Strong isotropic flux pinning in solution-derived YBa2Cu3O7-x nanocomposite superconductor films

Microstructural characterization of TFA-MOD processed Y(₁-x)Gd(x)Ba₂Cu₃O(₇-y) with BaZrO₃

Y(₁-x)Gd(x)Ba₂Cu₃O(₇-y) with BaZrO₃ film was fabricated on CeO₂ buffered LaMnO₃/ion beam assisted deposition-MgO/Gd₂Zr₂O₇/Hastelloy C276TM substrates by the metal organic deposition process using trifluoroacetates. Both microstructural and analytical characterizations were performed by transmission electron microscopy with selected area electron diffraction method and energy dispersive X-ray spectroscopy. The thickness of Y(₁-x)Gd(x)Ba₂Cu₃O(₇-y) with BaZrO₃ film was found approximately 700 nm and the film was composed of c-axis oriented Y(₁-x)Gd(x)Ba₂Cu₃O(₇-y) grains. Several types of particles, (Y,Gd)₂Cu₂O₅, CuO and BaZrO₃, were formed within the Y(₁-x)Gd(x)Ba₂Cu₃O(₇-y) film, whose sizes were about 100-200 nm for (Y,Gd)₂Cu₂O₅ and CuO particles, and about 10-20 nm for BaZrO₃ particles, respectively. In the Y(₁-x)Gd(x)Ba₂Cu₃O(₇-y) film, (Y,Gd)₂Cu₂O₅ and CuO particles were dispersed heterogeneously, whereas BaZrO₃ nanoparticles were found dispersed homogeneously with random orientation. In addition, electron tomographic observation was carried out to attain the three-dimensional information of polyhedral BaZrO₃ nanoparticles.

Influences of calcination temperature on growth and superconducting properties of GdBa₂Cu₃O₇-δ films fabricated by fluorine-free metal organic deposition method

GdBa2Cu3O7-δ is one of the best candidates for the superconducting coated conductors because it has high critical temperature and high critical current density under high magnetic fields. In this study, superconducting GdBa2Cu3O7-δ films were fabricated by fluorine-free metal organic deposition method via two different calcination temperatures, 723K and 873K, to examine the influence of calcination temperature on film growth and superconducting characteristics. Critical temperatures and critical current densities of the sample calcined at 873K were superior to the sample calcined at 723K. In the case of the sample calcined at 723K, the mixture of amorphous and crystalline phases was observed, and that of a- and c-axis oriented grains after the crystallization. In the case of the sample calcined at 873K, the randomly oriented crystalline phases were observed, and the mixture of c-axis oriented grains and (Gd, Al)2BaO4 phase after the crystallization. These microstructural changes caused the differences in superconducting characteristics.

Nanoscale strain-induced pair suppression as a vortex-pinning mechanism in high-temperature superconductors

Boosting large-scale superconductor applications require nanostructured conductors with artificial pinning centres immobilizing quantized vortices at high temperature and magnetic fields. Here we demonstrate a highly effective mechanism of artificial pinning centres in solution-derived high-temperature superconductor nanocomposites through generation of nanostrained regions where Cooper pair formation is suppressed. The nanostrained regions identified from transmission electron microscopy devise a very high concentration of partial dislocations associated with intergrowths generated between the randomly oriented nanodots and the epitaxial YBa(2)Cu(3)O(7) matrix. Consequently, an outstanding vortex-pinning enhancement correlated to the nanostrain is demonstrated for four types of randomly oriented nanodot, and a unique evolution towards an isotropic vortex-pinning behaviour, even in the effective anisotropy, is achieved as the nanostrain turns isotropic. We suggest a new vortex-pinning mechanism based on the bond-contraction pairing model, where pair formation is quenched under tensile strain, forming new and effective core-pinning regions.

Imprinting nanoporous alumina patterns into the magneto-transport of oxide superconductors

We used oxygen ion irradiation to transfer the nanoscale pattern of a porous alumina mask into high-T(C) superconducting thin films. This causes a nanoscale spatial modulation of superconductivity and strongly affects the magneto-transport below T(C), which shows a series of periodic oscillations reminiscent of the Little-Parks effect in superconducting wire networks. This irradiation technique could be extended to other oxide materials in order to induce ordered nanoscale phase segregation.

Self-organized Ce(1-x)Gd(x)O(2-y) nanowire networks with very fast coarsening driven by attractive elastic interactions

Assembling arrays of ordered nanowires is a key objective for many of their potential applications. However, a lack of understanding and control of the nanowires' growth mechanisms limits their thorough development. In this work, an appealing new path towards self-organized epitaxial nanowire networks produced by high-throughput solution methods is reported. Two requisites are identified to generate the nanowires: a thermodynamic driving force for an unrestricted elongated equilibrium island shape, and a very fast effective coarsening rate. These requirements are met in anisotropically strained Ce(1-x)Gd(x)O(2-y) nanowires with the (011) orientation grown on the (001) surface of LaAlO(3) substrates. Nanowires with aspect ratios above ≈100 oriented along two mutually orthogonal axes are obtained leading to labyrinthine networks. A very fast effective nanowire growth rate (≈60 nm min(-1)) for ex-situ thermally annealed nanostructures derives from simultaneous kinetic processes occurring in a branched network. Ostwald ripening and anisotropic dynamic coalescence, both promoted by strain-driven attractive nanowire interaction, and rapid recrystallization, enabled by fast atomic diffusion associated with a high concentration of oxygen vacancies, contribute to such an effective growth rate. This bottom-up approach to self-organized nanowire growth has a wide potential for many materials and functionalities.

Sol-gel-derived epitaxial nanocomposite thin films with large sharp magnetoelectric effect

Nanostructures of multiferroic materials have drawn increasing interest due to the enhanced magnetoelectric coupling and potential for next-generation multifunctional devices. Most of these structures are typically prepared by thin film evaporation approaches. Herein, however, we report a novel sol-gel-based process to synthesize epitaxial BaTiO(3)-CoFe(2)O(4) nanocomposite thin films via phase separation and enhanced heterogeneous nucleation. The magnetoelectric coupling effect is investigated by examining the temperature-dependent magnetization of the composite film, which manifests as a sharp and significant drop (>50%) of the magnetization at the vicinity of a BaTiO(3) ferroelectric phase transition. We propose that the phase transition in BaTiO(3) is mediated by the tensile strain due to intimate coupling to CoFe(2)O(4) phase, which has rarely been reported before. The significant coupling effect is attributed to the small substrate clamping, and the large areal distribution of intimate heteroepitaxial interfaces between the three-dimensionally distributed ferroelectric and magnetic nanostructured phases.

Reversible resistive switching and multilevel recording in La0.7Sr0.3MnO3 thin films for low cost nonvolatile memories

On the basis of a scanning probe microscopy strategy, we propose a combined methodology capable to program nonvolatile multilevel data and read them out in a noninvasive manner. In the absence of the common two-electrode cell geometry, this nanoscale approach permits, in addition, investigating the relevance of inherent film properties. We demonstrate the feasibility of modifying the local electronic response of La(0.7)Sr(0.3)MnO(3) to obtain nanostructures with switchable resistance embedded in low cost oxide thin films, which constitutes a promising approach for fabricating high density nonvolatile memories.

Orientational ordering of solution derived epitaxial Gd-doped ceria nanowires induced by nanoscratching

When one-dimensional nanostructures are epitaxially grown on a substrate a key goal is to control the nanowire's position and orientation. Nanoscratching of single crystalline (001)- LaAlO(3) substrates is demonstrated to be extraordinarily effective in directing the self-assembly of Ce(0.9)Gd(0.1)O(2-y) epitaxial nanowires grown by chemical solution deposition. The local anisotropic elastic strain field imposed by the indentation lines is responsible for the breaking of the pre-existing orientation energy degeneracy and selects the nanowires' orientation parallel to the lines to an extent that can reach 100%.

Synergetic combination of different types of defect to optimize pinning landscape using BaZrO(3)-doped YBa(2)Cu(3)O(7)

Retaining a dissipation-free state while carrying large electrical currents is a challenge that needs to be solved to enable commercial applications of high-temperature superconductivity. Here, we show that the controlled combination of two effective pinning centres (randomly distributed nanoparticles and self-assembled columnar defects) is possible and effective. By simply changing the temperature or growth rate during pulsed-laser deposition of BaZrO(3)-doped YBa(2)Cu(3)O(7) films, we can vary the ratio of these defects, tuning the field and angular critical-current (Ic) performance to maximize Ic. We show that the defects' microstructure is governed by the growth kinetics and that the best results are obtained with a mixture of splayed columnar defects and random nanoparticles. The very high Ic arises from a complex vortex pinning landscape where columnar defects provide large pinning energy, while splay and nanoparticles inhibit flux creep. This knowledge is used to produce thick films with remarkable Ic(H) and nearly isotropic angle dependence.

Vortex breaking and cutting in type II superconductors

In technological superconductors, the Lorentz force on the flux vortices is opposed by inhomogeneous pinning and so the critical current may be controlled by a combination of vortex entanglement, cutting, and cross-joining. To understand the roles of these processes we report measurements of structures in which a weak pinning layer is sandwiched between two strongly pinning leads. Quantitative modeling of the results demonstrates that in such systems the critical current is limited by the deformation of individual vortices and not by subsequent cross-joining processes.

Materials science challenges for high-temperature superconducting wire

Twenty years ago in a series of amazing discoveries it was found that a large family of ceramic cuprate materials exhibited superconductivity at temperatures above, and in some cases well above, that of liquid nitrogen. Imaginations were energized by the thought of applications for zero-resistance conductors cooled with an inexpensive and readily available cryogen. Early optimism, however, was soon tempered by the hard realities of these new materials: brittle ceramics are not easily formed into long flexible conductors; high current levels require near-perfect crystallinity; and--the downside of high transition temperature--performance drops rapidly in a magnetic field. Despite these formidable obstacles, thousands of kilometres of high-temperature superconducting wire have now been manufactured for demonstrations of transmission cables, motors and other electrical power components. The question is whether the advantages of superconducting wire, such as efficiency and compactness, can outweigh the disadvantage: cost. The remaining task for materials scientists is to return to the fundamentals and squeeze as much performance as possible from these wonderful and difficult materials.