO single textured grain samples can be prepared by the joining of numerous monoliths of RE-Ba-Cu-O bulk superconductors. Most researches indicated that the small pressure loading is needed to get a perfect joint. In this study, the loading free join technique in YBCO single crystal bulk is fabricated, due to the capillary force during melting, which used the 1mm thick Y211 pellets as a joint material. Part of the texture zone still exists during the melting profile, which will be taken as a seed to grow new texture crystal for nucleation during cooling. The trapped magnetic field distribution result show a symmetric profile was found in the as-grown sample, which implicated the perfect joint is made without weak link effect. Polarized optical microscope images showed the continuous Y123 phase across the junction, with the (100) orientation perpendicular to the bulks surface measured by one dimension XRD analysis. However, misorientation inner the textured grains need to investigate, which is suggested by two dimension X-ray diffraction. All of orientation information of the samples after bulk-joined process is reported in this study, displayed in the pole figures with (110), (100), and (001) as zone axis. It shows that misorientation inner the textured grains of our sample are smaller than 8 degree.
This study was supported by the National Science Council, Taiwan, under Contract NSC 100-2627-E-006 -002 and NSC 101-2627-E-006 -001

In this study, a superior J_c(H, T) with a peak effect was found in CeO₂ doped Y-Ba-Cu-O (YBCO) bulk materials grown by using infiltration growth (IG) method [1]. Magnetic susceptibility measurements by SQUID showed that the J_c(3T, 65K) of IG-YBCO sample with the addition of CeO₂ could reach 10⁵ A cm^-2, which was two times higher than that of CeO₂ doped top seeded melt textured (TSMT) YBCO sample. The Sm which dissolved from the SmBCO seed diffused into the bulk to form compositional fluctuations of (Y, Sm)BCO and was correlated to the effective pinning in high field regions (or peak effect) to improve the J_c(H, T). However, the spatial dependence of peak effect in IG growth YBCO bulk was also reported and attributed to the variation of microstructure throughout the bulk[2]. The ICP-MS results showed that the concentration of Sm ranged from 5.8×10^-2 to 7.0×10^-3 wt% throughout the bulk. Peak effect was only observed in regions where the concentration of Sm was higher than 1.5×10^-2 wt%. The spatial distribution of Sm was attributed to the way of liquid (BaCuO₂ and CuO) flow in the melting process of IG technique. Nano-scale Sm₂O₃ particles were added into the precursor powders to suppress the spatial dependence of composition. The concentration of Sm could be enhanced to higher than 1.5×10^-2 wt% in the bulk after doping nano-scale Sm₂O₃ particles. Therefore, peak effect was obtained throughout the bulk material. In addition, the maximum trapped field value and trapped field profile of the CeO₂ doped sample grown by IG were larger than that of sample grown by TSMT using the same diameter of precursor pellets.
Ref
[1] P.W. Chen, I.G. Chen*, S.Y. Chen, and M.K. Wu, Superconductor Science and Technology, 24, 085021(8pp) (2011)
[2] P.W. Chen, S.Y. Chen, I.G. Chen*, and M.K. Wu, Journal of the American Ceramic Society, 95 [10], 3109-3114 (2012)
This study was supported by the National Science Council, Taiwan, under Contract NSC 100-2627-E-006 -002 and NSC 101-2627-E-006 -001

Since the discovery of superconductivity in LaFeAs(O,F) in 2008^[1], Fe-based superconductors have attracted great attention as new high-temperature superconducting materials. AEFe₂As₂ (122) superconductors are promising materials for applications because they have simple crystal structure, small anisotropy and relatively high T_c by partial substitution of K for the AE site. Recently, it was reported that RE (RE = La, Ce, Pr, Nd) doped CaFe₂As₂ showed superconducting transition at approximately 40 K^[2]. Although reported single crystalline RE-doped Ca122 samples have very small superconducting volume fraction at 40 K, we have succeeded in the synthesis of polycrystalline samples with relatively large volume fraction^[3]. However, our polycrystalline samples contained relatively large amount of unidentified phases.
Based on these backgrounds, we have attempted to clarify the origin of superconductivity and improve superconducting properties in Ca-Pr-Fe-As system. Polycrystalline samples with several starting compositions were synthesized under various heat-treatment conditions in evacuated or Ar filled quartz ampoules. Synthesized samples were kept in air or pure H₂O or annealed under O₂ atmosphere.
Sample prepared from stoichiometric starting composition Ca:Pr:Fe:As = 0.86:0.14:2:2 contained only small amount of Ca122 phase and did not show superconductivity. On the other hand, samples synthesized from As rich starting compositions such as Ca:Pr:Fe:As = 0.9:0.1:1.8:2.3 showed large superconducting volume fraction. These samples contained unidentified phase. The samples were kept in air, pure H₂O or annealed under O₂ atmosphere. As a result, volume of unidentified phase of samples kept in air or pure H₂O was increased and superconducting volume fraction up to 40 K was also increased. Almost single phase Ca122 sample was obtained from nominal composition of Ca_0.9Pr_0.1Fe_1.1As_1.6. However, this sample showed very weak diamagnetism, while substitution of Pr was confirmed from the lattice constant. On the other hand, a sample with nominal composition of Ca_0.9Pr_0.1Fe_1.1As_1.6O_0.5 showed superconductivity, though peaks due to the Ca122 phase were not found in its powder XRD pattern. These results suggest that the unidentified phase might be one of the origins of superconductivity and H₂O affects superconducting properties of Ca-Pr-Fe-As system. Identifications of the composition and crystal structure of the unidentified phase are ongoing.
[1] Y. Kamihara et al., J. Am. Chem. Soc. 130 (2008) 3296
[2] S. R. Saha et al., Phys. Rev. B 85 (2012) 024525
[3] H. Yakita et al., MRS 2012 Spring Meeting and Exhibit I5. 18

The high transition temperature (39K) and high upper critical field (~15T at Tasymp;0) of MgB₂ make it a potential enabling technology for low-cost, high-efficiency superconducting industrial accelerators. These accelerators would be based on Cu or Al accelerating structures coated with a thin film of MgB₂. Many mature fabrication and coating techniques for MgB₂ exist; however, a low-temperature technique is required to allow application to Cu or Al cavity structures. Here we describe progress towards such a low-temperature technique that is based on energetic deposition of fast ions from a high-vacuum cathodic arc using a non-stoichiometric cathode. This deposition process delivers 50-150 eV ions of Mg and B to a substrate that is heated to < 400 °C. The non-equilibrium energetic condensation of Mg and B ions via subplantation creates a high crystal quality film of MgB₂. The stoichiometry of this film is controlled by adjusting the cathode stoichiometry and deposition temperature. Due to the different sticking coefficients of Mg and B on the substrate, the plasma at the substrate must be Mg rich to get the desired stoichiometry. Here we present the results of MgB₂ deposited on crystal and metal substrates. RRR/T_c measurements show that this method enables growth of superconducting films even on relatively low temperature substrates. This approach might offer a path to use of Cu or Al accelerator structures cooled by 10 K cryo-coolers for future superconducting industrial accelerators.
*Work supported by the Department of Energy

Superconducting iron chalcogenide thin films are deposited on amorphous substrates, i.e. glass substrates by a pulsed laser deposition (PLD) technique. Microstructural characterizations show that the films are highly textured along (00l) with good crystallinity. The superconducting critical transition temperature (Tc) is around 10 K. The in-field critical current density (Jcin-field) decreases slowly under high magnetic field confirmed by both transport and magnetization measurements. The growth of high quality superconducting FeSexTe1-x thin films on amorphous substrates demonstrates a low cost architecture for future Fe-based superconductor coated conductors.

There are many reports on the fabrication of high quality epitaxial thin film of iron-based superconductors, due to their attractive properties including high critical temperatures (Tc), high critical current density (Jc) and high upper critical fields (Hc2). Here, we report studies of high quality FeSe0.5Te0.5 (FST) thin films, made by pulsed laser deposition (PLD) method, on c-axis tilt SrTiO3 bicrystal substrates, with and without CeO2 buffer layer at various misorientation angles. It was found that critical current density across 24 degree grain boundaries was suppressed remarkably and modulated by the magnetic field perpendicular to the film surface, exhibiting the features of a typical resistive-shunted Josephson junction. CeO2 buffer appears to enhance the critical current densities across the grain boundaries. Implication of this study to iron-based superconducting wires and electronics will be discussed.

The transformation of grain boundary pinning to point pinning in MgB2-based materials (observed for the first time) with increasing manufacturing temperature from 800 to 1050 oC under pressures from 0.1 to 2 GPa is well correlated with an increase in critical current density in low external magnetic fields and with the redistribution of boron and the admixture of oxygen in the material structure, which influence pinning. As the manufacturing temperature increases (under 2 GPa), the discontinuous oxygen enriched layers transform into separately located Mg-B-O inclusions and the size and amount of inclusions of higher magnesium borides MgBX (X>2) are reduced. The effect of oxygen and boron redistribution can be enhanced by Ti or SiC adding.
The volume pinning force, Fp(max), increases with increasing synthesis or sintering pressure (0.1 MPa - 2 GPa) in materials prepared at high temperature (1050 oC) while it stays practically unchanged in those prepared at low temperature (800 oC). The position of Fp(max) can be shifted to higher magnetic fields by: (1) increasing the manufacturing pressure or decreasing the temperature (2) additions (Ti, SiC or C, for example) and (3) in-situ preparation.
The high pressure (2 GPa) - high temperature synthesized MgB2 bulk materials are characterized by near theoretical density (1-2% porosity), 80-98% connectivity, extremely high critical current densities reaching at 20 K, in 0-1 T jc=1.3-1.0 MA/cm2 (with 10% SiC) and jc= 0.92-0.73 MA/cm2 (without doping), irreversibility fields (Birr(18.4 K) =15 T and Birr (0 K) = 32.5 T) and upper critical fields (Bc2 (22 K) =15 T and Bc2(0 K) ~ 42.1 T (in the materials synthesized at 2 GPa, 600 oC, 1h). The spark plasma synthesized (SPS) material (50 MPa, 600-1050 oC, 1.3 h, without additions and 3-10 % porosity) demonstrated at 20 K, in 0 -1 T jc= 0.45-0.4 MA/cm2. Hot pressed materials (at 30 MPa, 800-1000 oC, 2 h, with 10 % of Ti additions and 8-15 % porosity) exhibited jc= 0.34 -0.23 MA/cm2. Usually the jc of in-situ prepared materials (from MgB2 powder) is higher than of ex-situ prepared ones (from Mg and B powders).

Superconductivity (SC), ferromagnetism (FM) and topological Insulators (TI) are the ingredients in the actual field of research in novel compounds in nanometer scale. SC-FM nano-composites can be fabricated, for example, by depositing in-beam prepared well-defined FM clusters of nanometer size and trapping them in SC matrix by depositing them onto a cold substrate. This technique was successfully employed with SC Pb wherein FM Co particles have been embedded. In these experiments the formation of spontaneous vortices without the application of an external magnetic field or any macroscopic magnetization will be described.
The same preparation system is used to deposit an amorphous Bi thin film layer at 4.2 K on top of layer of Co clusters with a mean size of ~ 4.5 nm. Since amorphous Bi is SC with Tc ~ 6K we can explore the competition between Mott insulator-like (IS) behavior and SC, in as prepared samples, and competition between SC and TI in annealed samples Co-clusters/Bi bi-layers. The study is based on in-situ transport measurements performed between 2 and 100 K. After heating up to asymp; 60 K the amorphous Bi films crystallize showing typical behavior of a two-dimensional metal with weak localization due to some disorder and no indication of superconductivity. The crystallized Bi films on top of the Co-clusters, on the other hand, show a strong increase in resistivity ρ with decreasing temperature and the sharp drop in ρ at Tc. This result could due to an opening of a gap in the density of states (DOS) at the Co/Bi interface layer of the Bi film due to the presence of Co magnetic moments. The results will be compared with films composed of layer of Bi on top or below normal Co metallic film prepared in the same cryostat evaporator.

Superconductors offer powerful opportunities for increasing capacity, reliability, and efficiency of the electricity grid. The mismatch between variation of renewable energy resources and electricity demand makes it necessary to capture electricity for later use. Developing affordable, large-scale energy storage systems would be a game-changing advance for the grid. Superconducting magnet energy storage systems (SMES) use magnetic fields in superconducting coils to store energy with near zero energy loss, and have instantaneous dynamic response (a US Dept. of Energy, ARPA-E project). Superconducting coils can also provide a lower cost alternative to the rare-earth permanent magnet or geared systems for high power (> 10 MW) direct-drive wind turbines (US Dept. of Energy, ARPA-E projects). In this presentation, I will first discuss current status of superconducting materials research at Brookhaven National Lab and briefly at Center of Emergent Superconductivity (an Energy Frontier Research Center of US Dept. of Energy, Office of Basic Energy Science, including Brookhaven and Argonne National Labs, and University of Illinois). Then, I will discuss the major challenges for energy applications of superconductivity, followed by highlighting several application projects currently being carried out at Brookhaven Lab in collaboration with leading superconducting wire manufactures and power system industry.
(The work at Brookhaven Lab was supported by DOE Office of Basic Energy Science and ARPA-E.)

We have successfully fabricated superconducting YBa₂Cu₄O₈ (Y124) thin films via coprecipitation. Y124 is a promising material for superconducting applications because of a structural stability which derives from no oxygen vacancy and an improvement of T_c by an elemental substitution [1]. Y124 is generally synthesized by using a high-pressure apparatus [2]. On the other hand, in the coprecipitation method, Y124 is easily obtained under ambient pressure without large-scale devices and instruments [3]. Therefore, this process is suitable for the fabrication of Y124 thin films, compared to other methods such as chemical vapor deposition [4] and plasma sputtering [5]. However, no one has been obtained Y124 thin film by this method yet.
We made the coprecipitate by a mixture of a reducing agent (as a precipitant) and a solution with stoichiometric amounts of Y, Ba and Cu nitrates (Y:Ba:Cu = 1:2:4). The coprecipitate was coated on a STO (100) substrate and heated several hours in an oxygen atmosphere. We performed XRD and SQUID measurements of the film. The film shows XRD pattern of Y124 with a c-axis orientation and T_c around 80 K. We will present the detailed information of this method.
[1] T. Miyatake, S. Gotoh, N. Koshizuka and S. Tanaka, Nature341 (1989) 41.
[2] J. Karpinski, H. Schwer, K. Conder, E. Jilek and E. Kaldis, Appl. Supercond.1 (1993) 333.
[3] J.S. Ho, C.T. Chang, R.S. Liu, P.P. Edwards, Appl. Phys. Lett.58 (1991) 2426.
[4] H. Hayashi, Y. Yamada, K. Sugawara, Y. Shiohara and S. Tanaka, Jpn. J. Appl. Phys.30 (1991) L352.
[5] Y. Yoshida, J.G. Wen, N. Watanabe, W. Ito, N. Koshizuka and T. Morishita, Appl. Phys. Lett.65 (1994) 3014.

We present an atomic-resolution study of the effects that a 48 hour bake at 120 °C in vacuum has on the high-field properties of Nb-based SRF cavities. It was previously shown that this bake results in a significant increase in the high-field quality factor, Q, and several mechanisms have been proposed, including an increased NbOx surface layer thickness and the precipitation of NbHy. However, it was also shown that baking at 800 °C for 2 hours will significantly reduce the Hc3/Hc2-ratio, resulting in a decreased performance of the Nb cavities. To date, a complete model explaining these distinct phenomena remains elusive.
Using a combination of atomic-resolution Z-contrast imaging and electron energy-loss spectroscopy with in-situ heating and cooling experiments, we examine the atomic and electronic structures of Nb and related oxides/hydrides near the cavity surface. We have developed a methodology to quantify the local Nb valence and employ this method to assess the oxygen diffusion towards the cavity surface during the low temperature bake. In addition, we will demonstrate that hydrogen atoms incorporated into the Nb crystal, forming β-NbH precipitates, can be directly visualized using annular bright field imaging in our aberration-corrected JEOL ARM-200CF.
We will show that at temperatures below 100 K, hydrogen precipitates into a variety of different NbHy phases within a single grain. We propose that this ordering is governed by the local hydrogen and point defect concentration. Finally, the effects of the 800 °C baking process on the local hydrogen and other impurity concentrations will be examined using both in-situ heating and cooling experiments. Our results will be combined with atom-probe tomography to develop a 3-dimensionsal impurity and phase profile of Nb near the SRF cavity surface.
Acknowledgment:
The authors acknowledge funding from and URA Visiting Scholars Program at Fermi National Accelerator Laboratory and the National Science Foundation [DMR-0959470] for the acquisition of the UIC JEOL JEM-ARM200CF, and thank the UIC Research Resources Center for assisting with the experiments.

LiFeAs, a stoichiometric superconductor with Tc~18K, presents an exceptional opportunity for surface sensitive studies owing to the non-polar cleaving plane between Li layers and the fact that it is superconducting without chemical substitution. Scanning tunneling microscopy (STM) and spectroscopy (STS) were used to characterize the gap structure and both the temperature and spatial dependence of the gaps. Two fully opened gaps, with Δ 1=5.3meV and Δ 2=2.5meV, were observed which close at the bulk Tc indicating the surface properties echo the bulk. Defects do not dramatically influence the superconducting gap with the larger gap varying less than 2% across large areas of the sample surface [1]. Five common defects were observed, each with unique topographical and energetic signatures. The most common defect, centered at an iron lattice site, preserves lattice symmetry and shows only one bound state energy at the edge of the small gap. Three others are clearly pair breaking, with bound states inside the small gap [3]. Energy resolved spatial maps surrounding these defects show complex and highly extended patterns in addition to quasiparticle interference. Although the chemical identity of the defects, required to make definitive statements regarding phase changes of the order parameter, is so far unknown, the energetic structure of the bound states is consistent with recent predictions for a multiband s+/- superconductor [3]. One of the most striking features of this material is that spectroscopy shows a pronounced dip-hump feature reminiscent of that observed in the cuprates. Here, the energy scale of the features most closely matches that of a resonant spin excitation that was recently reported from neutron scattering [4]. Despite being a multiband superconductor, the clean surface and bulk of this material while still presenting features of strong coupling make it an a relatively uncomplicated playground in which to study unconventional superconductivity.
[1] S. Chi, et al. PRL 109, 087002 (2012)
[2] S. Grothe, et al. PRB (in press) arXiv: 1207.4249
[3] Zhang, PRL 103, 186402 (2009); Tsai, PRB 80, 064513 (2009); Kariyado, JPSJ 79, 083704 (2010)
[4] Qureshi, et al. PRL 108,117001 (2012), Taylor, et al. PRB 83,220514 (2012)

The discovery of iron pnictide superconductors [1] opened a new possibility of very high field magnets owing to their high Tc and Hc2. However global intergranular supercurrent in polycrystalline iron pnictides is compromised by granularity due to structural defects and/or possible intrinsic weak-link [2]. To find the effective ways for improving Jc, investigations on the weak-link characteristics at grain boundaries using high purity and single-phase polycrystals would be very helpful. In the present study we have optimized the synthesis conditions of Co doped Ba122 polycrystalline bulks and observed an improved intergranular current transport property. Ba(Fe0.9Co0.1)2As2 bulks were synthesized from Ba, FeAs and CoAs with stoichiometric molar ratio. The powder was mixed under Ar atmosphere, put in Nb tube, sealed in vacuum quarts tube and followed by heating at 750-1200°C for 24 h. The samples heated at above 800°C showed high Tconset (~25 K) with high reproducibility. On the other hand, multiphase, i.e. 122 and FeAs phases, porosities and cracks were observed in their microstructure. An increase in the first heating time to 48 h at 900°C and an introduction of the second sintering at 900°C for 24 h with controlled cooling process were found to be effective for improving phase purity. Samples with the modified process showed an enhanced purity of the 122 phase with minor FeAs, decreased porosities and disappearance of cracks. Normal-state resistivity of the samples was largely decreased, and zero resistance temperature increased to ~23 K. Moreover sharp resistive transition was maintained under high fields up to 9 T. These suggest that the intergranular coupling in the superconducting state is also much improved.
[1] Y. Kamihara et al. JACS 130 (2008) 3296. [2] A. Yamamoto et al., SuST 21 (2008) 095008.

In this contribution we are going to present experimental results related to the synthesis and characterization of alkali intercalated FeSe prepared by means of liquid ammonia reactions. This kind of approach can be used to enhance the Tc of FeSe up to about 50 K by introducing, within the FeSe layers, "nude" alkali metals or amide-aklai metals complexes. By tuning the synthetic procedure it is possible to change the nature of the intercalating species and change the structural and physical properties of the material. This is true for Li intercalation (leading to Tc up to 40 K) while for bigger alkali metals the intercalating species is always the ion itself.

It has been more than 4 years since the discovery of FeSe superconductor. Through the efforts of many outstanding research groups unprecedented advancement in the field has been achieved. High quality single crystals of FeSe and related compounds have been successfully prepared by various techniques, thus allowing us to explore in details the physical properties of this class of materials. Detailed structure and properties characterizations of these crystals have provided critical information for better understanding the origin of superconductivity in FeSe. It has been clearly demonstrated that the occurrence of superconductivity is directly associated with the low temperature structure distortion. Several anomalous behaviors are also found to accompany the structural distortion. Recent measurements on quasiparticle and acoustic phonon dynamics with respect to the orbital modification in FeSe suggested the opening of an energy gap below 130 - 140 K, accompanying with a coincident transfer of the optical spectral weight in the visible range and alterations in the transport properties. These observations provide convincing evidence that the modification of the electronic structure is prior to the lattice distortion. These results further suggest that the high-T gap and the lattice symmetry breaking are driven by short-range orbital and/or charge orders.

Since the discovery of superconductivity in iron-based materials significant progress has been made in the fabrication of high quality bulk and thin film materials to explore their intrinsic properties and evaluate novel device applications. Artificial layered pnictide superlattices offer unique opportunity towards tailoring superconducting properties and understanding the mechanisms of superconductivity by creating model structures which do not exist in nature. For high field applications, very high critical current density (Jc) and irreversibility field (Hirr) are indispensable along all crystal directions. On the other hand the development of superconducting devices such as tunnel junctions requires multilayered heterostructures. We have domonstrated that artificially engineered undoped Ba-122 / Co-doped Ba-122 compositionally modulated superlattices produce ab-aligned nanoparticle arrays by layering and self-assembled c-axis aligned defects that combine to produce very large Jc and Hirr enhancements over a wide angular range. We also demonstrate a structurally modulated SrTiO3 (STO) / Co-doped Ba-122 superlattice with atomically sharp interfaces. Success in superlattice fabrication involving pnictides will serve to spur progress in heterostructured systems exhibiting novel interfacial phenomena and device applications.
This work has been done in collaboration with S. Lee, C. Tarantini, P. Gao, J. Jiang, J. D. Weiss, F. Kametani, C. M. Folkman, Y. Zhang, X. Q. Pan, E. E. Hellstrom, and D. C. Larbalestier. The work at the University of Wisconsin was supported by funding from the DOE Office of Basic Energy Sciences under award number DE-FG02-06ER46327.

Most coil-based applications of HTS are being targeted in a temperature range of 4.2 K - 50 K and magnetic fields of 2 - 30 T. In order for HTS tapes to reach commercial cost ($/kA-m) viability in these applications, their critical current performance needs to be improved by several fold at these operating conditions. A focus of our research recently has been on improvement of critical current of coated conductors fabricated by metal organic chemical vapor deposition (MOCVD) at the operating conditions of the superconducting coils. MOCVD precursor chemistries involving dopants and rare-earths have been developed to vastly improve the lift in critical current (‘lift factor&’) at the superconducting device operating conditions. With the optimum precursor chemistries, the processing of thicker MOCVD films has been improved to achieve high critical currents over a range of temperatures, fields and orientations. Critical currents at 4.2 K as high as 3400 A/cm at 5 T and 1900 A/cm at 10 T have been achieved in the orientation of field perpendicular to the tape surface. Additionally, the mechanical properties of these conductors are being improved for robustness in coil applications at high-fields and low temperatures.
This project was funded by Advanced Research Projects Agency-Energy (ARPA-E) award DE-AR0000196.

AMSC&’s Second Generation (2G) High Temperature Superconductor (HTS) wire manufacturing technology is based on the RABiTSTM/MOD process. AMSC&’s 2G wire (Amperium®), which carries up to 200 A (500 A/cm-w) at 77K, self-field, has been optimized for applications operating at high temperatures and low fields, such as cables and fault current limiters (FCL). In addition to cable and FCL applications, 2G wire is a leading alternative to rare-earth based permanent magnets (PM) in rotating machines operating at a temperature of 20 - 40K in a magnetic field of 1-3 T. Although the current Amperium wire carries over 750A/cm-w at 30K, 1.5T (H//c-axis) studies have shown that the pining microstructures designed for 77K applications are not ideally suited for lower temperature applications.
In this presentation, we will review the performance of AMSC&’s Amperium wire at various temperatures and fields and describe progress in optimizing the pinning microstructure of the MOD-based YBCO to maximize Ic at temperatures and fields applicable to coil applications.

Recently, long-length high-J_c GdBa₂Cu₃O_7-δ (GdBCO) coated conductors (CCs) have been successfully fabricated by the Reactive Co-Evaporation Deposition & Reaction (RCE-DR) process. In this process, an amorphous precursor film was firstly deposited by RCE using e-beam evaporation, and then converted into GdBCO crystalline phase within a minute by a subsequent annealing at high temperature of ~860°C in oxygen pressure of ~100 mTorr. Based on the stability phase diagram of GdBCO in low oxygen pressures (PO₂), we tried to optimize the processing parameters of the RCE-DR process for the improvement of J_c and also for the enhancement of flux pinning properties of GdBCO CCs. The major processing parameters controlled include the temperature and PO₂ to form Gd₂O₃ plus liquid, and also the additives to form the second phase nanoparticles within the GdBCO matrix. In this presentation, we will present detailed effects of the processing parameters and additives on the superconducting properties of GdBCO CCs.

Solution-derived YBCO nanocomposites have emerged as excellent low cost and scalable superconducting materials which properties can be finely tuned through processing to reach high performance long length coated conductors at high magnetic fields. We have demonstrated that the impressive properties arise from the strains associated to the network of intergrowths emerging from the spontaneously segregated oxide nanoparticles, and a novel pinning mechanism has been proposed coupling this lattice strain with superconducting pairing [1]. STEM and XRD investigations have been crucial to identify the highly dense defect structure involved like short Cu-O double and triple chains and corresponding nanostrain generated. In this contribution, we report on YBCO nanocomposites with a variety of second phase nanoparticles and off-stoichiometries where processing has enabled to control the density, length, stoichiometry and type of intergrowth. The corresponding interaction and strain has been evaluated by STEM and XRD. In particular, Y-O chains, combinations and mutual interactions of Y-O and Cu-O chains as well as atypical Cu-O chains have been observed and analyzed. The influence of the variety of intergrowths on vortex pinning has been evaluated through angular dependent transport critical current measurements. We conclude that by proper tuning intergrowths characteristics, the vortex pinning landscape of CSD YBCO nanocomposites can be modified and optimized.
[1] Nature Mater.11, 329-336 (2012)

Among the major high-temperature superconductors, Bi2Sr2CaCu2Ox is unique in that it can be fabricated into a multifilamentary round wire that carries large electric current in strong magnetic fields without tedious texture engineering, as is necessary for REBCO, for example. It could enable a new class of superconducting magnets for particle colliders and nuclear magnetic resonance spectrometers. The hope for a new magnet technology based on Bi2Sr2CaCu2Ox has been diluted for twenty years by many issues associated with high temperature melt-processing of long length Bi2Sr2CaCu2Ox conductors. Recent years have seen significant new understanding of the physics and materials science of Bi2Sr2CaCu2Ox, which has been followed by significant improvement in conductor performance and magnet engineering. Notably, the engineering current density JE of the state-of-the-art wire at 4.2 K and 20 T was recently raised above 600 A/mm2, a value that is practical for major magnet applications, and problems with extending this performance to long-length wires are being understood and overcome. With Bi-2212 emerging as a viable magnet technology, this talk will discuss the material science, conductor manufacturing methods, and major magnet processing issues of Bi2Sr2CaCu2Ox for very high field magnet applications.

Significant advances in the transport properties of Bi2Sr2CaCu2O8+X (Bi2212) round wires have made it a strong candidate for high field magnet applications. Bi2212 is a brittle material which shows an irreversible reduction in Jc with strain beyond a critical value. Ag/0.2-wt.%Mg alloy is currently the most commonly used alloy for the outer sheath of the Bi2212 conductor while pure silver is used for the inner sheath. Through internal oxidation during partial melt processing in oxygen, Mg strengthens Ag by forming MgO precipitates within the Ag matrix. Mg is not completely inert to Bi2212, however, it could react with Cu, depleting Cu from the Bi2212 cores and reducing Jc. A practical limit of about 0.2-wt.% Mg is typically used to reduce reactivity.
We have studied a high strength dispersion strengthened (DS) silver/alumina alloy as an alternative to Ag-Mg in order to enhance the mechanical strength of Bi2212 wires and reduce its strain sensitivity. Dispersion strengthening can effectively produce a fine grain metallurgical structure that is resistant to softening during partial melt processing. In this work, DS Ag/Al alloys with various Al compositions (0.5, 0.75, 1.0, and 1.25 -wt% Al) are fabricated via powder metallurgy and are compared with Ag/0.2-wt%Mg. We describe the method used to make a silver alloy containing various amount of alumina, the manufacture of silver alloy tubes, and the fabrication of a new Bi2212/AgAl round wire with DS Ag/Al alloy as both the outer and inner sheath. A Bi2212/AgMg round wire with the same geometry is used for comparison. Room temperature, 77 K and 4.2 K tensile tests, fracture studies, Vickers micro-hardness, electrical resistivity, optical microscopy, FIB ion channeling contrast imaging, field emission scanning electron microscopy and transmission electron microscopy are used to characterize the alloys and conductors. Transport measurements and Ic- strain behavior at 4.2 K are reported, including measurements at self-field and 5 T
This work was supported in part by the U.S. DOE Office of High Energy Physics SBIR Phase II Grant DE-SC0004268.

Microstructural observation is informative for studying the superconductivity of novel superconductors and fabricating their wires and tapes. Nano-scale characterizations of crystal structure, crystal defects, dopant distribution, atomic-bonding states, etc. are informative for understanding the nature of superconductivity of the novel superconductors. Conventional observations to clarify the morphology and crystallographic texture of polycrystalline grains, the distribution of impurity phases, etc. are also informative for developing the fabrication process of superconductor wires and tapes. However, it is often difficult to rationalize conclusively the relationship between superconducting properties and microstructure. Here, we will discuss how to make microstructural observations more effective for the superconducting materials research, by demonstrating our recent results. The proposed solutions are as follows: (i) using film-fabrication techniques to control microstructure of an objective superconductor (e.g. Kitaguchi et al.: Appl. Phys. Lett., 85, 2842 (2004)); (ii) statistical and quantitative characterization of microstructure (e.g. Kim et al.: MPG Asia Materials, 4, e3 (2012)); (iii) imaging both material properties and microstructure at the same field of view using different microscopy techniques (e.g. applications to Sm-123 and Bi,Pb-2223 tapes).

After the discovery of the high temperature superconductors (HTS) in the 80's, the properties of these materials began to be extensively studied. Actually, the principal mean of studying changes in the various properties of superconducting materials is through of the addition or substitution of doping elements in small quantities in its crystalline lattice. One of the most studied superconducting oxides system is the BSCCO (Bi-Sr-Ca-Cu-O) which was discovered in the 80's, with Tc between 7K and 110K directly related with the four superconducting phases presented by such system. Those phases differ from itself according to the number of CuO₂ and Ca planes in its crystalline structure. This work had the purpose to study the BSCCO superconducting system with stoichiometric formula Bi_1,6Pb_0.4Sr_2-xLa_xCa₂Cu₃O_10+δ doped with the rare earth element La in stoichiometric concentrations of x = 0, 0.5, 1.0, 1.5 and 2.0, in order to study the doping effects on the structural, electrical, morphological, magnetic properties of the BSCCO system. The samples were prepared based on the Pechini&’s method for which a resinous material was obtained and then submitted to thermal treatments of 200°C/10h, 400°C/6h, 600°C/6h , 800°C/6h and 810°C/68h . The samples were characterized structural, electric, magnetic and morphologically by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX), electric transport and ac magnetic susceptibility, respectively. X-ray diffraction patterns showed the presence of more than one superconducting phase of the BSCCO system. No significant changes in the characteristic peaks of this system for small concentrations of dopants were observed. For samples with high stoichiometric concentrations presented some changes in the characteristic of the peaks when compared to sample with x=0, this suggest a change in lattice parameters and the solubility limit of La in the structure of the material. The electrical resistance measurements showed superconducting behavior to the concentrations with x=0 and x=0.5, with a decrease of the Tc and the enlargement of the transition of x=0.5 when compared to the samples with x=0. For higher doping concentrations, xge;1.0, the superconductor material has lost its features, showing insulating properties. The SEM micrographs showed changes in morphological characteristics of the material with increasing the amount of dopant, i.e, we could observed an decrease of grain size. EDX indicated the average chemical composition it is near the expected values for the material produced. The AC magnetic susceptibility measurements showed degradation of the magnetic properties of the doped samples, i.e., the transition temperatures of the inter and intragranular parts are highly affected by the presence of La in Sr sites. Thus, the results showed a degradation of all the properties when the dopant concentration was higher than x=0.5.

An important challenge in applications of YBCO films is increasing critical current jc under an applied magnetic field. Self-assembled, c-axis extended BaSnO3 (BSO) nanorods as well as randomly distributed Y2O3 nanoparticles or PrBa2Cu3O7-x thin layers were introduced as artificial pinning centers (APCs) into YBa2Cu3O7-x (YBCO) thin films by pulsed laser deposition. Single doping of BSO nanorods with the amount of 4 wt % into YBCO produced the maximum global pinning force Fpmax = 28 GN/m3 at 77 K and B || c; however, the jc values are limited by the depinning of vortices from nanorods. The transition of a vortex line from one nanorod to another can take place via a thermally activated kink configuration, which throws a vortex segment onto an adjacent nanorod. Once kink excitation has nucleated, the kinks will run off easily by the Lorentz force, so the vortex spreads from one rod to the next. It is necessary to block the movement of the kink parts by the additional pinning enters for further increasing jc. The possible method is the insertion of the Y2O3 nanoparticle doped YBCO layers into the YBCO films containing BSO nanorods, resulting in the blocking of kink parts by nanoparticles. Another method inserts PrBa2Cu3Ox layers instead of Y2O3 doped layers. The kink parts can be blocked by PBCO layers. Additional pinning layers are very effective at lower temperatures and higher magnetic fields: the maximum global pinning force (Fpmax) became double at 40 K, 65 K compared with the usual BSO-doped YBCO films, but small enhancement at 77 K. Furthermore, the field angular dependence of Jc approach isotropic. These results indicate the double doping of APCs into YBCO is the promising way for enhancing Jc.

We present a detailed study carried out on oxide buffer layers grown by Metal-Organic Decomposition (MOD) on metallic substrates for YBa2Cu3O7-x (YBCO) coated conductor applications. Precursor solutions have been made starting from acetates or pentanedionates and characterized by means of Differential Scanning Calorimetry (DSC) and Thermogravimetric (TG) analyses coupled with Fourier Transform Infra-Red spectroscopy (FT-IR). A special attention has been devoted to the organic component remaining after high temperature annealing which strongly affects the buffer layer quality. Results obtained on thin films grown by spin-coating and dip coating on substrates made from different materials (Ni-alloy and Cu-alloy) with different aspect ratios will be reported. X-ray diffraction spectra (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM) techniques have been employed in order to optimize buffer layers in terms of film microstructure and surface quality, with the final aims of producing a suitable template for YBCO growth. Depth profiling analyses performed by Auger spectroscopy and Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) has been also been used to investigate the amount and the distribution of chemical species inside films before and after the deposition of the YBCO layer. It will be shown that the optimization of the recrystallization process can lead to high quality buffer layer allowing the growth of YBCO films showing good superconductive properties. In addition, the simplification of the buffer layer deposition process can be also achieved, which is an important issue to obtain a scalable and cost-effective coated conductors production process.
This work has been carried out in the framework of the Italian National Project FIRB called SURE:ARTYST funded by MIUR (Ministero Istruzione, Universitagrave; e Ricerca).

Epitaxial strain in thin films has a significant influence on the functional properties of superconducting materials. To study this effect, thin films are typically prepared on substrates with a different lattice parameter inducing a biaxial tensile or compressive strain. Unfortunately, this approach is often restricted to very thin films. Furthermore, it is difficult to correlate strain and superconductivity directly, as the preparation conditions and the resulting microstructure may severely affect the superconducting properties.
We use two alternative approaches to study the interplay between superconductivity and strain: Firstly, Fe-based superconductors are prepared on single-crystalline piezoelectric substrates enabling a dynamical variation of the induced strain by applying an electric field on the substrate. Secondly, these materials are grown on highly textured IBAD-MgO templates, which are based on flexible metal tapes. In this case, compressive or tensile strain is applied by in-situ bending of the substrate at low temperatures. Both methods were already successfully used to study the strain dependence of superconducting properties in cuprates.
Therefore, thin epitaxial BaFe_2-xCo_xAs₂ (122) and FeSe_1-xTe_x (11) films were prepared on piezoelectric (001) Pb(Mg_1/3Nb_2/3)_0.72Ti_0.28O₃ (PMN-PT) substrates as well as on IBAD-MgO/Hastelloy tapes. An undisturbed epitaxial growth of the superconductors was observed on both templates similar to the application of standard ceramic substrates. A reversible shift of the superconducting transition of about 10 K/% strain was measured for the 122 as well as for the 11 compound using the piezocrystal approach. Furthermore, first results on the in-situ bending of 122 as well as on 11 films grown on IBAD-MgO will be presented.
The supply of the IBAD-MgO template by V. Matias (now at iBeam Materials) is gratefully acknowledged.

In type II superconductors with extremely short coherence lengths xi; and large uniaxial crystallographic anisotropy, the Cooper pair density may be modulated along the crystallographic c-direction or even be restricted to superconducting planes. In such a case, the magnetic flux may be pinned intrinsically by this modulation. This is well known for the high-T_c superconducting cuprates. The recently discovered pnictide superconductors as well full-fill the conditions for intrinsic ab-plane pinning, however, up to now it has not been reported.
This contribution will show the signature of intrinsic pinning on transport properties of two types of pnictide thin films, FeSe_0.5Te_0.5 and Sm-1111. FeSe_0.5Te_0.5 has been deposited as temperature series between 350 °C and 550 °C. The film deposited at 350 °C shows a reduced T_c and low H_c2 values - the coherence length is larger than the interplanar distance. The film deposited at 550 °C shows a large density of strongly pinning planar defects ||ab, and a possible intrinsic pinning contribution is hidden. For the film deposited at 450 °C finally we found clear signature of intrinsic pinning at magnetic fields above 7 T and temperatures below 4 K. The critical current density is constant over B in this region and the n-value shows a strong dip in its angular dependence for field orientations near ab. For the lowest temperatures, n is rising again. This behavior is explained by trapping and locking-in of flux lines and the presence of double-kink excitations.
A Sm-1111 thin film with very clean microstructure and a T_c of 55.7 K has been investigated in high static magnetic fields. This compound shows the same intrinsic-pinning behavior in magnetic fields above 35 T - constant J_c and a dip of the n-value in the angular dependence.
The results on pnictide thin films will be compared to literature data on YBCO and torque measurements.
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-0654118, the State of Florida, and the U.S. Department of Energy.

Angular resolved transport measurements on Co-doped Ba-122 thin films were performed by means of a two-axis goniometer. The 80 nm thick films were grown by PLD (pulsed laser deposition) onto MgO single crystalline substrates with iron buffer layers. The films are very clean, do not contain strong pinning centres and have a high transition temperature of 24.7 K.
While the current is confined to the ab-planes by the geometry of the film, the direction of the field was varied with respect to the applied field and to the direction of the current (variable Lorentz force). The critical current was found to depend predominantly on the angle between the crystallographic c-axis and the applied magnetic field but hardly on the angle between the current and the field, which varies the Lorentz force; thus all data can be mapped onto the maximum Lorentz force curve. This indicates that the critical current is limited by the in-plane component of the Lorentz force only, but independent of the out-of-plane component. In the extreme case of rotating the field in the ab-plane (in-plane scans), the critical current is nearly constant, since the in-plane component of the Lorentz force results only from the field generated by the currents (self field) in this geometry. It can be concluded that flux pinning is extremely anisotropic in these films (despite the low effective mass anisotropy of this material) and that pinning along the crystallographic planes (or by parallel correlated defects) by far exceeds any random or c-axis correlated contributions. Only at low temperatures and high magnetic fields a significant influence of the out-of-plane Lorentz force was observed. The critical currents in a film grown on an IBAD template were lower and did not show any scaling with the out-of-plane Lorentz force.

Four years since the discovery by the Hosono&’s group of Fe-based superconductors, an enormous number of compounds, belonging to several different families have been discovered and fundamental properties have been deeply investigated in order to clarify the interplay between magnetisms and superconductivity in these compounds. Significant effort has been also put in preparing single crystals, thin films, wires, and polycrystalline bulk materials in view of future practical application.
Fe-based superconductors are midway between high temperature superconductors (HTS) and MgB2. In Fe-based superconductors the critical current is rather independent of the field, similarly to HTS, as a consequence of the exceptionally high upper critical field and strong pinning associated with nm-scale local modulations of the order parameter. They exhibit low anisotropy of the critical current with respect to the crystalline directions, as in the case of MgB2, which allows current flow along the c-axis. However, measurements on thin films grown on bicrystals indicate that Jc exponentially decreases with the misorientation angle, with a behavior reminiscent of that already observed in HTS thus suggesting intrinsic limitations in achieving high transport current in bulk materials.
Indeed early Fe-based superconductor polycrystalline materials exhibited strong electromagnetic granularity, like the HTS, but in the last four years an exponential grow of the transport critical current of wires and tapes have been obtained thank to a continuous improvement of the synthesis processes and fabrication techniques. All these issues are addressed within the Europe-Japan collaboration project SUPER-IRON which joined the efforts of the European partners from Austria, Germany, Italy and Switzerland and the Japanese partners (University of Tokyo, Kyushu University, the National Institute of Advanced Industrial Science and Technology, and the National Institute for Materials Science). The project is devoted to explore the potential of Fe-based superconductors and to assess them, setting the basis for future collaborations between Japan and EU . In this talk the main objectives and some of the results of the project will be resumed.

In order to understand the potential of new superconductors for applications, several properties, like H_c2, H_Irr, γ, J_c, grain boundary transparency and pinning properties, have to be investigated. Fe-based superconductors (FBS) quickly showed promising intrinsic properties: H_c2 (0) ranges from 55T to over 100T, H_Irr is close to H_c2, anisotropy is low (γ~1-7) compared to cuprates, intragrain J_c is close to 10⁷A/cm². However ensuring strong vortex pinning and sufficiently transparent grain boundary (GB) are really what need investigating. These two aspects will be considered here.
We investigated the field and angular dependent J_c in a wide temperature range and up to 45T in Co-doped BaFe₂As₂ (Ba122) thin films with high density of self-assembled nanorods and artificially introduced precipitates obtained by multilayer deposition. In high-T_c superconductors non-superconducting phases exceeding 3-4%vol strongly suppress T_c, whereas in Ba122 case even a density greater than 15-20%vol does not compromise the matrix properties. In case of high density of c-axis aligned nanorods, J_c(H//c) overcomes J_c (H//ab) up to ~20T producing a weak angular dependence and J_c(4.2K,20T) exceeding 10 ⁵ A/cm². Moreover the pinning force density F_p(4.2K) reaches 45-50GN/m³ at 15-20T, close to H_Irr/2. Similar results are achievable in multilayer films: because of a lower nanorod density but a high density of flat precipitates parallel to the ab-planes, a more isotropic J_c enhancement is obtained. The origin of such strong pinning has been identified by high resolution TEM that reveals in both cases complex microstructures.
First results on FBS polycrystals had shown that GBs are not in general transparent and the global J_c is suppressed by orders of magnitude because of secondary phases acting as current-blockers. Experiments performed on clean GBs in Ba(Fe_1-xCo_x)₂As₂ bicrystals had also revealed an exponential J_c decrease with increasing misorientation angle raising the question whether an intrinsic degradation of the superconducting properties at GB similar to the cuprates occurs in all FBS phases or not. More recent results reveal that a careful low temperature synthesis method prevents the formation of wetting phases at GB in (Ba_0.6K_0.4)Fe₂As₂ powders and bulk, as verified by high resolution TEM. Moreover magneto-optical images show a high global J_c in bulk materials. In untextured round wires fabricated using the so-obtained powders transport measurements reveal that J_c (4.2 K,s.f.) exceeds 0.12 MA/cm² (10 times larger than any previous results) suggesting that weak link behavior at GB is not typical of all FBS phases. Low anisotropy and high intragrain pinning appear to be the key points on this result.

Long-length GdBa2Cu3O7-δ (GdBCO) coated conductors (CCs) of high critical current (Ic) are routinely fabricated by a new high throughput process of RCE-DR (Reactive Co-Evaporation Deposition & Reaction) in SuNAM Co., Korea. Record-high Icxlength values of ~1.5 mu;m-thick GdBCO CCs/LaMnO3/Epi-MgO/IBAD-MgO/Y2O3/Al2O3 on the metal substrates of hastelloy and stainless steel are 421.7A x1,000m = 421,700 Am and 355A x 920m = 326,600Am, respectively, at 77.3 K in self field. At first, I will suggest the fast conversion mechanism of GdBCO film from the amorphous layer responsible for the high throughput RCE-DR process on the basis of the GdBCO stability phase diagram. Next, recent efforts for improving Jc(B,T,theta;) properties of GdBCO CCs will be presented. Finally, I will discuss the critical points for a successful application of our ex-situ conversion process to the fabrication of other high performance REBCO CCs.

Second generation (2G) superconducting wires offer a compelling value proposition for large electrical generator and motor applications. Current designs of 2G wire based motors and generators envision operation of a superconducting rotor at a temperature of 20 - 40K in a magnetic field of 1-3 T created by a copper-iron stator. The pinning physics in this operation range is different from the well-studied 65-77 K region. The primary difference between 30 K and 77 K operation is that at 30 K the role of thermal fluctuations is reduced at and point-like pinning centers become increasingly important.
Here we report the latest results on improving the low-temperature performance of thick YBCO layers manufactured by reel-to-reel metal-organic deposition (MOD). The pinning-improvement strategy has two elements: (i) maximization of the in-plane stain by adjusting the processing conditions; (ii) incorporation of pre-formed pinning centers. The flux pinning is realized primarily by chain oxygen ions displaced from equilibrium positions by the in-plane strain fields. An important element of the strategy is reliable measurement of in-plane structure of 2G wires and identification of the processing conditions that maximize the in-plane stain. In this work in-plane structure is determined by gazing-incidence diffraction using high-intensity synchrotron radiation. High-resolution in-plane diffraction is shown to provide important information regarding the in-plane strain and orthorhombic distortion of YBCO layer. The in-plane strain level is strongly affected by the processing parameters. Systematic changes of the in-plane structure and YBCO grain size are mapped with respect to the YBCO stability line and the Cu2O-CuO line on the Bormann-Hammond diagram. We show that the strain level in optimally doped films is limited by the orthorhombic distortion present in the sample after the high-temperature growth stage. Processing below the Cu2O-CuO line allows synthesizing samples with practically zero initial distortion and the maximum level of the in-plane strain after the low-temperature annealing. It is demonstrated that the optimum critical current density is the result of a trade-off between YBCO grain coupling and the strain-induced pinning. The optimized samples exhibit Ic&’s in excess of 600 A/cm at 30 K, 1.5 T H||c.
This work has been performed under contract with ARPA-E, U. S. Department of Energy.

Metal-organic decomposition has been established as the versatile methodology to grow low cost, scalable, high performance epitaxial YBa2Cu3O7 films for coated conductors. Nowadays, there is an important push in moving towards eco-friendly precursor solutions while reaching the same results as for the Trifluoracetate route. In this respect, the community is still far away from identifying the best formulation. In this work, we review our understanding on solution synthesis, deposition, pyrolysis and growth process of precursor solutions ranging from 54% up to 100% reduction of fluorine content. Methanolic solutions with non-fluorine precursors (acetates, ethylhexanoates) in different amounts of additives (thriethanolamine, propionic acid) have been stabilized and their rhehology modified for substrate wetability. Thermal decomposition analysis performed directly in films, have revealed the inherent differences in decomposition and pyrolysis rates up to 25C/min have been optimized. An in-situ growth resistivity technique has determined the growth rates and oxygenation step. Upon optimization of growth process parameters, Tc and Jc(77K)of around 90 K and 3 MA/cm2 are obtained.

Chemical solution deposition (CSD) techniques are a very competitive low cost method to achieve coated conductors. Recently, fluorine-free CSD methods have made a great progress and face the requirements for industrial implementation. By elucidating the reaction mechanism behind this approach (finally giving an answer to the question why it is possible to fabricate high quality YBCO films without TFA), different processing routes can give rise to high quality YBCO films (>1MA.cm-2).
Each route has it's own benefits. One specific route offers the opportunity to tune the crystallographic orientation. By changing 1 process parameter, a shift from complete c-axis to complete a-axis orientation is observed. This can be very usefull for e.g. Josephson Junctions.
We particularly investigated the fundamental reaction mechanism of each reaction route, with the focus on the corresponding barium compound. New insights in the fluorine-free approach arose from HT-XRD, IR and TGA-DTA measurements.
Nevertheless, a borderline thickness of ~500nm, which could be correlated to the high crystallization rates in fluorine-free YBCO films, limits further industrial implementation. In order to avoid these issues, a new reaction pathway is investigated and preliminary results are also included within the presentation.
Finally, it should be mentioned that these results were obtained by using water as primary solvent for the CSD precursor, resulting in a sustainable YBCO film.

The study of high quality YBa₂Cu₃O_7-x (YBCO) based superconducting films is a fundamental issue to be addressed when dealing with the realization of efficient coated conductors with large current carrying capacity. In this perspective the investigation of innovative buffer layers structures able to allow epitaxial YBCO film grow on metallic substrates and to prevent contamination and degradation issues holds a central role.
In this work we thoroughly study the properties of YBCO films grown by means of pulsed lasers deposition using MgO/CeO₂ and MgO/Pd/CeO₂ based buffer layers on Ni-W5% cubic textured metallic substrates. Due to its high chemical and temperature stability the MgO layer, produced by electron beam evaporation, reduces the oxygen diffusion effects. On the other hand, the CeO₂ layer, pulsed laser deposited, prevents the YBCO film from metallic contamination and facilitates its epitaxial growth. Morphology and crystalline structure of buffer layers and superconductors film are investigated by using scanning electron, transmission electron and atomic force microscopies (SEM, TEM and AFM), X-ray and electrons back-scattered diffraction techniques (XRD and EBSD) and X-ray photoelectrons spectroscopy (XPS).
The YBCO films, deposited using of a Nd:YAG solid state laser radiation, show very good superconductors properties and very poor a-axis oriented grains. Good critical temperature values (Tc > 90K) with sharp transitions are measured. High critical current densities in self-field (Jc > 1MA/cm²) are reported for the the best YBCO films. These encouraging results make our structures promising candidates in the realization of high quality YBCO based coated conductors.

In the current manufacturing process, HTS tapes such as YBa₂Cu₃O_7-x on Hastelloy substrate (YBCO/Hastelloy) often have in-situ quality control of surface morphology by high-speed cameras that detect and flag defects and by a X-ray diffractometer that accesses and adjusts the tape texture. However, these tools do not provide direct information on the expected critical current performance of the superconductor tape. Since, kilometers long 2G tapes are manufactured by the MOCVD process, an ideal solution would use a non-contact and non-destructive technique capable of providing real time information about both the buffer and superconducting layers at each point of the manufacturing process. Microwave material characterization techniques have been used in a large number of applications and are classified as a nondestructive and non-contact evaluation (NDE) method. We have used a microwave dielectric resonator (DR) technique to characterize complex conductivity of three YBCO tapes of different quality with the aim to evaluate such technique as a potential quality control method of the fabricated YBCO tapes.
The tapes were deposited at different temperatures on Hastelloy-supported oxide buffers using the MOCVD technique. The buffer stack consists of aluminum oxide (Al₂O₃), yttrium oxide (Y₂O₃), and textured IBAD-MgO and LaMnO₃ layers. Two configurations of DRs i.e. the single post (SPDR) consisting of high-permittivity barium zirconium titanate (BZT) ceramic operating at 9.4 GHz in quasi TE_01δ and the rod (RDR) consisting of rutile operating at 13 GHz in TE₀₁₁ mode were designed to measure the complex conductivity of the superconductor at the normal and superconducting states, respectively.
The multilayered HTS tape with metallic substrate and dielectric buffer layers further creates a complicated electromagnetic (EM) problem. In order to extract complex conductivity from Q-factor and resonant frequency shift measurements, a non-standard multilayered rigorous EM modeling was carried out using a commercial EM simulator HFSS (finite elements analysis). Experimental values of resonance frequency shifts and Q-factor were matched with the theoretical curves to extract the conductivity values of the samples. The effective Rs of pure Hastelloy, Hastelloy/buffer/YBCO with and without Ag overlayer were measured as 200, 35, 230 mOmega; and 120, 9 and 4 mOmega;, at 295 K and 77 K, respectively.
There have been several studies aimed at analyzing the normal/superconducting state and dc/high frequency properties of superconductor tapes. There are also a few reports on existing correlations between normal state resistivity and critical current J_c as well as on correlation between microwave and dc Jc and their dc magnetic field dependences. We will show an optimized design of the dielectric resonator for normal state YBCO/Hastelloy characterization. Also we will discuss measured Rs(T) normal state slopes values and their correlation with corresponding J_c values.

In order to understand and resolve practical performance issues of superconducting forefront materials, we have developed spatially resolved measurement methods for local electromagnetic properties such as flux flow loss and current flow in various kinds of superconducting materials. Critical current density, Jc, is one of the most important parameters of superconductors. Four-probe transport measurement and/or magnetization measurement are widely used to estimate Jc assuming that the sample is homogeneous in macroscopic scale. In many cases, however, local current blocking obstacles influence current flow significantly and therefore limit Jc in macro scale. We can hardly detect such local obstacles by the conventional global measurements. Namely, it is very crucial to investigate local electromagnetic properties inside the samples to understand current transport properties. In this talk, we will review our recent advancement of these characterization technologies such as visualization of local flux flow dissipation, measurement of spatial Jc distribution, their temperature- and magnetic-field-dependences, and scale up to the reel-to-reel measurement system in 100's meter length scale. These analyses allow us to gain knowledge about the critical current limiting mechanisms and can give the guideline to improve the property in various kinds of superconducting forefront materials including RE-123 coated conductor, Bi-2223 tapes, MgB2 multi-filamentary round wire and iron-based superconductors.
This work was partly supported by the “New Energy and Industrial Technology Development Organization (NEDO) as the Project for Development of Materials & Power Application of Coated Conductors, M-PACC”, “Japan Science and Technology Agency (JST) as Advanced Low Carbon Technology Research and Development Program, ALCA“, “JST as Strategic Promotion of Innovative Research and Development Program, S-INNOVATION“, and “Japan Society for the Promotion of Science (JSPS) as KAKENHI (24360122, 23760263)”.

The application of atomic layer deposition (ALD) to the synthesis of superconducting films opens the way for large-scale utilization of thin film coatings in a variety of applications, including improving the performance and decreasing the cost of high energy particle accelerators, superconducting wires for energy storage, and bolometers for radiation detection. ALD is a scalable gas-phase deposition technique which utilizes sequential self-limiting surface chemical reactions to deposit films in an atomic-layer-by-layer manner. The ALD process therefore offers the possibility of uniformly coating complex shapes with precise, layered structures with tightly constrained morphology and chemical properties. The unmatched conformality characteristic of ALD enables facile integration of superconducting thin films in device architectures which are otherwise limited by line-of-sight constraints (e.g. depositing uniform coatings on prefabricated magnet coils or inside bulk superconducting radio frequency (SRF) resonator cavities). Furthermore, the atomic-scale thickness control afforded by ALD enables the study of superconductivity and correlated electron phenomena in homogeneous layers in the ultra-thin film limit. Recent work in our group demonstrates that MoN and NbTiN films with superconducting properties equivalent to those deposited by traditional methods (e.g., reactive magnetron sputtering) are achieved by ALD [1]. Additionally, ALD processes have been demonstrated for the synthesis of a growing number of superconducting materials including transition metal silicides, carbides, carbonitrides, and nitrides and covering a wide range of critical temperatures up to 14 K [1-5]. In this respect, we will present results of a comprehensive study of ALD-grown superconductors and multilayer superconductor-insulator (S-I) heterostructures. We will discuss the ALD growth processes and the results of characterization of film structure, morphology, and superconducting properties as well as performance in working SRF structures.
[1] Proslier, Th., et al. ECS Trans. 41, 237 (2011).
[2] Proslier, Th., et al. J. Phys. Chem. C 115, 9477 (2011).
[3] Klug, J. A., et al. J. Phys. Chem. C 115, 25063 (2011).
[4] Greer, F., et al. ECS Trans. 41, 263 (2011).
[5] Driessen, E. F. C., et al. Phys. Rev. Lett. 109, 107003 (2012).

Development of new low-cost and scalable synthetic pathways to energy materials is a crucial step for the next generation of energy solutions. Recently discovered Fe-based superconductors exhibit common structural fragment, FeX (X = As, Se) layer, where each iron atom is tetrahedrally coordinated by non-metal elements, X. Such FeX layers are often separated by cation Aⁿ⁺ (A = Na-Cs, Ca-Ba) or oxide LnO⁺ (Ln = La-Lu) layers. Typical synthesis of these compounds requires high temperatures of ~1200 K. We have developed low temperature synthetic route to iron selenides. This approach allows us to combine superconductor structural fragment, chains of FeSe₄ tetrahedra, with unsaturated [FeL₂]²⁺ transition metal complex. Such combination leads to the formation of three-dimensional Fe-Se anisotropic framework. Crystal structure of Fe₃Se₄L₂ consists of Fe^IIISe₂ chains running parallel to each other along [001] direction. These chains are connected into 3D framework through additional Se-Fe^II-Se bridges. ⁵⁷Fe Mössbauer spectroscopy studies reveal presence of two different oxidation states of Fe atoms as well as magnetic ordering of Fe centers at 4 K.Fe₃Se₄L₂ exhibits metallic-like temperature dependence of the electrical conductivity. Magnetic studies indicate ferrimagnetic nature of Fe spins ordering at low temperature. Details of the crystal structure and magnetic properties of Fe₃Se₄L₂ will be discussed.

A major impediment for use of high temperature superconducting (HTS) RE-Ba-Cu-O (REBCO, RE=rare earth) coated conductors (CC) in AC applications is extensive AC losses generated by the geometry. Although different conductor geometries have been suggested such as round wire to reduce AC losses, based on the current manufacturing techniques, the only viable way to reduce AC losses is to striate the conductor into small filaments. We have recently developed a selective electroplating method combined with a cost-effective mechanical striation to create fully-filamentized conductors. Magnetization AC loss behaviors of such multifilamented conductors with different stabilizer thicknesses as well as a reference sample were investigated. AC loss measurements were performed in varying magnetic fields of frequencies between 40 and 500 Hz at ~77 K. Results showed that selectively-electroplated stabilizer layer was extended across the striations by increasing stabilizer thickness and it merged from both sides of the striation tracks beyond a certain thickness. This results immense increase in coupling component of AC losses while there is no influence of stabilizer thickness on AC losses up to 30 µm which appears to be enough for most of the targeted AC applications of coated conductors known to the authors. The details of AC loss performance, microstructures and physical properties of mechanically scribed and selectively electroplated HTS CCs will be presented.

Vortex pinning landscape engineering is foreseen as the route to high performance YBCO coated conductors at high fields. Solution-derived nanocomposites with randomly oriented second phase nanoparticles (BaZrO3, Y2O3, BaCeO3 and Ba2(TaY)O6 ) were shown to be an excellent low cost processing option with huge isotropic pinning forces associated to a highly dense defect structure (mainly extra Cu-O intergrowths). We find that the local strain ensuing from the partial dislocation associated to intergrowths break the vertical coherence of twin boundaries (TB) and strongly reduce the TB spacing (le;10 nm).
The lack of twin boundary coherence will affect their role as pinning centers or channels for easy vortex flow. Transport measurements at different temperatures and magnetic fields realized in tracks with different crystallographic orientations has enabled us to determine the effect of twin boundaries in a quantitative manner and establish their contribution (on vortex pinning and/or channeling) in a H-T phase diagram.
We show that the anisotropic pinning contribution coming from TBs has a minor role compared with the huge enhancement of isotropic pinning in nanocomposites. On the contrary, we demonstrate that the reduction of the TB vertical coherence has a relevant effect on precluding vortex channelling at low temperatures and thus avoiding a Jc suppression for field parallel to the c-axis.

Control of the dynamic behavior of vortices has a broad relevance for reducing losses in superconducting wires and microwave devices. One of the outstanding obstacles in widespread application of high-temperature superconductors has been related to methods of reducing vortex flow at high magnetic fields when the system is in vortex liquid state. Here we demonstrate vortex jamming effect through long-range manipulation of Abrikosov vortices in mesoscopic superconducting wires. Reduction or complete termination of vortex flow due to strong vortex-vortex interaction in constricted (funnel) geometries leads to a reduction of dissipation due to vortex movement. Similar to the case in granular materials, the affinity to jam in funnel geometries is most pronounced at high particle densities, i.e. in our case at high magnetic fields. We report on dynamics of non-local Abrikosov vortex flow in mesoscopic superconducting Nb channels terminated with funnel geometry constrictions. Magnetic field dependence of the non-local voltage induced by the flux flow shows that vortices form ordered vortex chains. Voltage asymmetry (rectification) with respect to the direction of vortex flow is evidence that vortex jamming strongly influences vortex dynamics in mesoscopic constriction geometries. The findings can be applied to superconducting devices exploiting vortex dynamics and vortex manipulation, including superconducting wires with self-assembled engineered pinning centers.
G. Karapetrov et al. Phys. Rev. B 86 (2012) 054524
The use of the Center for Nanoscale Materials and the Electron Microscopy Center at Argonne National Laboratory were supported by UChicago Argonne, LLC, Operator of Argonne National Laboratory (``Argonne''). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357.

Traditionally, ceramic materials such as yttrium barium copper oxide (YBCO) require high temperatures, pressures and lengthy processing to synthesise. The products of these so called “heat-and-beat” syntheses are generally inhomogeneous in morphology, and consist of many irregularly-sized and orientated crystallites. Top down fabrication techniques are now of sufficient quality to create nanowires of YBCO and other high temperature superconductors (HTSC), however these all rely on thin film deposition to create the YBCO crystals which can then be fabricated into nanowire arrays using pattern transfer and etching techniques. Using biological polymers we have shown that HTSC materials can be created without the need for highly controlled conditions, and in morphologies which would otherwise be unobtainable, via a bottom-up process. Further to this, recent advances in nanoscale characterisation techniques have allowed, for the first time, measurements of conductivity along substrate- and strain-free HTSC nanowires for temperatures at and below the transition temperature for YBCO, around 90 K.
Bottom-up fabrication of YBCO nanowires were first reported in 2006, through the use of a biopolymer templating technique. The template used, sodium alginate, was a polysaccharide consisting of two distinct monomers, polyguluronate (-G-) and polymannuronate (-M-). Depending on their ratio, the monomer blocks determine the structural properties of the seaweed from which the polymer is derived. -G- blocks are known to strongly sequester divalent metal cations and so a high ratio of -G- to -M- blocks will create a water soluble polymer capable of taking up divalent cations from the surrounding environment. Upon mixing with aqueous precursor ions of yttrium, barium and copper, the sodium alginate forms a gel, with the metal cations acting as linkers between the polymer strands. This is particularly true of the barium ions which, upon heating, form nanoparticles of barium carbonate. At 811 degrees C these particles melt and can form a eutectic, into which the other intermediates can melt, effectively reducing the melting point of the entire mixture of intermediate oxide phases. The particles have shown to act as sites for growth via, it was postulated, vapour-liquid-solid outgrowth, forming nanowires. The HTSC properties of this new morphology have only been investigated in bulk.
Recently, through new observations performed using in situ heating on a transition electron microscope, we have been able to examine the outgrowth process directly, and provide conclusive proof of the exact mechanism by which the wires form. Single nanowires superconductivity measurements have also been performed for the substrate-free nanowires using a nanoscale 4-point probe to temperatures below the superconducting transition. These observations give an unprecedented level of detail into biopolymer templated syntheses in terms of both growth mechanism and physical properties.