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We report on electrical transport properties of epitaxial Ba(Fe1-xNix)2As2 thin films grown by pulsed laser deposition in static magnetic fields up to 35 T. The thin film shows a critical temperature of 17.2 K and a critical current density of 5.7x10^5 A/cm^2 in self field at 4.2 K while the pinning is dominated by elastic pinning at two-dimensional nonmagnetic defects. Compared to single-crystal data, we find a higher slope of the upper critical field for the thin film at a similar doping level and a small anisotropy. Also an unusual small vortex liquid phase was observed at low temperatures, which is a striking difference to Co-doped BaFe2As2 thin films., Comment: Accepted for publication in Applied Physics Letters, 5 figures

The current transport capability of YBa 2 Cu 3 O 7-x (YBCO) based coated conductors (CCs) is mainly limited by two features: the grain boundaries of the used textured template, which are transferred into the superconducting film through the buffer layers, and the ability to pin magnetic flux lines by incorporation of defined defects in the crystal lattice. By adjusting the deposition conditions, it is possible to tailor the pinning landscape in doped YBCO in order to meet specific working conditions (T, B) for CC applications. To study these effects, we deposited YBCO layers with a thickness of about 1-2 μm using pulsed laser deposition on buffered rolling-assisted biaxially textured Ni-W substrates as well as on metal tapes having either an ion-beam-textured YSZ buffer or an MgO layer textured by inclined substrate deposition. BaHfO 3 and the mixed double-perovskite Ba 2 Y(Nb/Ta)O 6 were incorporated as artificial pinning centers in these YBCO layers. X-ray diffraction confirmed the epitaxial growth of the superconductor on these templates as well as the biaxially oriented incorporation of the secondary phase additions in the YBCO matrix. A critical current density J c of more than 2 MA/cm 2 was achieved at 77 K in self-field for 1-2 μm thick films. Detailed TEM (transmission electron microscopy) studies revealed that the structure of the secondary phase can be tuned, forming c-axis aligned nanocolumns, ab-oriented platelets, or a combination of both. Transport measurements show that the J c anisotropy in magnetic fields is reduced by doping and the peak in the J c (θ) curves can be correlated to the microstructural features.

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

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 addition of mixed double perovskite Ba2Y(Nb/Ta)O6 (BYNTO) to YBa2Cu3O7−δ (YBCO) thin films leads to a large improvement of the in-field current carrying capability. For low deposition rates, BYNTO grows as well-oriented, densely distributed nanocolumns. We achieved a pinning force density of 25 GN/m3 at 77 K at a matching field of 2.3 T, which is among the highest values reported for YBCO. The anisotropy of the critical current density shows a complex behavior whereby additional maxima are developed at field dependent angles. This is caused by a matching effect of the magnetic fields c-axis component. The exponent N of the current-voltage characteristics (inversely proportional to the creep rate S) allows the depinning mechanism to be determined. It changes from a double-kink excitation below the matching field to pinning-potential-determined creep above it.

The incorporation of nanoscaled pinning centers in superconducting YBa 2 Cu 3 O 7-δ (YBCO) films is one of the core topics to enhance the critical current density J c (B, Θ) of coated conductors. The mixed double-perovskite Ba 2 Y(Nb/Ta)O 6 (BYNTO) can be grown in nanosized columns parallel to the YBCO c-axis and in steplike patterns, making it customizable to meet specific working conditions (T, B, Θ). We compare a 1.6 μm thick film of pure YBCO and a similar film with additional 5 mol% of BYNTO, grown by pulsed laser deposition with a growth rate of 1.6 nm/s on chemically buffered biaxially textured Ni-5at.% W tape. Our doped sample shows nanosized BYNTO columns parallel to c YBCO and plates in the ab-plane containing Y, Nb, and Ta. An improved homogeneity of the critical current density J c over the sample was evaluated from trapped field profiles measured with a scanning Hall probe microscope. The mean J c in the rolling direction of the tape is 1.8 MA/cm 2 (77 K, self-field) and doubles the value of the undoped sample. Angular-dependent measurements of the critical current density, J c (Θ), show a decreased anisotropy of the doped film for various magnetic fields at 77 K and 64 K.

GdBa 2 Cu 3 O 7-x (GdBCO) films doped with up to 13.6 mol.% BaHfO 3 (BHO) have been grown by electron beam physical vapor deposition (EB-PVD). A critical current density J c at 30 K, 3 T of 2.34 MA/cm 2 has been achieved for a doping level of 10.6 mol.%. This is the first report of artificial pinning centers being successfully incorporated into REBa 2 Cu 3 O 7-x (REBCO, RE = Y, Gd, Dy . . .) via EB-PVD. The samples were grown on Hastelloy tapes coated with a MgO buffer layer deposited by inclined substrate deposition. The J c of the samples was characterized at 77 K, 1 T and at 30 K, 3 T. An increase in J c (30 K, 3 T) upon Hf addition has been observed in the whole angular range α. The level of Hf addition was found to heavily influence the kind of nanoprecipitate formed, with low levels of Hf causing a reduction in the anisotropy of J c (77 K, 1 T) and higher levels of Hf resulting in the formation of nanoprecipitates elongated parallel to the GdBCO ab-plane, causing an increase in the anisotropy of J c (77 K, 1 T). The largest J c (77 K, sf) measured was 0.63 MA/cm -2 .

Scanning electron microscope imaging and electron backscatter diffraction are applied to 400 nm thick YBCO films grown on Ni-9at.%W and ABAD-YSZ tape. On Ni-9at.%W tape, the orientation of YBCO strongly varies from grain to grain, which is attributed to the different orientations of the underlying substrate grains with regard to the surface normal. On ABAD-YSZ, the structures causing the orientation variations are observed on a micrometer scale only, which is attributed to the granularity of the template. In contrast to Ni-9at.%W where no preferred misorientation axis is notable within single substrate grains, the misorientation of YBCO on the ABAD-YSZ tape is primarily caused by lattice rotations about the sample normal.

Reversible control of magnetism at the metal/electrolyte interface offers great application potential for multifunctional magnetic devices. Here, the electrode–electrolyte interactions and the charging of FePt alloy films in LiPF 6 in dimethyl carbonate(DMC)–ethylene carbonate (EC) are analysed in comparison to the pure metals Fe and Pt. The main electrode processes are Li deposition and dissolution, Fe dissolution and redeposition, and reduction as well as oxidation processes of impurities and electrolyte. Fe is found to be more stable against anodic dissolution in FePt films. Pt attenuates electrode passivation and catalyses impurity reduction reactions. This results in a cathodic potential limit of 1.8 V vs. Li/Li + for charging FePt films without side reactions. Subsequently, ultrathin textured FePt films promising large voltage induced magnetic surface effects where used as electrodes. Magnetic properties during charging were monitored by an in situ setup based on the anomalous Hall effect. Marginal Fe dissolution at the surface above 3 V vs. Li/Li + is detected by the in situ measurement and limits the upper charging potential. In consequence, a potential window of 1.2 V for charging the FePt films is obtained. In this potential range a large reversible change of magnetic moment is detected, which is attributed to reversible redox processes in the native iron oxide surface layer.

Ion Beam Assisted Deposition (IBAD) offers the opportunity to prepare highly textured templates for YBa2Cu3O7-δ (YBCO) coated conductors. We investigated a new simplified buffer layer architecture on mechanically polished stainless steel tapes with an amorphous Ta0.75Ni0.25 bed layer. Cube textured TiN layers have been prepared by ion beam assisted pulsed laser deposition on these substrates. Consequently, highly textured YBCO layers were obtained with an (103) in-plane alignment of Δφ = 5.9° using an additional SrZrO3 buffer layer. A maximum critical current density of about 2.9 MA/cm2 was measured at 77 K in self-field. The films showed an improved Jc(H)-behavior compared to standard YBCO coated conductors.

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

We discuss implementation of reactive co-evaporation (RCE) to deposit high temperature superconducting films on metal tapes. Our method utilizes alternating high-rate deposition and oxygenation and can be extended to coating of moving tape in a continuous fashion. We have demonstrated our method with YBa2Cu3O7 films on 8 cm long ion-beam assisted deposition textured template tapes. Critical current densities of approximately 2.5 MA/cm2 at 75.5 K were achieved in HTS films with 2 mum in thickness, and a maximum measured self-field critical current of 950 A/cm-width was achieved in a 6 mum thick film. We discuss some practical possibilities for manufacturing superconducting wire and discuss a cost model for this RCE process. We make the case that the cost of HTS deposition using RCE in manufacturing can be less than $5/kAmiddotm.

We examined crystalline-texture evolution during ion-beam-assisted deposition (IBAD) of MgO thin films. We have demonstrated for the first time that in-plane crystalline texturing in IBAD of MgO scales with deposition rate. At high ion currents an in-plane texture full width at half-maximum (FWHM) of 10° can be achieved in less than 1 s, and 6° in 2.2 s. MgO texture further improves with thickness of a homoepitaxial layer deposited on top. We have developed an empirical quantification of the texture evolution in both IBAD and homoepitaxial layers. The best texture attained thus far in the MgO layer on polished Hastelloy tape has an in-plane FWHM of 1.6°. The high deposition rates demonstrated here make high-throughput manufacturing of IBAD textured templates a practical and cost-effective concept.

To measure the critical current of a long coated conductor tape as a function of angle, Ic(thetas), where thetas is the angle between the magnetic field B and the c-axis direction, we have solved the problems of measurement system geometry, size, and cost by developing a cryogenic rotator containing permanent magnets to change the angle of the applied field. Central magnetic fields of 0.3 T and 0.53 T have been generated. For position dependent measurements, a device, similar in design to a large audio cassette, translates the superconductor tape through the magnetic field region. To demonstrate the usefulness of this device, we present results on the position-dependent critical current anisotropy at 75 K for a 5 m long YBCO coated conductor.

In this paper we present initial results on growing YBCO/IBAD-MgO/YBCO stacks. Amorphous Y2O3 layers, deposited by sol-gel, are used to level the YBCO surface. Since the YBCO films prepared by pulsed laser deposition are relatively rough, preparing a smooth surface on top of YBCO for IBAD texturing is a major challenge for this structure. For all substrates studied-polished and unpolished metal tape and YBCO - we were able to reduce the roughness of the substrate using the sol-gel layer, with multiple coatings being more effective. First it is demonstrated that high quality YBCO films can be deposited on sol-gel-Y2O3/IBAD-MgO templates. Second it is shown that the sol-gel and the IBAD process do not affect the superconducting properties of the underlying YBCO film. Finally, a superconducting YBCO bilayer stack is demonstrated. Improvements in the properties of the top YBCO layer is expected when the surface roughness is reduced further for deposition of subsequent IBAD-textured layers. This new technology should enable novel designs of coated conductors for reduced AC losses and increased Je.

At higher magnetic fields, the critical current density in coated conductors based on biaxially textured metal substrates is limited by the intragrain pinning properties. Therefore, artificial pinning centers were introduced in Y123 films using on-axis as well as off-axis pulsed laser deposition. The nanosized particles were prepared using a quasi-multilayer approach where Hf metal layers having a thickness less than one unit cell were incorporated into Y123 films. Due to the oxidizing deposition atmosphere and solid state reaction with the Y123 phase, nanometer sized perovskite precipitates, which are biaxially textured with a well-defined relationship towards the Y123 lattice were formed. The pinning properties and the anisotropy were characterized using magneto-transport measurements at various fields and temperatures. A significant improvement of as well as irreversibility field B irr could be observed in the off-axis deposited quasi-multilayers.

A thin-film YBa2Cu3O7−δ sample has been investigated with regard to its irreversibility field at low temperatures. For these investigations, the magnetic-field dependence of the electrical resistance R(H) was measured at various temperatures in pulsed magnetic fields up to 50 T. These curves show an unusual, very rapid increase of R even at low fields. We describe a method of extracting Hirr that is comparable to measurements in static magnetic fields. The irreversibility line shows the same, nearly linear behaviour at low temperatures as bulk YBa2Cu3O7−δ samples and a power-law dependence at higher temperatures. These two regions are also clearly visible in pinning force plots.

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

Chemical solution deposition (CSD) is a promising way to realize REBa2Cu3O7−x (REBCO; RE = rare earth (here Y, Gd))-coated conductors with high performance in applied magnetic fields. However, the preparation process contains numerous parameters which need to be tuned to achieve high-quality films. Therefore, we investigated the growth of REBCO thin films containing nanometre-scale BaHfO3 (BHO) particles as pinning centres for magnetic flux lines, with emphasis on the influence of crystallization temperature and substrate on the microstructure and superconductivity. Conductivity, microscopy and x-ray investigations show an enhanced performance of BHO nano-composites in comparison to pristine REBCO. Further, those measurements reveal the superiority of GdBCO to YBCOe.g. by inductive critical current densities, J c, at self-field and 77 K. YBCO is outperformed by more than 1 MA cm−2 with J c values of up to 5.0 MA cm−2 for 265 nm thick layers of GdBCO(BHO) on lanthanum aluminate. Transport in-field J c measurements demonstrate high pinning force maxima of around 4 GN m−3 for YBCO(BHO) and GdBCO(BHO). However, the irreversibility fields are appreciably higher for GdBCO. The critical temperature was not significantly reduced upon BHO addition to both YBCO and GdBCO, indicating a low tendency for Hf diffusion into the REBCO matrix. Angular-dependent J c measurements show a reduction of the anisotropy in the same order of magnitude for both REBCO compounds. Theoretical models suggest that more than one sort of pinning centre is active in all CSD films.

To study the possibility of enhancing the pinning forces in YBa2Cu3O7?? films through the introduction of nanosized precipitates, quasi-multilayers of YBa2Cu3O7?? and a transition metal (TM = Ti,Zr,Hf) were deposited on single-crystal SrTiO3 substrates using pulsed laser deposition. The transition metal layer thickness was chosen to be less than one unit cell, resulting in separated nanoscale islands that form inclusions with perovskite structure BaTM O3 during film growth. These inclusions grow biaxially textured within the film. Whereas the Ti-doped films show a very strong decrease in the critical temperature Tc and, hence, a strong decrease in the critical current density Jc with increasing TM amount for all temperatures, Hf and Zr doping show an increase in Jc for the smallest amounts of doping. An irreversibility field as high as 10.3?T at 77?K was observed in the case of low Hf content.

Cube textured NiO layers were prepared by surface oxidation epitaxy on microalloyed Ni tapes. Different perovskite buffers were successfully grown on these substrates using pulsed laser deposition. Among them, BaZrO3 and SrZrO3 buffers show an excellent epitaxial growth on NiO with an in-plane orientation similar to the underlying NiO. The subsequent deposition of YBa2Cu3O7−x on top of these buffers requires a thin intermediate SrTiO3 layer and results in epitaxial layers with a Tc of about 89 K and Jc values up to 1.6 MA cm−2. Alternatively, epitaxial SrRuO3 was grown on NiO using a thin intermediate BaZrO3 layer leading to YBCO films of similar quality. The investigation of the magnetic field dependence of the critical current density shows the major influence of the grain boundary network in the low-field region.

By transport measurements, we demonstrate that critical current density and irreversibility field are improved in the mixed rare earth NEG123 films for both field directions of H/spl par/c and H/spl perp/c. Angular dependence of the enhanced flux pinning force at low temperature and field is characterized by two peaks at H/spl par/c and H/spl perp/c, respectively. This may be attributed to favorite distribution of stress fields in such two directions due to 2D lattice misfits. With increasing temperature and field, the strongest improvement of flux pinning take places only at the field perpendicular to the (a, b) plane where intrinsic flux pinning is absent, thus allowing the stress field, such an uncorrelated disorder, to be most effective by collective pinning behavior.

The critical current density in RABiTS (rolling assisted biaxially textured substrates) based coated conductors is limited by the network of small-angle grain boundaries up to a texture dependent crossover field H/sub co/. The current flow through this network is percolative in nature and thus depends on misorientation angle distribution, the width and length of the tape, and on the grain shape. These dependencies were simulated using a fast and simple limiting path algorithm on real grain boundary networks obtained by electron backscattering diffraction (EBSD) and cross-checked with transport measurements. A strong dependence of J/sub c/ on conductor width below 20 grains and a large increase in J/sub c/ for elongated grains was found. H/sub co/ of tapes with very sharp cube textures are around 1 T at 77 K. Hence, the intra-grain pinning must be increased for a further improvement of coated conductors applied in higher magnetic fields. With transport measurements on YBa/sub 2/(Cu/sub 1-x/Zn/sub x/)/sub 3/O/sub 7-/spl delta// single crystalline thin films, the possibility of a J/sub c/ increase due to Zn doping was investigated. Monolayer films with Zn contents up to 0.2%, however, showed a decrease in J/sub c/ and H/sub irr/, whereas multilayer films with x=0.017% and x=0.025% Zn) showed an increase in J/sub c/ at 77 K.

The conductor width dependence of the critical current density Jc in the grain boundary network occurring in RABiTS based coated conductors is simulated using experimentally obtained electron backscattering diffraction (EBSD) maps and the exponential dependence of the critical current density on the misorientation angle. It is found that a conductor width of around 20 grains (of average size) is sufficient to pass 90% of the maximum current density, independent of texture quality and grain aspect ratio in the current direction. The aspect ratio does however influence the absolute value of Jc, giving higher Jc values for higher aspect ratios. These results are in good agreement with Jc simulations based on purely statistical grain boundary distributions.

Magnetoresistivity of YBa 2 Cu 3 O 7−δ films with thicknesses between 7 and 100 nm deposited on CeO 2 and PrBa 2 Cu 3 O 7−δ buffer layers on sapphire substrate has been measured to analyze the temperature dependence of the second critical magnetic field B c2 . To define B c2 , the mean-field transition temperature T c was evaluated by fitting the resistive transition in zero magnetic field with the fluctuation conductivity theory of Aslamazov and Larkin. At T → T c the B c2 ( T ) dependence shows a crossover from downturn to upturn curvature with the increase in film thickness.

We report on the growth of epitaxial Fe1+δ Se0.5Te0.5 thin films on 0°, 5°, 10°, 15° and 20° vicinal cut CaF2 single crystals by pulsed laser deposition. In situ electron and ex situ x-ray diffraction studies reveal a tilted growth of the Fe1+δ Se0.5Te0.5 films, whereby under optimized deposition conditions the c-axis alignment coincides with the substrate [001] tilted axis up to a vicinal angle of 10°. Atomic force microscopy shows a flat island growth for all films. From resistivity measurements in longitudinal and transversal directions, the ab- and c-axis components of resistivity are derived and the mass anisotropy parameter is determined. Analysis of the critical current density indicates that no effective c-axis correlated defects are generated by vicinal growth, and pinning by normal point core defects dominates. However, for Hab the effective pinning centers change from surface defects to point core defects near the superconducting transition due to the vicinal cut. Furthermore, we show in angular-dependent critical current density data a shift of the ab-planes maxima position with the magnetic field strength.

Highly textured NdFeAs(O,F) thin films have been grown on ion beam assisted deposition (IBAD)-MgO/Y2O3/Hastelloy substrates by molecular beam epitaxy. The oxypnictide coated conductors showed a superconducting transition temperature (Tc) of 43 K with a self-field critical current density (Jc) of 7.0 x 104 A/cm2 at 5 K, more than 20 times higher than powder-in-tube processed SmFeAs(O,F) wires. Albeit higher Tc as well as better crystalline quality than Co-doped BaFe2As2 coated conductors, in-field Jc of NdFeAs(O,F) was lower than that of Co-doped BaFe2As2. These results suggest that grain boundaries in oxypnictides reduce Jc significantly compared to that in Co-doped BaFe2As2 and, hence biaxial texture is necessary for high Jc., 4 pages, 4 figures

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., Comment: 8 pages, 6 figures

The discovery of superconductivity (SC) with a transition temperature, Tc, up to 65K in single-layer FeSe (bulk Tc =8K) films grown on SrTiO3 substrates has attracted special attention to Fe-based thin films. The high Tc is a consequence of the combined effect of electron transfer from the oxygen-vacant substrate to the FeSe thin film and lattice tensile strain. Here we demonstrate the realization of SC in the parent compound BaFe2As2 (no bulk Tc) just by tensile lattice strain without charge doping. We investigate the interplay between strain and SC in epitaxial BaFe2As2 thin films on Fe-buffered MgAl2O4 single crystalline substrates. The strong interfacial bonding between Fe and the FeAs sublattice increases the Fe-Fe distance due to the lattice misfit which leads to a suppression of the antiferromagnetic spin density wave and induces SC with bulk-Tc ?10K. These results highlight the role of structural changes in controlling the phase diagram of Fe-based superconductors., http://www.nature.com/ncomms/2013/131206/ncomms3877/full/ncomms3877.html

The reel-to-reel coated conductor facilities at Los Alamos National Laboratory enable electropolishing, ion-beam assist deposited (IBAD) MgO texturing, buffer layer deposition, REBaCuO pulsed laser deposition and position dependent critical current measurements, all in a continuous fashion. These systems allow development of long-length coated conductors as well as high-throughput experimentation by linear combinatorial design. We discuss how we utilize the deposition systems to obtain thickness profiles and study the properties of the films as a function of thickness. In particular we demonstrate that texture improves with thickness of epitaxial layers, whether homoepitaxial MgO or heteroepitaxial YBCO.

The anisotropy of the critical current density Jc depends in general on both the properties of the flux lines (such as line tension, coherence length and penetration depth) and the properties of the defects (such as density, shape, orientation etc.). Whereas the Jc anisotropy in microstructurally clean films can be scaled to an effective magnetic field containing the Ginzburg-Landau anisotropy term, it is in general not possible (or only in a limited field range) for samples containing extended defects. Here, the Jc anisotropy of a Co-doped BaFe2As2 sample with 45{\deg} [001] tilt grain boundaries (GBs), i.e. grain boundaries created by 45{\deg} in-plane rotated grains, as well as extended Fe particles is investigated. This microstructure leads to c-axis correlated pinning, both due to the GBs and the Fe particles and manifests in a c-axis peak in the Jc anisotropy at low magnetic fields and a deviation from the anisotropic Ginzburg-Landau scaling at higher fields. Strong pinning at ellipsoidal extended defects, i.e. the Fe particles, is discussed, and the full Jc anisotropy is fitted successfully with the vortex path model. The results are compared to a sample without GBs and Fe particles. 45{\deg} GBs seem to be good pinning centers rather than detrimental to current flow., Comment: 8 pages, 7 figures, CEC-ICMC 2013 proceeding, accepted for publication in Advances in Cryogenic Engineering (Materials)

We report on the transport properties of clean, epitaxial Fe(Se,Te) thin films prepared on Fe-buffered MgO (001) single crystalline substrates by pulsed laser deposition. Near Tc a steep slope of the upper critical field for H||ab was observed (74.1 T/K), leading to a very short out-of-plane coherence length, \xi c, of 0.2 nm, yielding 2\xi c(0) approximately 0.4 nm. This value is shorter than the interlayer distance (0.605 nm) between Fe-Se(Te) planes, indicative of modulation of the superconducting order parameter along the c-axis. An inverse correlation between the power law exponent N of the electric field-current density (E-J) curve and the critical current density, Jc, has been observed at 4 K, when the orientation of H was close to the ab-plane. These results prove the presence of intrinsic pinning in Fe(Se,Te). A successful scaling of the angular dependent Jc and the corresponding exponent N can be realized by the anisotropic Ginzburg Landau approach with appropriate \Gamma values 2~3.5. The temperature dependence of \Gamma behaves almost identically to that of the penetration depth anisotropy., Comment: 8 figures

We report on the development of room temperature THz bolometers made from semiconducting PrBa2Cu3O7-δ (PBCO) thin films on MgO substrate. PBCO thin films show a high temperature coefficient of resistance (TCR ≈ 1-2%/K) at low resistivity values (ρ ≈ 2000 μΩcm) in comparison to other semiconducting materials like amorphous silicon or vanadium oxide. A low resistivity enables efficient coupling to an integrated planar antenna required to couple THz radiation to micrometer sized detector elements. A detailed electrical characterization as well as radiation measurements at 0.65 THz of the 70-100 nm thick PBCO film microbridges embedded into log-spiral planar antennas have been performed. An electrical responsivity up to S=33 V/W and a noise equivalent power of NEP=1.52t10-10 W/√Hz at a modulation frequency of 10 kHz limited by the measurement setup have been achieved.

Compared with mono-rare earth 123 films, ternary rare earth (Nd,Eu,Gd)Ba2Cu3O7−δ (NEG123) films show higher critical current density (Jc) and improved irreversibility field (Hirr), but no increase in the characteristic field corresponding to a crossover from a low-field plateau to a linear region in a log Jc–log H plot. At intermediate fields, Jc vs H scales as H(−0.5±0.05) for NEG123, in contrast to H(−0.73±0.05) for mono-rare earth samples such as Gd123. The slow power decay of Jc vs H together with the improved Jc and Hirr strongly implies that extra flux pinning centers exist in NEG123, which are thought to be noncorrelated stress fields induced by lattice mismatch.

High-temperature superconducting YBa2Cu3O7-d (YBCO) thin-film detectors with improved responsivities were developed for fast time-domain measurements in the THz frequency range. YBCO thin films of 30 nm thickness were patterned to micro- and nanobridges and embedded into planar log-spiral THz antennas. The YBCO thin-film detectors were characterized with continuous wave radiation at 0.65 THz. Responsivity values as high as 710 V/W were found for the YBCO nanobridges. Pulsed measurements in the THz frequency range were performed at the electron storage ring ANKA from the Karlsruhe Institute of Technology (KIT). Due to the high responsivities of the nanobridges no biasing was required for the detection of the coherent synchrotron radiation pulses achieving very good agreement between the measured pulse shapes and simulations.

The photoresponse of YBaCuO thin film microbridges with thicknesses between 15 and 50 nm was studied in the optical and terahertz frequency range. The voltage transients in response to short radiation pulses were recorded in real time with a resolution of a few tens of picoseconds. The bridges were excited by either femtosecond pulses at a wavelength of 0.8 μm or broadband (0.1–1.5 THz) picosecond pulses of coherent synchrotron radiation. The transients in response to optical radiation are qualitatively well explained in the framework of the two-temperature model with a fast component in the picosecond range and a bolometric nanosecond component whose decay time depends on the film thickness. The transients in the THz regime showed no bolometric component and had amplitudes up to three orders of magnitude larger than the two-temperature model predicts. Additionally THz field-dependent transients in the absence of DC bias were observed. We attribute the response in the THz regime to a rearrangement of vortices caused by high-frequency currents.

Superconducting and normal state transport properties in iron pnictides are sensitive to disorder and impurity scattering. By investigation of Ba(Fe1-xCox)2As2 thin films with varying Co concentration, we demonstrate that in the dirty limit the superconducting dome in the electronic phase diagram of Ba(Fe1-xCox)2As2 shifts towards lower doping concentrations, which differs significantly from observations in single crystals. We show that especially in the underdoped regime superconducting transition temperatures higher than 27 K are possible., Comment: 7 pages, 7 figures

A low and almost temperature independent resistance in the normal state and an anomalous peak effect within the normal-superconducting transition have been observed in BaFe1.8Co0.2As2/Fe bilayers, prepared on ion beam assisted deposition-MgO/Y2O3 buffered technical substrates. A resistor network array sufficiently reproduces this effect, assuming an increase of the electrical conductance between tape and film with decreasing buffer layer thickness. Based on this model, we evaluated the influence of this effect on the critical current density and successfully reconstructed the superconducting transition of the bilayer.

This work deals with experiments on the electrical transport of epitaxial Bi 2 Sr 2 Ca 2 Cu 3 O x (Bi2223) thin films fabricated by pulsed laser deposition on (1 0 0) oriented SrTiO 3 single crystal and bicrystalline substrates with [0 0 1] tilt grain boundaries. The current–voltage characteristics and the temperature dependence of the critical current density J c ( T ) across grain boundaries of thin film Bi2223 bicrystals have been measured as a function of tilt angle ( Θ ). For Θ =0–36.8°, the critical current density decreases exponentially with increasing tilt angle. This orientation dependence is very similar to that observed for YBa 2 Cu 3 O 7 bicrystals.

We have investigated the influence of the superconducting layer thickness, d, on the structural and transport properties of Co-doped BaFe2As2 films deposited on Fe-buffered MgO substrates by pulsed laser deposition. The superconducting transition temperature and the texture quality of Co-doped BaFe2As2 films improve with increasing d due to a gradual relief of the tensile strain. For d>90 nm an additional 110 textured component of Co-doped BaFe2As2 was observed, which leads to an upward shift in the angular dependent critical current density at H // c. These results indicate that the grain boundaries created by the 110 textured component may contribute to the c-axis pinning., 6 figures

We demonstrate the growth of epitaxial Fe/Ba(Fe(1-x)Co(x))2As2 (Fe/Ba-122) bilayers on MgO(001) and LSAT(001) single crystal substrates using Pulsed Laser Deposition (PLD). By exploiting the metallic nature of the FeAs tetrahedron in the Ba-122 crystal structure, we achieve a coherent interfacial bond between bcc iron and Co-doped Ba-122. Tc values for both bilayers were close to that of the PLD target. Direct observation of interfacial bonding between Fe and the Ba-122 FeAs sublattice by atomic resolution transmission electron microscopy implies that this bilayer architecture may work for other iron pnictide systems and pave the way for the fabrication of superconducting/ferromagnetic heterostructures., 3 pages, 4 figures

Investigating the anisotropy of superconductors permits an access to fundamental properties. Having succeeded in the fabrication of epitaxial superconducting LaFeAs(O,F) thin films we performed an extensive study of electrical transport properties. In face of multiband superconductivity we can demonstrate that a Blatter scaling of the angular dependent critical current densities can be adopted, although being originally developed for single band superconductors. In contrast to single band superconductors the mass anisotropy of LaFeAs(O,F) is temperature dependent. A very steep increase of the upper critical field and the irreversibility field can be observed at temperatures below 6K, indicating that the band with the smaller gap is in the dirty limit. This temperature dependence can be theoretically described by two dominating bands responsible for superconductivity. A pinning force scaling provides insight into the prevalent pinning mechanism and can be specified in terms of the Kramer model., 7 pages, 13 figures

The angular-dependent critical current density, ${J}_{\text{c}}(\ensuremath{\Theta})$, and the upper critical field, ${H}_{\text{c}2}(\ensuremath{\Theta})$, of epitaxial $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}$ thin films have been investigated. No ${J}_{\text{c}}(\ensuremath{\Theta})$ peaks for $H\ensuremath{\parallel}c$ were observed regardless of temperatures and magnetic fields. In contrast, ${J}_{\text{c}}(\ensuremath{\Theta})$ showed a broad maximum at $\ensuremath{\Theta}=90\ifmmode^\circ\else\textdegree\fi{}$, which arises from intrinsic pinning. All data except at $\ensuremath{\Theta}=90\ifmmode^\circ\else\textdegree\fi{}$ can be scaled by the Blatter plot. ${H}_{\text{c}2}(\ensuremath{\Theta})$ near ${T}_{\text{c}}$ follows the anisotropic Ginzburg-Landau expression. The mass anisotropy increased from 1.5 to 2 with increasing temperature, which is an evidence for multiband superconductivity.

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, d(Fe). The well-textured growth of the Fe/Ba-122 bilayer with d(Fe) = 15 nm results in a high J(c) of 0.45 MA cm(-2) at 12 K in self-field, whereas a low J(c) value of 61 000 A cm(-2) is recorded for the bilayer with d(Fe) = 4 nm at the corresponding reduced temperature due to the presence of grain boundaries. (C) 2010 American Institute of Physics. [doi:10.1063/1.3509418]

We report a new type of carbon material-porous colossal carbon tubes. Compared with carbon nanotubes, colossal carbon tubes have a much bigger size, with a diameter of between 40 and 100 mum and a length in the range of centimeters. Significantly, the walls of the colossal tubes are composed of macroscopic rectangular columnar pores and exhibit an ultralow density comparable to that of carbon nanofoams. The porous walls of colossal tubes also show a highly ordered lamellar structure similar to that of graphite. Furthermore, colossal tubes possess excellent mechanical and electrical properties.

We present a new reel-to-reel method for growth of high temperature super-conducting (HTS) films by reactive co-evaporation on flexible metal tapes. We have demonstrated proof of principle for this method with a small laboratory-scale setup using 8 cm long tape pieces. YBa2Cu3O7-δ is deposited on ion-beam assisted deposition textured MgO layers on top of flexible polycrystalline metal tapes. Critical current densities at 75.5 K of over 2 MA/cm2 have been achieved in HTS films with over 2 μm in thickness, yielding a self field critical current of 450 A/cm-width. A 4.5 μm thick film had a self field critical current of 590 A/cm. We discuss some practical possibilities for manufacturing of superconducting wire using this process and present new areas of research that are still needed.

Coated conductor samples, prepared by reactive co-evaporation, are investigated with respect to the hole-doping dependence of the critical current density. The samples are annealed in an atmosphere of variable oxygen content after which critical currents, critical temperature and the c-axis lattice spacing are measured. The lattice spacing increases with decreasing oxygen content, consistent with literature data. These co-evaporated samples show hole overdoped behavior with respect to the maximum Tc. The achievable range of hole doping in these samples seems to depend on surface coverage. Both self-field and in-field Jc at 75.5 K have a maximum in the overdoped region but at less than maximum oxygen content. The reason for the overdoping of these samples is discussed briefly in terms of Y-Ba disorder.

We examine the influence of various substrate preparation procedures for ion-beam assist deposition (IBAD) texturing of MgO. IBAD-MgO nano-texturing is very sensitive to the nucleation surface, and surface roughness has an important influence on the texture of the MgO layer. We studied Hastelloy C-276 metal alloy as the substrate. The untreated substrate is leveled by either electropolishing, mechanical polishing or solution deposition. All three methods are applied to continuously moving tapes in long lengths. The RMS surface roughness decreases from 20-50 nm for the untreated substrate to 0.5 nm, 0.3 nm and 1 nm respectively. The in-plane and out-of plane crystalline alignment of the MgO layer improves as the roughness is decreased below 2 nm.