Pinning Enhancement for YBa2Cu3O7-δ (YBCO) and Iron Chalcogenide Thin Films

Over the past two decades, YBa2Cu3O7-δ(YBCO) has aroused great research interests, owing to its high transition temperature (Tc) of above 90 K and some other advantages. However, most of the practical applications of YBCO coated conductors require high electrical current transport properties under magnetic field with little or even no losses, which means high in-field critical current density (Jc) is needed. Therefore, tremendous research has been focused on Jc improvement for YBCO thin films. One effective way is to introduce defects by designed nanostructure landscapes, so far, 0-D nanoparticles, 1-D nanopillars and 2-D nanolayers have been reported for effective defect pinning enhancement. Magnetic pinning is another method to enhance the Jc values for YBCO thin films, by the interaction between the fluxons and the magnetic inclusions. However, there are very limited reports on the combination of defect and magnetic pinning together to further improve the Jc values. In this thesis, both defect and magnetic pinning are introduced by incorporating designed vertically aligned nanocomposite (VAN) layers, which include magnetic portion. (CoFe2O4)x(CeO2)1-x and (La0.67Sr0.33MnO3)x(CeO2)1-x are introduced as either cap layer or buffer layer into YBCO thin films for the pinning enhancement, as CoFe2O4 and La0.67Sr0.33MnO3 are both magnetic materials. Furthermore, VAN/YBCO mulitlayers are also successfully grown for effective pinning enhancement. By these doping methods and designed architectures, both magnetic and defect pinning are involved, and the superconducting properties are improved. On another side, several iron based superconductors have been discovered, and FeSe with the simplest structure and a transition temperature (Tc) around 8 K arouses much research interest. Up to date, most of the research efforts in this field are to improve the Tc value of iron chalcogenide thin films. In this thesis, the pinning effects are studied for the superconducting FeSexTe1-x thin films by nanoinclusions, such as CeO2 nanolayer, which was proved to be able to create effective defect pinning centers for FeSexTe1-x thin films. In addition, (CoFO2O4)0.1(CeO2)0.9 VAN layer is also incorporated for both defect and magnetic pinning. Last but not the least, FeSexTe1-x thin films have been deposited on various kinds of substrates, including single crystal STO, amorphous glass, Si with or without a SiOx protection layer, and even metal substrate without a complicated set of buffer layers. Surprisingly, the FeSexTe1-x thin films can be grown along the c-direction on all the different substrates, even on amorphous glass and metal substrates, which demonstrates a very simplified and cost-effective approach of this Fe-based coated conductor for potential high field applications

Inherent-opening-controlled pattern formation in carbon nanotube arrays

We have introduced inherent openings into densely packed carbon nanotube arrays to study self-organized pattern formation when the arrays undergo a wetting–dewetting treatment from nanotube tips. These inherent openings, made of circular or elongated hollows in nanotube mats, serve as dewetting centres, from where liquid recedes from. As the dewetting centres initiate dry zones and the dry zones expand, surrounding nanotubes are pulled away from the dewetting centres by liquid surface tension. Among short nanotubes, the self-organized patterns are consistent with the shape of the inherent openings, i.e. slender openings lead to elongated trench-like structures, and circular holes result in relatively round nest-like arrangements. Nanotubes in a relatively high mat are more connected, like in an elastic body, than those in a short mat. Small cracks often initialize themselves in a relatively high mat, along two or more adjacent round openings; each of the cracks evolves into a trench as liquid dries up. Self-organized pattern control with inherent openings needs to initiate the dewetting process above the nanotube tips. If there is no liquid on top, inherent openings barely enlarge themselves after the wetting–dewetting treatment

Tailoring physical functionalities of complex oxides by vertically aligned nanocomposite thin-film design

Abstract: Self-assembled nanocomposite thin films couple two materials into a single film, typically, in the form of vertically aligned nanopillars embedded in a matrix film. High-density vertical heterointerfaces provide a great platform for engineering new physical properties and novel multifunctionalities, as well as for nanoscale device integration. Tremendous research efforts have been devoted to developing different nanocomposite systems. In this article, we summarize recent progress on vertically aligned nanocomposite thin films for enhanced functionalities such as ferroelectricity, tunable magnetoresistance, multiferroicity, dielectricity, magnetic anisotropy, perpendicular exchange bias, novel electrical/ionic properties, interfacial conduction, and resistive switching. Using specific examples, we discuss how and why the fundamental physical properties can be significantly tuned/improved in vertically aligned nanocomposites. Finally, we propose future research directions to achieve further enhanced performance as well as practical devices

Strongly enhanced dielectric and energy storage properties in lead-free perovskite titanate thin films by alloying

Lead-free perovskite oxide thin films prepared by alloying of titanates and materials with lower melting points are shown to have enhanced ferroelectric and dielectric properties. BaTiO3 (or SrTiO3) with 25% addition of BiFeO3 has much improved crystalline perfection because of the lower melting point of the BiFeO3 giving enhanced growth kinetics. The maximum dielectric peak temperature of BaTiO3 is increased by similar to 200 degrees C and leakage currents are reduced by up to a factor of similar to 100. The loss tangent reduces up to 300 degrees C, with a factor of > 14 reduction at room temperature. The dielectric breakdown strength is higher by a factor of similar to 3 (> 2200 kV cm(-1)) and from room temperature up to 500 degrees C the dielectric constant is > 1000. Also, a low variation of dielectric constant of similar to 9% from room temperature to 330 degrees C is obtained, compared to similar to 110% for BaTiO3. The maximum polarization (P-max) is double that of BaTiO3, at 125.3 mu C cm(-2). The film has high energy storage densities of > 52 J cm(-3) at 2050 kV cm(-1), matching Pb-based ferroelectric films. The strongly improved performance is important for applications in energy storage and in high temperature (up to 300 degrees C) capacitors as well as wider application in other electronic and energy technologie

Upper Critical Field and Kondo Effects in Fe(Te0.9Se0.1) Thin Films by Pulsed Field Measurements

The transition temperatures of epitaxial films of Fe(Te(0:9)Se(0:1)) are remarkably insensitive to applied magnetic field, leading to predictions of upper critical fields B(c2)(T = 0) in excess of 100 T. Using pulsed magnetic fields, we find B(c2)(0) to be on the order of 45 T, similar to values in bulk material and still in excess of the paramagnetic limit. The same films show strong magnetoresistance in fields above B(c2)(T), consistent with the observed Kondo minimum seen above T(c). Fits to the temperature dependence in the context of the WHH model, using the experimental value of the Maki parameter, require an effective spin-orbit relaxation parameter of order unity. We suggest that Kondo localization plays a similar role to spin-orbit pair breaking in making WHH fits to the data

Transformational dynamics of BZO and BHO nanorods imposed by Y2O3 nanoparticles for improved isotropic pinning in YBa2Cu3O7−δ thin films

An elastic strain model was applied to evaluate the rigidity of the c-axis aligned one-dimensional artificial pinning centers (1D-APCs) in YBa2Cu3O7-δ matrix films. Higher rigidity was predicted for BaZrO3 1D-APCs than that of the BaHfO3 1D-APCs. This suggests a secondary APC doping of Y2O3 in the 1D-APC/YBa2Cu3O7-δ nanocomposite films would generate a stronger perturbation to the c-axis alignment of the BaHfO3 1D-APCs and therefore a more isotropic magnetic vortex pinning landscape. In order to experimentally confirm this, we have made a comparative study of the critical current density Jc (H, θ, T) of 2 vol.% BaZrO3 + 3 vol.%Y2O3 and 2 vol.%BaHfO3 + 3 vol.%Y2O3 double-doped (DD) YBa2Cu3O7-δ films deposited at their optimal growth conditions. A much enhanced isotropic pinning was observed in the BaHfO3 DD samples. For example, at 65 K and 9.0 T, the variation of the Jc across the entire θ range from θ=0 (H//c) to θ=90 degree (H//ab) is less than 18% for BaHfO3 DD films, in contrast to about 100% for the BaZrO3 DD counterpart. In addition, lower α values from the Jc(H) ∼ H-α fitting were observed in the BaHfO3 DD films in a large θ range away from the H//c-axis. Since the two samples have comparable Jc values at H//c-axis, the improved isotropic pinning in BaHfO3 DD films confirms the theoretically predicted higher tunability of the BaHfO3 1D-APCs in APC/YBa2Cu3O7-δ nanocomposite films

Effect of Surface Morphology Changes on Optical Properties of Silicon Nanowire Arrays

The optical properties of silicon nanowire arrays (SiNWs) are closely related to surface morphology due to quantum effects and quantum confinement effects of the existing semiconductor nanocrystal. In order to explore the influence of the diameters and distribution density of nanowires on the light absorption in the visible to near infrared band, we report the highly efficient method of multiple replication of versatile homogeneous Au films from porous anodic aluminum oxide (AAO) membranes by ion sputtering as etching catalysts; the monocrystalline silicon is etched along the growth templates in a fixed proportion chemical solution to form homogeneous ordered arrays of different morphology and distributions on the surface. In this system, we demonstrate that the synthesized nanostructure arrays can be tuned to exhibit different optical characteristics in the test wavelength range by adjusting the structural parameters of AAO membranes