In the real life, almost all materials are imperfect and contain impurities introduced intentionally to enhance the properties of materials. The controlling of impurities so that the second phases caused by them are of great importance in materials science and engineering. It is therefore that the nanoscale characterizations of these impurities and second phases in three-dimensions (distribution, morphology, volume, etc.) are of worthwhile for improving the property and further design of materials. Among characterization methods to achieve three-dimensional information at nanoscale resolution, three-dimensional electron tomography, by combination of transmission electron microscopy and computed tomography (TEM-CT), has been applied for materials science in recent years. During my talk, three-dimensional nano-characterization of REBCO-123 type high-temperature superconductors (HTSCs) containing artificial pinning centers in various dimensions, will be presented. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as the Materials and Power Application of Coated Conductors Project (M-PACC Project).

Many research efforts have been attempted to introduce nano-sized non-superconductive particles to act as artificial pinning centers (APCs) in REBa₂Cu₃O_y superconductive layers on a metallic substrate with a textured buffer layer. These coated conductors with APCs are indispensable to develop industrial applications. One of the promising processes is to control nano-sized BaZrO3 (BZO) particles in Y_xGd_1-xBa₂Cu₃O_y (YGdBCO) layers by trifluoroacetates-metal organic deposition (TFA-MOD). Recently two-step crystallization process in the TFA-MOD has been found to be very effective to enhance in-field critical currents significantly. However, the effects on the microstructure and the mechanism for the improvement have not yet been understood. In order to optimize the process conditions, we need to understand the structural change in the two-step crystallization process. In this study, we prepared precursor films, for which both MOD solution coating and drying process was repeated using reel-to-reel tape transfer system, quenched films during the two-step crystallization process and a final crystallization film. The nanostructures of these films were investigated sequentially by transmission electron microscopy. Multi-layered structures corresponding to the coating process of the TFA-MOD were found in the precursor film, which was composed of tiny grains. The boundary of each layer in the precursor film had Cu-rich content. During the interim annealing at 848K, the tiny grains grew larger, and BZO particles were gathered around the boundaries of the Cu-rich regions. When these films were heated up at 1038 K, c-axis oriented YGdBCO grains formed on the buffer layer. Numerous BZO particles with an average size of 20 nm formed in the precursor film, and some BZO particles were trapped in the c-axis oriented grains. Finally, a dense YGdBCO layer was formed, which was mainly composed of c-axis oriented grains. In addition, the BZO particles, which had formed in the precursor, were distributed in the YGdBCO layers. Though some of the BZO particles were aligned perpendicularly to the substrate normal, these particles were homogenously distributed in the YGdBCO layer. On the contrary, larger elliptical pores were formed in an YGdBCO layer fabricated by conventional one-step crystallization process, and BZO particles clamped together in the layer. Based on these insights, we can obtain design guidelines to produce high-performance superconductive layer in high magnetic fields. This work was supported by New Energy and Industrial Technology Development Organization (NEDO) as the project for Development of Materials & Power Application of Coated Conductors.

Using multiple microscopy techniques, we have carried out combined analysis to reveal both the superconducting and micro-structural properties in Sm₁Ba₂Cu₃O_7-δ (SmBCO) coated conductors (CCs). Al₂O₃, Y₂O₃, IBAD-MgO and LaMnO₃ layers were deposited sequentially on a Hastelloy substrate to obtain bi-axially textured template. Successively, 2 μm thick SmBCO layers have been deposited by batch type co-evaporation process using drum in dual chambers method. In order to investigate current limiting mechanism, we first carried out laser induced thermoelectric effect (LITE) imaging by laser scanning microscopy at room temperature to visualize in-plane distribution of CuO₂ tilting domains and defects in SmBCO layers. It is followed by the low temperature laser scanning microscopy (LTLSM) cooling down the sample to superconducting state. We successfully visualized local flux flow dissipation as a function of bias current. From these analysis, we can determine the position of current limiting obstacles with μm resolution. Lastly, micro-sampling transmission electron microscopy (TEM) has been carried out. It allows us to investigate crystallographic structure at the obstacles in nm scale. We found that the amplitude of LITE signal shows very good linearity as a function of CuO₂ tilting angle which was determined from the coordinated cross-sectional TEM observation. Since the LITE has a good compatibility with the LTLSM and also has a robustness against surface contamination, it is very effective method to analyze the crystallinity in the SmBCO layers. TEM analysis also clearly shows that current blocking obstacles in the SmBCO layer in the present sample is originated from deformation of the Hastelloy substrate such as grain boundaries and high density dislocations. Namely, the quality of Hastelloy surface is crucial to improve superconducting performance of these CCs. This work was partly supported by JSPS: KAKENHI (20360143).

To enhance a global critical current in a superconductor, it is indispensable to understand current limiting factors and their influence on such a critical current. From this point of view, we have investigated in-plane distribution of local critical current density and its dependence on electric field criterion in a thin-film superconductor by using scanning-Hall probe microscopy. In a remanent state after the application of sufficiently high magnetic field to a sample, current flows at critical current density according to the critical state model. Such distribution of current density was estimated from that of measured magnetic field by considering Biot-Savart law. Furthermore, the corresponding electric field criterion was evaluated from the relaxation of such remanent magnetic field by considering Faradayâ?Ts law. This means that we could estimate in-plane distribution of local critical current density as a function of electric field criterion in a nondestructive manner. This characterization method would be very helpful for finding current limiting factors in a thin-film superconductor and their influence on its global current density versus electric field properties which would usually be obtained by 4-probe method.
This work was supported by "The New Energy and Industrial Technology Development Organization (NEDO), â?oJSPS: KAKENHI (23760263)â? and â?oThe Nakajima Foundation".

Manufacturing low AC loss high temperature superconductor (HTS) coated conductors (CCs) required for AC applications, such as fault current limiters, transformers, and generators have always been challenging. Subdividing CCs into filaments has been shown as a viable method to reduce AC losses. In this regard, different techniques have been developed to construct desired filamentized configuration of CCs to obtain minimal AC losses. Complexity, difficulty to control, limited throughput, and high cost are some of the drawbacks regarding with these techniques. We have implemented a simple, cost effective and readily scalable method for making filaments on our REBCO CC made by metal organic chemical vapor deposition (MOCVD). We have also successfully fabricated fully filamentized conductors after electrodeposition of copper stabilizer layer. We have observed that the copper stablizer provides no-contribution to AC losses while significant contribution to AC losses was observed in case of copper in the non-filamentized conductor. AC losses of striated conductors as well as a reference non-striated conductor both before and after electrodeposition were measured in AC magnetic fields normal to the conductor at various frequencies. After 11 striations, at least 11 fold reduction in AC losses, which is very close to the theoretically estimated value, was obtained at AC field of around 40 mT, and 77K. In this presentation, AC loss measurements, physical properties, and microstructure of mechanically multifilamented conductors will be discussed.

Almost all high Tc superconductors such as YBCO, Bi-2223 or the recently discovered pnictides possess grain boundaries that tend to block current. In the case of planar grain boundaries, an exponential decay of the critical current density Jc with increasing misorientation angle θ is observed. For polycrystalline versions of these materials to have high Jc we must develop a uni- or biaxial textured architecture. However there is one interesting exception to this rule, Bi2212. Macroscopically untextured Bi2212 round wire can achieve a high Jc of ~105 A/cm2 at 4.2 K and 45 T. In order to explore how such a high Jc is possible where many high angle grain boundaries occur in the round wires, we have been combining high resolution SEM imaging, EBSD-OIM, and FIB-extraction to local Jc measurements. Firstly the experiments have been carried out on much denser Bi2212 bulk with high intergrain Jc. By measuring the V-I characteristics of extracted individual filaments, we certainly can see that weak-link-free current paths exist in filaments and we are now focusing on individual grain boundaries of varied misorientation angles. The goal of our experiments is to understand those factors which make Bi2212 grain boundaries special. Our hope is that such information will enable better grain boundary current transfer in other high Tc materials such as YBCO.

Twenty five years have passed since the first discovery of high-Tc superconductors (HTS). Today, HTS applications are truly expected to revolutionize energy technologies such as DC and/or AC power transmission cables, highly efficient and compact motors and generators, etc. The most important development toward realizing innovation using HTS will be to improve the current carrying capacity of HTS wires. (Bi,Pb)2Sr2Ca2Cu3Ox(Bi-2223) tapes are now commercially produced and used in various proto-type applications. DI-BSCCO is the high performance silver-sheathed Bi-2223 wires produced with the controlled-overpressure (CT-OP) sintering technique. The present commercial DI-BSCCO can provide the uniform high critical current up to 180 A (@77K, self-field) over 1km. Continuing efforts have been performed in fundamental HTS materials science as well as wire processing aimed at doubling the current carrying capacity of Bi-2223 wires. Some short length test pieces exhibit 250 A (@77K, self-field) as the highest critical current (corresponding to 600 A per 1 cm-width). The performances of DI-BSCCO can meet the growing needs for various HTS applications. In this presentation, current carrying performance of recent Bi-2223 wires will be reported.

In the most of cuprate superconductors, the post annealing process is well recognized to be quite important for controlling nonstoichiometric oxygen composition, which affects both intragrain and intergrain characteristics. However, very few attentions have been paid for the effects of post annealing on the cation composition of the layered cuprates in spite of its serious influence on the critical current properties of the material. Tc of Bi2223 was found to increase up to ~118 K by post annealing at ~1000 K for a long time accompanying an increase in the c-axis length in our previous study. This result gave a hint to increase critical current of polycrystalline Bi2223 tapes by enhancement of Tc, i.e. an increase in condensation energy, which was suppressed somehow by nonstoichiometric cation composition. In the present study, we have systematically studied the post annealing effect on the grain coupling characteristics of various cuprate superconductors. In the case of the randomly oriented Bi2223 sintered bulk specimen, the intergrain Jc enhanced three times by addition of post annealing process under moderately reduced atmospheres at ~1000 K prior to the low temperature annealing to control the oxygen content. Effects of post annealing on the grain boundary characteristics of high-Tc superconductors, such as Bi2223, RE123 and other compounds, will be shown.

Formation mechanism of nanorods and nanoparticles in YBa2Cu3O7-x (YBCO) films is still uncertain though a huge enhancement of Jc of the films has been reported by introduction of nano-inclusions such as BaZrO3 and Y2O3. In this study, we report on the new finding concerning the formation mechanism of nanorods and nanoparticles in the YBCO films obtained by using a molecular dynamics (MD) simulation. The MD calculation was executed by assuming the film deposition condition, and using a large-scale nanoparticle/YBCO interface model. The interfacial energy was calculated from the internal energy after MD simulation, and the strain distribution in the YBCO area was estimated from the local structural changes. As a result, it was clarified that the interfacial energy was more predominant than the strain energy in the formation mechanism of nanorods and nanoparticles. We also report the comparative study of nanoinclusions/YBCO interfaces by using transmission electron microscopy (TEM) and X-ray diffraction.

The crystal structure of YBa2Cu4O8 resembles that of YBa2Cu3O7-x, the difference being a doubling of the so called chains with respect to YBa2Cu3O7-x, causing the c axis to more than double. Due to this double chain all the oxygen atoms have a strong bond with a copper atom, resulting in fixed and stable oxygen content, so that YBa2Cu4O8 is essentially oxygen-stoichiometric and is expected as a thermally stable compound. However, it is practically difficult to produce the YBa2Cu4O8 phase because of its limited stability region in the phase diagram of the Y-Ba-Cu-O system. YBa2Cu4O8 phase is thermodynamically stable at low temperatures and higher oxygen pressures than YBa2Cu3O7-x. Since controlling of higher oxygen pressures is difficult for growing films by physical vapor deposition processes, post annealing is necessary. We report the preparation of REBa2Cu4O8 (RE=Y, Yb) thin films by using pulsed laser deposition and chemical solution deposition processes. YBa2Cu4O8 precursor films deposited on single crystal substrates by pulsed laser deposition were solid-state reacted in the post annealing processes. The epitaxially grown films showed the superconductivity, but the c-axis of the films was in-plane oriented. YbBa2Cu4O8 films were prepared on single crystal substrates by chemical solution process. The films heat treated at ambient oxygen pressure showed metallic behavior and sharp superconducting transition. These films were stable under the high temperatures and low oxygen pressures. The growth mechanism of the films will be discussed.

During the last years in scientific literature row of reports about atomic-scale perturbation charges in high temperature superconductors (HTSC) have appeared. The question is about the so-called stripes and checkerboard patterns which appear as a result of electronic states modulation. The occurrence of such anomalies in the division of the electronic states causes the special interest because can be related to creation of NTSCâ?Ts high-temperature superconductivity state. The charge increasing means strong correlations of the spins and charge states of HTSC. Thus, there is a question to these modulations origin nature and their influence on high- temperature superconductivity state. With the purpose of physical nature investigation of the highpointed anomalies X-ray and photoemission studies of some ternary compounds of the R-Cu-Si system (R-rare-earth metal) and high-temperature superconductor had been performed. Such objects selection is caused fact that all named compounds are cuprates, in which the dehibridization phenomenon had been found. The dehibridization of electronic states especially evidently shows up in X-ray emissions spectra, consideration of which peculiar attention are spared in this investigation. It is important to note that in the case of HTSC the role of dehibridization factor is carried out hybrid orbitals - type, which form plattigu. Oxygen, which is a strong oxidizer, destroys 3d-shell and forms power strong pointed up orbitals. Thus, power stableness of Cu3d10 shells caused that of - hybrid orbitals. Exactly the Cu-O planes are basic objects (in the sense of transition to superconducting state) in the structure of HTSC, which are accountable for superconducting state origin. On our mind the dehibridization process causes occurrence stepped off electronic states. The quantum interference such states can result in stripes and checkerboard patterns.

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 in the past with SC Pb wherein FM Co particles have been embedded [1,2]. In these experiments the formation of spontaneous vortices without the application of an external magnetic field or any macroscopic magnetization has been observed. In the present work we show some evidence for the competition between Mott insulator (IS) like behavior and SC in as prepared samples and competition between SC and TI in annealed samples Co-clusters/Bi bi-layers. We deposited an amorphous Bi film at 4.2 K with a thickness between 3 and 7 nm on top of former deposited Co clusters having a mean size of â?^ 4.5 nm. Total amount of deposited clusters corresponds to a mean layer thickness between 0.7 and 5.5 nm. In-situ transport measurements were performed between 2 and 100 K. Electrical resistance measurements on as prepared samples show hopping (tunneling) conductivity as Ïf = Ïf₀ exp[-(T₀/T)^1/2] above the superconductor transition (T_c) and the re-entrance into the normal state with hopping (tunneling) conductivity again below T_c. After heating up to â?^ 60 K the amorphous Bi crystallized. Pure crystallized Bi films show the 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 T_c. This can be explained 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. Such an opening of gap in the DOS has been predicted at the surface of traditional TI like Bi₃Se₂, decorated with magnetic impurities [3,4]. [1] Y. T. Xing et al. Phys. Rev. B 78, 224524 (2008). [2] Y. T. Xing et al. Phys. Rev. B 80, 224505 (2009). [3] Hai-Zhou Lu et al. Phys. Rev. Lett. 107, 076801 (2011). [4] Hai-Zhou Lu et al. Phys. Rev. B 84, 125138 (2011).

We report the evolution of the pressure-temperature phase diagrams of the Ba(Fe_1-xM_x) ₂As₂ (M = Co, Cu, Ru, Mn) single crystals grown from In-flux. We compare the effects of the applied pressure in samples with different concentrations of M but with the same temperature for the onset of the spin density wave (T_SDW) at ambient pressure by following the evolution of the magnetic and uperconducting (SC) phases as a function of the pressure. The electrical resistivity measurements under pressure for samples that belong to the under-doped region of the phase diagram showed that T_SDW decreased and the SC phase is achieved at high pressures with T_c) creaching a maximum value of 28 K for P â?^ 16 kbar for x = 0.03 of M = Co doping. We also discuss how the critical pressure for inducing SC depends on the particular M-doping with and also the influence of the growth route by comparing our pressure-temperature phase diagram with one constucted form the samples grown using the self-flux method.

Recently iron-based superconductors have attracted attention as new high-T_c superconductors. They have alternate stacking structure of iron-pnictide layers and blocking layers. In particular, we have focused on iron pnictides with perovskite-type blocking layer, because they have chemical and structural flexibilities. Recently, we found iron arsenide superconductors with very thick perovskite-type blocking layers such as (Fe₂As₂)(Ca_n+1 (Sc,Ti)_nO_y) (n = 3,4,5), (Fe₂As₂)(Ca_n+1 (Mg,Ti)_nO_y) (n = 3,4) and (Fe₂As₂)(Ca_n+2 (Al,Ti)_nO_y) (n = 2,3,4)[1]. They exhibit superconductivity at higher than 40 K without intensive carrier doping. Up to now, their carrier-doping state is not clear while cation/anion nonstoichiometry such as oxygen deficiency or M/Ti ratio is expected. Control of these nonstoichiometry and investigation of their effects on physical propereties have been attempted in the present study. All samples were synthesized by the solid-state-reactions. Starting reagents were mixed and pelletized in a glove box filled with argon gas. These pellets were sealed in evacuated quartz ampoules and heated at 1100°C for 72 h. Constituent phases and the lattice constants were determined by powder X-ray diffraction. Temperature dependence of magnetization was measured by a SQUID magnetometer. Electric resistivity was measured by the AC four-point-probe method. There was no significant difference in lattice constants of (Fe₂As₂)(Ca₅(Sc,Ti)₄O_y) with different Sc/Ti ratio in starting compositions. This indicated that nonstoichiometry in Sc/Ti ratio of this material is not large. On the other hand, lattice constants of (Fe₂As₂)(Ca₆(Al,Ti)₄O_y) were changed by different starting Al/Ti ratios. Their Tc were also changed with Al/Ti ratio and it seemed that there is correlation between T_c and c-axis length. On the other hand, temperature dependence of resistivity of (Fe₂As₂)(Ca₆(Al,Ti)₄O_y) with different Al/Ti ratio indicated similar behavior with each other in normal state, suggesting that there is no significant change in carrier-doping state with Al/Ti ratio. This suggested that local structure of Fe₂As₂ layer such as As-Fe-As bond angle or pnictogen height are determining factors of Tc for these compounds. Effect of oxygen annealing on (Fe₂As₂)(Ca₅(Sc,Ti)₄O_y) and (Fe₂As₂)(Ca₆(Al,Ti)₄O_y) will be also reported. [1] H. Ogino et al., Supercond. Sci. Technol.24 (2011) 085020.

Since high Tc superconductivity in LaFeAs(O,F) was discovered in 2008[1], many new superconductors which contain anti-fluorite type Fe₂Pn₂ layer have been reported. However, further increase in Tc with less toxic elements is required for practical applications. Therefore, it is necessary to explore new iron-based superconductors. Among the iron-based superconductors, a series of compounds containing perovskite-type blocking layer have high chemical and structural flexibility at the perovskite-type layer. In this system, a lot of iron arsenides, such as (Fe₂As₂)(Ca_n+1(Sc,Ti)_nO_y) with large variety of crystal structures and compositions are reported. On the other hand, only three iron phospides, (Fe₂P₂)(Sr₄Sc₂O₆) (T_c ~17 K)[2], (Fe₂P₂)(Ca₄Al₂O₆) (T_c ~17.1 K) and (Fe₂P₂)(Ca₃Al₂O_5-y) (T_c ~16.6 K)[3] are found in this system so far. Based on these backgrounds, we have explored new layered iron phosphides in this system and discovered (Fe₂P₂)(Sr₃Sc₂O₅). Samples were synthesized by solid-state reaction under ambient pressure or high pressure synthesis starting from FeP, FeAs, Sr, SrO and Sc₂O₃ powders. Since some of starting materials are sensitive to moisture or oxygen in air, powders were mixed and pelletized under an inert gas atmosphere in a glove box. Samples were reacted at 1200 ~ 1250°C for 120 h in evacuated quartz ampoules or at 1600 °C for 2 h under 3.5 ~ 4.5 GPa. Phase identification and evaluation of lattice constants were carried out by X-ray diffraction (XRD) using silicon powder as the internal standard. Magnetic susceptibility was measured by a SQUID magnetometer and electric resistivity was studied by the AC four-point-probe method. (Fe₂P₂)(Sr₃Sc₂O₅) was formed as a main phase with impurity phases under only high pressure. Lattice constants of the compound were a = 4.038 Ã. and c = 26.37 Ã.. The a-axis length of the compound is longer than that of (Fe₂P₂)(Sr₄Sc₂O₆) (a = 4.016 Ã.) and the longest among the layered iron phosphides. On the other hand, superconducting transition was not observed above 2 K, indicating that superconducting properties of iron phosphides change greatly by a small difference in the local crystal structures. In addition, substitution of As for P site of (Fe₂P₂)(Sr₃Sc₂O₅) was attempted. (Fe₂(P1-xAsx)₂)(Sr₃Sc₂O₅) (x = 0.1 ~ 0.5) were formed under ambient pressure while (Fe₂P₂)(Sr₃Sc₂O₅) was only formed under high pressure. Their a and c-axis length increased with increasing As ratio. The samples for x = 0.1 ~ 0.33 exhibited superconductivity and the highest T_c was 15 K for x = 0.2 in magnetization. [1] Y. Kamihara et al., J. Am. Chem. Soc. 130 (2008) 3296. [2] H. Ogino et al, Supercond. Sci. Technol. 22 (2009) 075008. [3] P. M. Shirage et al, J. Am. Chem. Soc. 133 (2011) 9630.

Since the discovery of new superconductor LaFeAs(O,F) (T_c = 26 K)^[1] in 2008, Fe-based pnictides or chalcogenides have attracted great attention as new high-temperature superconducting materials. Fe-based superconductors are promising materials because of their high T_c (~55 K) and very high upper critical field. AEFe₂As₂ (122) superconductors are particularly examined for applications, such as wire, rods and thin films, due to their simple crystal structure, small anisotropy and relatively high T_c, which is achieved by partial substitution of K for the AE site. Recently, it was reported that CaFe₂As₂ show superconducting transition at approximately 40 K by partial substitution of RE (RE = La, Ce, Pr, Nd) for Ca^[2]. However, superconductivity is reported only for single crystalline samples and their superconducting volume fraction is not large. Based on these backgrounds, we have attempted to improve superconducting properties of the Ca-Pr-Fe-As system. Single crystalline and polycrystalline samples with several starting compositions were synthesized under various heat-treatment conditions in evacuated quartz ampoules. Excess FeAs was added for the single crystal growth. Constituent phases and lattice parameters of the resulting samples were analyzed by the X-ray diffraction (XRD) method. Magnetic susceptibility measurement was performed by a SQUID magnetometer and electric resistivity was measured by the AC four-point-probe method. The single crystalline samples of CaFe₂As₂ with nominal compositions of Ca _1-xPr_xFe₄As₄ (x = 0.14, 0.20) showed superconducting transition at 35 K though superconducting volume fraction was small. On the other hand, all polycrystalline samples with nominal compositions of Ca_0.9Pr_0.2Fe₂As₂ and Ca _1-xPr_xFe₄As₄ (x = 0.14, 0.20) sintered under various conditions did not show any sign of superconductivity. A polycrystalline sample with a nominal composition of Ca_0.8Pr_0.2Fe_1.3As_1.8 sintered at 1200°C for 48 h showed superconducting transition at approximately 20 K and its superconducting volume fraction estimated from ZFC magnetization was large enough to suggest bulk superconductivity. This sample consists of 122 phase, FeAs, PrAs and some unidentified phases. Identifications of the superconducting phase and its chemical composition and optimization of synthesis procedures are ongoing. [1] Y. Kamihara et al., J. Am. Chem. Soc. 130 (2008) 3296 [2] S. R. Saha et al., arXiv Cond-mat 1105.4798v1

BaFe2As2 turns into a superconductor by doping either Co, K, or P. Many studies on thin film preparation have already been reported for Ba(Fe,Co)2As2 and (Ba,K)Fe2As2 by several groups. However, there had been no report on thin film preparation of BaFe2(As,P)2, although it has a high Tc up to 31 K, which is higher than Ba(Fe,Co)2As2. In this work, therefore, we studied the thin film growth of BaFe2(As,P)2. BaFe2(As,P)2 thin films were grown on LaAlO3 (LAO) or (La,Sr)(Al,Ta)O3 (LSAT) substrates by molecular beam epitaxy. All elements were supplied from solid sources charged in Knudsen cells; Ba, Fe, As, and GaP. The obtained thin films were characterized by x-ray diffraction (XRD), electron probe micro-analysis (EPMA), atomic force microscope (AFM), resistivity, and magnetic susceptibility. First, a non-doped BaFe2As2 thin film was grown on a LAO substrate. The thin film was single phase and c-axis oriented, and the temperature dependence of the resistivity showed an anomaly at about 140 K, as expected for a non-doped sample. Phosphorus flux was then supplied without changing any other growth parameters, and BaFe2(As,P)2 thin films were obtained. Only (00l) peaks of the BaFe2(As,P)2 phase were observed in the XRD patterns of these thin films, indicating that they were single phase and c-axis oriented. The lattice parameter c of the thin films decreased with increasing the vapor pressure of P, indicating that P was successfully substituted for As. Thin films were also grown with various vapor pressures of As, but the P content was almost independent to the vapor pressure of As and depended mostly on the vapor pressure of P. The transition temperature (Tc) varied according to the P content similar to bulk samples. The optimal thin film showed an onset Tc at 30 K with a sharp transition width of 1.5 K, and a smooth surface with a RMS roughness of 1.06 nm.

Introduction of nanorods into REBa2Cu3Oy (REBCO, RE denotes rare earth element) thin films is very effective to improve critical current density (Jc) in magnetic fields applied parallel to the c-axis. We have reported that critical current properties in ErBCO thin films with BaNb2O6 (BNO) inclusions prepared by pulsed laser deposition (PLD) [1, 2] were strongly affected by the deposition temperature (Ts), which is due to the change in morphology of nanorods by Ts. In this study, we focused on the Ts dependence of critical current properties in REBCO matrix with two different RE ions. BNO-doped YBCO and ErBCO thin films were grown at various substrate temperatures on SrTiO3 single crystal substrates by PLD using 4th harmonic Nd:YAG-laser. The vortex-Bose-glass-like behavior of irreversibility lines (ILs) emerged for the both BNO-doped YBCO and ErBCO films. The crossover filed (Bcr), which is defined as the border magnetic field between positive curvature and linear portions on ILs, increased with increasing Ts for BNO-doped YBCO. However, in the case of BNO-doped ErBCO, Bcr decreased with increasing Ts. Ts dependences of Jc-B characteristics were also quite different between YBCO and ErBCO system. We will discuss origin of the different Ts dependence of critical current properties from microstructures of nanorods and current-voltage characteristics. This work partially supported by Grants-in-Aid for Scientific Research (No. 23760020) from the Ministry of Education, Culture, Sports, Science and Technology of Japan, and the Research Foundation for the Electrotechnology of Chubu. [1] S. Horii et al., APL 93 (2008) 152506., [2] M. Haruta et al., Physica C 471 (2011) 944

High quality thin films are essential for many applications of superconductors. In our previous studies, we have succeeded in in-situ growth of superconducting NdFeAs(O,F) thin films by molecular beam epitaxy, which showed clear superconducting transitions at high critical temperatures. Here, we report on our study of further improving the thin film quality. We addressed two issues, both important particularly for junction fabrication. First, the surface roughness was significantly reduced because a very thin barrier layer has to be formed on it to fabricate a junction. Secondly, a new method for fluorine doping was developed, because a non-superconducting NdOF cap layer was necessary to form on the NdFeAs(O,F) phase in our previous studies, which was, however, an obstacle for junction fabrication. All the thin films were grown by molecular beam epitaxy. Fe, As, Ga, NdF3 and Fe2O3 charged in Knudsen cells were used as source materials. Here, Fe2O3 was used as an oxygen source. Some of the thin films were grown using O2 gas instead of Fe2O3. First, we investigated the correlation between the growth parameters and the surface roughness of the thin films grown on GaAs substrates. We found that the surface roughness depended strongly on oxygen flux. RMS roughness of a typical non-doped NdFeAsO thin film could be as high as 6.4 nm, which is obviously too large for junction fabrication. By carefully adjusting the oxygen flux, however, the RMS roughness reduced significantly. The optimal condition was very narrow, but a smooth surface with a RMS roughness of about 1 nm was successfully obtained. Thin films with a similarly small RMS roughness were also obtained for MgO and CaF2 substrates. We also developed a new method for fluorine doping in this study, namely in-situ fluorine annealing. A solid source, FeF3, was used to supply fluorine because we found that FeF3 supplies fluorine by thermal decomposition. After growing a non-doped NdFeAsO thin film on a MgO substrate, the thin film was exposed in-situ to fluorine flux at 700°C for 30 min. XRD analysis confirmed that the thin film remained a single phase after fluorine annealing. Both a- and c-axis lengths of the thin film decreased with fluorine annealing, and the resistivity showed a clear superconducting transition at about 40 K. The susceptibility measurement confirmed that fluorine was doped to the whole thin film because the volume fraction was almost 100%. In addition, the surface roughness of the F-annealed thin film was still sufficiently small for junction fabrication. We also prepared several thin films and confirmed that doping can be controlled by the annealing condition. These results indicate that a single phase, superconducting thin film can be obtained by in-situ fluorine annealing.

To obtain the MgB₂ with high critical current properties, the pre-treatment process of boron (B) powder such as mechanical milling and glycerin carbon (C) doping, and low temperature solid-state reaction of 550 or 600^oC were attempted to in situ powder-in-tube processed MgB₂/Fe wires. The mechanical milling and liquid glycerin treatment of the B powder were to reduce the grain size of the MgB₂ and to achieve homogeneous C incorporation into the MgB₂, respectively. The MgB₂ phase formation, the full width at half maximum, the critical temperature (T_c), the field dependence of critical current density (J_c) and the temperature dependence of upper critical field (H_c2) were evaluated for MgB₂ wires using pre-treated B powders. It was found that the MgB₂ phase formation in the wires using the 2 h milled B powder was more activated when compared to the wires using the un-milled B powder under the same heat-treatment conditions, because of the improved reactivity of the B powder after milling. The H_c2 and J_c of MgB₂ wire using the milled B powder were enhanced due to a smaller grain size and an increased volume of the superconducting phase. In addition, the low temperature heat-treatment also contributed to the enhanced superconducting properties by minimizing the grain growth for an increased grain boundary pinning when compared to a conventional high temperature heat-treatment of 800^oC. The MgB₂ wires heat-treated at 600^oC for 40 h using ball-milled and glycerin-treated B powder showed the highest transport J_c values at 4.2 K over the entire applied field regime. It was revealed that the grain boundary density was higher and the C substitution also occurred by a low temperature heat-treatment process, which led to a higher J_c. A solid-solid diffusion reaction with the pre-treated B powder resulted in a poor crystallinity with small grain size and an increased lattice disorder, which enhanced the H_c2 and J_c. Acknowledgments; This work was supported by the Power Generation & Electricity Delivery of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by Korea government Ministry of Knowledge Economy. Part of this work was also supported by the National R&D Program of the Ministry of Education, Science and Technology (MEST), Republic of Korea.

MgB₂ superconductor is one of candidate for practical use such as MRI magnets and cable at 10 â?" 20 K of liquid hydrogen temperature. The improvement of critical current density (Jc) is important for practical use. In the present study, the temperature dependence and the anisotropy of Jc of powder In tube (PIT) tapes have been investigated. The MgB₂ tapes were fabricated by conventional PIT method. We fabricated two kinds of tapes with pure MgB₂ and carbon doped tapes using the mixed powders of MgH₂ (99.8%), amorphous B(99.99%), nano size SiC(30nm) and the ethyl toluene. These powders were packed in the Fe tube of 6.3mmÏ? in outside diameter and 3.5mmÏ? in the inside diameter, and rolled using grove rolling and flat rolling. Each tapes heat treated at 630^oC for one hour in the Ar gas flowing. The XRD peak strength ratio of I₀₀₂/I₁₁₀ in the MgB₂ powder was about 0.7. On the other hand, the peak ratio of I₀₀₂/I₁₁₀ of tapes was almost 2.0 that the value was much higher than those of powder. It was suggested that MgB₂ in-situ tapes had an oriented texture in comparison with powder. From the results of the temperature magnetic field dependency of Jc in the pure and C-doped tapes, we found Jc strongly depended on the direction of the magnetic field in a low temperature and high magnetic field. We didnâ?Tt find the big difference between pure and C-doped tapes. We will also discuss about angular and temperature dependence of MgB₂ tapes.

PIT and the related processes have been developed intensively for fabricating MgB2 superconducting wires, because of their simplicity and potentially low manufacturing cost. However, since the PIT process is based on powder sintering, it is difficult to attain clean grain boundaries or biaxial crystal alignment, which is crucial for achieving high transport critical current density, Jc. Though recent Jc of PIT MgB2 wires well above 10,000 A/cm2 at 4.2 K and 10 T have been reported, the Jc values are still lower than those of the commercial metallic superconductors Nb-Ti and Nb3Sn. The PIT process needs to be improved or a different process needs to be developed for MgB2 to be used in practical applications. In this paper, we will propose a potential alternative. The MgB2 layer without biaxial crystal orientation deposited by an electron beam evaporation technique on an Al tape revealed excellent Jc of about 10 MA/cm2 at 10 K in self field. In contrast to YBCO, as neither biaxial orientation nor high deposition temperature are required for synthesis of the MgB2 layer, it is easy to thicken and to deposit the MgB2 layer on a round shape Al wire, and to enhance the deposition rate. We may expect fairly low production cost of the MgB2 coated conductor.

A centennial after the discovery of superconductivity by H.K. Onnes, more than never before, we can observe how much important it is in our lives (that its higher importance.). Images from the inner human body can be obtained with high resolution predicting as soon as possible diagnosis for some diseases, electrical energy can be transported for long distance almost or without any lost, people can be transported using trains that levitate is almost a reality. All of these examples are becoming a reality due a lot of researches developed looking for new superconductor materials. Then, in this work we have developed a study about how to obtain superconductor thin films of the Bi-system using microwave oven instead electrical furnace or conventional furnace in the thermal treatments. Although, thin films have been prepared in the way to obtain a certain kind of device such as Josephson junctions. Bridge junctions were prepared using optical microscopy and special tools developed in our labs. Electric and magnetic characterization were carried out using dc four probe and PPMS/VSM equipment at 4K-100 K indicating at least two superconducting transitions around 80 K and 100 K, associated to 2212 and 2223 phases, respectively. Structural measurements were also made using, X-ray diffractometry and scanning electron microscopy.

Superconductivity research involving applications have grew a lot in last few years, particularly in the transport of energy, had a promising advance over the years. This study aims to manufacture superconducting wires using the method Powder-In-Tube (PIT). Preliminary studies were done on pellets made of BSCCO powder obtained by polymeric precursors method, and after some results were prepared the superconducting wires. Characterization measurements were made among them the X-ray diffraction, electrical measurements by the method of dc four probes and type of microscopy (FEG-SEM) equipped with EDX to determine the morphology and chemical composition of the sample. X-ray diffraction results showed the formation of the desired phases of the BSCCO system such as Bi-2212, Bi-2223 and Bi-2234 phases. The electrical measurements have detected a sudden drop in electrical resistance of the wires showing the range of superconducting transition with good accuracy. It was possible to perform the calculation of current density in the wires made based on a criterion of 1 μV/cm were obtained and values of around 1,30 â?" 3,90 KA/m2. The values are consistent with the specifications of the samples and within the limits of our research.

Bi2223 thin films on SrTiO₃(100) fabricated by DC and RF sputtering have been investigated. According to results of the resistivity-temperature curvatures of as-grown film, the T_c was 72 K, and the temperature transition, Î"T, was also wide. These values were due to the low crystallinity, a kind of deviation in the composition of Bi2223. To obtain the Bi,Pb2223 phase, we annealed the material surrounded with Bi,Pb2223 pellets. On the other hand, T_c = 105 K was obtained in (Bi,Pb)2223 thin film after post-annealing. This T_c value was almost the same value as that of bulk materials of Bi,Pb2223. Furthermore, the transition was also sharper than that of as-grown films. These results suggest that the sputtering deposition and post-annealing process were very effective for obtaining high quality of Bi,Pb2223 thin films. We also checked grain alignments. We observed strong 00l peaks and a sharp quadrupole in X-ray θ-2θ and Ï?-scan measurements, suggesting that this film had a single phase of Bi,Pb-2223 and a biaxial orientation. The maximum J_c value of 3.3 x 10⁵ A/cm² at 77 K was obtained in Bi,Pb-2223 thin film. These J_c values were much higher than that of commercialized Bi,Pb-2223 tapes, suggesting that Bi,Pb-2223 tapes still have the possibility of improvement and enhancement of J_c by controlling the microstructures. Furthermore, from the J_c-B-angle properties, Bi,Pb2223 has very strong anisotropy. 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.

Bi2SexTe3-x system is known for years because of its excellent thermoelectric properties. Recently, its application as a topological insulator has been theoretically predicted and experimentally demonstrated. The bulk of topological insulator is insulating with an energy gap while the surface is metallic with a Dirac cone. This unique property has attracted great attention for investigation of novel quantum phenomena in the field of fundamental physics as well as for potential application in quantum computing. The compound Bi2Se0.3Te2.7 was synthesized in a pre-cleaned quartz ampoule using commercial starting materials of Bi, Te, and Se (5N purity). Further purification of the material was achieved through multiple translations in a horizontal zone-refinement system. The final crystal growth was carried out using Bridgman technique. X-ray diffraction (XRD) techniques were used to characterize the structure of the Bi2Se0.3Te2.7 crystal. The cell parameters as well as the composition of the ternary were measured from the Bragg-Brentano geometry x-ray diffraction measurements. The crystal structure was identified by both Phi-Scan and pole figure measurement. Rocking curve and reciprocal space mapping were also used to get the details of interested crystal planes. The results will be presented and thin film topological insulator using this Bi2Se0.3Te2.7 crystal through pulsed laser deposition will be discussed. Acknowledgment: The work at Hampton University was supported by the National Science Foundation (HRD-0630372 and HRD-1137747).

REBa₂Cu₃O_y (REBCO) thin films including BaMO₃ (BMO, M = Zr, Sn) nano-rods have been reported by many groups. The BMO nano-rods improve the performance of REBCO thin films in magnetic fields. However, they are effective only for magnetic fields applied parallel to the c-axis of the REBCO. In this study, to enhance J_c in the magnetic fields for all applied angles, we fabricated multilayered films consisting of Sm_1.04Ba_1.96Cu₃O_y (SmBCO) and BaSnO₃ (BSO) doped SmBCO layers on LaAlO₃ (LAO) substrates (multilayered SmBCO film) by PLD method and measured the magnetic field angular dependence of J_c. It was reported that average diameter of BSO was about 10 nm in many studies. Therefore, the thickness of each layer was fixed about 10 nm to form BSO nano-dots. We fixed the number of layers to 32 layers and the total thickness of the film was about 320 nm. We introduced BSO by alternating BSO target and pure SmBCO target in depositing BSO doped SmBCO layers. Typical T_c and J_c of the multilayered SmBCO film were 91.6 K and 3.39 MA/cm² at 77 K in self-field, respectively. These values were higher than those of a BSO doped SmBCO film without multilayered structure (SmBCO+BSO film) in which BSO was doped by the same method with the multilayered SmBCO film. The typical T_c and J_c of the SmBCO+BSO films were 90.4 K and 2.20 MA/cm², respectively. In magnetic field angular dependence of J_c (B = 1 T, 77 K), the multilayered SmBCO film had peaks of J_c for B//c and B//ab. The J_c values were 1.15 MA/cm² (B//c) and 1.53 MA/cm² (B//ab). The minimum value of J_c was 0.92 MA/cm². On the other hand, the SmBCO+BSO film had a peak of J_c only for B//c. The J_c value was 1.23 MA/cm² (B//c) and the minimum value of J_c was 0.51 MA/cm². Here, we define a as (J_c^MAX - J_c^min) / J_c^MAX to express the anisotropy of J_c. The a values (B = 1 T, 77 K) are 0.399 for the multilayered SmBCO film and 0.585 for the SmBCO+BSO film. This result shows that the J_c anisotropy of the multilayered SmBCO film was smaller than the SmBCO+BSO film. In From TEM images, there were BSO nano-dots dispersed at random in the multilayered SmBCO film. It seems that the BSO nano-dots act as 3D-APC at low magnetic field. This work was partly supported by a Grant-in-Aid for Scientific Research (19676005 and 23226014).

Recently, we have reported that YBCO thin films are grown by PLD method using a forth harmonic Nd:YAG pulsed laser and superconducting properties of the films are comparable to those of films prepared by conventional excimer-PLD method. We concluded that the Nd:YAG-PLD method is promising for coated conductor process. In order to enhance J_c of YBCO thin films under magnetic fields, artificial pinning centers (APC) are doped into the films. For example, BaMO₃ (BMO, M=Sn, Zr etc.) is useful as APC due to a formation of nano-rods within the films. However, excess doping of APC reduces the superconducting properties. Therefore, there is an optimal content to maximize the flux pinning performance. Based on above, we used combinatorial Nd:YAG-PLD (C-PLD) method. The C-PLD is an efficient way to screen the optimal APC content, by making it possible to create a film with continuously changing composition across a single substrate. In this study, we investigated a quick screening of the optimal content of APC and an exploration of new APC. At first, we optimized the content of BaSnO₃ (BSO) by using the C-PLD method. As a result, we concluded that the optimal content of BSO was around 3.2 vol.%. This value was similar to an optimal value obtained by other groups who prepared their films by using BSO-YBCO mixed targets. From the cross-sectional TEM image of the optimal BSO-doped film fabricated by C-PLD, we confirmed the presence of nanorods made of BSO. Next, in order to explore new APC materials, we doped Ba-based oxides such as Ba₃Cu₃In₄O₁₂ (334) and BaTbO₃ (BTO) into YBCO thin films by the C-PLD method. We measured J_c in magnetic fields at various temperatures. At 77 K, the J_c in magnetic fields at 77 K were not so high compared with the BSO doped YBCO films and a pure-YBCO thin film. However, the J_c-B curves were improved with decreasing measurement temperature and became higher than that of the pure-YBCO thin film. This means that the 334 and BTO doped films include small precipitates made of 334 and BTO and the small precipitates would become effective for flux pinning at low temperatures since the coherence length becomes small at low temperature. This work was partly supported by a Grant-in-Aid for Scientific Research (19676005, 20686065 and 23226014).

A superconducting YBaCu3O7-δ (YBCO) thin film has been produced at 890 oC substrate temperature by pulsed electron deposition (PED) which is an essential and low-cost physical deposition technique of high quality superconducting films. For production of superconducting film, Ni-W textured substrate and SrTiO3 (STO) single buffer layer (instead of multi-layer buffer to reduce the number of processing steps and simplify manufacturing) have been used. The Ni-3%W alloy tapes were used as the metal substrate. The STO targets used in the experiment were prepared by a conventional sintering process. A commercially available YBCO targets was used for deposition of superconducting layer. The texture information was obtained from the XRD analysis. The crystalline structures of Ni-W substrates and YBCO/STO/Ni-W films have been investigated by x-ray diffractometer (XRD). XRD patterns of the substrates consist of (200) and (220) peaks, but with annealing the intensity of (220) peak which means deformation texture decreased, and the intensity of (200) peak which means cube texture increased. It indicates that most of the (200) texture was formed during annealing. The X-ray diffraction analysis suggested that the STO buffer layer and YBCO superconducting layer consisted of randomly oriented polycrystallites. The epitaxial growth of (001)-oriented STO on Ni-W surface was inhibited by the formation of (111) NiO at the oxide/metal interface. The surface morphology and microstructure of YBCO/STO/Ni-W thin film have been characterized with atomic force microscope (AFM) and Scanning Electron Microscope (SEM) .The root-mean-square (Rms) and average (Ra) surface roughness are about 13 nm and 9.6 nm, respectively.

High-Jc GdBa₂Cu₃O_7-δ (GdBCO) coated conductors (CCs) was produced by the RCE-DR (Reactive Co-Evaporation Deposition & Reaction) process on LaMnO₃-buffered IBAD MgO template. In this study, we have carefully analyzed the microstructural features of GdBCO CCs, including grain boundaries, growth morphologies, interfacial reactions and etc. From these analyses, a large number of inclusions, identified as Gd₂O₃ second phase particles, were clearly observed in the GdBCO matrix. Even though these particles are potential pinning sites, they are not very effective for the improvement of in-field critical current density ( J_c), which is attributed to rather large sized inclusions. Further details of microstructure analyses of GdBCO CCs by the RCE-DR process will be presented and their effects on the field dependency and anisotropy of J_c, J_c(B,θ) values will be discussed.

Recent magnetic orientation techniques enable alignment of three crystallographic axes of substances with structurally low symmetry by usage of a modulated rotating magnetic field (MRF). The technique using MRF is a candidate of production processes of high critical temperature cuprates with orthorhombic structure. Furthermore, another advantageous point in the magneto-scientific process is a room-temperature process without the epitaxial growth. In practice, our group has achieved tri-axial orientation of RE₂Ba₄Cu₇O_15-y (RE247, RE=Y) powder in 12T of MRF in epoxy resin at room temperature[1]. Therefore, the magnetic orientation technique is fascinated from the viewpoints of practical production of superconducting bulks and cables. However, in the case of twinned REBa₂Cu₃O_y (RE123) superconductors, inplane magnetic anisotropy probably disappears in a grain level by the introduction of twin microstructure. In the present study, we focus on a twinned Er123 superconductor, and studied the effects of pulverization time and applied conditions of MRF on the degrees of tri-axial orientation. Consequently, pulverization using ball-milling[2] is found to be a useful process for the achievement of bi-axial orientation of RE123 powders. This work was partly supported by the Asahi Glass Foundation, and the Noguchi Institute. [1] Fukushima, Horii et al., Appl. Phys. Express 1 (2008) 111701. [2] Horii et al., Supercond. Sci. Technol. 25 (2011) 055001.

Recent advances in high temperature superconductor (HTSC) synthesis have lead to the creation of a raft of novel morphologies using biopolymers as a means to template crystal growth. In particular, the morphologies created using branched glucan polymers such as dextran have been shown to directly template the crystal growth during calcination to only the areas of the underlying template and nowhere else. We have used dextran to create novel spherical morphologies of yttrium barium copper oxide (YBCO) and bismuth strontium calcium copper oxide (BSCCO) phases. This is the first time that isotropic multi-micron-sized spherical particles of YBCO have been synthesised. Interestingly, despite the non-classical macroscopic form of the superconducting particles, they are crystallographically identical to bulk single crystals. We are able to achieve this morphology through use of beads of a commercially available gel filtration matrix (Sephadex). The beads of Sephadex range from 20-50 μm in diameter and we have shown that the YBCO phase can be templated directly by the beads which are then calcined to leave the perfect YBCO replica. In addition, owing to the outgassing of the dextran during calcination, the spheres are hollow and porous which opens up the possibility for back-filling with a second functional material, thereby allowing us the possibility to explore complementary physico-chemical synergistic effects. On a more immediate level, this novel synthetic route shows a great deal of promise in the rational design of non-classical superconductor morphologies, which may allow us to better address the existing problem of poor grain boundary alignment, which leads to a decrease in critical current and is presently one of the main limiting factors in the application of YBCO-based superconductors.

Nanoscale defects or imperfections (chemical or structural) within YBa2Cu3O7-δ (YBCO) superconductor play a central role in â?opinningâ? or immobilizing magnetic flux-lines (or vortices), the motion of which induces dissipative current and consequently, limits the critical current density, Jc. Hence, we have conducted extensive research to improve, control, and optimize the pinning and Jc characteristics for the wide range of magnetic fields and temperatures as well as for the field orientations via controlled introduction of various nanoscale defects in the superconducting films. The compositions of these nanoscale defects range from simple rare-earth (RE) oxides and Ba-based perovskites (BaMO3, M=Zr, Sn, etc.) to double perovskite rare-earth tantalates and niobates (Ba2RETaO6 and Ba2RENbO6). In this talk, we will discuss how density, morphologies, and composition of these nano-engineered defects influence the pinning efficacy and Jc in various temperature, field and angular regimes. Detailed microstructural, and superconducting properties of the films having such nano-engineered defects will be presented. This research was sponsored by the U.S. DOE Office of Electricity Delivery and Energy Reliability - Superconductivity Program for Electric Power Systems Advanced Cables and Conductors.

An important challenge in applications of YBa2Cu3O7-x (YBCO) films is achieving maximum critical current jc under an applied magnetic field. The introduction of artificial pinning centers whose dimensions are controlled is indispensable to achieve high jc in the YBCO films. Self-assembled nanorods of c-axis aligned BaZrO3, BaSnO3 etc. in the films are very effective pinning centers for increasing jc in magnetic fields. 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 double-kink configuration, which throws a vortex segment onto an adjacent nanorod. Once double-kink excitation has nucleated, the kinks will run off in the opposite direction, 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 centers for further increasing jc. The possible method is the insertion of the non-superconducting PrBa2Cu3O7-x layers into the YBCO film including the BaZrO3 or BaSnO3 nanorods at appropriate intervals, resulting in the blocking of kink parts by the non-superconducting layers. Another method inserts the YBCO layer with Y2O3 nanoparticles instead of the PrBa2Cu3O7-x layers. The kink parts can be blocked by the nanoparticles as well. Owing to these techniques, the higher jc values were achieved and the more isotropic jc angular dependences were observed.

In-field current transport properties of BaHfO₃ doped PLD-GdBCO coated conductors (CCs) have been studied as a function of temperature and magnetic field. It has been shown that BaHfO₃ addition allows us to improve practical performance of CCs significantly. Not only the improvement of critical current density, Jc, itself but also over all Jc can be enhanced effectively because thickness dependence of Jc is quite constant at least up to around 3 μm in thickness. Therefore, in-field critical current, Ic, reached practical value of 90 A/cm-w even at 77 K and 3 T in c-axis direction. Angular dependence of Ic also shows isotropic behavior, namely the minimum Ic is still kept at 80 A/cm-w in the all field angles. Performance at lower temperature is more promising to wide variety of practical applications. If we compared with pure GdBCO CCs, in-field Ic value has reached almost double than that of 600 A/cm-w (77K, s.f.) class CCs. The enhancement can be confirmed over a wide range of operation conditions even at low temperatures around 20 K. Detailed comparison with pure- and BZO-doped- GdBCO will also be discussed at the presentation. This work was supported by New Energy and Industrial Technology Development Organization (NEDO) as the Project for Development of Materials & Power Application of Coated Conductors, M-PACC and JSPS: KAKENHI (20360143).

We numerically compute the critical current of a layered extreme type-II superconductor in tilted external magnetic field with respect to randomly located line pins oriented perpendicular to the layers. A film geometry is assumed. Intra-layer vortex dynamics is computed in parallel by assigning each layer to a dedicated central processing unit (CPU). Inter-layer vortex dynamics is achieved through communication between the CPU's. We test for the existence of a threshold film thickness, below which line pins are no longer effective. This cross-over is studied for two characteristic arrangements of c-axis line pins: frozen gas and frozen liquid. We seek to minimize the threshold film thickness and to maximize the critical current.

Precipitates in YBCO thin films yield a significant improvement of the magnetic flux pinning properties and a reduced anisotropy of the critical current density in coated conductors. Therefore, controlling precipitates in size and density, but also understanding their associated strain fields is of crucial importance for applications. YBa₂Cu₃O_7-δ (YBCO) superconducting films containing nanosized BaHfO₃ precipitates were prepared by Metal Organic Deposition (MOD). Strain-fields of crystal defects associated with these precipitates were measured quantitatively by High-Angle Annular Dark-Field (HAADF) aberration-corrected Scanning Transmission Electron Microscopy (STEM). Experimental images were evaluated with respect to the strain tensor components by a geometrical phase analysis and a model-based parameter estimation method. Interfacial BaHfO₃ nanoinclusions grow quasi-epitaxially on SrTiO₃ and LAlO₃ substrates. A coexistence of coherent and semi-coherent BaHfO₃ precipitates was found in the YBCO matrix for films annealed at 700-800°C. Such films yield a maximum pinning force of 6 GN/m³ and a critical current density of >5 MA/cm² at 77 K and zero field. The nanosized BaHfO₃ precipitates are strongly facetted along {110} and {100} crystallographic planes. Arrays of misfit edge dislocations of type b = [1 1 0] and b = [1 0 1/3] were identified at the BaHfO₃-YBCO interfaces and their corresponding strain-fields could be directly imaged by HAADF-STEM. Y124 intergrowth was found to terminate/nucleate at the intersection of facets surrounding the BaHfO₃ particles yielding a nanostrained region. The possibilities of the applied techniques and their limitations with respect to strain levels will be discussed.

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 were shown to be an excellent low cost processing option with huge isotropic pinning forces. We will report on YBCO-TFA nanocomposites with BaZrO3, Y2O3, BaCeO3 and Ba2(TaY)O6 second phase nanoparticles, which evidence that these nanocomposites do experience a highly effective novel pinning mechanism, coupling superconducting pairing to lattice strain. The nanoscale strain evaluated from X-ray diffraction line broadening, correlates with the enhanced pinning forces and a vanishing anisotropy of the vortex pinning landscape is achieved. Still we demonstrated that the YBCO intrinsic mass anisotropy is preserved as was evidenced by high field Hc2(T) measurements. TEM investigation evidences a ramified shape of inhomogeneously distributed nanostrained regions associated to a highly dense defect structure (mainly extra Cu-O chains and partial dislocations interacting with twin boundaries). We propose that the inhomogeneously distributed nanostrained regions locally suppress Cooper pair formation according to a Bond Contraction Pairing model. In addition, the progress achieved in the use of colloidal solutions with pre-formed size-controlled nanoparticles will be discussed. * We acknowledge support from MAT2008-01022, Nanoselect, SGR-770, and XaRMAE

Chemical solution deposition (CSD) is becoming 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. In this presentation we will report on our recent advances concerning the analysis of advanced processing methodologies of YBCO films and bilayers grown through the Trifluoroacteate approach on top of LAO single crystals and different sorts of doped CeO2 cap layers grown either on YSZ single crystals or ABAD metallic substrates. We have particularly used in-situ resistance measurements to investigate the influence of processing parameters on nucleation, growth and oxygenation. Ag salts addition to the metallorganic precursors has appeared as a powerful route to tune the balance among competing nucleation orientations as well as to modify films planarities. Finally, we will show that the YBCO films mesostrain, as determined from X-ray diffraction breath analysis, influences the self-field percolating critical currents. We correlate this finding to the grain boundary healing process occurring during the coalescence of islands nucleated through the Volmer - Weber mechanism.

Solution deposited-YBa₂Cu₃O₇ (YBCO) nanocomposites appear as a very robust approach to prepare low cost coated conductors with strongly improved superconducting properties. Recently, we have prepared novel TFA-YBCO nanocomposites with spontaneous segregation of double perovskite Ba₂YTaO₆ (BYTO) secondary phase. X-ray diffraction and STEM studies indicated the presence of high percentage of randomly oriented BYTO nanoparticles within the epitaxial YBCO matrix that produce non homogeneous interfaces and therefore localized nanostrained areas. This non-homogeneous strain is suggested to be closely related with the enhancement of isotropic vortex pinning contribution. Therefore, controlling nanoparticle size and distribution emerge as a key challenge to tune vortex pinning landscape. We have also studied the morphological and chemical composition evolution of BYTO phase during the TFA-YBCO growth from quenched studies. Angle-dependent critical current measurements indicate that these nanocomposites present strong isotropic pinning contribution. This study supports the potential of this novel phase to act as an effective pinning center for YBCO coated conductors. We acknowledge the financial support from MICINN(CONSOLIDER, MAT2008-01022)

We have recently developed a new high throughput process, named RCE-DR (Reactive Co-Evaporation Deposition & Reaction), for GdBa2Cu3O7-δ (GdBCO) coated conductors (CCs) on the IBAD template with the architecture of LaMnO3/Epi-MgO/IBAD-MgO/Y2O3/Al2O3/Electro-polished Hastelloy or SUS substrates. Now, SuNAM is routinely producing a long-length (piece length > 500 m) GdBCO CCs having the critical current over 300 A/cm-width at 77 K in self-field with very high throughput speed exceeding 360 m/h (4 mm width equivalent). In this process, an amorphous GdBCO precursor film is first deposited by RCE on the IBAD template using an electron gun at room temperature, and then passed through a tube furnace held at 860C to grow 1.0~1.5 μm thick GdBCO films, where the oxygen pressures within the tube furnace are controlled relatively low (~10-2 mTorr) and high (~100 mTorr) in series. The amorphous precursor film passing through the tube furnace is fast converted into the GdBCO film within a minute by a peritectic recombination reaction of Gd2O3 + liquid formed at an intermediate stage, which can be well understood on the basis of experimentally determined stability diagram for the GdBCO phase. In this talk, I will present the following subtopics related to GdBCO CCs produced by the RCE-DR process for a discussion; the trapping mechanism of biaxially textured Gd2O3 nanoparticles within the GdBCO matrix, the flux pinning characteristics of GdBCO CCs including Gd2O3 nanoparticles, and recent efforts for the development of stronger pinning sites.

One important drawback in the production of YBCO superconductor tapes by TFA route using Chemical Solution Deposition is the presence of high quantities of fluorine in the metaloorganic precursors and, as a consequence, in the evolved gases produced during the thermal treatment. Thus, efforts have been devoted to reduce, as much as possible, the fluorine content in the precursor solution keeping the minimum amount of fluorine to form the BaF2 after the initial decomposition step. This is because the evacuation of HF controls the growth rate in the TFA route. Here, we will present our recent results in the complete elimination of the fluorine in the precursor chemical solution, preparing â?ofluorine freeâ? solutions leading to fully epitaxial YBCO layers with high critical temperatures (Tc=91.3K) and critical current densities (Jc (77K)= 3.5MA/cm2). We have used an in-situ growth resistivity technique to characterize the growth rate as function of the process parameters and results have been compared to those of the TFA route. A deep understanding of the thermal decomposition process has enabled us to avoid the undesired formation of barium carbonate and the preservation of the stoichiometry by controlling the copper sublimation.

Vortex physics has been a topic of interest since the discovery of the oxide high temperature superconductors (HTS). The complex vortex phenomena in these materials arise from the strong influence of thermal fluctuations, which is a consequence of the small superconducting coherence length (ξ) and the large crystalline anisotropy (γ). Although this behavior contrasts with the simpler behavior in conventional low temperature superconductors, there is no sharp boundary between them. The discovery of the iron-based superconductors provided a chance to "bridge the gap" by studying vortex matter in a new family of materials with broad ranges of Tc and γ, where the small ξ in some of them results in large fluctuation effects similar to those found in the oxide HTS. On the other hand, the multi-band superconductivity in the Fe-based compounds introduces a new level of complexity, requiring a re-evaluation of the concept of anisotropy in the vortex behavior. Valuable information can also be obtained from MgB2, a chemically simpler two-band superconductor where ξ and γ can be modified by doping. I will present an overview of the current understanding of vortex pinning mechanisms and regimes in oxide, iron-based and MgB2 superconductors, which may arise from correlated or uncorrelated disorder, or from mixed pinning landscapes with combinations of both. Then I will discuss our recent studies of vortex matter in thin films and single crystals of these materials. I will present a comparative analysis of the vortex dynamics and the characteristics of the depinning excitations. This work was supported by the US DOE, Office of Basic Energy Sciences, Materials Sciences and Engineering Division.

Since the discovery of superconductivity in LaFeAs(O,F) with T_c ~26 K, various kinds of iron-based superconductors (IBSs) have been discovered. Not only polycrystalline samples of IBSs, but also single crystals became available soon after the discovery of each compound. Even wires and tapes of IBSs have been fabricated. We have grown high-quality single crystals of 122- and 11-type IBSs, and characterized their superconducting properties in terms of magneto-optical imaging (MOI) technique. A Bi-doped garnet film with in-plane anisotropy is used to visualize local variation of stay fields from the superconducting sample. In the case of 122-type IBSs, the homogeneity of the crystal is generally good except for the presence of mechanical cracks, which are sometimes invisible from the sample surface [2]. In Fe(Te,Se) single crystals, a long-term annealing is essential to make a uniform superconducting sample with an appreciable critical current density [3]. We also apply MOI to characterize polycrystalline samples, wires, and tapes of IBSs. In the case of polycrystalline samples, intergranular current is much smaller than the intragranular current, which can be inferred by the fact that the roof-top field profile covering the whole sample is weak or negligible [4]. We also fabricated superconducting wires of (Ba,K)Fe₂As₂ using powder-in-tube method and FeSe tapes by the diffusion method. In (Ba,K)Fe₂As₂ wire, transport critical current density, J_c, of 1.3x10⁴ A/cm² at T = 4.2 K under self-field is achieved [5]. However, MOI shows presence of grains with much larger intragranular current density reaching to 5x10⁵ A/cm². In FeSe tape, transport J_c is still low with ~ 600 A/cm² at T = 4.2 K under self-field. However, here again, intragranular J_c estimated by MOI reaches to 1.3x10⁴ A/cm² [6]. In layered superconductors, the shape of the crystal is a thin plate. When the magnetic field is applied parallel to the c-axis, a novel turbulent feature of vortices have been observed at the interface of vortices and anti-vortices in YBa₂Cu₃O_7-δ [7]. Our MOI allows us to observe a very similar phenomenon also in Co-doped BaFe₂As₂, which is the first report among any other superconductors other than 123-type cuprate superconductors. [1] Y. Kamihara et al., J. Am. Chem. Soc. 130 (2008) 329. [2] Y. Nakajima, T. Taen, and T. Tamegai, J. Phys Soc. Jpn. 78, 023702 (2009). [3] T. Taen, Y. Tsuchiya, Y. Nakajima, and T. Tamegai, Phys. Rev. B 80, 092502 (2009). [4] T. Tamegai, Y. Nakajima, Y. Tsuchiya, A. Iyo, K. Miyazawa, P. M. Shirage, H. Kito, and H. Eisaki, Physica C469, 915 (2009). [5] Q. P. Ding, T. Prombood, S. Mohan, Y. Nakajima, T. Tamegai, submitted. [6] Q. P. Ding, S. Mohan, Y. Tsuchiya, T. Taen, Y. Nakajima, and T. Tamegai, submitted. [7] V. K. Vlasko-Vlasov,et al., Phys. Rev. B 56, 5622 (1997). [8] S. Mohan, Y. Tsuchiya, Y. Nakajima, and T. Tamegai, to be published in Phys. Rev. B.

The recently discovered iron-based superconductors attract great interest in both their fundamental properties and potential applications. Among various iron-based superconductors, doped BaFe₂As₂ is thought to be one of the key materials for application, because of rather high T_c ~ 23 K, high H_c2 (0)~50-70 T, phase stability, and large flexibility in doping. It is known that the parent compound, BaFe₂As₂, is not superconducting: superconductivity appears either by carrier doping or by adopting external pressure. From the application point of view, we have to clarify the effect of element substitution on the critical current properties. It is likely that the substitution causes a local disturbance in the superconducting electron system. Such a local disturbance may provide pinning force, if it is strong enough to cause a local minimum in the condensation energy. However, the disturbance is too strong, it will weaken the superconductivity of entire matrix, and pinning effect is largely suppressed. So we have to control the amount of substitution to obtain the best performance of pinning effect. Hence, in the present study, we studied the magnetization behavior of BaFe₂As₂ single crystals with element substitution at different sites; i.e. substitution of Fe by Co (Co-substituted Ba122) and As by P (P-substituted Ba122). In particular, we investigated influence of substitutions on the critical current characteristics and flux creep behavior. All Co-substituted Ba122 crystals showed a very pronounced â?opeak effectâ? irrespective of doping state. On the other hand, J_c-B curve of P-doped Ba122 is well characterized by a monotonic power-law decay of J_c. From the analysis of pinning force, the peak effect of Co-substituted Ba122 is likely to be caused by so-called delta-T_c pinning mechanism, which is inactive in P-doped case. We also observed a depression of normalized flux creep rate in Co-substituted Ba122 at high magnetic field where the delta T_c pinning is thought to be active. In the last, we will discuss about the difference in the pinning phase diagrams between Co- and P-substituted Ba122. This work supported by Japan Society for the Promotion of Science (JSPS) through â?oFunding for World-Leading Innovative R&D on Science and Technology (FIRST) Programâ?.

Numerous experiments on the layered cuprate superconductors revealed that their critical current properties strongly depend on the interlayer distance of superconducting CuO2 plane, i.e. thickness of the blocking layer, which directly affects the electromagnetic anisotropy. On the other hand, a large number of Fe-based superconductors have been discovered since 2008 and they have large variety in the crystal structure at the blocking layer. The interlayer distance of Fe-plane, d(Fe-Fe), varies from approximately 0.4 nm for the 11 system to above 2 nm for the layered iron arsenides with very thick blocking layers. In the present study, we have studied the relationship between the d(Fe-Fe) and critical current properties of Fe-based superconductors to clarify how d(Fe-Fe) affects electromagnetic anisotropy. A series of sintered bulk samples with various d(Fe-Fe) were prepared by the solid-state reaction. Superconducting properties were studied by a SQUID magnetometer. Since the Tcâ?~s of the samples were different, such as 27 K for (Fe2As2)(Sr4V2O6), 42 K for (Fe2As2)(Ca4(Mg,Ti)3O8), 33 K for (Fe2As2)(Ca5(Sc,Ti)4O11), 32 K for (Fe2As2)(Ca6(Al,Ti)4O12) and 34 K for (Fe2As2)(Ca8(Mg,Ti)6O18), their magnetization properties were compared under the normalized temperature by Tc. Powder X-ray diffraction analyses revealed that d(Fe-Fe) values of these five compounds were 1.57, 1.67, 2.06, 2.27 and 3.00 nm, respectively. We found following two features of Fe-based superconductors through the systematic experiments. One is that the intragrain critical current density tends to decrease with an increase in d(Fe-Fe) and this tendency is more prominent at high temperatures. The other is that the irreversibility field dramatically decreases with an increase in d(Fe-Fe) as in the case of cuprate superconductors. However, irreversibility fields of the Fe-based superconductors are higher than that of the layered cuprates when we compare them at the same interlayer distance between superconducting layers. This means that the electromagnetic anisotropy originated from the layered crystal structure is intrinsically smaller in the Fe-based superconductors than in the cuprate superconductors.

We have grown NdFeAs(O,F) (Nd-1111) and BaFe2(As,P)2 (P-Ba122) thin films by molecular beam epitaxy (MBE). LnFeAs(O,F) (Ln=lanthanide) possesses the highest critical temperature Tc among the iron-based superconductors discovered to date, which makes these materials very attractive for applications. Previously, we have succeeded in an in-situ growth of superconducting thin films of Nd-1111, but there remained a large room for improvement. Here, we report on our recent progress, in particular about the fluorine doping method and on the improvement of the surface roughness. In our previous study, fluorine was doped to the Nd-1111 phase by forming a NdOF layer on top of it. Superconducting thin films with Tc values comparable to bulk samples can be readily obtained with this method. However, a fluoride layer exists always on top of the Nd-1111 phase. This will be an obstacle for some applications, for instance, planar-type junctions. Here, we report on an alternative method of fluorine doping. We have exposed the thin films in-situ to fluorine flux after growing a non-doped NdFeAsO layer. FeF3 was used as the source of fluorine, which releases fluorine at elevated temperatures. Fluorine was successfully doped to NdFeAsO with this new method, and a clear superconducting transition was observed. The dependence of the c-axis length and Tc on the fluorine flux, annealing temperature, and annealing time were thoroughly studied, which showed that doping can be well controlled by the annealing condition. We also carried out a detailed study on the correlation between the surface roughness and the growth parameters, because the surface roughness is important for fabricating superconductor junctions. We found that the surface roughness depends strongly on O2 flux. While the window for optimal O2-flux was very narrow, the surface roughness can be significantly reduced by adjusting the O2 flux, and thin films with RMS roughness of about 1.0 nm were reproducibly obtained. We also report on the thin film growth of P-Ba122. Studies on single crystals show that P-Ba122 possesses a relatively high Tc up to 31 K with a very good crystalline quality, but there had been no report of thin film preparation of this superconductor so far. After optimizing the flux carefully, we obtained single phase BaFe2(As1-xPx)2 thin films with various x on LaAlO3 and (La,Sr)(Al,Ta)O3 substrates. The x dependence of the c-axis length as well as the temperature dependence of resistivity for different x showed a good agreement with the data reported for polycrystalline samples. The thin film with the highest Tc showed an onset transition at 30 K and zero resistance at 28.5 K with a RMS roughness of 1.06 nm.

The recent discovery of high-Tc superconductivity in iron pnictides pioneered by Kamihara et al. [1] has triggered worldwide research efforts in both science and technology. Ever since the discovery, many articles have been published reporting the physical properties of these compounds in order to elucidate the superconducting mechanism. However, most of the measurements have been performed on bulk polycrystals. High-quality epitaxial thin films as well as high-quality sizable single crystals are needed to obtain more reliable experimental results. There are several families in Fe-based superconductors: Fe(S,Se,Te) (FeCh-11, Ch = chalcogen) with Tc ~ 15 K, LiFeAs (FeAs-111) with Tc of 18 K, AEFe2As2 (FeAs-122, AE = alkaline-earth element) with the highest Tc of 38 K, and REFeAsO (FeAs-1111, RE = rare-earth element) with the highest Tc of 55 K. Among these families, the growth of highest-Tc FeAs-1111 films may be the most challenging. Kawaguchi et al., however, reported success in growing superconducting NdFeAs(O,F) by molecular beam epitaxy (MBE) [2]. In their process, a cap layer of NdOF plays an important role in achieving superconductivity since it supplies F to a film by diffusion and also blocks the desorption of F. In this presentation, we report the MBE growth of high-Tc and high-Jc SmFeAs(O,F) films. In our process, non-superconducting F-free SmFeAsO films were first prepared and subsequently converted to superconducting films by F diffusion from an overlayer of SmF3. By this simple process, the films on CaF2 showed record high Tc, namely, Tcon (Tcend) = 57.8 K (56.4 K). Furthermore these films showed high critical current density over 1 MA/cm2 in the self-field at 5 K. On the other hand, the films grown on LaAlO3(001) showed Tcon (Tcend) = 52 K (48.5 K), slightly lower than the bulk Tc. The difference in the structure and superconductivity between SmFeAs(O,F) films grown on fluoride and oxide substrates is discussed. [1] Y. Kamihara et al., J. Am. Chem. Soc. 130 (2008) 3296. [2] T. Kawaguchi et al., Appl. Phys. Lett. 97, 042509 (2010).

Since the discovery of iron-based superconductors, epitaxial thin films has significantly advanced potential device applications and the understanding of the fundamental physical properties of these new superconductors. In particular, we recently have grown high quality epitaxial Co-doped BaFe2As2 (Ba-122) thin films using template engineering which generated c-axis aligned, self-assembled, second phase nanorods. For high field applications of Co-doped Ba-122 thin films, very high critical current density (Jc) and irreversibility field (Hirr) are indispensable along all crystal directions. On the other hand the development of superconductor-based devices such as junctions, SQUIDs and integrated circuits requires multilayer deposition with atomically smooth and uniform barriers. We have achieved success in both aims. We show that artificially engineered undoped Ba-122 / Co-doped Ba-122 compositionally modulated superlattices structures produces ab-aligned nanoparticle arrays and self-assembled c-axis aligned defects that combine to produce very large Jc and Hirr enhancements. We also demonstrate atomically sharp interfaces in a structurally modulated SrTiO3 (STO) / Co-doped Ba-122 superlattice that can serve as the basis for electronic uses of Co-doped Ba-122. Success in superlattice fabrication involving pnictides will serve to spur progress in heterostructured systems exhibiting novel interfacial phenomena and device applications.

The recent discovery of high-Tc superconductivity in iron-based superconductors pioneered by Kamihara et al. has attracted great scientific and technological interests. From a technological perspective, one of the urgent subjects is to explore the possibility of electronics application using Fe-based superconductors. Epitaxial thin films are highly desirable to develop superconducting devices, such as Josephson junctions and SQUIDs. Especially, one can envisage a large scope for Fe-based superconductor electronics because of the chemical and structural compatibility with III-V compound semiconductors. This compatibility together with well-developed III-V semiconductor technologies may produce ideal superconductor/semiconductor multilayer structures with enhanced functionalities. Toward such purposes, it is necessary to establish the technique to grow high-quality epitaxial thin films. In this presentation, we report on our molecular beam epitaxy (MBE) growth of superconducting (Sr,K)Fe2As2 and (Ba,K)Fe2As2. Several articles have already been published on the PLD growth of high-quality FeAs-122 films with the ThCr2Si2 structure, but almost all are on the growth of Co-doped 122 films (Sr(Fe,Co)2As2 or Ba(Fe,Co)2As2) with rather low Tc (� 25 K), except for one report on the growth of (Ba,K)Fe2As2 with Tc ~ 40 K, in which Ba-Fe-As amorphous precursor films were annealed at 700°C under the vapor of K in a quartz tube[1]. There has been no report on in situ growth of high-Tc K-containing FeAs-122 films where the volatility of K presents a serious concern. Here we present our MBE growth of high-quality single-crystalline films of (Sr,K)Fe2As2 and (Ba,K)Fe2As2. We used r-cut sapphire as substrates. All the elements (Sr, Ba, Fe, As) except for K were supplied from pure metal sources by resistive heating. Elemental K is very reactive and is quite difficult to handle in air. Hence we have employed a relatively stable and safe-to-handle In-K alloy (In8K5) for supplying K flux. No contamination by In is likely as the vapor pressures of In and K at 350°C are vastly different. The key to incorporating volatile K in films is low-temperature (300 - 350°C) growth in reduced As flux [2]. The highest Tc of 122 films so far obtained are Tcon (Tcend) = 33.4 K (31.0 K) and 38.3 K (35.5 K) for (Sr,K)Fe2As2 and (Ba,K)Fe2As2, respectively. [1] N. H. Lee et al. , Appl. Phys. Lett. 96 (2010) 202505. [2] S. Takeda et a.l, Appl. Phys. Express. 3 (2010) 093101.

High critical current density (Jc) YBa2Cu3O7 (YBCO) superconducting films have been grown on cube-textured metal tapes for the purpose of developing second generation superconducting wires for high temperature, high magnetic field applications. The RABiTS process is used to introduce a high degree of grain alignment in the metal substrate, that is epitaxially transferred into the buffer and YBCO film layers. In the standard RABiTS approach, a pure Ni or a Ni alloy substrate is used to fabricate long lengths of YBCO coated conductor. However, the ferromagnetism of pure Ni or Ni alloy substrates makes them unfavorable for use in magnetic fields or for AC applications. Therefore, the use of a non-ferromagnetic substrate such as Cu is highly desirable. Additional advantages of Cu-based substrates include potentially lower material cost, possibly easy formation of a sharp cube texture and high electrical and thermal conductivity. The primary disadvantage, however, is that Cu has poorer resistance to oxidation than a pure Ni and Ni a alloy substrate, especially in an oxygen atmosphere at the high temperatures necessary for the fabrication of the YBCO superconducting layer. For overcoming this problem, we introduced an electroplated Ni layer as an oxygen barrier between the Cu substrate and oxide buffer layers, and demonstrated high Jc (> 2 MA/cm2) of YBCO films on CeO2/YSZ/CeO2 buffered Ni-electroplated Cu tapes. In this paper, we report the fabrication of high Ic YBCO films with high Jc on the Ni-electroplated Cu/S316. Cube-textured Cu tapes of 30 μm thickness were obtained from high purity (99.99%) Cu bars by cold-rolling and subsequent heat treatment for recrystallization. The Cu tape was laminated with a stainless steel (SUS316) tape of 100 μm thickness by a SAB (surface activated bonding) technique, and then the surface of the Cu/SUS316 tape was polished to reduce the surface roughness. For reducing the oxidation rate of the Cu tape, a Ni layer was added on top of the Cu layer by electroplating. The thickness of the electroplated Ni layer was 500 nm. The first CeO2, Y2O3, the second CeO2 and the YBCO layers were deposited epitaxially on the Ni-electroplated Cu/SUS316 (Ni/Cu/SUS) tape by a pulsed laser deposition (PLD) method. After depositing the 1 μm thick YBCO films, they were annealed at 450°C for 100 hours in the flow of O2. From Ni {111} pole figure, the Ni overlayer of the Ni/Cu/SUS tape was confirmed to have a single cube-on-cube texture with the FWHM value of 4.6° in the Ï? scan. On this Ni/Cu/SUS substrate, CeO2, Y2O3, CeO2 and YBCO were deposited and then a 1 μm YBCO film was grown on the CeO2/Y2O3/CeO2 buffered Ni/Cu/SUS tape. The YBCO film had biaxial crystal orientation and the FWHM value of YBCO (103) was 6°. The Jc of the 1 μm thick YBCO layer measured at 77 K in self field revealed 2.1 MA/cm2 and the corresponding Ic was 210 A/cm-width.

High quality REBCO coated conductors have been deposited on NiW tapes by pulsed laser deposition process. Although REBCO films have a very high critical current density Jc of 4.0x10 ⁶ A/cm ² (at 77 K, in zero magnetic field), superconducting critical current under two tesla applied magnetic field perpendicular to film surface becomes about one tenth of zero magnetic field Ic value. In order to improve superconducting properties of REBCO films under applied magnetic field, various flux pinning centers were introduced into the films by adding other oxide particles. XRD analysis showed that doped REBCO films still had pure c-axis orientation at optimized substrate temperatures. At low dose doping range, zero field superconducting property degradation was not observed. However, superconducting critical current under two tesla applied magnetic field perpendicular to film surface increased to one sixth of zero magnetic field Ic value. Our experiment results showed doped REBCO coated conductors on NiW tapes have excellent superconducting transport properties under applied magnetic field.

Epitaxial growth on (001) surfaces of cubic zirconate substrates is an essential feature in both of the leading processes for mass production of high-temperature superconducting tape. In the Rolling-Assisted Biaxially Textured (or RABiT) tapes, a ceria-capped layer of yttria-stabilized zirconia (YSZ) typically serves as a buffer between the textured metal base and the superconducting layer of YBCO (yttrium-barium-copper oxide). In the other process, Ion Beam Assisted Deposition (IBAD) is used to grow a highly textured layer of gadolinium zirconates on a rolled metal base, and this textured zirconate layer in turn serves as the substrate for the YBCO layer. In order to optimize the current-carrying capacity of the YBCO layer, it is desirable to introduce localized, nano-meter scale, flux-pinning structural defects in the YBCO. For these purposes, we propose to take advantage of a process of patterning YSZ-(001) surfaces by the formation of self-assembled arrays of epitaxial nanoislands, thereby providing a nano-scale relief to the (001) surface without altering its crystal structure. Surface patterning of yttria-stabilized-zirconia (YSZ) single crystals can be done by doping with rare earths at 1100oC, producing nanoisland arrays on the surface. A suspension of gadolinia-doped ceria (GDC) powder in water was simply applied to the YSZ-(001) surface. The substrate was dried and then air-annealed at 1100 oC, to form arrays of nanoislands surrounding the powder particles on the surface. The remnant powder particles can be removed easily from the patterned substrate surface by sonication, leaving a surface modified with nanoislands only. The powder based method is quite flexible, and we have demonstrated that nanoislands on YSZ can be formed from powders of GDC, as well as gadolinia, samaria and neodymia. YBCO thin films were deposited on YSZ-(001) substrates by pulsed laser deposition (PLD) and the parameters optimized to attain maximum Tc. The aim of this study is to develop a different technique (to the one already known) for introducing structural defects in YBCO that does not alter the superconductor composition and does not rely on any particular method of growing the YBCO layer.

Jc properties for the nanorods introduced Gd123 coated conductors were investigated at low temperatures under high magnetic fields. The large peak in the field angle dependence of critical current density, Jc(θ), for θ =0° (B//c), which usually appears at high temperature near 77K, is clearly observed at a low field and high temperature region but its peak height decreases with decreasing temperature and then disappears below 20 K. At low temperatures, the field dependency of Jc is small below the matching field but becomes large above the matching field in comparison with that of the Gd123 tape without nanorods. These behaviors of Jc at low temperatures can be described by the cooperation model of the c-axis correlated and random pinning centers. Therefore, the nanorods are effective even at low temperature but its efficiency is drastically reduced due to the limitation of flux pinning force by the matching field. Increase of naonorod density is necessary to improve the high field properties of Jc for B//c. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) through ISTEC.

Coated conductors are multilayer/nanostructured RE_1+xBa_2-xCu₃O_y (REBCO) film on the metallic tapes. From recent works, the fabrication for high quality epitaxial REBCO layer using buffer layer and seed layer and multilayer/nanostructured coated conductor were reported by many groups for the high I_c coated conductor at magnetic field. SmBa₂Cu₃O_y (SmBCO) and GdBCO films have significant advantages in several aspects of coated conductor applications, especially in terms of higher T_c and J_c at magnetic field. In this presentation we study the flux pinning and microstructure of SmBCO films fabricated by PLD method alternatively ablating SmBCO and BaSnO₃ targets. Now we study two approach; the controlling the composition ratio in SmBCO film for high T_c and the doping artficial pinning center (APC) technique for high J_c. In the case of SmBCO+BSO process using a premixed target and a sectored target, a T_c value decreased as increasing the BSO content such as YBCO+BSO films. On the other hand a decreased T_c value in SmBCO+BSO prepared by the alternating targets is inhibited. The optimum level of BSO doping was found to be 4.8 vol%, which achieved T_c=89.9K and J_{c, self}=3.07MA/cm2 at 77K. As the BSO content was increased the J_c at higher magnetic field of the films was found to be increased. J_c and F_p values reach 1.35 MA/cm² and 25GN/m² at B=1.8T at 77K, respectively. We will discuss about TEM images and superconducting properties at low temperature and high magnetic field of SmBCO+BSO film. This research was partially supported by â?oJapan Society for the Promotion of Science (JSPS): Grant-in-Aid for Scientific Research (19676005, 23226014)â? , â?oJapan Science and Technology Agency (JST): Advanced Low Carbon Technology Research and Development Program (ALCA)â?.

Reversible dependences of critical current J_c on strain in BiSCO based tapes are quasi-linear and asymmetric by sign, while such dependences in YBCO-based coated conductors and PLD films are approximately parabolic. Moreover, they are asymmetric, can be non-monotonic and occasionally have two peaks. Therefore, a simple natural account of T_c dependence on strain is insufficient to explain these features, especially in applied magnetic field. In literature they were empirically described by a power law but neither sound physical meaning was attributed to the extracted parameters. We have suggested a model clarifying physical nature of the observed effects. Two key features of YBCO are taken into account: twin domain structure of YBCO in orthorhombic phase with alternating directions of a and b axes in adjacent domains and the anisotropy of uniaxial strain dependence of T_c in this phase. The twin domain structure is being formed at tetra-ortho transition during cooling and/or oxygen annealing. It is important that twinning proceeds after other defect structure, chains of edge dislocations forming low-angle boundaries, impurity precipitates, etc, are already formed in tetra-phase during deposition. The sample volume is divided into a multitude of coherent twin blocks. The formed structure is characterized by one of two possible orthogonal directions of twin plane normal (about ±45° to a and b axes) and by a distribution of twin domain width W. The twin domain length L is determined by size and shape of the coherent blocks. According to TEM images the typical L is few hundreds of nanometers, while W is an order of magnitude less. We considered only 2D-picture in ab-plane for simplicity. Assuming that pinning occurs at dislocations in low angle boundaries (below the transition to Abrikosov - Josephson regime, that is <1-2°), applied strain changes elementary pinning force of the defects and results in redistribution of vortices. In combination with the T_c dependence on strain this leads to approximately parabolic strain behavior of J_c. We have obtained analytical expressions for the â?oinitial strainâ?, which actually is due to naturally appearing misbalance between numbers of grain boundaries separating a and b oriented domains, as well as for â?ostrain sensitivityâ?, which is determined by T_c dependence on uniaxial a and b strains and by the effective redistribution of vortices. The non-monotonic strain behavior of J_c in the framework of the model is explained by additional pinning on inhomogeneities within grains due to twin domain structure and applied strain. This work is supported by the Global Initiative for Proliferation Prevention (GIPP) program of the Department of Energy, USA, through Project BNL 368, by NAS of Ukraine and by the Ministry of Education and Science of Ukraine.

Recent advance in the processing of bulk RE-Ba-Cu-O (RE: rare-earth elements) high-temperature superconductors are described. Different techniques, such as thin film seed crystals, top-seeded melt growth (TSMG) and infiltration growth (IG) methods, and modification of micro/nano structures have been developed to grow large single grained bulk superconductors with strong pinning centers at high field. Reports had shown that the processing techniques led to the production of single-grain RE-Ba-Cu-O superconductors > 10cm with the trapped magnetic fields as high as 4T and 17T at 77K and 29K respectively in between a gap of two pieces of Y-Ba-Cu-O single-grained materials. The significant enhancement of Jc (H,T) is found in REBCO bulks with the addition of nano-scaled precursors. Microstructure analysis reveals that the crystalline defects which related to del-l pinning at low magnetic fields and nano-scale periodic structure which related to the field induced del-Tc pinning centers at high magnetic fields were observed in the REBCO bulks with the addition of nano-scaled precursors. In addition, the dR/dT of Resistivity-Temperature-Magnetic field (H) curves exhibits two peaks represent that there are two types of effective pinning mechanisms at different magnetic fields respectively. This study was supported by the National Science Council, Taiwan, Republic of China, under Contract NSC 100-2627-E-006 -002

The combination of high magnetic field and Resonant X-ray Scattering (RXS) would be of substantial value for investigating strongly correlated electron systems such as HTS, non-Fermi liquid and heavy-Fermion systems, CMR compounds, structural, magnetic and quantum phase transitions, frustrated and low-dimensional systems. As National Synchrotron Radiation Research Center (NSRRC) of Taiwan is establishing a new beam line (BL07A) with high spatial and energy resolution. This provides an opportunity to incorporate x-ray scattering and a high magnetic field for the advance study of material science. Based on a 2003 Nature report, Y-Ba-Cu-O (YBCO) HTS bulks with top-seeded melt-textured process (TSMT) are capable of trapping magnetic flux density up to 17 Tesla at 29 K. We intend to build a new portable HTS superconducting magnet system for this purpose. Preliminary results show that 4-6 Tesla can be trapped at a gap of 4-6 mm at 25 K when field cooled at 9 Tesla within the assembly of two pieces of TSMT bulk (1.6 cm diameter each). The detailed design principle and working plan will be reported. This study was supported by the National Science Council, Taiwan, Republic of China, under Contract NSC 100-2627-E-006 -002

The time-resolved response of the high-temperature superconductor YBa₂Cu₃O_7-δ to the transmission of intense single-cycle THz pulses was investigated. By using high-power THz time-domain spectroscopy (THz-TDS), YBa₂Cu₃O_7-δ thin films were probed with peak THz electric field strengths of tens of kV cm^-1. We were able to look at the nonlinear THz field-dependence of YBa₂Cu₃O_7-δ for the first time, an aspect that has remained elusive in spite of the extensive research conducted on YBa₂Cu₃O_7-δ with conventional THz-TDS. Intense single-cycle THz pulses were generated by means of optical rectification in a LiNbO₃ crystal. Thereby, the tilted-pulse-front method was used to achieve a high pump-to-THz generation efficiency. A tight focusing of the high-power THz beam onto the YBa₂Cu₃O_7-δ thin films ensured a strong peak electric field, while a set of rotatable wire grids allowed for a variation of the field strength. We find that the superconducting properties of the YBa₂Cu₃O_7-δ thin films clearly depend on the THz electric field strength. For strong electric fields, polarized parallel to the a-b plane of the superconductor, there is a sharp drop in the imaginary part of the conductivity for frequencies between 0.2 and 0.8 THz. Similarly, the superfluid density decreases strongly for high field strengths and our results suggest that increasing THz electric fields lead to a reduction of the carriersâ?T effective mass. In the case of weak fields, the frequency and temperature dependence agrees with previous studies of YBa₂Cu₃O_7-δ. Acknowledgment: We are grateful to M. Nagai (Osaka U.), S. Tani (Kyoto U.), K. Tanaka (Kyoto U.) and I. Katayama (Yokohama National U.) for insight and inspiring discussions. This work is partially supported by Grant-in-Aid A22246043, JSPS, and Industry-Academia Collaborative R&D, JST.

A common assumption is that the parent compounds of high-Tc cuprates are antiferromagnetic charge transfer insulators. Recently, however, we have demonstrated Tâ?T-RE2CuO4 (RE: rare-earth ions, Pr, Nd, Sm, Eu, and Gd) prepared by metal organic decomposition (MOD) to be superconducting with a transition temperature (Tc) even exceeding 30 K when they are optimally synthesized and annealed [1]. The contradiction between published and our results originates from impurity oxygen atoms at the apical site (Oap). Impurity Oap atoms in Tâ?T cuprates act as a scatterer as well as a Cooper-pair breaker. Inherent properties of the Tâ?T-cuprates are exposed only upon thorough removal of impurity Oap atoms. Early works published almost two decades ago on Nd2-xCexCuO4, which presumably contained impurity Oap atoms, show a wide antiferromagnetic (AF) region (x = 0.00 â?" 0.14) and a very narrow superconducting region (x = 0.14 â?" 0.18). A special recipe at low-temperatures allowed us to evacuate interstitial oxygen atoms from the Oap sites while preserving regular oxygen (O1) and (O2) sites occupied (O1 and O2 are the regular oxygen sites in the CuO2 layer and in the RE2O2 layer, respectively). In our Nd2-xCexCuO4 films, superconductivity has been observed for 0.2 â?¥ x â?¥ 0, and for lower x, higher Tc has been observed. Furthermore we have performed a systematic investigation on the RE dependency of superconductivity in Tâ?T-RE2CuO4. Superconductivity with a transition temperature (Tc) â?¥ 30 K is achieved for RE = Pr and Nd. By contrast, Tconset is at highest 20 K for RE = Gd. Tb2CuO4 does not show superconductivity. Our results indicate that the induction of superconductivity into Tâ?T-RE2CuO4 cuprates strongly depends on the RE3+ ionic size. This trend can be explained from the viewpoint of RE-dependent thermodynamic stability of Tâ?T-RE2CuO4 [2]. For smaller RE3+ ions, the thermodynamic boundary conditions to evacuate interstitial oxygen atoms from the Oap sites become tighter. Based on this RE dependence, the nature of superconductivity in parent Tâ?T-RE2CuO4 cuprates is discussed. [1] O. Matsumoto et al., Phys. Rev. B 79 (2009) 100508. [2] A. Ikeda et al., Physica C 471 (2011) 686.

Transition metal oxide films grown by molecular beam epitaxy (MBE) together with the misfit strain imposed by an underlying substrate allows to strain them up to high levels. High levels of strain are used to enhance the mobility of transistors [1] and increase ferromagnetic [2,3] , ferroelectric [4,5] and superconducting [6] transition temperatures. Moreover, MBE allows the stabilization of phases typically unstable in their bulk form, e.g., Tâ?T-La_2-xCe_xCuO₄ [7]. Cuprate films are typically grown in a relaxed mode and external parameters e.g., temperature and oxygen pressure, determine the lateral crystal grain dimensions. For electron doped cuprates, the copper environment is square planar coordinated and the impact of biaxial strain on the electronic properties remain elusive. Especially for the recently reported Tâ?T-La_2-xY_xCuO₄ (LYCO) superconductors (T_câ?^22 K) [8], where superconductivity appears in the limit of zero doping, the Cu-O bond length is associated to the electron filling. For bulk synthesized Tâ?T-cuprates, highest superconducting transition temperatures are reported for Tâ?T- PrLa_0.85Ce_0.15CuO₄ (PLCCO) [9] as Tâ?T- La_2-xCe_xCuO₄ as well as underdoped PLCCO are unavailable. Since the constituent elements do not carry any magnetic moment, Tâ?T-LYCO is a suitable candidate for the determination of the magnetic ground state of under doped and dopant free Tâ?T cuprates. We have grown Tâ?T-LYCO thin films coherently on rare earth scandate substrates. We show, that the tuning of the superconducting transition temperature is closely related to the annealing conditions after the growth. Moreover, the average crystal grain dimensions of coherently grown Tâ?T-LYCO films are significantly larger compared to films grown in a relaxed mode which in turn significantly influences the annealing conditions. [1] R. Chau et al., Nature Materials 6, 810 (2007). [2] R. S. Beach et al., Phys. Rev. Lett. 70, 3502 (1993). [3] D. Fuchs et al., Phys. Rev. B 77 014434 (2008). [4] K. J. Choi et al., Science 306, 1005 (2004). [5] J. H. Lee et al., Nature 466, 954 (2010). [6] H. Sato et al, Phys. Rev. B 61, 12447 (2000). [7] M. Naito et al. Jpn. J. Appl. Phys. 39, L485 (2000) [8] A. Tsukada et al. Solid State Commun. 133, 427 (2005) [9] H. J. Kang et al. Nature Materials 6, 224 (2007)

In type-II superconductor, energy dissipation is associated with the motion of vortex lattice in a form of vortex flow or creep. This dissipation is reduced by the presence of crystalline defects, which block the vortex motion by â?opinningâ?. An important challenge in application of type-II superconductors is achieving optimal critical current density Jc(B) under an applied magnetic field B. Below Jc the vortices are pinned and can support supercurrent without dissipation. The Jc decreases as magnetic field increases and consequently the pinning efficiency should be optimized. However, the detailed particular of the depinning of vortices are complex, involving the nonequilibrium dynamics of an elastic lattice through a disordered medium. Simulation study of vortex dynamics by solving the time-dependent Ginzburg-Landau (TDGL) equations, where the vortex-vortex and vortex-pinning interactions are completely characterized, is effective approach to reliably describe the details of complex depinning transition. We employ two-dimensional TDGL simulation with GL parameters of 8 and 16 to make qualitative measurements of the V-I curve and the dependence of Jc on the density and size of pinning centers. A maximum Jc equal to around 5% of the departing current density was achieved under the conditions: pinning size of 1.5 times of coherence length and pinning density of around 20%. The complex pinning transition will be visualized based on the TDGL calculation.

Biaxially textured Ba(Fe1â^'xCox)2As2 (Ba-122) and Fe(SexTe1-x) thin films have been realised on Fe buffered MgO substrates. A careful transmission electron microscopy study revealed a clean and coherent interface between Fe and Ba-122 (1). Whenever the body centered cubic Fe is grown epitaxially with a smooth and closed surface, biaxial textured Ba-122 layers can be formed. For instance, epitaxial Ba-122 films with superior properties can be grown on MgO bicrystals, MgAl2O4 and even ion-beam assisted deposition MgO coated conductor templates by employing Fe buffer layers (2). These results demonstrate that Fe works as a generic buffer layer for epitaxial growth of Ba-122. In this talk, structural and superconducting properties of Fe/Ba-122 bilayers on those substrates as well as recent results obtained for Fe(SexTe1-x) thin films will be presented (3). We acknowledge financial support by the EU (IRON-SEA and SUPER-IRON) under project Nos- FP7-283141 and FP7-283204. (1)T. Thersleff et al, Appl. Phys. Lett. 97 (2010) 022506. (2)K. Iida et al, Appl. Phys. Express 4 (2011) 013103. (3)K. Iida et al, accepted for publ. in Appl. Phys. Lett.

Iron chalcogenides are of great interest for both basic physics and applications. Although their superconducting transition temperatures are typically lower than those of iron pnictides, iron chalcogenides exhibit lower anisotropies with very high upper critical field slopes near the superconducting transition temperatures. In this presentation, I will discuss recent progress in the field of superconducting thin films and coated conductors of iron chalcogenides, including substantial works done at the Brookhaven Lab on FeSe_1-xTe_x (0 â?¤ x â?¤ 1). Of particular interest is that some of them exhibit significantly higher superconducting transition temperatures than those of bulk polycrystalline samples and single crystals over the entire doping regime. Upon the incorporation of small amounts of oxygen, superconductivity is seen to emerge in a thin film of FeTe (parent compound of the iron chalcogenides), but not in the bulk. Advances in superconducting-tape fabrication of iron chalcogenides at the Brookhaven are also described since the very high upper critical fields and critical current densities of these films suggest that they are prospective candidates for high field and energy applications. As examples, I will highlight two R&D effort currently being carried out at Brookhaven: superconducting magnetic energy storage (SMES system) for GRIDs, and superconducting wires for large wind turbine generators.

Grain connectivity is a serious problem that limits superconducting critical current in the AeFe2As2 (122; AE = alkaline earth) superconductors. The 122 grain boundaries are often wetted by metallic Fe-As phase, which has a low melting temperature. We developed a new method to synthesize (Ba0.6K0.4)Fe2As2 powders using a mechanochemical reaction that reduces the negative effects of this wetting phase. In this mechanically activated process, a stoichiometric mixture of the elements is subjected to high-energy ball milling. A mechanically-activated, self-sustaining reaction (MSR) initiates during the first few minutes of milling. After milling, x-ray diffraction shows that the 122 phase forms. The powder that comes out of the mill, referred to as MSR powder, has an average grain size less than 1 μm. It is much easier to from denser, more phase pure bulk material during subsequent heat treatments using MSR powder than 122 made by hand mixing or low-energy mixing of the elements. The MSR powder was heated in a Hot Isostatic Press (HIP) which densifies the material and completes the reaction to Ba-122. The MSR powder could be densified in the HIP at lower temperatures. Although some Fe-As impurity was still present in the HIPped bulk samples made from MSR powder, it did not appear to wet the grain boundaries as it did in bulk samples made with the standard synthesis technique. We fabricated powder-in-tube Ag-sheathed (Ba0.6K0.4)Fe2As2 round wires using MSR powder. After drawing, the wires were heated in a HIP to sinter and densify the MSR powder. Monocore and 7 filament wires have been made. Critical current densities Jc ~120 kA/cm2 (4.2 K, SF) and 7 kA/cm2 (15 T, 4.2 K) have been measured in these wires.

Since the discovery of superconductivity in iron-based superconductors, great efforts have been devoted to the preparation of films and wires for practical applications. Among these iron-based superconductors, FeSe has several practical advantages over the pnictide. FeSe has the simplest structure and less toxicity compared to the other As-based compounds. They also possess a high upper critical field Hc2 and extremely low Hc2 anisotropy. Furthermore, FeSe with Tczero ~8 K and Tconset ~10 K shows that the application of pressure leads to a significant enhancement of Tc up to 37 K under high pressure. These aspects make FeSe to be more promising materials for application to high Jc superconducting wires. We succeeded in fabricating mono- and multi-core wires of FeSe using the Fe-diffusion powder-in-tube (PIT) method. The seven-core superconducting wires showed a transport Jc as high as 1027 A/cm2. What is more interesting is that the transition temperature Tczero was observed at 10.5 K[1], which is about 2 K higher than that of bulk. These results indicate that the Fe-diffusion PIT method is highly suitable for fabricating FeSe superconducting wires. Recently, K0.8Fe2Se2 superconductor with Tc ~30 K was discovered. K0.8Fe2Se2 could be a potential material for practical applications under high magnetic fields because of the high Hc2 and small anisotropy. We also attempted to fabricate K0.8Fe2Se2 superconducting wires. [1] T. Ozaki, et al, arXiv: 1103.3602, J. Appl. Phys (to be published)

Local fluctuations in the distribution of dopant atoms are thought to cause the nanoscale electronic disorder or phase separation in pnictide superconductors. Atom probe tomography has enabled the first direct observations of dopant species clustering in a K-doped 122-phase pnictide. Frequency distribution and nearest neighbor analysis conclusively show that K is non-random distributed compared to the same layer of Ba where K tend to form high solute regions at atomic scale. Strong correlation between location of K with electronic property and crystal structure, indicating the non uniform impurity-state can account for major structure and electronic state distortion in its vicinity. Our result also provides alternative explanation for nano-scale electronic state disorder in the K doped 122 systems which are basically due to the inhomogeneous of dopant atom distributions.

Amid the flurry of activity on K_1-xFe_2-ySe₂ superconductors, it remains established that the stoichiometric compound K₂Fe₄Se₅ is an antiferromagnetic semiconductor. This raises the question of whether subtle Fe^1+/3+ doping causes K_1-xFe_2-ySe₂ to become a bulk superconductor, and if so, can structural differences between these two phases distinguish superconducting from non-superconducting samples? We have grown K_1-xFe_2-ySe₂ samples that show superconductivity with T_C = 31 K, even when growth conditions are starkly different from those reported in the literature. Here we present high-resolution synchrotron X-ray diffraction measurements, alongside single-crystal x-ray and electron diffraction, to elucidate the phase space in this system. Combined with magnetometry, heat capacity, and transport measurements, our structure-property relations help prescribe how chemical composition and heat treatment induce superconductivity and vacancy ordering in the K_1-xFe_2-ySe₂ system.

We have investigated the global and local magnetic properties of Ba(Fe_1-xCo_x)₂As₂ (Ba-122) thin films by use of scanning SQUID microscopy (SSM). Epitaxial Fe/Ba-122 bilayers and Ba-122 single layers have been deposited on IBAD-MgO template on Hastelloy and bare (La,Sr)(Al,Ta)O₃ single crystalline substrates, respectively, by pulsed laser deposition technique. The critical temperature of these films is around 22.5 K. SSM allows us to vitalize the 2D magnetic field distribution on the sample surface with high sensitivity and high spatial resolution. We successfully observed global shielding currents at sample edges. From the field penetration, we obtained the critical current density in comparison with theoretical analysis. We also observed trapped vortices in the film. Studying temperature dependence across the critical temperature, we will discuss the influence of Fe buffer layer on the vortex dynamics. Details of measurement and analysis will also be presented in this talk. This research is supported by Strategic International Collaborative Research Program (SICORP), Japan Science and Technology Agency. We also acknowledge the EU (SUPERIRON) under Project no. FP7-283204.

It is well known that critical properties of superconducting films in a great extent are determined by their grain and subgrain structure. Relatively high-angle grain boundaries serve as easy motion channels for Abrikosov â?" Josephson vortices thus limiting critical current density. In a contrary, really low-angle boundaries between columnar domains (subgrains) consist of well separated edge dislocations and provide strong pinning for Abrikosov vortices. Therefore, the nature of such a subgrain structure and mechanisms of its formation are extremely important for comprehending and development of new superconducting materials. It has been recently established that properties of new Fe-based superconductors are also strongly affected by the boundaries. Mismatch between the film and the substrate crystal lattices is a principal reason of subgrain structure formation. The free energy of mismatched interface can be reduced either by misfit dislocations in the interface plane or by rotational relaxation of strains with formation of threading edge dislocations. In the first case atoms of the film are displaced longitudinally along the principal crystallographic directions. However, at certain conditions the film atoms may displace transversally resulting in rotation of the lattice around the c-axis and the film fragmentation reducing the free energy. We have upgraded our earlier suggested model of mismatched film â?" substrate interface to expand its range of applicability to less anisotropic materials, such as ferropnictides. The model describes formation of threading edge dislocation arrays by the mechanism of rotational relaxation of interface stresses. The calculations have shown that the center of a growing cluster should be displaced at certain cluster sizes, because the free energy of the film â?" substrate interface is oscillating with the cluster size. Therefore, the system can transfer to a metastable rotated state. Thus, we have shown that rotational relaxation occurs due to finite size of clusters and to non-equilibrium effect of the film growth. The updated interface energy calculation more accurately takes into account local strains in the film compared with previous calculations. Favorable cluster shape is also taken into account. We have predicted the real subgrain size and the expected rotation of domains for different combinations of practically used superconductors and substrates. We have also made a statistical computer simulation of possible merging of domains leaving no any visible boundaries between them. Thus, we modeled formation of the whole subgrain structure â?" actual arrangement of low-angle boundaries and edge dislocations, which can provide strong pinning. The computed patterns are consistent with the nanostructures observed by TEM and EBSD. This work is supported by the Global Initiative for Proliferation Prevention (GIPP) program of the Department of Energy, USA, through Project BNL 368, by NAS of Ukraine and by the Ministry of Education and Science of Ukraine.

We have studied the magnetism in superconducting single crystals of EuFe2 As1.4 P0.6 by using the local probe techniques of zero-field muon spin rotation/relaxation and 151 Eu/57 Fe Mössbauer spectroscopy. All of these measurements reveal magnetic hyperfine fields below the magnetic ordering temperature Tm = 18 K of the Eu2+ moments. The analysis of the data shows that there is a coexistence of ferromagnetism, resulting from Eu2+ moments ordered along the crystallographic c-axis, and superconductivity below Tsc â?^ 15 K. We find indications for a change in the dynamics of the small Fe magnetic moments (â^¼ 0.07 μB ) at the onset of superconductivity: below Tsc the Fe magnetic moments seem to be â?ofrozenâ? within the ab-plane.

We have performed detailed 57Fe Mössbauer spectroscopy measurements on Ba0.78K0.22Fe2As2 and BaFe2â^'xNixAs2 single crystal mosaics showing SDW magnetic ordering below TN and superconductivity below TC. Analysis of the Mössbauer spectra shows a decrease in the magnetic hyperfine (hf) field but no change in the magnetic volume fraction below TC. This clearly indicates the coexistence of magnetism and superconductivity in these compounds. The decrease in the magnetic hf field below TC depends on the difference between TN and TC, being the largest for TN close to TC. Two different explanations for this observation are given. We also find that the non-magnetic volume fraction below TN correlates with the Ni doping x, being large for high TC and small for high TN.

The discovery of iron based superconductors provided an unanticiated new direction in superconductivity. The materials are unconventional superconductors near magnetism that are qualitatively different both from the cuprates and from other known superconducting families. This talk discusses the electronic and magnetic properties of these materials in relation to the cuprates, the basis for the sign changing s-wave state, and the properties of the superconducting state of these materials. This work is supported by the Department of Energy, Office of Basic Energy Sciences, Materials Science and Engineering Division.

The focus of my talk will be on the novel iron-pnictide superconductors, which set the stage for the discussion of relevant issues in ongoing solid state physics. I will present our thorough optical investigation over a broad spectral range and as a function of temperature of the charge dynamics in Ba(Co_xFe_(1-x))_2As_2 compounds for Co-doping ranging between 0 and 18%. Through spectral weight arguments, we give clear-cut evidence for moderate electronic correlations for 0> x <0.061, which then crossover to values appropriate for a regime of weak interacting and nearly-free electron metals for xâ?¥0.11. Furthermore, a renewed interest in the study of symmetry-breaking competing states in complex interacting systems followed the discovery of a broken rotational symmetry, due to stripe or nematic order, in the pseudogap phase of the copper oxide superconductors. In this context, we also investigate the optical conductivity with light polarized along the in-plane orthorhombic a- and b-axes of Ba(Co_xFe_(1-x))_2As_2 for x=0 and 2.5% under uniaxial pressure across their structural and magnetic transitions. The charge dynamics at low frequencies and temperatures on these detwinned, single domain samples reveals an enhancement of both the scattering rate and Drude weight of the charge carriers along the antiferromagnetic a-axis with respect to the ferromagnetic b-axis. Our findings also allow us to estimate the dichroism, which extends to high frequencies. These results reveal a substantial nematic susceptibility as well as demonstrate the electronic nature of the structural transition found in underdoped Fe-pnictides.

The recent discovery of superconductivity in iron-based materials triggered a strong interest in the experimental and theoretical study of their electronic, structural, magnetic and superconducting properties. These materials have a rich phase diagram as a function of temperature, doping and pressure, and show different magnetic, structural and superconducting phase transitions. To this end, validation of theoretical predictions is a fundamental step for a complete comprehension of materials' properties. In this talk, Density Functional Theory predictions will be compared with available experimental results in iron-based superconductors, analyzing many physical properties and their variation with pressure and doping. We show that DFT-based calculations are able to reproduce most of the observed experimental features, revealing at the same time interesting discrepancies which deserve a careful comprehension. We present results on the effects of hydrostatic and non-hydrostatic pressure calculating the complete pressure phase diagram and explaining the different experimental data reported in the literature for the critical pressure value where the structural transitions takes place in 122 compounds. Surface structural and electronic properties are investigated and compared with experimental evidences, demonstrating the possibility to stabilize different terminations and helping the interpretation of experiments. At the same time, we analyzed the role of doping in both 1111 and 122 compounds, LaOFeAs, SmOFeAs, BaFe2As2 and CaFe2As2, presenting predictions for different dopants and finding interesting aspects when we compare theoretical results with experimental data. In addition, we calculated the electronic and dynamical properties interpreting infrared, Raman and optical conductivity spectra from first-principles. The computational results presented in this talk will open discussions on the predicting power of DFT in the local density approximation for iron-based superconductors and on the interpretation of the experiments.

Low carrier densities and short coherence lengths in the recently discovered semi-metallic Fe-based superconductors (FBS) can result in interesting behaviors at strong magnetic fields due to the interplay of multiband superconductivity, unconventional pairing symmetry and the Zeeman and orbital pairbreaking. I this talk I will discuss these effects in FBS, particularly the anomalous temperature dependencies of the upper critical field Hc2(T) which often extrapolate to very high values > 100 Tesla at low temperatures. The materials features of FBS and the multiband pairing symmetry with the sign change of the order parameter on different sheets of the Fermi surface can significantly increase Hc2(T) and facilitate the Fulde-Ferrel-Larkin-Ovchinnikov transition to the state with a spatially modulated order parameter. Moreover, the low Fermi energies and short coherence lengths in multiband FBS offer a new opportunity of tuning Hc2 by doping and strains, which appears to be more effective than the conventional way of increasing Hc2 by introducing nonmagnetic disorder.

We report magneto-transport measurements of H_c2(T) at very high dc and pulsed magnetic field up to 85T in Ba_1-xK_xAs₂Fe₂ single crystals varying doping level and in FeSe_1-xTe_x stressed thin films. Optimal doped Ba_1-xK_xAs₂Fe₂ show high H_c2 extrapolating to â^¼100T whereas the stressed FeSe_1-xTe_x thin films with enhanced T_c have extremely high slopes up to 20T/K for H//c and 500T/K for H//ab, value never reported before. Those properties indicate significant Pauli pair breaking and a possibility of the Fulde-Ferrel-Larkin-Ovchinnikov state. The transitions remain sharp also at the highest field showing an irreversibility field close to H_c2, property particular important for application. In the Co-doped BaFe₂As₂ thin films we investigated field and angular dependences of J_c down to 4.2 K in high magnetic field up to 45T. We found strong correlated pinning can be introduced in this material by a high density of nanorods and/or nanoparticles arranged along both the main crystallographic directions. Nanorods and nanoparticles are introduced by self-assembling, changing the growth conditions, or by multilayer deposition alternating superconducting and non-superconducting layers. Differently from high temperature superconductors as YBCO, where non-superconducting phases exceeding 3-4%vol strongly suppress T_c, in the Co-doped BaFe₂As₂ thin films the high density of non-superconducting phase well over 10%vol does not affect the matrix properties. We verified J_c for field along the c-axis may exceed J_c for H//ab up to ~20T, inverting the expectation of the H_c2 anisotropy and showing a weak J_c angular dependence. J_c at 20T and 4.2K is still over 10⁵ A/cm². Moreover the maximum of the pinning force density F_p(4.2K) can exceed 45-50 GN/m³ at 15-20T depending on the field configuration. High resolution TEM was performed in order to identify the origin of such strong pinning and to correlate the microstructure to the transport properties.

After the discovery of Fe-based superconductors, fabrications of epitaxial Fe-based superconducting thin films on various single crystal substrates have been carried out. Highly textured thin films were deposited on oxide single crystals substrates such as MgO, SrTiO₃, LaAlO₃, etc and higher T_c and higher J_c exceeding 1 MA/cm² were reported by using arsenic-contained 111 and 122 Fe-based superconducting films on these substrates. But, very recently, high-J_c Fe-Te-Se films have been demonstrated on non-oxide single crystal substrate CaF₂. J_c of the films at 4.5 K almost reached 1 MA/cm² at low magnetic fields. These results indicate that the fluoride substrates have high potential for the growth of Fe-based superconductors in comparison with popular oxide substrates. To make clear the effect of CaF₂ substrates on the superconducting properties, epitaxial Fe-Te-Se films were prepared on the substrates by pulsed laser deposition. Tc, zero nearly 16 K with a sharp normal-superconducting transition and high irreversibility fields were measured. We are now working to enhance the transport properties of Fe-Te-Se films by nano-engineering approach. The order of magnitude of ξ(0) in the films belonging to Fe-based superconductor family suggests that the introduction of nanometric artificial defects for the pinning of quantized vortices should significantly increase J_c, similarly to the case of YBCO. Possible approaches can be the modulation of interfacial strain by introduction of oxide buffer layers and the insertion of artificial pinning centers by means of surface-modified targets. New results obtained on nano-engineered doped Fe-based superconductor films will be reported at the conference.

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 misfit 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. An alternative approach is the preparation of superconducting films on single crystalline piezoelectric substrates enabling a dynamical variation of the induced strain by applying an electric field on the substrate. This method was successfully used to study the strain dependence of superconducting properties in cuprates. More recently, 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 using additional buffer layers if required. An undisturbed epitaxial growth of the superconductors on the piezocrystals was observed 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.

The crystallographic lattice constants and superconducting critical temperatures of FeSe_0.5Te_0.5 thin films have been found hardly dependent on the in-plane lattice constants of the substrates when grown on oxide substrates. However, various correlations were observed between structural and transport properties of the films themselves, and it has been gradually recognized that the penetration of oxygen from the substrates suppresses the critical temperatures.^{1, 2} Thus we need to choose substrates by considering the effects of chemical properties of the substrate, especially the â?omobilityâ? of oxygen. Because one of the main driving forces of moving oxygen is supposed to be a valence change from Fe²⁺ to Fe³⁺, substrate materials consisting of only typical elements will work better for the growth of iron chalcogenide superconducting thin film. Based on this idea, fluoride substrate is a good candidate for the growth of iron-based superconductors. We grew FeSe_0.5Te_0.5 thin films on CaF₂ substrates and obtained films with sufficient reproducibility that show the superconducting critical temperature above 15 K. The critical current density at T = 4.5 K also show high values as 5.9 x 10⁴ Acm^-2 at μ₀H = 10 T, and 4.2 x 10⁴ Acm^-2 at μ₀H = 14 T.³ Other fluoride substrates, SrF₂ and BaF₂, also exhibit a good result, which means that the idea to exclude oxygen from substrates actually works. Upon characterization of the substrate materials in more detail, we can summarize the conditions of the substrate for successive growth of iron-chalcogenide superconductor thin films as follows: 1) its crystal structure should not have a vacancy that would permit electronegative elements to migrate, and 2) it should be composed only of typical elements, in contrast to popularly used substrates that contain transition-metal elements. ¹ M. Hanawa et al. Jpn. J. Appl. Phys. 50 (2011) 053101. ² Y. Imai et al. Appl. Phys. Express 3 (2010) 043102. ³ I. Tsukada et al. Appl. Phys. Express 4 (2011) 053101.

Fe-based superconductor is expected to be a new series of high-Tc superconductors. Among them, the FeSe so called 11 system with Tc ~10 K has the simplest crystal structure among the iron-based superconductors. Surprisingly, the superconducting transition temperature raised from 10K to 37K under the pressure of around 4-6GPa [1]. We found that this increase of Tc is closely related to anion height from Fe layer [2]. As mentioned above, FeSe have high potential for applications. However, synthesis of high quality FeSe film is very difficult. Then, new easy and low-cost methods are required. Recently, we have succeeded to synthesizes superconducting FeSe films using electrochemical technique. At my presentation, the detailed condition of the electrochemical synthesis of FeSe will be presented. [1]Y. Mizuguchi et al., APL 93, 152505 (2008) [2]Y.Mizuguchi et al., SUST 23, 054013 (2010)

The precise nature of unconventional superconductivity in iron chalcogenides is a matter of heavy debate. In FeSe, an additional complication is represented by the level of Fe excess or selenium deficiency, which introduce disorder and vacancies in the superconducting scenario. Due to the high temperature synthesis conditions, considering the phase transition from the insulating high-temperature hexagonal phase into the bad metallic low-temperature tetragonal phase is mandatory. In this talk we present our recent efforts [1] to understand the role of this phase transition at the microscopic level, including its impact on the transport [2,3] and superconducting transition temperature. We use chemical transport techniques, X-ray diffraction combined with HR-TEM imaging to characterize the intermediate steps of the transitions, on the eve of the formation of the superconducting state. We also provide transport and magnetic data for the normal and supeconducting phases. We discover a bipartite behavior in the transport properties, as a function of the degree of magnetic order, which allows for a further refinement of the phase diagram of FeSe. [1] S. Leoni, M. Brando, L. Craco, in preparation [2] L. Craco, S. Leoni, EPL 92, 67003 (2010) [3] L. Craco, S. leoni, EPL, 91, 27001 (2010).

Intercalation of alkaline and alkaline-earth metal atoms into hydrocarbon system produces a new family of superconductors [1, 2, 3]. The superconducting transition temperature, Tc, reaches 18 K in K3picene, which is highest in organic superconductors reported so far, but two different superconducting phases, which show Tc of 7 and 18 K, in K3picene are observed. In Kxpicene, we have investigated a softening of Raman shift to determine exact composition for superconducting phase. Comparing with theoretical calculation, we failed to identify difference in composition for two phases with different Tc, but found that a number of metal atoms doped is exactly three. Our theoretical study also suggests that, in tri-anion of picene systems, strong intra-molecular electron phonon coupling leads to high Tc of 18 K [4]. Recently, Wang et al. discovered phenanthrene superconductors with the Tc being 4 - 7 K. The shielding fraction is more than 20% [3] and the maximum value reaches 60%. Thus, hydrocarbons provide a new stage for the research of superconductivity. In this symposium, recent progress of study on hydrocarbon superconductors are presented. [1] R. Mitsuhashi et al., Nature 98, 2021 (2010). [2] Y. Kubozono et al., Phys. Chem. Chem. Phys., 13, 16476 (2011). [3] X. F. Wang et al., Nat. Commun. 2:507 doi:10.1038/ncomms1513(2011). [4] T. Kato et al. Phys. Rev. Lett. 107, 077001 (2011).

This talk will focus on experimental search and discovery of novel forms of quantum order in metallic and insulating magnets, intercalated compounds, ferroelectric systems and multi-ferroic materials. Particularly discussed will be the pressure-induced superconductivity and critical phenomena in the vicinity of quantum phase transitions. Materials tuned to the neighbourhood of a zero temperature phase transition often show the emergence of novel quantum phenomena. Much of the effort to study these new emergent effects, like the breakdown of the conventional Fermi-liquid theory in metals has been focused in narrow band electronic systems. But Spin or Charge ordered phases in insulating systems can also be tuned to absolute zero using hydrostatic pressure. Close to such a zero temperature phase transition, physical quantities like resistivity, magnetisation and dielectrics constant change into radically unconventional forms due to the fluctuations experienced in this region giving rise to new kinds ordered states including superconductivity in the metallic systems.

Boron (B)-rich solids have a framework crystal structure built up from B12 icosahedral clusters. These solids are called boron-icosahedral cluster solids (B-ICSs). One of the extraordinary characteristics of B-ICSs is the high degeneracy of the electronic states owing to the high symmetry of the icosahedron. Such degeneracy brings a relatively high density of states (DOS), and if one can adjust the Fermi energy (EF) to this high-DOS position, B-ICS could be a high-Tc superconductor. Another notable characteristic of B-ICSs is their comparatively large interstitial sites; therefore, there have been some attempts to dope other elements into B-ICSs with the aim of adjusting EF. α-rhombohedral boron (α-B) is the simplest polymorphism of elemental B. It consists of only B12 icosahedral cluster. In spite of various theoretical and experimental efforts, there is no reliable report on superconductivity in metal-doped α-B. There were two problems in the previous experiments. The first problem was that the crystallized α-B powder included impurity phases of B6O, β-B and amorphous boron (am-B), which covered the surface of α-B particles and obstructed effective Li doping. The second problem was that Li reacted not only with α-B but also with the quartz tube used for sealing. In this study, we developed a new superconductor in Li-doped α-B system. A metal transition and superconductivity were observed in Li-doped α-B. Li-doped α-B contained glittering metallic particles, and Meissner effects were observed for some samples (TC = 3.2 ~ 7 K). The temperature dependence of the electrical conductivity is metallic and a drop in the electrical resistivity is detected near the Meissner temperature. The existence of Li and the Fermi edge in Li-doped α-B crystals were verified by a transmission electron microscope-electron energy loss spectroscopy (TEM-EELS) analysis. Lattice expansion, which is a well-known indicator of metal doping into a crystal, was also observed. Thus, Li doping into α-B was successful for the first time.

Strongly correlated Ca _2-xSr_xRuO₄ (CSRO) has attracted much attention for its rich physical properties such as Mott metal-insulator (MI) transition, antiferromagnetism (AFM), orbital ordering, and spin-triplet superconductivity caused by complex interplay between the charge, spin, orbital, and lattice degrees of freedom. These properties can be tuned through modulating the active lattice degrees of freedom such as the tilting, rotation, and flattening of RuO₆ octahedra. Such modulation of RuO₆ octahedra could be achieved by epitaxial strain and isovalent substitution in epitaxial thin films of CSRO. We have grown epitaxial thin films of CSRO with very high crystalline qualities by pulsed laser deposition. The Ca₂RuO₄ films with thickness ranging from 20-200 nm shows strong compressive strain on LaAlO₃ (001) substrates leading to an itinerant ferromagnetic (FM) phase coexisting with insulating AFM phase in ground state and a suppressed broad and gradual MI transition. This is in sharp contrast to bulk Ca₂RuO₄ with AFM Mott-insulating ground state and sharp MI transition. Modulation of RuO₆ tilting state caused by substrate confinement is responsible for the suppression of MI transition and the low temperature itinerant FM. Our 20~100 nm thick films with x=0.1, 0.5 on LaAlO₃ (001) also exhibit tetragonal symmetry with a coherent strain. The MI transition and itinerant FM are partially suppressed in x=0.1 films and fully suppressed in x=0.5 films. Such evolution of electrical and magnetic properties with increasing Sr content in our CSRO films can be explained by the subsititution induced suppression of rotation, tilting, and flattening of RuO₆ octahedra and consequently the widening of 4d bandwidth. In contrast to x=0, 0.1, 0.5 films, our systematic studies on 20~200 nm thick Sr₂RuO₄ films show that they are not susceptible to strain on various perovskite substrates. These films generally show comparable metallic behavior without superconductivity due to the lack of RuO₆ tilting state modulation and the disorders generated from the lattice mismatch. This research is supported by DOD ARO under Grant No. W911NF0910530 and the LA-SiGMA program.

Electric field effect on superconductors has long history more than 50 years. There have been many reports on electrostatic modulation of superconducting properties, electrostatically induced superconductor-insulator transition and device applications. However, all of these studies have used well-known superconductor materials as a channel or an electrode. Electric field induced superconductivity in an insulator has remained a challenge, since dielectric breakdown of gate insulator had always prevented us from accumulating high density carriers and from inducing superconducting state in a channel. An electric double layer transistor (EDLT), a sort of field-effect transistor with a liquid electrolyte gate [1], recently attracts considerable attention due to its capability to induce high charge carrier densities above 10¹⁴ cm^-2. In this talk, we present the emergence of superconductivity in two dimensional electron systems at the electrolyte/insulator interfaces in EDLTs. The first example is the superconductivity in an insulating SrTiO₃. With inducing a sheet carrier density more than 10¹³ cm^-2, we could induce superconductivity purely by electric field for the first time [2]. Superconducting critical temperature Tc was almost constant at 0.4 K regardless of gate voltage (V_G), which is different from a phase diagram in a chemically doped SrTiO₃. Critical magnetic field showed a quite good agreement to the two dimensional Ginzburg-Landau theory, indicating that this system behaves as a typical two dimensional superconductor. As a second example, we show inducing superconductivity in KTaO₃ that is known as a non-superconducting material down to 0.01 K even if making it metallic with chemical doping [3]. Maximum charge carrier density in KTaO₃ EDLT is one order of magnitude larger than the density that can be achieved with chemical doping. Superconductivity appears in KTaO₃ below 50 mK for a V_G of 5 V [4]. This result clearly demonstrates the potential of electrostatic doping to achieve higher charge carrier densities than what is possible with chemical doping, which could lead to the discovery of new superconducting compounds. This work was conducted in collaboration with H. Aoki, Y. Iwasa, M. Kawasaki, N. Kimura, S. Nakamura, T. Nojima, A. Ohtomo, H. Shimotani. [1] H. Shimotani, et al., Appl. Phys. Lett. 91, 082106 (2007). [2] K. Ueno, et al., Nature Mater. 7, 855 (2008). [3] J. R. Thompson, J. Low Temp. Phys. 47, 467 (1982). [4] K. Ueno, et al., Nature Nanotech. 6, 408 (2011).

Elucidating the interplay between electronic and lattice degrees of freedom is one of the central problems in the strongly correlated electron materials. The interaction between different degree of freedom results in a variety of phenomenon such as metal-insulator transitions, high temperature superconductivity and colossal magneto-resistance found in highly correlated oxides. One of the very interesting aspects is the formation of phases with charge/orbital order and their relationship to the materialsâ?T functionality such as electrical conductivity. Magnetite (Fe3O4) was the first correlated electron system where such a relationship between electronic ordering and transport was established. It has become a model system for understanding the conductivity decrease by two orders of magnitude on crossing the metal-insulator transition (TV=123K) with decreasing temperature. This metal to insulator transition is accompanied by a structural change from monoclinic to cubic symmetry. It has been proposed that charge and orbital order (CO/OO), via Fe+3 and Fe+2 charge disproportionation, also play a crucial role. Even after 70 years of discovery this phase transition still defies complete understanding. We studied the structural as well as charge/orbital dynamics at ultrafast timescale by using pump-probe soft X-ray scattering technique; pump being a 50 fs pulse from optical laser and probing with intense and short pulses of X-ray Free Electron Laser (LCLS) at SLAC National Accelerator Laboratory. Measuring at off-resonance on the high temperature forbidden (001) lattice reflection, we find a lattice response on time-scales t< 250 fs, displaying a pump fluence threshold indicative of a phase transition. This surprisingly fast response of lattice strongly suggest coupling to certain low energy phonon mode. We also measured structurally forbidden (00½) peak at resonance which allows us to study charge/orbital order. Upon excitation the charge gap of 200 meV was quenched on resolution limited time-scale < 250 fs, while we still observe a residual CO/OO signal. This suggests that both lattice and charge/orbital degrees of motion are coupled to each other, while the band gap collapse clearly indicates it to be a metal-insulator transition. This also indicates the existence of a new transient state of matter, displaying charge and orbital order in coexistence with metallic behavior.

The electron-doped infinite-layer (IL) Sr1-xLaxCuO2 (SLCO) (a0 = 0.3949 nm) is one of the most attractive high-Tc cuprates due to its unique features: crystal structure is the simplest among the high-Tc cuprates and Tc is the highest (Tc = 43K at x = 0.1) among the electron-doped cuprates. However, it can be stabilized in a bulk form only by means of high-pressure (2.5 GPa) synthesis method. Therefore, epitaxial films of SLCO have been of considerable interest. However, there are very few papers on the SLCO thin films. Karimoto et al. reported on c-axis SLCO films with the highest Tc = 43K on almost lattice-matched (110) DyScO3 substrates (a0 ~ 0.3943 nm) whereas, the superconducting properties degrade in the SLCO films on (100) SrTiO3 substrates (a0 = 0.3905 nm) with significantly larger lattice mismatch. The identical trend is also reported by Sugii et al., who examined an effect of Pr2CuO4 buffer layers (a0 = 0.3958 nm). These results suggest that the lattice mismatch between IL films and substrates, namely, the lattice strain effects to the IL films make large influence on their superconducting properties. However, the detail of the lattice strain effects to the IL films has not been well understood. BaxSr1-xTiO3 (BSTO) buffer layers may provide a good opportunity for investigating the lattice strain effects to the IL films since the lattice constant can be systematically controlled by changing Ba/Sr ratio, from 0.3905 nm (x = 0) to 3.992 nm (x = 1) following Vegardâ?Ts law. We have succeeded in epitaxial growth of c-axis oriented BSTO (x = 0.4 and 0.7) buffer layers on LAO substrates by sputtering. The in-plane lattice constant of BSTO was estimated to be 0.3936 nm (x = 0.4) and 0.3969 nm (x = 0.7), and the lattice mismatch between SLCO (x = 0.1) and BSTO is -0.3% for x = 0.4 and 0.5% for x =0.7. The 50-nm-thick SLCO (x = 0.1) were deposited on BSTO buffer layers on LAO substrates. All SLCO films on LAO//BSTO were c-axis oriented. However, for these as-grown films, metallic behaviors and superconducting transitions were not observed due to excess apex-oxygens. In order to remove apex-oxygens and obtain superconducting SLCO films, reductive annealing of the as-grown films was performed in 30 min in Ar + 5%H2. The SLCO films on BSTO (x = 0.4) showed superconductivity with TcONSET of 24 K after the annealing procedure. On the other hand, the SLCO films on BSTO (x = 0.7) showed semiconducting behaviors with no trace of superconductivity. The c-axis lattice constant (c0) of the SLCO films on BSTO (x = 0.7) (0.3444 nm) is larger than c0 of the SLCO films on BSTO (x = 0.4) (0.3418 nm), indicating that excess apex-oxygens still remains in the films. Therefore, we consider that the degree of removal of excess apex-oxygens strongly depends on the strain states of the SLCO films on BSTO buffer layers. In conclusion, the present study suggests that the degree of removal of excess apex-oxygens may be systematically controlled by using BSTO buffer layers.

Morphological control of cuprate-based superconductors has been a major challenge to manufacturers because as ceramics, ductility and malleability are practically absent. However, certain critical properties such as Tc and Jc of superconductors are intimately linked to their size and shape [1]. Nanowire morphology would minimise the disorientations at grain boundaries with concomitant improvement in Jc owing to the natural tendency for anisotropic particles to self-align [2]. The recent application of biopolymers to the sol-gel synthesis of YBCO has opened the hall-way to the synthesis of various morphologies of a range of functional high temperature ceramic materials. The mechanism behind the nanowire growth is attributed to an invariant inorganic phase acting as discrete catalytic sites for the outgrowth of nanowires of the desired product [3-4]. Using the same synthetic protocol, we present here the first reported nanowires of superconducting Pr-BCO (Pr123, Pr124, Pr247 and Pr358). The samples were characterized using powder X-ray diffraction, electron microscopy and SQUID magnetometry. The results showed that Pr123 produced the largest quantity of wire-like crystallites followed by Pr124, Pr358 and Pr247. Generally, there was an observed trend in thickness, paramagnetism and magnetic susceptivity as the metal ion ratios were increased. 1. Y. lijima; K. Onabe; N. Futaki; N. Tanabe; N. Sadakata; Kohno J. Appl. Phys. 1993, 74, 1905. 2. S.R.Hall Advanced Materials 2006, 18, 487. 3. Z.A.C. Schnepp; S. Mann; S.C. Wimbush; Hall., S. R. Adv. Mater. 2008, 20, 1782. 4. S.R. Hall; S.C. Wimbush; Y. Shida; Ogasawara, W. Chemical Physics Letters 2011, 507, 144.

The advent of the MgB2 superconductor adds a new entry in the competition between the High and Low Temperature Superconductors and offers new opportunity to tailor the superconducting material for the applications. The versatility of the MgB2 derives from its possibility to be used either as bulk material either in wire, tape or film form. In any case, their crucial applicative properties, other than the critical temperature, are the mechanical strength and the behavior in high magnetic field: both very important to approach the high power applications. In these years, the development of the MgB2 has demonstrated a substantial improvement of its characteristics by acting either on the preparative process and on the quality of the raw materials either by changing the chemical nature of the material by an appropriate doping. Among the preparative processes it will be compared the various conventional solid state reaction processes with a new process that was discovered during the first attempts to produce high density material: the Reactive Liquid Mg infiltration (Mg-RLI) [1]. Regarding the doping effects, up to now the best results are obtained by a partial boron substitution by carbon atoms in the lattice, but several other heterogeneous additions of Nano size dopant phases are claimed to enhance a lot the critical magnetic fields. To give a more comprehensive comparison between some typical doping solutions, we have applied the same preparative process (Mg-RLI) to different doping agents and the results will be reviewed. Further improvements of the MgB2, along the way to obtain high performance superconductors, are expected to be introduced by operating on the manufacturing aspects of the superconducting systems. The possibility to create superconducting metallic composites of the MgB2 with Mg or other metals that are inert with respect to the superconducting properties, opens the way to improve also the mechanical characteristic of this intrinsic brittle material. [1] G.Giunchi - Int. Jour. Mod. Phys. B17, Nos. 4, 5 & 6, 453-460 (2003)

After more than ten years of incessant development, ex-situ MgB₂ wires have reached a significant level of reliability and homogeneity over very long lengths. Many real examples, both prototypes and commercial products based on these conductors, including MRI systems, have been successfully operated continuously and flawlessly, in some cases being under operation since about five years. In spite of the significant progress in industrial manufacturing and commercialization of ex-situ MgB₂ wires, the performance which has been achieved on long lengths is still clearly below the somewhat optimistic expectations resulting from initial R&D efforts that followed the discovery occurred in 2001. The aim of this paper is to present a detailed analysis of the microstructural and transport properties of ex-situ MgB₂ wires, and to interpret these results highlighting the main current limiting factors, as lack of grain connectivity, inhomogeneity in the phase formation and pinning properties, reacdtion with the sheath material, as well as the presence of impurities. Considerations about suitable processes to further enhance ex-situ MgB₂ wire performances will be finally given, bearing in mind that the ambitious low-cost target of about 1 â,¬/kAâ-m has to be achieved in the short to medium term in order to enable full commercial exploitation of ex-situ MgB₂ superconductors.

MgB2 synthesis and heat treatment to high temperatures (1000-1700 C) and under high pressure (up to 1500 psi) was investigated with the use of a high pressure vessel used in conjunction with RF heating. The temperature for the peritectic reaction MgB2 -> MgB4 + Mg was determined to be 1450 C (under 1500 psi). This was established by the observation (after heating above 1450C and re-cooling) of a microstructure of MgB4 cores with MgB2 skins in combination with excess Mg, as expected for a diffusion limited re-conversion. EPMA analysis was used to determine the phases and their compositions accurately. In addition, we investigated the high temperature addition of C to the structure, finding the limits for C substitution incorporated under these extreme conditions. Finally, the addition of Zr, Nb and Ti to MgB2 was studied, in an attempt to investigate whether substantial doping could occur on the Mg site, and the level of homogeneity possible. High resolution XRD with Rietveld refinement was used to extract the lattice parameters. Bc2 and Birr values were measured to 14 T as a function of field, and the slope of dHc2/dT was extracted and compared. The level of homogeneity was further observed with the aid of heat capacity measurements looking at the sharpness of the specific heat jump. This work was supported by the U.S. Dept. of Energy, Office of High Energy Physics, under Grant No. DE-FG02-95ER40900

Magnesium diboride (MgB2) represents an exciting possibility as a superconductor due to its high critical temperature (Tc ~40 K) and potential for higher critical current density, Jc, than other proposed materials. The measured Jc of current MgB2 materials, however, is much lower than the theoretically estimated Jc, reducing its practicality. Here, we present a microstructural investigation of this material to better understand the reasons for the discrepancy. By combining transmission electron and x-ray microscopy, the microstructural imperfectness is revealed. These observations, carried out on MgB2 wires fabricated a standard in-situ powder-in-tube method, are believed to explain the reasons for differences in measured vs. predicted Jc. Through implementations of these microstructure investigations, new fabrication techniques may be tested for Jc values to produce next-generation MgB2 materials that better agree with theory.

In this study, the critical current density (Jc) of MgB2 was optimized via different routes including variation of heat treatment and additions of excess Mg and nano-SiC particles. Firstly, polycrystalline MgB4 powders were synthesized and the presence of MgO as impurity was minimized by using acid leaching. The MgB4 powders were then well mixed with Mg for reaction over a wide temperature range. Rietveld refinement analysis of the x-ray diffraction data shows that the weight fraction of MgB2 increases with the sintering temperature up to 750°C. For the addition of excess Mg into MgB4 for reaction, a considerable increase in the weight fraction of MgB2 together with the presence of unreacted Mg for flux pinning enhances Jc significantly. For the samples reacted with nano-SiC, the decrease in the a-axis lattice parameter indicates carbon substitution in the B site. The form of field dependence of Jc is improved with only a slight decrease in Tc.

Various potential applications for bulk superconducting magnet have emerged owing to recent progresses in superconducting bulk materials with high critical current density and mechanical strength and performance of cooling systems which much easily provide low temperature environments. MgB2 with Tc = 39 K has several attractive natures for bulk superconducting magnet, such as low cost of materials, light weight (â?¦2.6 g/cm3), weak-link-free homogeneous current flow on a bulk scale and great design flexibility of magnet shapes. In the present study we have fabricated disk shape MgB2 bulk superconducting magnets (10-60 mm in diameter, 5, 10, 20 mm in thickness) from magnesium and boron powders. Electron microscopy analyses and electromagnetic measurements showed the obtained MgB2 bulk samples have single-phase, homogeneous microstructure and high critical current density at 15-30 K. The bulk samples were cooled down by a cryocooler and magnetized by a superconducting magnet under the field-cooling condition. Trapped field of the bulk magnets at ~20 K was measured by a transversal cryogenic hall sensor. We observed a trapped magnetic field of over 3 Tesla, promising for new compact bulk superconducting magnets operating at 15-30 K with cryocooler. Detailed relationship between size of the bulk and trapped field will be discussed.

Arsenic, through its unique and specific interaction of As 4p orbital with transition-metal d orbital, exhibits a remarkable bond breaking and bond making. The first focus of this talk is a tunable As-As distance in the iron-based superconductors with a ThCr2Si2-type structure. In the parent material, CaFe2As2, the As-As distance between the Fe2As2 layers is long, indicating the absence of the chemical bond. The As-As distance shrinks in a first-order fashion when a tiny amount of transition metal, such as Ru, Rh, or Pd, is substituted for Fe. The resultant distance is characteristic of As-As single bond. We demonstrate that superconductivity up to 14 K emerges when the As-As bond is absent, while superconductivity disappears when the As-As bond are formed between the adjacent Fe2As2 layers [1]. We discuss physical consequences on the bond making and breaking on CaFe2As2. The second focus of this talk is the formation of As-As bond within the ab plane. We partially substitute Pt for Fe. Then, what we obtain is not a ThCr2Si2-type material but new material expressed by a chemical formula Ca10(Pt4As8)(Fe2-xPtxAs2)5. It consists of Fe2As2 layers interspaced with Ca and Pt4As8 layers. The Pt4As8 layers are characterized by As-As dimmers with As-As single bond. Superconductivity up to 38 K emerges in this material when Fe is partially replaced by Pt with x = 0.36 [2]. We present structural details of this material as well as its physical properties. [1] M. Danura, K. Kudo, Y. Oshiro, S. Araki, T. C. Kobayashi, and M. Nohara, J. Phys. Soc. Jpn. 80 (2011) 103701. [2] S. Kakiya, K. Kudo, Y. Nishikubo, K. Oku, E. Nishibori, H. Sawa, T. Yamamoto, T. Nozaka, and M. Nohara, J. Phys. Soc. Jpn. 80 (2011) 093704.

A number of iron-based superconductors have been identified since the discovery of superconductivity with a transition temperature (T_c) of 26 K in the fluorine-doped iron oxypnictide LaFeAsO_1-xF_x [1]. All materials identified so far consist of alternating two-dimensional Fe₂As₂ layers and â?~â?~spacer layersâ?Tâ?T [2]. The central issues in realizing higher T_c are to seek novel spacer layers and engineer them to tune the electronic states of Fe₂As₂ layers in which high-T_c superconductivity emerges. Spacer layers previously reported so far consist of ionic chemical bonds. In this paper, we report the discovery of novel iron-based superconductors with spacer layers made of covalent Pt arsenides. Using X-ray diffraction and chemical analyses, we identified the compounds to be Ca₁₀(Pt₄As₈)(Fe_2-xPt_xAs₂)₅ (referred to as alpha-phase) and Ca₁₀(Pt₃As₈)(Fe_2-xPt_xAs₂)₅ (referred to as beta-phase). Both compounds crystallize in triclinic structures with the space group P-1 (#2). The alpha-phase exhibits superconductivity at T_c = 38 K, the beta-phase at T_c = 13 K [3]. The most notable feature of the present compounds is the presence of Pt_nAs₈ layers. These platinum arsenide layers are characterized by a distorted square lattice of corner-sharing PtAs₄ squares. Rotation of the PtAs₄ squares result in the formation of As₂ dimers. Such As₂ dimers are observed in pyrite-type structures; the Pt₄As₈ layers can be derived from the ab-plane of pyrite PtAs₂. The size of the Pt square lattice (with a Pt-Pt distance of approximately 4.4 Ã.) is by far larger than the size of the Fe₂As₂ square lattice (approximately 3.9 Ã. for CaFe₂As₂). This lattice mismatch leads to the formation of the 5 x 5 superstructure in the ab-plane in the present compounds. There exist Pt vacancies in the platinum arsenide layers of the beta-phase to form the Pt₃As₈ layers. This finding demonstrated that a variety of spacer layers are possible in iron-based superconductors. Engineering of this novel spacer layer will lead to higher superconducting transition temperatures in this class of materials. [1] Y. Kamihara et al., J. Am. Chem. Soc. 130 (2008) 3296. [2] D. C. Johnston, Adv. Phys. 59 (2010) 803. [3] S. Kakiya et al., J. Phys. Soc. Jpn. 80 (2011) 093704.

The superconductivity in the iron-based pnictides emerges in close proximity to a magnetically ordered state. Uncovering the character of the magnetic fluctuation is crucial for understanding the mechanism of superconductivity. CaFe₂As₂ is a unique system that experiences a first order phase transition from the uncollapsed-tetragonal (ucT) phase to the collapsed-tetragonal (cT) phase under either an applied hydrostatic pressure [1] or chemical substitutions [2,3]. This transition is characterized by the shrinkage of the c axis by approximately 10% without breaking symmetry. Band calculations predicted the lifting of the Fermi surface nesting [2,4] along with the disappearance of the iron magnetic moment [5] in the cT phase. This gives us a unique opportunity to investigate the relation between magnetism and superconductivity. In this paper, we demonstrate that the lattice collapse transition takes place in Ca(Fe_1-xTM_x)₂As₂ (TM = Ru, Rh). The Rh doped one undergoes successive phase transition with increasing x in the T = 0 limit. The antiferromagnetic-metal (AFM) phase with orthorhombic (O) structure at 0.00 â?¤ x â?¤ 0.020 is driven to a superconducting phase with ucT structure at 0.020 â?¤ x â?¤ 0.024; a non-superconducting phase with cT structure takes over at x â?¥ 0.024. On the other hand, the Ru doped one exhibits that AFM/O phase directly changes to the nonsuperconducting cT phase at x â?¥ 0.020. Upon either aliovalent Rh or isovalent Ru doping, no superconductivity is observed in the cT phase. We found that T-linear paramagnetic susceptibility in the ucT phase abruptly changes into a T-independent Pauli paramagnetic behavior in the cT phase. Moreover, we observed that non-Fermi liquid transport in the ucT phase changes to Fermi liquid transport in the cT phase. All of these observations suggest that the magnetic fluctuation in the ucT phase is lifted by the lattice collapse transition. Our result points toward the important role of magnetic fluctuations in the occurrence of superconductivity. [1] A. Kreyssig et al., Phys. Rev. B 78 (2008) 184517. [2] S. Kasahara et al., Phys. Rev. B 83 (2011) 060505(R). [3] M. Danura et al., J. Phys. Soc. Jpn. 80 (2011) 103701. [4] D. A. Tompsett and G. G. Lonzarich, arXiv:0902.4859. [5] T. Yildirim, Phys. Rev. Lett. 102 (2009) 037003.

Since the discovery of high-T_c superconductivity in REFeAsO[1], development of new materials containing iron tetragonal lattice becomes subject of study to find new superconductors with high T_c. So far, several new superconductors containing anti-fluorite iron pnictide or chalcogenide layers have been developed. Especially, large number of layered iron pnictides composed of antifluorite-type iron pnictide layers and perovskite-type oxide layers have been found. Because there are chemical and structural flexibility at the perovskite-type structure, there are possibilities to find new materials in this system. Recently several new compounds having extremely thick blocking layers were found in this system[2]. In this presentation we will present development of these compounds and their physical properties. The structure of the compounds consists of stacking of FeAs layers and perovskite-type oxide layers. Such kind of compounds were discovered in Fe-As-Ca-(Sc,Ti)-O, Fe-As-Ca-(Mg,Ti)-O and Fe-As-Ca-(Al,Ti)-O systems. These compounds have up to 6 sheets of perovskite-type layers between iron pnictide layers, and interlayer iron-plane distances of these compounds are the longest among layered iron pnictides. Their structures can be classified as (Fe₂As₂)(Ca_n+1 M_nO_y) and (Fe₂As₂)(Ca_n+2 M_nO_y) depending on M cations. Similar to LiFeAs, these compounds showed bulk superconductivity without intensive carrier doping, and the T_c of the compounds are up to 47 K in (Fe₂As₂)(Ca₃(Mg,Ti)₃O_y). On the other hand, The relationship between T_c and the iron-plane interlayer distance suggested that superconductivity due to the mono Fe₂As₂ layer is substantially 40 K-class. This fact indicates T_c of the iron based superconductor is basically determined by the local structure of Fe₂As₂ layer. It is thought that there is still considerable room for the development of new layered iron-based compounds, due to the structural and chemical flexibility of the blocking layer. However, the results suggest that optimization of the local structure of Fe₂As₂ layers and the dimensionality of the crystal structure may not lead to further enhancement of T_c in iron pnictides. Other challenging approaches will be required for a new breakthrough in T_c. [1] Y. Kamihara et al., J. Am. Chem. Soc.130 (2008) 3296. [2] H. Ogino et al., Supercond. Sci. Technol.24 (2011) 085020.