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We investigate the iron-based superconductor Ba(Fe 1−x Co x ) 2As 2 (Ba-122) regarding its superconducting properties and possible applications. Therefore, Ba-122 thin films are used as base electrode to prepare different kinds of hybrid Josephson junctions with a counter electrode of the conventional superconductor Pb. Additionally, we use both c-axis and a b-plane transport geometries and different kinds of barriers like interface-engineered surfaces, sputtered titanium oxide and gold layers. Temperature dependent I– V characteristics as well as magnetic field dependence and microwave response of the junctions are shown. The examined I– V characteristics and I c R n– T behaviours of each junction type are compared and described according to the electrical behaviour of the respective normal conducting or insulating barrier. While the I c R n product of the interface-engineered barrier junction was 12 μV and the planar junction with Au barrier showed 18 μV, we could increase the I c R n to 90 μV for planar TiO x barrier junctions.

Surface properties of Co-doped BaFe 2 As 2 (Ba-122) thin films prepared by pulsed laser deposition on MgO with Fe buffer layer and CaF 2 substrates were inspected by atomic force microscopy, scanning spreading resistance microscopy, scanning tunneling microscopy, X-ray photoelectron spectroscopy, auger electron spectroscopy/microscopy and point contact spectroscopy (PCS). Selected PCS spectra were fitted by extended 1D BTK model. The measurements were done on as-received as well as ion beam etched surfaces. Our results show that the substrate is considerably influencing the surface properties of the films, particularly the topography and surface conductivity distribution, what can affect results obtained by surface-sensitive techniques like PCS.

We demonstrate the efficiency of using interference effects in multilayer structures for a quantitative determination of submillimeter (submm) electrodynamic characteristics of materials using quasioptical spectroscopy based on backward-wave oscillators (BWOs). We present the results of studying fundamental physical properties of the conducting and superconducting objects of several types. Strong absorption in iron-based superconductors at “sub-gap” frequencies has been detected. In the temperature behavior of submm conductivity in LaNiO3/LaAlO3 heterostructures we found a metal–insulator phase transition that is observed in the case of thin (two-unit-cell) LaNiO3 layers and is absent in the case of thicker (four-unit-cell) LaNiO3 layers. In Ge/Si heterostructures with Ge quantum dots (QDs) we discovered the submm conductivity that is strongly enhanced compared with the conductivity of the structures containing the same amount of germanium, but not organized into a QD array.

We have systematically investigated the grain boundary (GB) angle dependence of transport properties for NdFeAs(O,F) fabricated on [001]-tilt symmetric MgO bicrystal substrates. In our previous study, NdFeAs(O,F) bicrystal films showed a weak-link behaviour even at a GB angle of 6{\deg}. However, this was caused by an extrinsic effect originating from the damage to both NdFeAs(O,F) and MgO substrate by excess F-diffusion along the grain boundary. To investigate the intrinsic nature of grain boundaries, we minimized the damage to NdFeAs(O,F) and MgO by reducing the deposition temperature of NdOF over-layer needed for F-doping. The resultant NdFeAs(O,F) bicrystal films have a critical angle of 8.5{\deg}, above which Jc starts to decrease exponentially. This critical angle is almost the same as those of other Fe-based superconductors., Comment: Accepted in Superconductor Science and Technology

To apply tensile or compressive uniaxial strain to functional thin films, we propose a novel approach in combining a piezoelectric-based device and a technical metallic substrate used widely in second generation coated conductors (i.e. superconducting tapes). A strain-induced shift of the superconducting transition temperature of 0.1 K for Co-doped BaFe2As2 was observed along the [100] direction, corresponding to a uniaxial pressure derivative dT c/dp 100 = −4 K/GPa. For Mn3CuN, a uniaxial strain derivative along the [100] direction of the Curie temperature dTC/depsilon 100 = 13 K/% was observed. The current approach is applicable to various functional thin films in a wide range of temperatures., ファイル公開：2020-01-04

In this work we present multilayers consisting of Fe and Co-doped BaFe 2 As 2 fabricated by UHV pulsed laser deposition. Employing the advantage of a coherent interfacial bonding between the FeAs tetrahedra of the iron pnictide and bcc Fe enables the growth of epitaxial bi- and multilayers of Fe and the new Fe-based superconductors with high crystalline quality. Epitaxial growth of the multilayers was controlled in situ by reflection high energy electron diffraction (RHEED) and after film growth by X-ray diffraction and TEM analysis. The stoichiometry determined by Auger electron spectroscopy (AES) indicated As-deficient layers of Co-doped Ba-122. The multilayers exhibit critical temperatures of T c,90 = 29 K which is so far the highest critical temperature reported for Co-doped Ba-122. We propose that the T c -enhancement is caused by As-vacancies.

For superconducting films, the optical intra-gap conductivity provides valuable information on the superconductivity mechanism, in particular on the gap symmetry, quasiparticles, and superconducting carrier dynamics. Experimentally, however, it is extracted with rather large uncertainty caused by the huge negative value of the dielectric constant. We have performed systematic numerical analysis of Fabry–Perot resonators, which consist of two identical superconducting iron pnictide thin films on dielectric substrates that are positioned face-to-face to each other and are separated by a spacer. We demonstrate that such a Fabry–Perot arrangement can significantly enhance the accuracy to the dynamical conductivity σ 1 of the films, as the efficiency of interaction of the probing radiation with the superconducting films is improved. Using a coherent source terahertz spectrometer (4–45 cm−1) with a Mach–Zehnder interferometer, we have measured the complex transmissivity of three Fabry–Perot resonators composed by identical pairs of Ba(Fe0.9Co0.1)2As2 films with the thicknesses: 25, 30, and 50 nm. We show that the experimental spectra can be well described by a corresponding five-layer model based on Fresnel’s equations and analyze the advantages and challenges of the Fabry–Perot resonant technique for iron pnictide films.

We report on the systematic characterization of Ba(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ epitaxial thin films on CaF$_2$ substrate in view of their possible use for superconducting electronic applications. By using different and complementary techniques we studied the morphological characteristics of the surface, the structural properties, the magnetic response, and the superconducting properties in terms of critical temperature, critical current, and energy gaps. Particular attention was paid to the homogeneity of the films and to the comparison of their superconducting properties with those of single crystals of the same compound., 19 pages, 7 figures. Preprint version

The Hall effect is a powerful tool for investigating carrier type and density. For single-band materials, the Hall coefficient is traditionally expressed simply by $R_H^{-1} = -en$, where $e$ is the charge of the carrier, and $n$ is the concentration. However, it is well known that in the critical region near a quantum phase transition, as it was demonstrated for cuprates and heavy fermions, the Hall coefficient exhibits strong temperature and doping dependencies, which can not be described by such a simple expression, and the interpretation of the Hall coefficient for Fe-based superconductors is also problematic. Here, we investigate thin films of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with compressive and tensile in-plane strain in a wide range of Co doping. Such in-plane strain changes the band structure of the compounds, resulting in various shifts of the whole phase diagram as a function of Co doping. We show that the resultant phase diagrams for different strain states can be mapped onto a single phase diagram with the Hall number. This universal plot is attributed to the critical fluctuations in multiband systems near the antiferromagnetic transition, which may suggest a direct link between magnetic and superconducting properties in the BaFe$_2$As$_2$ system., Comment: Accepted for publication in Scientific Reports, 6 main figures plus Supplemental Information (8 figures)

The Fe-based superconductors (FBS) present a large variety of compounds whose properties are affected to different extents by their crystal structures. Amongst them, the $\it{RE}$FeAs(O,F) ($\it{RE}$1111, where $\it{RE}$ is a rare earth element) is the family with the highest critical temperature $T_c$ but also with a large anisotropy and Josephson vortices as demonstrated in the flux-flow regime in Sm1111 ($T_c$ $\sim$ 55 K). Here we focus our attention on the pinning properties of the lower-$T_c$ Nd1111 in the flux-creep regime. We demonstrate that for H//c critical current density $J_c$ at high temperatures is dominated by point-defect pinning centres, whereas at low temperatures surface pinning by planar defects parallel to the $\it{c}$-axis and vortex shearing prevail. When the field approaches the $\it{ab}$-planes, two different regimes are observed at low temperatures as a consequence of the transition between 3D-Abrikosov and 2D-Josephson vortices: one is determined by the formation of a vortex staircase structure and one by lock-in of the vortices parallel to the layers. This is the first study on FBS showing this behaviour in a full temperature, field, and angular range and it demonstrates that, despite the lower $T_c$ and anisotropy of Nd1111 with respect to Sm1111, this compound is substantially affected by intrinsic pinning generating a strong $\it{ab}$-peak in $J_c$., Comment: Final version available on Scientific Reports (Open access)

The results of infrared reflectivity measurements for the iron-based high-temperature superconductor Ba(Fe0.9Co0.1)2As2 are reported. The reflectivity is found to be close to unity at frequencies w lower than 2Delta/h (2Delta 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(Fe0.9Co0.1)2As2 manifest opening of several superconducting gaps at temperatures lower than critical Tc., Comment: 4 pages, 2 figures

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 ≈ 80 nm) 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 (≈ 5 nm) 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 10 nm 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.

Terahertz and infrared spectra of the conductivity, σ(ν), and dielectric constant, ɛ(ν), of a Ba(Fe0.9Co0.1)2As2 film (T c = 20 K) have been analyzed together with previous specific-heat and angular resolved photoelectron spectroscopy data. It has been shown that the spectra σ(ν) and ɛ(ν) of Ba(Fe0.9Co0.1)2As2 in the superconducting phase at T = 5 K, as well as the magnetic field penetration depth, can be described well using the standard Bardeen-Cooper-Schrieffer (BCS) model with an additive contribution of electron and hole bands. It has been found that the measured temperature dependence of the magnetic field penetration depth in a wide temperature range 5 K < T < T c can be described only with the introduction of interband pairing interaction. The coupling constant of electron and hole bands, λ1, 2 = 0.1, as well as the temperature dependences of superconducting gaps in the electron and hole subsystems, has been determined using the model of two-band superconductivity developed earlier for MgB2.

We have successfully grown epitaxial, superconducting films of Ba(Fe1-xCox)2As2 (Ba-122)with x ~ 0.1. The films grow without observable correlated defects parallel to the c-axis, as confirmed by TEM. This is also reflected in the absence of a caxis peak in Jc({\theta}). In contrast to cuprate high-Tc superconductors such as YBa2Cu3O7-d or even Bi2Sr2Ca2Cu3O10-d, the pnictides show a rather low anisotropic behavior in their Jc({\theta}) behavior as well as in their upper critical fields, Hc2. As a multiband superconductor, Ba-122 exhibits a temperature dependent electronic mass anisotropy., Comment: contribution at ASC2010, accepted for publication in IEEE Trans. Appl. Supercond

Planetary milling technique has been a very promising way to obtain bulk superconductors with very high critical current density, Jc, albeit a detail characterisation of milled secondary phase precursor powders in particular has not been reported to date. Hence we report systematic studies of the effect of addition of planetary milled Gd2BaCuO5 (Gd-211) on the final microstructures and superconducting properties of air-processed Gd–Ba–Cu–O/Ag bulk samples. Average size of Gd-211 precursor particles, which were planetary milled with 1.0 mm ZrO2 beads, has been observed to decrease significantly from 1.03 μm to 0.52 μm with increasing milling duration. Besides the size distribution of milled Gd-211 was narrow compared to that of the reference powder. A small amount of Zr was detected, however, in the milled Gd-211 powder by the inductively coupled plasma-optical emission spectroscopy (ICP-OES) and its content was increased with increasing milling period, which led to an inhomogeneous bulk microstructure. Significantly, the average size of Gd-211 particles milled for 45 min has been observed to decrease from 0.73 μm to 0.48 μm without severe contamination of Zr when the diameter of the beads were reduced from 1.0 mm to 0.3 mm. Trapped magnetic field of single grain Gd–Ba–Cu–O/Ag bulk sample with 32 mm in diameter prepared from almost Zr free Gd-211 fine particles recorded over 1.5 T at 77 K, which was almost 1.3 times greater than that of the reference sample. Nevertheless the repulsive force of both bulk samples showed around 57 N at a gap of zero between the sample surface and SmCo5 permanent magnet.

Large, single grains of Y–Ba–Cu–O (YBCO) have been batch-processed to date by the top seeded melt growth (TSMG) process using NdBCO or SmBCO seed crystals. It has proved difficult, however, to economically batch-process light rare earth (LRE) LRE–Ba–Cu–O bulk high temperature superconductors, which have higher critical current densities and irreversibility fields than YBCO, and therefore greater potential for high field engineering applications. In this paper, we report a novel batch-process based on a cheap, readily available generic seed crystal, developed recently at Cambridge, and a TSMG melt processing technique based on cold seeding in air for the batch fabrication of Gd–Ba–Cu–O–Ag single grains. The superconducting properties of the (LRE)BCO single grains fabricated by this process are, in all respects, equivalent to those processed more conventionally in a reduced oxygen atmosphere.

The effect of $\alpha$-particle irradiation on a NdFeAs(O,F) thin film has been investigated to determine how the introduction of defects affects basic superconducting properties, including the critical temperature $T_c$ and the upper critical field $H_{c2}$, and properties more of interest for applications, like the critical current density $J_c$ and the related pinning landscape. The irradiation-induced suppression of the film $T_c$ is significantly smaller than on a similarly damaged single crystal. Moreover $H_{c2}$ behaves differently, depending on the field orientation: for H//c the $H_{c2}$ slope monotonically increases with increasing disorder, whereas for H//ab it remains constant at low dose and it increases only when the sample is highly disordered. This suggests that a much higher damage level is necessary to drive the NdFeAs(O,F) thin film into the dirty limit. Despite the increase in the low temperature $H_{c2}$, the effects on the $J_c$(H//c) performances are moderate in the measured temperature and field ranges, with a shifting of the pinning force maximum from 4.5 T to 6 T after an irradiation of $2\times10^{15} cm^{-2}$. On the contrary, $J_c$(H//ab) is always suppressed. The analysis demonstrates that irradiation does introduce point defects acting as pinning centres proportionally to the irradiation fluence but also suppresses the effectiveness of c-axis correlated pinning present in the pristine sample. We estimate that significant performance improvements may be possible at high field or at temperatures below 10 K. The suppression of the $J_c$(H//ab) performance is not related to a decrease of the $J_c$ anisotropy as found in other superconductors. Instead it is due to the presence of point defects that decrease the efficiency of the ab-plane intrinsic pinning typical of materials with a layered structure., Comment: 12 pages, 9 figures

Y 2 Ba 4 CuMO (Y-2411, where M is a transition metal element) has been recognized as a potential candidate for enhanced flux pinning in Y–Ba–Cu–O (YBCO) bulk superconductors. However, the non-uniform distribution of Y-2411 in the superconducting phase matrix and the presence of a – c micro-cracks in the sample microstructure limit the magnetic field trapping ability of bulk YBCO containing these inclusions. Recent attempts to add Ag to the YBCO matrix have been observed to enhance the microstructural properties of Y-2411 containing materials. In this work we report the fabrication of high performance YBCO composite containing Y-24Nb1 (Y 2 Ba 4 CuNbO y ) and metallic Ag. The average value of critical current density, J c , is compared for samples containing different concentrations of Y-24Nb1. The microstructural features of these samples have been compared with a YBCO single grain processed with Y-24Nb1 and metallic Ag.

A practical processing route for the fabrication of LRE-Ba–Cu–O single grain superconductors has been developed at the University of Cambridge based on a generic, Mg-doped Nd-123 melt textured seed and suppression of the formation of the solid solution phase in air by enriching the precursors with higher Ba concentration. The processing of high performance Gd–Ba–Cu–O single grains using this processing route is described. The Mg-doped generic seed crystal has been used effectively to promote heterogeneous nucleation via a cold-seeding process. The Gd/Ba solid solution has been suppressed by enriching Gd–Ba–Cu–O precursor powders with two different Ba-rich compositions. This involved adding BaO2 and GdBa6Cu3Oy (Gd-163) (a novel Ba-rich second phase) to the precursor powders, respectively. The Gd-163 phase has been observed not only to suppress formation of the solid solution phase, but also to promote increased heterogeneous grain size. A detailed further study has been carried out with an initial aim of optimizing the BaO2 and Gd-163 phase content of the precursor composition to produce a single grain almost free of solid solution. Based on the optimized parameters, large single grain Gd–Ba–Cu–O superconductors have been fabricated in an air atmosphere and demonstrated to exhibit record trapped magnetic fields for this material melt processed in air in relatively small single grain samples. The trapped fields of samples produced in air atmosphere are at least comparable to those processed under reduced oxygen partial pressure.

A new melt processing route for the fabrication of large grain Gd-Ba-Cu-O (GdBCO) bulk superconductors has been developed based on the use of novel GdBa4Cu3O8−δ (Gd-143) and GdBa6Cu3O10−δ (Gd-163) precursor compositions. The new processing route enables the fabrication of large single grains from precursor powders that contain high concentrations of Ba. The superconducting properties and microstructures of GdBCO single grains with extra Ba fabricated via this new processing route are reported. Most importantly, the possible formation of a new form of Gd1+xBa2−xCu3O7−δ solid solution (Gd-123ss) with x < 0 in single grains fabricated from the Ba-rich precursor are discussed for the first time based on the superconducting, chemical, and structural properties of the large GdBCO grains.

Nano-scale Sm 2 Ba 4 CuMo y (Sm-2411) phase inclusions have been introduced into the Sm-123 single grain using a top seeded melt growth technique. Microstructural features of SmBa 2 Cu 3 O 7 containing Sm 2 Ba 4 CuMo y phase inclusions and their influence on the J c (B) performance of single grain composites have been presented in this paper. The presence of Sm-2411 nano-scale inclusions is observed generally to improve both J c over a wide range of magnetic fields and the irreversibility field of bulk Sm-Ba-Cu-O superconductors.

Single grain, Ag-doped Y–Ba–Cu–O bulk superconductors of up to 30 mm in diameter have been fabricated successfully by a seeded infiltration and growth (IG) technique. These samples exhibit low porosity compared with samples grown by conventional top seeded melt growth (TSMG). The density of voids within as-processed IG samples can be reduced further by adding metallic Ag, rather than Ag2O, to the Y2BaCuO5 (Y-211) pre-form. An inhomogeneous distribution of Y-211 has been observed in these samples. Finally, a maximum trapped magnetic field of 0.5 T was observed for Ag-doped Y–Ba–Cu–O of diameter 30 mm at liquid nitrogen temperature (77 K).

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. Y2Ba4CuWOy) 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.

Presence of various nano-scale Y2Ba4CuMOy phase inclusions in YBa2Cu3O7−δ phase matrix are shown to improve magnetic flux pinning over wide range of magnetic fields. We first fabricate single phase of Sm2Ba4CuBiOy (Sm-2411) using a solid-state reaction and then introduced them into SmBCO single grains. Top seeded melt growth (TSMG) has been used to grow SmBCO single grains in air atmosphere. A significant improvement in Jc is observed, over wide magnetic fields, for single grains containing Sm-2411nano-phase inclusions when compared to that of SmBa2Cu3O7−δ/Sm2BaCuO5 composites. When compared to YBCO nano-composites, SmBCO composites are shown to exhibit high Jc at medium range of magnetic fields (1–2 T) at 77 K.

The thermodynamic stability of MgO-doped Nd 1+ x Ba 2− x Cu 3 O y ( x = 0–0.5) has been investigated as a function of solid solution level ( x ) under various oxygen partial pressures for use as a seed crystal composition in the melt growth of (LRE)-Ba–Cu–O (where LRE = light rare earth) bulk superconductors. The gradient of decomposition temperature, T p , against solid solution level increases significantly with decreasing oxygen partial pressure, indicating that MgO-doped Nd–Ba–Cu–O (NdBCO) seed crystals should be grown under reduced p O 2 to obtain seeds with high T p . Based on these results, the optimum cold-seeded processing conditions for the fabrication of single grain GdBCO bulk superconductors containing silver (GdBCO/Ag) under a controlled oxygen partial pressure of 0.01 atm have been determined. The onset T c of GdBCO/Ag samples grown in air with the optimized seed crystal composition is improved significantly with increasing Ag content, indicating that silver addition to the precursor powder suppresses the formation of Gd–Ba solid solution.

Silver-doped Y?Ba?Cu?O (YBCO) bulk superconductors have been fabricated successfully in the form of large, single grains by a seeded infiltration and growth (IG) technique. A uniform distribution of silver in the superconducting YBa2Cu3O7?? (Y-123) phase matrix was achieved by adding Ag2O to both the Y2BaCuO5 (Y-211) pre-form and a liquid source precursor pellet. Although Ag can also be dispersed well in Ag-doped YBCO fabricated by the top-seeded-melt-growth (TSMG) technique, bulk samples prepared by the IG technique typically exhibit significantly lower porosity due to the absence of the formation of oxygen during the peritectic decomposition process within the Y-211 pre-form. This yields samples with potentially enhanced current-carrying ability. The density of voids in bulk samples can be decreased further by adding metallic Ag, rather than Ag2O, to the Y-211 pre-form in the IG process. The distribution of Y-211 in the Y-123 matrix in samples fabricated by both IG and TSMG is rather inhomogeneous. As a result, the superconducting properties of individual small samples cut from large parent grains are similar for samples fabricated by both IG and TSMG.

Insulating RE2BaCuO5 (RE-211) second-phase inclusions embedded within the REBa2Cu3O7−δ (RE-123) superconducting phase matrix are known to contribute significantly to flux pinning in melt-processed superconductors. The critical current density in these materials has been observed to increase significantly in the presence of low external magnetic fields, in particular due to the presence of the RE-211 phase. Jc at higher applied magnetic field, however, is unaffected by these inclusions. Recently we have introduced successfully nanoscale (RE)2Ba4CuMOy (RE-2411, where M = W,Nb,Ag and Bi) phase inclusions into the RE-123 phase matrix using a top-seeded melt growth (TSMG) technique. Here we report the flux pinning behaviour of various REBa2Cu3O7−δ/(RE)2Ba4CuMOy nanocomposites and compare their properties with those of REBa2Cu3O7−δ/RE2BaCuO5 single-grain composites. A direct correlation between increased Jc and increased concentration of nanoscale inclusions is observed. Finally, the presence of RE-2411 nanoscale inclusions is observed generally to improve both Jc and the irreversibility field of bulk RE–Ba–Cu–O superconductors processed by TSMG.

A generic Mg-doped Nd-Ba-Cu-O seed crystal has been developed recently for the fabrication of any type of rare earth (RE) based (RE)-Ba-Cu-O single grain bulk superconductor in air. The new generic seed simplifies significantly the top seeded melt growth (TSMG) process for light rare earth based (Nd, Sm, Gd, or mixed rare earth elements) bulk superconductors, in particular. GdBCO single grains have been fabricated successfully in air using the new seed in a cold-seeding process. In this study, precursor powders were enriched with different amounts of BaO2 to investigate the extent of substitution of Gd for Ba in the Gd1+xBa2-xCu3O7-delta solid solution phase. The growth process of large single grains in air was investigated at various growth temperatures under isothermal processing conditions. Crystal growth rate as a function of under-cooling and BaO2 content has been determined from these experiments. The spatial variation of Tc and transition temperature width for applied field aligned along the a/b and c-axis of grains fabricated with different BaO2 content has also been investigated in order to understand the extent of the formation of Gd/Ba solid solution with varying growth temperature and precursor composition. These results have been used to establish the optimum conditions for fabricating solid solution-free, large single grains of GdBCO in air.

A multi-seeding process has been developed to fabricate single Y-Ba-Cu-O (YBCO) grains containing strong artificial grain boundaries. Multi-seeding of heterogeneous YBCO grains with controlled orientation was achieved using large Sm-Ba-Cu-O (SmBCO) single crystal seeds of rod-like geometry with slots of various widths (up to 13 mm) cut into their bottom surface (i.e. parallel to the c-axis of the seed) to produce a bridge-like structure. Several YBCO grains with artificial grain boundaries were fabricated from these seed crystals and used to investigate the effect of varying the distance between the individual grain nucleation sites and the grain orientation (in-plane and out of plane) on the nature of grain boundaries. The measured local magnetic critical current density (Jc) and the magnitude of the trapped field across these artificial grain boundaries indicate that seed alignment is a key parameter in achieving strongly-coupled grain boundaries in multi-seeded grains.

Flux pinning sites are most effective if their size is comparable to the superconducting coherence length, which is on the nano-meter scale for RE-Ba-Cu-O superconductors [RE = rare earth element]. Introducing nano-phase inclusions directly into the bulk superconducting material has only been partially successful to date, however, due primarily to the absence of chemically stable phases that can co-exist with RE-Ba-Cu-O without suppressing its key superconducting properties. We have identified novel isostructural phases based on (RE)2Ba4CuMOy (where M = W, Zr, Nb, Ag and Bi) and have fabricated successfully superconducting bulk nano-composites with a high current carrying capability. The average size of the nano-inclusions is observed to vary from 20 nm to 300 nm depending on element M. An observed improvement in Jc under low and high external magnetic fields at 77 K correlates directly with an increased density of nano-inclusions in the superconducting matrix.

Consecutive proton irradiation of iron pnictides reduces the superconducting transition temperature to decrease monotonically. Our systematic optical THz investigations of Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ thin films reveal, however, that the low-energy superconducting gap is first suppressed, but recovers for higher irradiation doses. At the same time the decrease of the superfluid density with disorder comes to a halt. These observations are in excellent agreement with theoretical calculations, which predict a disorder-induced transition of the order parameter from $s_{\pm}$ to $s_{++}$ symmetry. Thereby we solve the longstanding puzzle of the iron pnictides gap symmetry in favor of $s_{\pm}$-symmetry and demonstrate THz spectroscopy as a tool to determine the symmetry of the superconducting order parameter.

The upper critical field Hc2 anisotropy of Ca10(PtnAs8)(Fe2-xPtxAs2)5 (n = 3, 4) single crystals with long FeAs interlayer distance (d) was studied by angular dependent resistivity measurements. A scaling of the angular dependent resistivity was realized for both single crystals using the anisotropic Ginzburg-Landau (AGL) approach with an appropriate anisotropy parameter {\gamma}. The AGL scaling parameter {\gamma} increases with decreasing temperature and reaches a value of about 10 at 0.8Tc for both single crystals. These values are much larger than those of other iron-based superconductors (FeSCs). Remarkably, the values of {\gamma}2 show an almost linear increase with the FeAs/FeSe interlayer distance d for FeSCs. Compared to cuprates, FeSCs are less anisotropic, indicating that two dimensionality of the superconductivity is intrinsically weak., Comment: Accepted in Applied Physics Letters. 5 figures

FeSe0.5Te0.5 thin films were grown by pulsed laser deposition on CaF2, LaAlO3 and MgO substrates and structurally and electro-magnetically characterized in order to study the influence of the substrate on their transport properties. The in-plane lattice mismatch between FeSe0.5Te0.5 bulk and the substrates shows no influence on the lattice parameters of the films, whereas the type of substrates affects the crystalline quality of the films and, therefore, the superconducting properties. The film on MgO showed an extra peak in the angular dependence of critical current density Jc({\theta}) at {\theta} = 180{\deg} (H || c), which arises from c-axis defects as confirmed by transmission electron microscopy. In contrast, no Jc({\theta}) peaks for H || c were observed in films on CaF2 and LaAlO3. Jc({\theta}) can be scaled successfully for both films without c-axis correlated defects by the anisotropic Ginzburg-Landau (AGL) approach with appropriate anisotropy ratio {\gamma}J. The scaling parameter {\gamma}J is decreasing with decreasing temperature, which is different from what we observed in FeSe0.5Te0.5 films on Fe-buffered MgO substrates., Comment: accepted for publication in SUST

A systematic characterization of Co-doped BaFe2As2(Ba-122) thin films has been carried out. Two samples were available, one grown on CaF2substrate and the other on MgO with an Fe buffer layer. The goal was to investigate films’ magnetic and superconducting properties, their reciprocal interplay, and the role played by the Fe buffer layer in modifying them. Morphological characterization and Energy Dispersive X-ray analyses on the Fe-buffered sample demonstrate the presence of diffused Fe close to the Co-doped Ba-122 outer surface as well as irregular holes in the overlying superconducting film. These results account for hysteresis loops obtained with magneto-optic Kerr effect measurements and observed at both room and low temperatures. The magnetic pattern was visualized by magneto-optical imaging with an indicator film. Moreover, we investigated the onset of superconductivity through a measure of the superconducting energy gap. The latter is strictly related to the decay time of the excitation produced by an ultrashort laser pulse and has been determined in a pump-probe transient reflectivity experiment. A comparison of results relative to Co-doped Ba-122 thin films with and without Fe buffer layer is finally reported.

Nanocomposite oxide high-temperature bulk superconductors can be used as “quasi-magnets.” Thanks to the recent progress of material processing, “quasi-magnet” with 26 mm diameter can generate a large field of 17.6 T at 26 K. These results are highly attractive for applications, involving levitation of permanent magnets on the bulk superconductors. Indeed, several other applications such as motors and magnetic resonance microscope using bulk superconductors have been proposed and demonstrated. In this chapter, we describe several techniques to improve the magnetic properties for bulk superconductors together with some basics such as phase diagrams and solidifications.

A Mg-doped Nd–Ba–Cu–O (NdBCO) generic seed crystal has been developed recently for the fabrication of any type of rare earth (RE) based (RE)-Ba–Cu–O single grain bulk superconductor. The new generic seed significantly simplifies the top seeded melt growth (TSMG) process for light RE based (Nd, Sm, Gd or mixed rare earth elements) bulk superconductors in particular. In this paper we report that single domain, bulk GdBCO samples have been fabricated successfully in air using the new seed in a cold seeding process. In this study, precursor powders were enriched with different amounts of BaO2 to investigate the extent of substitution of Gd for Ba in the Gd1+xBa2−xCu3O7−δ (Gd-123) solid solution phase. The particle size of the Gd2BaCuO5 (Gd-211) phase inclusions in the Gd-123 phase matrix and the spatial variation of Tc and ΔTc along the a/b and c-axes of single grains fabricated with varying amounts of externally added BaO2 have been investigated as a function of BaO2 content. Jc values as high as 7.2 × 104 A cm−2 at 77 K and 0 T have been observed in large GdBCO grains processed under air.

Large single grains of Y–Ba–Cu–O (YBCO) bulk superconductor of up to 32 mm in diameter have been fabricated by a seeded infiltration and growth (IG) technique. Small Y 2 BaCuO 5 (Y-211) particles are observed in the seeded IG sample microstructure, albeit with a rather inhomogeneous distribution in the superconducting YBa 2 Cu 3 O 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 0.42 T at 0.2 mm above the sample surface was observed at liquid nitrogen temperature (the actual mean temperature during measurement was around 78 K) in samples fabricated by seeded IG due to the increased J c , despite the relatively small YBCO grain size (25 mm diameter × 7.4 mm thickness). This value of trapped field is almost two times greater than that of samples fabricated by TSMG and of a similar size.

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 YBa 2 Cu 3 O 7− 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.

Nano-composite Y–Ba–Cu–O (YBCO) superconductors containing non-superconducting phase inclusions of composition Y2Ba4CuMOy (Y-2411), where M = Bi and Ag, embedded in the YBa2Cu3O7−δ (Y-123) superconducting matrix have been fabricated successfully. The Y-2411 phase, which typically has dimensions in the range 20–300 nm depending on the element M, is characterized by a double perovskite cubic crystallographic structure and forms effective flux pinning sites in bulk YBCO. A direct correlation between the Y-2411 phase inclusion content in the Y-123 matrix and increased magnetic flux pinning has been observed in these materials. The flux pinning force and the critical current densities increase by an order of magnitude for nano-composites compared to Y–Ba–Cu–O fabricated without nano-scale inclusions, confirming their potential for improved current carrying performance and for use in high field engineering applications.

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 Tc 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 77 K and higher trapped fields have been achieved in optimized (LRE)BCO superconductors fabricated in air for the first time.

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-δ 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-δ 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.

Nano-composite (RE)-Ba-Cu-O bulk superconductors containing non-superconducting phase inclusions of composition RE 2 Ba 4 CuMO y (the '2411' phase), where M = W, Nb, Ag and Bi, embedded in the REBa 2 Cu 3 O 7- d (RE-123) superconducting matrix have been fabricated successfully. The 2411-phase has typical dimensions in the range 10-300 nm and is stable under the substitution of any rare earth element on the Y site for all M elements. The 2411-phase is paramagnetic in nature and is characterized by a double perovskite cubic crystallographic structure. All phases within the (RE)-2411 family are chemically inert within the RE-123 matrix and do not coarsen appreciably at elevated temperatures during processing. Most significantly, the (RE)-2411 phase forms effective flux pinning sites within the superconducting matrix, since their size is comparable to that of the coherence length of RE-Ba-Cu-O. A direct correlation between the RE 2 Ba 4 CuMO y phase inclusions content in the RE-123 matrix and increased magnet...

We have succeeded recently in synthesizing a chemically stable, inert family of materials of composition Y2Ba4CuMOy (Y-2411 where M Nb, Ta, Mo, W, Zr, Hf) within the superconducting YBa 2Cu3O7-δ (Y-123) phase matrix that forms effective flux pinning centers of nano-scale dimensions. In this paper we report the synthesis of the Y2Ba4CuBiOy phase with nano-scale dimensions that is similarly compatible with the Y-123 matrix and which does not impair the properties of the bulk superconductor. YBa 2Cu3O7-δ/Y2BaCuO5 (Y-123/Y-211) precursor powders enriched with various amounts of Bi 2O3 and Y2Ba4CuBiOy have been fabricated successfully in the form of large, single grains by the top seeded melt growth (TSMG) process. Microstructural studies of these composites reveal the presence of nanometer-sized Y2Ba4CuBiO y and much larger Y-211 phase particles (∼1 νm) embedded in the Y-123 phase matrix. The critical current density of the nano-composites is observed to increase significantly compared to undoped YBCO. © 2006 IOP Publishing Ltd.

Large, single grains of Y?Ba?Cu?O (YBCO) bulk superconductor composed of a YBa2Cu3O7?? (Y-123) matrix with Y2BaCuO5 (Y-211) inclusions have been fabricated using a seeded infiltration and growth (IG) process. Y-211 precursor pellets prepared from powders containing different Y-211 particle sizes were processed by the seeded IG technique to investigate the effect of Y-211 size on the single grain microstructure. The superconducting properties of the single grains fabricated in this study are observed to correlate well with the variation in Y-211 particle size in the fully textured Y-123 phase matrix. The critical current density, Jc, in particular, has been observed to increase from 68?000?A?cm?2 for a reference sample to 94?000?A?cm?2 at 77?K in self-field for a sample prepared using refined Y-211 precursor powder with added platinum oxide.

Single-grain Gd–Ba–Cu–O (GdBCO) bulk superconductors have been grown by a seeded infiltration and growth (SIG) technique under a 1% O2+N2 atmosphere using a generic MgO-doped Nd–Ba–Cu–O (MgO–NdBCO) seed placed on the sample surface at room temperature (the so-called the cold-seeding method). Partial melting of the MgO–NdBCO seeds fabricated in air under notionally identical thermal processing conditions, however, limited the reliability of this bulk GdBCO single-grain process. The observed seed decomposition is attributed to the dependence of the peritectic temperature Tp of MgO-doped Nd1+xBa2−xCu3Oy solid solution (MgO-doped Nd-123ss, where ss indicates solid solution) compounds on both oxygen partial pressure during the melt process and the level of solid solution (x). The peritectic decomposition temperature of MgO-doped Nd-123ss, with x ranging from 0 to 0.5 under p(O2) = 1.00 atm, was observed to remain constant at 1120 °C. Tp was observed to decrease linearly as a function of solid solution level, on the other hand, under oxygen partial pressures of both p(O2) = 0.21 and 0.01 atm. Based on these results, MgO-doped NdBCO seed crystals should be grown under reduced oxygen partial pressure in order to obtain a stable MgO-doped NdBCO seed crystal suitable for cold-seeding processes of large-grain (RE)BCO bulk superconductors (where RE is a rare earth element).

We report a practical processing method for the fabrication in air of large, single grain LRE-Ba–Cu–O [(LRE)BCO, where LRE = light rare earth ] superconductors in a simple box-type furnace that exhibit high Tc and Jc. The technique is potentially economical, offers a significant degree of freedom in terms of both processing parameters and reproducibility in the growth of oriented single grains and yields bulk samples with significantly improved superconducting and field-trapping properties. The process is based initially on the development of a new type of generic seed crystal that can promote effectively the epitaxial nucleation of any (RE)-Ba–Cu–O system, and secondly on the suppression of the formation of (LRE)/Ba solid solution in a controlled manner within large (LRE)BCO grains processed in air. The characteristics of the generic seed are discussed in detail. In addition, the degree of homogeneity of large grain Nd–Ba–Cu–O, Sm–Ba–Cu–O and Gd–Ba–Cu–O superconductors fabricated in air by this novel processing technique has been investigated from the spatial distribution of the superconducting transition temperature and the critical current density along both the a and c growth sectors of each single grain. As a result, the optimum processing conditions for the fabrication of single grains with improved homogeneity and with high Jc have been established. Finally, Jc of 105 A cm−2 has been measured for large grains of Gd–Ba–Cu–O bulk superconductor at 77 K.

We report the seeded infiltration and growth (IG) of Gd–Ba–Cu–O (GdBCO) bulk superconductors for varying molten Ba–Cu–O liquid compositions using an Mg-doped Nd–Ba–Cu–O (Mg-NdBCO) melt-textured seed crystal developed recently at the IRC. All the samples were grown in the form of single grains, although the size of the grain was observed to decrease for an increasingly Ba-rich initial composition. The size of Gd2BaCuO5 (Gd-211) particles in the GdBa2Cu3Oy (Gd-123) phase matrix is almost constant for samples with different BaO2 content in the liquid source pellet. The volume fraction of Gd-211 in the Gd-123 matrix (Vf211), however, decreases significantly with increasing BaO2 content. The superconducting transition temperature, Tc, and transition width within the entire seeded IG bulk GdBCO grain are also extremely sensitive to BaO2 concentration. A maximum critical current density, Jc, of 88 000 A cm−2 at 77.3 K in self-field is observed in this study in the sample containing the greatest concentration of BaO2 in the liquid source pellet grown in air and without further Ar and/or low oxygen heat treatment (which would give further improvements in the superconducting properties of the samples). The highest value of Jc observed in this study of 115 000 A cm−2 at 77.3 K in self-field was obtained in the sample grown in a 1% O2+N2 atmosphere. These results indicate that the seeded infiltration and growth technique produces effective pinning centres in the bulk microstructure, at least in the self-field limit.

Nanocomposite Y?Ba?Cu?O superconductors containing a wide range of Y2Ba4CuMOy (Y-2411), where M = Nb, Ta, Mo, W, Zr, Hf, Ru, Ag, Sb, Sn?and?Bi, embedded in the YBa2Cu3O7?? matrix, have been fabricated successfully. The Y-2411 phase typically has dimensions in the range 20?50?nm and it is observed to form effective flux pinning sites even at higher magnetic fields. A direct correlation between the Y2Ba4CuMOy phase inclusion content in the YBa2Cu3O7?? matrix and increased magnetic flux pinning has been observed. The critical current density increases by an order of magnitude for a nanocomposite compared to that of Y-123 without inclusions.