Alberto Pomar

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.

Competing misfit relaxation mechanisms in epitaxial correlated oxides

Strain engineering of functional properties in epitaxial thin films of strongly correlated oxides exhibiting octahedral-framework structures is hindered by the lack of adequate misfit relaxation models. Here we present unreported experimental evidence of a four-stage hierarchical development of octahedral-framework perturbations resulting from a progressive imbalance between electronic, elastic, and octahedral tilting energies in La(0.7)Sr(0.3)MnO(3) epitaxial thin films grown on SrTiO(3) substrates. Electronic softening of the Mn-O bonds near the substrate leads to the formation of an interfacial layer clamped to the substrate with strongly degraded magnetotransport properties, i.e., the so-called dead layer, while rigid octahedral tilts become relevant at advanced growth stages without significant effects on charge transport and magnetic ordering.

Atomically Flat Surface: The Key Issue for Solution-Derived Epitaxial Multilayers

We present a general methodology to study the morphology of complex thin film surfaces by evaluating the percentage of flat area through the use of atomic force microscopy. We prove that this is the key parameter to assess the suitability of a layer and ensure the desired epitaxy of the subsequent layer in a heterostructure. The method has been verified by applying it to chemically deposited YBa2Cu3O7 (YBCO) superconducting films grown on buffer layers with different surface morphologies. We demonstrate that, indeed, YBCO self-field critical current density is controlled by the buffer layer flat area fraction.

Enhanced Vortex Pinning in YBCO Coated Conductors With BZO Nanoparticles From Chemical Solution Deposition

We present our latest results on the growth of chemical solution deposited YBa₂Cu₃O₇ - BaZrO₃ nanocomposites on metallic substrates. All chemical ^TFAYBCO-BZO/^MODCZO/^ABADYSZ/SS tapes with J_c(77K,sf) = 1.7 MA/cm² have been achieved with weaker field dependence of J_c than that of standard ^TFAYBCO tapes. Angular resolved measurements show isotropic enhancement of vortex pinning due to the presence of randomly oriented BZO nanoparticles in the YBCO matrix. Chemical routes are thus a promising way to efficiently increase vortex pinning in coated conductors and improve their capabilities for high field applications.

Growth rate control and solid–gas modeling of TFA-YBa2Cu3O7 thin film processing

The trifluoroacetate metal-organic decomposition route to YBa2Cu3O7 film growth was investigated in order to bring new insights in the growth mechanism and its dependence on processing conditions and critical current density. Precursor films were processed on LaAlO3 substrates at different total pressure, oxygen partial pressure, water vapor partial pressure, and volume gas flow rate keeping the growth temperature at 740??C. The influence of these various experimental parameters on the film growth rate, which was evaluated by in?situ electrical resistance measurements, was studied thoroughly. It was found that the growth rate is nearly independent of the oxygen pressure and proportional to the square root of the water pressure. Additionally, the growth rate increases with a decrease of the total pressure or an increase of the gas flow rate. An empirical multi-exponential model simulates the experimental data, however, a better understanding was achieved using a theoretical solid?gas reaction?diffusion model, including both the diffusion of the product HF gas from the surface and the chemical reaction kinetics at the YBCO/precursor interface. A complex cross-linked relationship between the film growth rate and the processing parameters has been properly verified from the experimental data. Finally we showed that the growth rate is an important parameter influencing the critical current densities of YBCO films because it controls the nucleation of non-c-axis oriented grains.

Self-Arranged Misfit Dislocation Network Formation upon Strain Release in La0.7Sr0.3MnO3/LaAlO3(100) Epitaxial Films under Compressive Strain.

Lattice-mismatched epitaxial films of La0.7Sr0.3MnO3 (LSMO) on LaAlO3 (001) substrates develop a crossed pattern of misfit dislocations above a critical thickness of 2.5 nm. Upon film thickness increases, the dislocation density progressively increases, and the dislocation spacing distribution becomes narrower. At a film thickness of 7.0 nm, the misfit dislocation density is close to the saturation for full relaxation. The misfit dislocation arrangement produces a 2D lateral periodic structure modulation (Λ ≈ 16 nm) alternating two differentiated phases: one phase fully coherent with the substrate and a fully relaxed phase. This modulation is confined to the interface region between film and substrate. This phase separation is clearly identified by X-ray diffraction and further proven in the macroscopic resistivity measurements as a combination of two transition temperatures (with low and high Tc). Films thicker than 7.0 nm show progressive relaxation, and their macroscopic resistivity becomes similar than that of the bulk material. Therefore, this study identifies the growth conditions and thickness ranges that facilitate the formation of laterally modulated nanocomposites with functional properties notably different from those of fully coherent or fully relaxed material.

Magnetic anisotropy and valence states in La2Co1−xMn1+xO6 ( x≈0.23 ) thin films studied by x-ray absorption spectroscopy techniques

X-ray absorption spectroscopy was used to determine the valence state in La

Macroscopic evidence of nanoscale resistive switching in La2/3Sr1/3MnO3 micro-fabricated bridges

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Nucleation Mechanism OF YBa2Cu3O7 by CSD using TFA Precursors

Co

Structural, chemical and strain features of misfit dislocation cores in ultrathin La0.7Sr0.3MnO3 epitaxial films deposited on LaAlO3

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