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We tested oxidized titanium layers as barriers for hybrid Josephson junctions with high IcRn-products and for the preparation of junctions for tunneling spectroscopy. For that we firstly prepared junctions with conventional superconductor electrodes, such as lead and niobium, respectively. By tuning the barrier thickness, we were able to change the junction's behavior from a Josephson junction to tunnel-like behavior applicable for quasi-particle spectroscopy. Subsequently, we transferred the technology to junctions using Co-doped BaFe2As2 thin films prepared by pulsed laser deposition as base electrode and evaporated Pb as counter electrode. For barriers with a thickness of 1.5 nm, we observe clear Josephson effects with IcRn ≈ 90 µV at 4.2 K. These junctions behave SNS'-like (SNS: superconductor-normal conductor-superconductor) and are dominated by Andreev reflection transport mechanism. For junctions with barrier thickness of 2.0 nm and higher, no Josephson but SIS'- (SIS: superconductor-insulator-superconductor) or SINS'-like (SINS: superconductor-normal conductor-insulator-superconductor) behavior with a tunnel-like conductance spectrum was observed. [ABSTRACT FROM AUTHOR]

Second phase nanoinclusions of composition Y2Ba4CuWOy (Y-24W1) have been incorporated into the YBa2Cu3O7-δ (Y-123) superconducting matrix to form effective artificial flux pinning sites. A remarkable improvement in critical current density Jc has been observed at both self- and higher applied fields in these samples, in contrast to the effect that the presence of Y2BaCuO5 (Y-211) second phase inclusions has on Jc, particularly at low field. The field trapping ability of samples containing the Y-24W1 phase has been improved by adding silver to the nanocomposite precursor composition in an attempt to reduce the formation of c-axis microcracks in the sample microstructure. The combined effect of the Y-24W1 artificial defects and the addition of silver produce enhanced trapped fields in these samples. [ABSTRACT FROM AUTHOR]

Angle-resolved transport measurements revealed that planar defects dominate flux pinning in the investigated Co-doped BaFe2As2 thin film. For any given field and temperature, the critical current depends only on the angle between the crystallographic c-axis and the applied magnetic field but not on the angle between the current and the field. The critical current is therefore limited only by the in-plane component of the Lorentz force but independent of the out-of-plane component, which is entirely balanced by the pinning force exerted by the planar defects. This one-dimensional pinning behavior shows similarities and differences to intrinsic pinning in layered superconductors. [ABSTRACT FROM AUTHOR]

Highlights: [•] Fabrication of multilayers and quasi-multilayers of high crystalline quality. [•] First fabrication of ferromagnetic/superconducting junctions using Co-doped BaFe2As2. [•] Highest reported superconducting critical temperature in the Co-doped BaFe2As2 – system. [•] Origin of the increase of the critical temperature may be a result of As-vacancies. [ABSTRACT FROM AUTHOR]

Surface properties of Co-doped BaFe2As2 epitaxial superconducting thin films were inspected by X-ray photoelectron spectroscopy, scanning spreading resistance microscopy (SSRM), and point contact spectroscopy (PCS). It has been shown that surface of Fe-based superconductors degrades rapidly if being exposed to air, what results in suppression of gap-like structure on PCS spectra. Moreover, SSRM measurements revealed inhomogeneous surface conductivity, what is consistent with strong dependence of PCS spectra on contact position. Presented results suggest that fresh surface and small probing area should be assured for surface sensitive measurements like PCS to obtain intrinsic properties of Fe-based superconductors. [ABSTRACT FROM AUTHOR]

Abstract: For the investigation of the superconducting properties of iron-based superconductors Josephson junctions as well as Andreev reflection studies are potential tools. Therefore, we prepared different kinds of junctions, based on transport in direction of c-axis and ab-plane, respectively. The first one was a planar hybrid superconductor-normal metal-superconductor (SNS’) thin film junction for transport studies along the c-axis. We used a BaFe1.8Co0.2As2 (Ba-122) thin film (thickness ≈ 80nm) prepared by pulsed laser deposition (PLD) as base electrode and a lead indium alloy as counter electrode, while the normal metal (barrier) layer was formed by thin gold films. By tuning the thickness of the gold layer and the area of the tunneling window, we were able to switch the behavior of our junction. For a thin gold layer (≈ 5nm) and greater areas, RCSJ-like Josephson behavior can be observed with a radiation dependent excess current and nearly linear temperature dependence of the critical current. Increasing the thickness of the gold layer to 10nm leads to conductance spectra, which are dominated by Andreev reflection and which show no remaining sign of Josephson effects. Secondly, an edge-type junction for transport measurements in the ab-plane of the pnictide layer, was prepared. It also is realized with PbIn as counter electrode, but with no artificial thin film. The temperature dependent conductance spectra showed some interesting features, which cannot be described within usual theoretical models. We introduced a third geometry, phase-sensitive corner junctions, which could ultimately test the pairing symmetry of Ba-122, if it is s± type. [Copyright &y& Elsevier]

The effect of Gd2Ba6Al4O15 powder addition on microstructure, field trapping capability and critical transition temperature, Tc, of GdBCO-Ag superconductors grown in air is reported. Single-grain GdBCO-Ag bulk superconductors with different amounts of Gd2Ba6Al4O15 powder were fabricated by the top seeded melt growth process. Addition of 0.3 wt% Gd2Ba6Al4O15 powder increased the maximum trapped field value, BZmax. Microstructure analysis confirmed that, up to 1.0 wt% addition of this powder, Al-based phase is not formed and Al is supposed to substitute Cu in the GdBa2Cu3O7-δmatrix. Addition of 2.0 wt% Gd2Ba6Al4O15 powder leads to formation of Pt, Al-based phase, which indicates that the solubility limit of Al in the studied system was exceeded. The critical transition temperature, Tc, decreases monotonically with increasing Gd2Ba6Al4O15 powder addition. [ABSTRACT FROM AUTHOR]

The results of infrared reflectivity measurements for the iron-based high-temperature superconductor Ba(FeCo)As are reported. The reflectivity is found to be close to unity at frequencies ω lower than 2Δ/ h (2Δ is the superconducting gap and h is Planck's constant). This is evidence for the s or s symmetry of the superconducting order parameter in the studied compound. The infrared reflectivity spectra of Ba(FeCo)As manifest opening of several superconducting gaps at temperatures lower than critical T. [ABSTRACT FROM AUTHOR]

Abstract: We report the fabrication of large (LRE)BCO single-grains with improved superconducting properties for LRE=Nd, Sm and Gd using a practical process via both conventional top seeded melt growth (TSMG) and seeded infiltration-growth (SIG). This process uses a new generic seed crystal that promotes heterogeneous grain nucleation in the required orientation and suppresses the formation of solid solution in a controlled manner within individual grains by the addition of excess BaO2 to the precursor powder. The spatial distribution of the superconducting properties of LRE bulk superconductors as a function of BaO2 addition for large (LRE)BCO grains fabricated in air by TSMG and SIG for LRE=Gd, Sm and Nd are compared. The optimum BaO2 content required to fabricate single-grain (LRE)BCO with high and homogeneous T c is determined from these experiments for each LRE system. The irreversibility fields of (LRE)BCO bulk superconductors processed in air are as high as those processed in reduced PO2. Critical current densities in excess of 105 A/cm2 at 77K and higher trapped fields have been achieved in optimized (LRE)BCO superconductors fabricated in air for the first time. [Copyright &y& Elsevier]

Abstract: Large single grains of Y–Ba–Cu–O (YBCO) bulk superconductor of up to 32mm in diameter have been fabricated by a seeded infiltration and growth (IG) technique. Small Y2BaCuO5 (Y-211) particles are observed in the seeded IG sample microstructure, albeit with a rather inhomogeneous distribution in the superconducting YBa2Cu3O y (Y-123) phase matrix. Values of magnetic critical current density, J c, measured in self-field along the a and c growth sectors of the bulk samples prepared by seeded IG appear higher than the corresponding values for samples fabricated by conventional top seeded melt growth (TSMG), despite the inhomogeneous distribution of Y-211 particles in the former. A maximum trapped field of 0.42T at 0.2mm above the sample surface was observed at liquid nitrogen temperature (the actual mean temperature during measurement was around 78K) in samples fabricated by seeded IG due to the increased J c, despite the relatively small YBCO grain size (25mm diameter×7.4mm thickness). This value of trapped field is almost two times greater than that of samples fabricated by TSMG and of a similar size. [Copyright &y& Elsevier]

Two Y–Ba–Cu–O bulk superconductors were joined by using Ag-added Y–Ba–Cu–O as a joining reagent. Two different Y–Ba–Cu–O blocks were prepared individually and subjected to the joining process. Here the joined surfaces were aligned parallel to the (1 1 0) plane. A Y–Ba–Cu–O/Ag plate was prepared via a solid state reaction. The plate was sandwiched by two Y–Ba–Cu–O blocks and heated to a temperature between the decomposition temperatures of Y–Ba–Cu–O and Y–Ba–Cu–O/Ag, and further annealed in flowing oxygen. The trapped field distribution of the joint using a Y–Ba–Cu–O/Ag solder with Y211 powders that were not refined by ball milling showed a small dip, corresponding to a decrease of the maximum trapped field values by 15%, while a single peak with no dip was obtained at the joint using a Y–Ba–Cu–O/Ag solder with finely refined Y211. [Copyright &y& Elsevier]

In this study, we have performed the tensile tests of Y–Ba–Cu–O blocks welded by Er–Ba–Cu–O superconducting solder at room temperature. For comparative study, melt-textured Y–Ba–Cu–O and Er–Ba–Cu–O fabricated by the top-seeded melt growth are also tested. Most welded samples fractured at the joined region and the average strength was lower than those of bulk samples. This is due to the segregation of foreign phases at the joint. [Copyright &y& Elsevier]

We have measured the thermal conductivity (κ) of bulk Y–Ba–Cu–O blocks welded with sintered Er–Ba–Cu–O and Y–Ba–Cu–O/Ag at temperatures between 5 and 300 K. For comparison, the thermal conductivity was also measured for melt-textured Y–Ba–Cu–O and Er–Ba–Cu–O. All the samples showed the enhancement of κ at around 50 K, which was commonly observed in high temperature superconductors. The maximum κ values were 90 mW/cm K for both welded Y–Ba–Cu–O and melt-textured Er–Ba–Cu–O and 150 mW/cm K for melt-textured Y–Ba–Cu–O. Such differences have originated from different microstructural features. [Copyright &y& Elsevier]

Joining of different Y–Ba–Cu–O bulk samples was performed using Er–Ba–Cu–O and Ag added Y–Ba–Cu–O sintered plates. In both cases, a small dip in the trapped field distribution was observed at the joint region due to the poor superconducting properties. However, the trapped field distribution of the sample joined with Ag added Y–Ba–Cu–O using refined Y2BaCuO5 powders showed a single peak, which was similar to that of a single grain bulk superconductor. [Copyright &y& Elsevier]

Two monoliths of single grain Y–Ba–Cu–O superconductors were successfully joined with Er–Ba–Cu–O superconductor, which was sandwiched by two Y–Ba–Cu–O plates. Microstructural analyses revealed that no segregation of the secondary phase and/or the residual liquid phase was observed at the joint with two (1 1 0) surfaces. We also found that we can produce a strongly coupled joint for Ag added Y–Ba–Cu–O blocks using a sintered Er–Ba–Cu–O/Ag bar as a welding reagent. [Copyright &y& Elsevier]

Two melt-textured Y–Ba–Cu–O superconductor blocks were successfully welded with Er–Ba–Cu–O solder having a lower peritectic temperature than that of Y–Ba–Cu–O. The key to a strongly coupled joint was a combination of the control of crystal growth along 〈1 1 0〉 direction and the employment of a highly dense sintered Er–Ba–Cu–O bar as a solder material. Microstructural analyses revealed that no segregation of secondary phase and/or residual liquid phase was observed at the joint interface. Furthermore, it was found through magneto-optical observations that magnetic field could not penetrate into the joint, indicating that the two superconductors were strongly connected with this method. [Copyright &y& Elsevier]

Examines the effect of oxygen partial pressure and the addition of CeO[sub 2] or Pt on the grain growth of Nd-422 particles in Nd-Ba-Cu-O bulk superconductors. Suppression of coarsening of Nd422 particles by reducing oxygen partial pressures or changing the initial composition toward the Ba-rich side.

The implementation of an Fe buffer layer is a promising way to obtain epitaxial growth of Co-doped BaFe2As2 (Ba-122). However, the crystalline quality and the superconducting properties of Co-doped Ba-122 are influenced by the Fe buffer layer thickness, dFe. The well-textured growth of the Fe/Ba-122 bilayer with dFe=15 nm results in a high Jc of 0.45 MA cm-2 at 12 K in self-field, whereas a low Jc value of 61 000 A cm-2 is recorded for the bilayer with dFe=4 nm at the corresponding reduced temperature due to the presence of grain boundaries. [ABSTRACT FROM AUTHOR]

Large, single-grains of (LRE)-Ba–Cu–O (LRE=light rare earth: Nd, Sm, and Gd) bulk high-temperature superconductors with significantly improved properties have been fabricated in an air atmosphere using a practical processing technique based on seeded infiltration growth. This process involves the use of a generic seed crystal, developed recently at the IRC in Superconductivity, to promote epitaxial grain nucleation. The formation of a solid-solution phase within the nucleated single grain is then suppressed effectively by providing excess Ba to the growth front under an air processing atmosphere. Critical current densities in excess of 105 A/cm2 at 77 K have been observed in the resulting large grain samples, with an associated and significant improvement in trapped magnetic field. This air-based seeded-infiltration-growth process offers a significant degree of freedom both in terms of the processing parameters and the reproducibility in growth of oriented single grains. [ABSTRACT FROM AUTHOR]

We have fabricated c-axis oriented Nd–Ba–Cu–O superconductors with the top-seeded melt-growth technique in air. The highest Jc reached 50,000 A/cm2 at 77 K and 0.9 T. The local superconducting properties strongly depended on the distance from a seed crystal. The onset Tc as well as the transition width was improved as the distance from a seed increased. The chemical analyses revealed that the substitution levels of Nd on the Ba site gradually decreased with increasing the distance from a seed. [Copyright &y& Elsevier]

Superconducting and epitaxially grown LaFeAsO1[?]xFx thin films were successfully prepared on (001)-oriented LaAlO3 substrates using pulsed laser deposition. The prepared thin films show exclusively a single in-plane orientation with the epitaxial relation (001)[100]\parallel (001)[100] and a full width at half-maximum value of 1deg. Furthermore, resistive measurement of the superconducting transition temperature revealed a Tc,90% of 25 K with a high residual resistive ratio of 6.8. The preparation technique applied, standard thin film pulsed laser deposition at room temperature in combination with a subsequent post-annealing process, is suitable for fabrication of high quality LaFeAsO1[?]xFx thin films. A high upper critical field of 76.2 T was evaluated for magnetic fields applied perpendicular to the c-axis and the anisotropy was calculated to be 3.3 assuming single band superconductivity. [ABSTRACT FROM AUTHOR]

We report the fabrication of Mg-doped NdBCO generic seed crystals, which have been developed recently for the fabrication of any rare earth (RE) based (RE)-Ba-Cu-O single-grain superconductor, with a wide range of Nd1+xBa2-xCu3-yMgyO7-d compositions. Three basic characteristics of the seed crystals required for effective seeding of bulk (RE)BCO materials were studied in detail. We report the chemical, crystallographic, microstructural, and superconducting properties of the seeds and demonstrate clearly their potential to process various (RE)BCO superconductors in single-grain form. The melting point, volume fraction of Mg-rich inclusions in the bulk microstructure, and the chemical composition of the Nd1+xBa2-xCu3-yMgyO7-d superconducting matrix of the seed crystals were studied as a function of initial MgO content. Finally, the suitable range of MgO-content within the generic seed crystal that controls effectively the orientation of the seeded grain without compromising its superconducting properties relative to those of the Mg-free compound is identified based on the wide range of seed crystal compositions investigated. [ABSTRACT FROM AUTHOR]

Abstract: We have reported recently the fabrication of high performance, nano-composite Y–Ba–Cu–O (YBCO) bulk superconductors containing nano-sized Y2Ba4CuMO (Y-2411, where M is typically a transition metal element) phase inclusions that contribute effectively to flux pinning. However, these studies have revealed that the field trapping capability of YBCO bulk superconductors containing the Y-2411 phase is limited due to the presence of micro- and macro-cracks in the bulk sample microstructure. Here, we report the fabrication of nano-composite YBCO bulk superconductors containing Ag and W-based Y-2411 (i.e. Y2Ba4CuWO y ) secondary phase inclusions (SPI''s) in an attempt to investigate the influence of Ag content on the formation and propagation of a/b and c- axis micro- and/or macro-cracks in the Y-123 matrix. The crack density for samples containing Ag is observed to be significantly lower than for samples fabricated without Ag. A detailed study of the microstructures and superconducting properties of Ag-doped, nano-composite bulk YBCO is presented. [Copyright &y& Elsevier]

Abstract: A Mg-doped Nd–Ba–Cu–O generic seed crystal has been developed for the fabrication of any type of rare earth (RE) based (RE)–Ba–Cu–O single grain superconductor by the top seed melt growth (TSMG) technique. Melt processed crystals of Nd–Ba–Cu–O enriched with various amounts of MgO exhibit increased melting points compared to other (RE)–Ba–Cu–O systems, whilst retaining a similar crystal structure. The Mg-doped Nd–Ba–Cu–O phase is stable with Ba–Cu–O liquid at elevated temperature and, as a result, can be used to seed the growth of any (RE)–Ba–Cu–O single grain. Key seed properties have been investigated as a function of MgO content, including T c, the distribution of MgO inclusions within the YBa2Cu3O7−δ (Y-123) matrix, melting point and composition. MgO doping results usually in a decrease in T c of the parent (RE)BCO compound. It is particularly important, therefore, to investigate fully any variation of T c associated with the diffusion of Mg into a grain grown from a Mg-containing seed. We report the superconducting and microstructural properties of YBCO, SmBCO and GdBCO single grains fabricated using various Nd–Ba–Cu–Mg–O generic seed crystals with a wide range of MgO content. Finally, we report the optimum composition of Nd–Ba–Cu–O generic seed crystal from a study of the variation of Nd–Ba–Cu–O melting temperature with concentration of Mg in the precursor seed composition. [Copyright &y& Elsevier]

Abstract: YBa2Cu3O7−δ /Y2O3/Y2BaCuO5 (Y-123/Y2O3/Y-211) precursor powders enriched with various amounts of Nb2O5 and Y2Ba4NbCuO y have been fabricated successfully in the form of large, single grain superconductors. Microstructural studies of these composites reveal the presence of nanometer-sized (as small as 10–20nm) Y2Ba4NbCuO y phase particles and relatively larger sized Y-211 phase particles (about a micron) embedded in the YBa2Cu3O7−δ phase matrix. The field trapping ability of these nano-composites is observed to increase significantly compared to undoped YBCO. [Copyright &y& Elsevier]

Bulk magnetization, transport, and neutron scattering measurements were performed to investigate the electronic and magnetic properties of a polycrystalline sample of the newly discovered ferromagnetic superconductor, CeO0.3F0.7BiS2. Ferromagnetism develops below TFM=6.54(8)K and superconductivity is found to coexist with the ferromagnetic state below TSC ~ 4.5K. Inelastic neutron scattering measurements reveal a very weakly dispersive magnetic excitation at 1.8 meV that can be explained by an Ising-like spin Hamiltonian. Under application of an external magnetic field, the direction of the magnetic moment changes from the c axis to the ab plane and the 1.8 meV excitation splits into two modes. A possible mechanism for the unusual magnetism and its relation to superconductivity is discussed. [ABSTRACT FROM AUTHOR]

Abstract: To investigate the transport properties of iron based superconductors, we prepared planar hybrid superconductor-normal metal-superconductor (SNS’) thin film junctions with BaFe1.8Co0.2As2 as base electrode. As counter electrode we used a lead indium alloy, while the normal metal layer was formed by thin gold films. The junction design allows us to characterize the electrodes as well as the junction itself. Temperature dependent Andreev reflection studies of the junction show gap-like features but were also strongly influenced by the properties of the electrodes. [Copyright &y& Elsevier]

A skilful combination of different types of pinning centers with the aim to optimize Jc(B) performance of bulk superconductors in a wide range of magnetic fields is demonstrated on melt-textured (Nd,Eu,Gd)Ba2Cu3O7−δ and NdBa2Cu3O7−δ materials. In this contribution we show (i) the distinct effect of different types of micrometer-size secondary phase particles on Jc(B) dependence at low fields; (ii) possibility to produce materials with either high-current densities at moderate fields or with a flat Jc(B) curve extending to high fields, via appropriate choice of Nd:Eu:Gd ratio in NEG-123 matrix; (iii) extremely high-irreversibility field (≈9 T at 77 K) in NEG-123 samples Eu/Gd>1 and a small content of NEG-211; (iv) twin structure effect in melt-textured Nd-123 doped by micron-sized Nd-211 particles resulting at low temperatures in a field independent Jc(B) extending to very high fields (at 40 K above 10 T). Vast majority of the data could be interpreted in terms of additive contributions of pinning on point-like and large defects. [ABSTRACT FROM AUTHOR]