Orientation dependence of grain-boundary critical currents in YBa2Cu3O7- delta bicrystals

Grain Boundary Plane Measurement Using Transmission Electron Microscopy Automated Crystallographic Orientation Mapping for Atom Probe Tomography Specimens

Grain boundaries are critical in determining the properties of materials, including mechanical stability, conductivity, and corrosion resistance. The specific properties of materials depend not only on the misorientation of the crystals, the three most commonly characterized parameters, but also on the angle of the grain boundary plane between the two crystals, the final two parameters in the five-parameter macroscopic description of the grain boundary. The method presented here allows for the direct measurement of all five parameters of the grain boundary in a transmission electron microscopy specimen of various morphologies. This is especially applicable to atom probe specimens, where only a single-tilt axis is generally available, allowing the crystallographic description to be matched to the detailed chemical data available in the atom probe tomography. This method provides a platform for efficient grain boundary analysis in unique samples, saving operator time and allowing for ease of acquisition and interpretation in comparison with traditional electron diffraction methods.

Stabilization and Self-Passivation of Grain Boundaries in Halide Perovskite by Rigid Body Translation

The physical properties of grain boundaries in halide perovskites, especially their atomic structure, have not been fully understood yet. We report that Σ5 [130] symmetrical tilt grain boundaries can be stabilized by rigid body translation which is moving one side of the grain parallel with respect to the adjacent grain. Such reconstruction passivates grain boundaries by removing Pb-Pb and I-I interactions that introduce shallow defect states in the band gap. Rigid body translation also stabilizes the [110] antiphase boundary as well in both CsPbI₃ and CsPbBr₃.

Analysing neutron radiation damage in YBa2 Cu3 O7-x high temperature superconductor tapes

Superconducting windings will be necessary in future fusion reactors to generate the strong magnetic fields needed to confine the plasma, and these superconducting materials will inevitably be exposed to neutron damage. It is known that this exposure results in the creation of isolated damage cascades, but the presence of these defects alone is not sufficient to explain the degradation of macroscopic superconducting properties and a quantitative method is needed to assess the subtle lattice damage in between the clusters. We have studied REBCO coated conductors irradiated with neutrons to a cumulative dose of 3.3×10²²  n*m^-2  that show a degradation of both T_c  and J_c values, and use HRTEM analysis to show that this irradiation introduces ∼10 nm amorphous collision cascades. In addition we introduce a new method for the analysis of these images to quantify the degree of lattice disorder in the apparently perfect matrix between these cascades. This method utilises Fast Fourier and Discrete Cosine Transformations of a statistically-relevant number of HRTEM images of pristine, neutron-irradiated, and amorphous samples, and extracts the degree of randomness in terms of entropy values. Our results show that these entropy values in both mid-frequency band FFT and DCT domains correlate with the expected level of lattice damage, with the pristine samples having the lowest and the fully amorphous regions the highest entropy values.  Our methodology allows us to quantify 'invisible' lattice damage to and correlate these values to the degradation of superconducting properties, and also has relevance for a wider range of applications in the field of electron microscopy where small changes in lattice perfection need to be measured. This article is protected by copyright. All rights reserved.

Electroceramics for High-Energy Density Capacitors: Current Status and Future Perspectives

Materials exhibiting high energy/power density are currently needed to meet the growing demand of portable electronics, electric vehicles and large-scale energy storage devices. The highest energy densities are achieved for fuel cells, batteries, and supercapacitors, but conventional dielectric capacitors are receiving increased attention for pulsed power applications due to their high power density and their fast charge-discharge speed. The key to high energy density in dielectric capacitors is a large maximum but small remanent (zero in the case of linear dielectrics) polarization and a high electric breakdown strength. Polymer dielectric capacitors offer high power/energy density for applications at room temperature, but above 100 °C they are unreliable and suffer from dielectric breakdown. For high-temperature applications, therefore, dielectric ceramics are the only feasible alternative. Lead-based ceramics such as La-doped lead zirconate titanate exhibit good energy storage properties, but their toxicity raises concern over their use in consumer applications, where capacitors are exclusively lead free. Lead-free compositions with superior power density are thus required. In this paper, we introduce the fundamental principles of energy storage in dielectrics. We discuss key factors to improve energy storage properties such as the control of local structure, phase assemblage, dielectric layer thickness, microstructure, conductivity, and electrical homogeneity through the choice of base systems, dopants, and alloying additions, followed by a comprehensive review of the state-of-the-art. Finally, we comment on the future requirements for new materials in high power/energy density capacitor applications.

Atomic origin of spin-valve magnetoresistance at the SrRuO3 grain boundary

Defects exist ubiquitously in crystal materials, and usually exhibit a very different nature from the bulk matrix. Hence, their presence can have significant impacts on the properties of devices. Although it is well accepted that the properties of defects are determined by their unique atomic environments, the precise knowledge of such relationships is far from clear for most oxides because of the complexity of defects and difficulties in characterization. Here, we fabricate a 36.8° SrRuO3 grain boundary of which the transport measurements show a spin-valve magnetoresistance. We identify its atomic arrangement, including oxygen, using scanning transmission electron microscopy and spectroscopy. Based on the as-obtained atomic structure, the density functional theory calculations suggest that the spin-valve magnetoresistance occurs because of dramatically reduced magnetic moments at the boundary. The ability to manipulate magnetic properties at the nanometer scale via defect control allows new strategies to design magnetic/electronic devices with low-dimensional magnetic order.

Weakly-Emergent Strain-Dependent Properties of High Field Superconductors

All superconductors in high field magnets operating above 12 T are brittle and subjected to large strains because of the differential thermal contraction between component parts on cool-down and the large Lorentz forces produced in operation. The continuous scientific requirement for higher magnetic fields in superconducting energy-efficient magnets means we must understand and control the high sensitivity of critical current density Jc to strain ε. Here we present very detailed Jc(B, θ, T, ε) measurements on a high temperature superconductor (HTS), a (Rare-Earth)Ba2Cu3O7-δ (REBCO) coated conductor, and a low temperature superconductor (LTS), a Nb3Sn wire, that include the very widely observed inverted parabolic strain dependence for Jc(ε). The canonical explanation for the parabolic strain dependence of Jc in LTS wires attributes it to an angular average of an underlying intrinsic parabolic single crystal response. It assigns optimal superconducting critical parameters to the unstrained state which implies that Jc(ε) should reach its peak value at a single strain (ε = εpeak), independent of field B, and temperature T. However, consistent with a new analysis, the high field measurements reported here provide a clear signature for weakly-emergent behaviour, namely εpeak is markedly B, (field angle θ for the HTS) and T dependent in both materials. The strain dependence of Jc in these materials is termed weakly-emergent because it is not qualitatively similar to the strain dependence of Jc of any of their underlying component parts, but is amenable to calculation. We conclude that Jc(ε) is an emergent property in both REBCO and Nb3Sn conductors and that for the LTS Nb3Sn conductor, the emergent behaviour is not consistent with the long-standing canonical explanation for Jc(ε).

Significant texture improvement in single-crystalline-like materials on low-cost flexible metal foils through growth of silver thin films

Low texture spreads of single-crystalline-like materials are critical for high performance of low-cost flexible semiconductors and second-generation high-temperature superconductors based on metal foils. For texture improvement, a single-crystalline-like Ag film is epitaxially grown on an ion-beam-assisted deposition TiN substrate using magnetron sputtering. Ultra-low texture spreads are found in the thin Ag film (∼330 nm), with an out-of-plane texture spread (Δω) of ∼1.03° and an in-plane texture spread (Δϕ) of ∼1.34°. Compared with the texture spreads of the TiN substrate, Δω and Δϕ of the Ag film are reduced by ∼42 and ∼79%, respectively. Applying this Ag buffer, the texture spreads of a single-crystalline-like Ge film are reduced by ∼37% (Δω) and ∼36% (Δϕ). Factors contributing to the texture improvement by Ag are studied using single-crystalline-like Ag films with various thicknesses.

Superconducting Quantum Interferometers for Nondestructive Evaluation

We review stationary and mobile systems that are used for the nondestructive evaluation of room temperature objects and are based on superconducting quantum interference devices (SQUIDs). The systems are optimized for samples whose dimensions are between 10 micrometers and several meters. Stray magnetic fields from small samples (10 µm–10 cm) are studied using a SQUID microscope equipped with a magnetic flux antenna, which is fed through the walls of liquid nitrogen cryostat and a hole in the SQUID’s pick-up loop and returned sidewards from the SQUID back to the sample. The SQUID microscope does not disturb the magnetization of the sample during image recording due to the decoupling of the magnetic flux antenna from the modulation and feedback coil. For larger samples, we use a hand-held mobile liquid nitrogen minicryostat with a first order planar gradiometric SQUID sensor. Low-T_c DC SQUID systems that are designed for NDE measurements of bio-objects are able to operate with sufficient resolution in a magnetically unshielded environment. High-T_c DC SQUID magnetometers that are operated in a magnetic shield demonstrate a magnetic field resolution of ~4 fT/√Hz at 77 K. This sensitivity is improved to ~2 fT/√Hz at 77 K by using a soft magnetic flux antenna.

Preferred Orientation Contribution to the Anisotropic Normal State Resistivity in Superconducting Melt-Cast Processed Bi₂Sr₂CaCu₂O8+δ

We describe how the contribution of crystallographic texture to the anisotropy of the resistivity of polycrystalline samples can be estimated by averaging over crystallographic orientations through a geometric mean approach. The calculation takes into account the orientation distribution refined from neutron diffraction data and literature values for the single crystal resistivity tensor. The example discussed here is a melt-cast processed Bi₂Sr₂CaCu₂O8+δ (Bi-2212) polycrystalline tube in which the main texture component is a <010> fiber texture with relatively low texture strength. Experimentally-measured resistivities along the longitudinal, radial, and tangential directions of the Bi-2212 tube were compared to calculated values and found to be of the same order of magnitude. Calculations for this example and additional simulations for various texture strengths and single crystal resistivity anisotropies confirm that in the case of highly anisotropic phases such as Bi-2212, even low texture strengths have a significant effect on the anisotropy of the resistivity in polycrystalline samples.

Vortices in high-performance high-temperature superconductors

The behavior of vortex matter in high-temperature superconductors (HTS) controls the entire electromagnetic response of the material, including its current carrying capacity. Here, we review the basic concepts of vortex pinning and its application to a complex mixed pinning landscape to enhance the critical current and to reduce its anisotropy. We focus on recent scientific advances that have resulted in large enhancements of the in-field critical current in state-of-the-art second generation (2G) YBCO coated conductors and on the prospect of an isotropic, high-critical current superconductor in the iron-based superconductors. Lastly, we discuss an emerging new paradigm of critical current by design-a drive to achieve a quantitative correlation between the observed critical current density and mesoscale mixed pinning landscapes by using realistic input parameters in an innovative and powerful large-scale time dependent Ginzburg-Landau approach to simulating vortex dynamics.

Supercurrent through grain boundaries of cuprate superconductors in the presence of strong correlations

Strong correlations are known to severely reduce the mobility of charge carriers near half filling and thus have an important influence on the current carrying properties of grain boundaries in the high-T(c) cuprates. In this Letter we present an extension of the Gutzwiller projection approach to treat electronic correlations below as well as above half filling consistently. We apply this method to investigate the critical current through grain boundaries with a wide range of misalignment angles for electron- and hole-doped systems. For the latter excellent agreement with experimental data is found. We further provide a detailed comparison to an analogous weak-coupling evaluation.

Interface Structure Of Iron Oxide Thin Films Grown On Sapphire And Single-Crystal MgO

The ability to tailor thin film interface structure is illustrated by the growth of thin ferric oxide films using different deposition parameters. Low-pressure chemical vapor deposition (CVD) has been used to grow α-Fe2O3 on sapphire while pulsed-laser ablation (PLA) followed by low temperature oxidation has been used to grow γ-Fe2O3 on MgO. The structure of the films and of the film-substrate interfaces has been characterized using transmission electron microscopy (TEM) and selected-area diffraction (SAD) in plan view. The different deposition techniques lead not only to the growth of two different polymorphs of ferric oxide but also to different growth mechanisms and film-substrate interface structures. In addition, changes in the thermodynamic conditions during deposition can give rise to different interface structures between the individual structural domains of the film.

Morphology and Defect Structure of Sputtered High-Quality In-Situ YBa2Cu3O7−δ Films

Transmission Electron Microscopy has been used to study the morphology and defect structure of sharp superconducting transition, high (2-6 ×107 A/cm2) critical current YBa2Cu3O7−δ films on MgO substrates. These were oriented such that the unit cell axes of the film aligned with those of the substrate, with some domains obeying a second orientation relationship rotated by 45° in the plane of the film, i.e. film <110> parallel to substrate <100>. The latter is not expected from simple lattice matching considerations. A strong influence of substrate surface topography on film microstructure was noted, leading to a high density of out-of-phase, low-angle tilt, and other boundaries near the substrate-film interface, which decreased with increasing distance from the substrate. Finally, the effects on film microstructure of two variables of specific interest in our sputtering system were investigated: the thickness of the deposited film, and the temperature at which a high oxygen pressure (500 torr) is introduced after deposition is complete. Increases in film thickness resulted in longer, more widely spaced twins, whereas lower oxygenation temperatures resulted in shorter twins.

Epitaxial Growth of Oxide Thin Films on (001) Metal Surfaces Using Pulsed-Laser Deposition

The epitaxial growth of CeO2 on various (001) metal surfaces using pulsed-laser deposition is discussed. In particular, the growth of (001) CeO2 on (001) Pd, Ag, and Ni is described. Emphasis is given to the specific deposition conditions which successfully alleviate the formation ornative oxides at the metal/metal oxide interface. The control of the epitaxial relationships between the oriented oxides films and the underlying noble and oxidizing metal surfaces is addressed.

Microstructural Defects in SrTiO3 Bicrystals and Their Influence on YBa2Cu3O7 Film Growth and Junction Performance

We use a near-field scanning optical microscope (NSOM) to investigate microstructural defects at the fusion boundaries of SrTiO3 bicrystal substrates. The optical transmission across the fusion boundary shows circular dark spots with diameters varying from 0.1 to 1 μm that are distributed non-uniformly along the boundary. After detailed characterization of the substrates, a thin YBa2Cu3O7 (YBCO) film (∼ 40 nm) was deposited on a 24° bicrystal. Combining NSOM and scanning force microscopy, we show that these substrate defects can cause the grain boundary of a YBCO thin film grown on the bicrystal to wander up to 0.8 micron in the film. Strain fields associated with these substrate defects are attributed to their influence on YBCO growth. The relation between these structural defects and the electrical characteristics of YBCO grain boundary junctions are also discussed.

Texture evolution of vertically aligned biaxial tungsten nanorods using RHEED surface pole figure technique

Vertically aligned biaxial tungsten nanorods with cubic A15 crystal structure were deposited by DC magnetron sputtering on native oxide covered Si(100) substrates with glancing angle flux incidence (theta approximately 85 degrees) and a two-step substrate rotation mode at room temperature. These vertical nanorods were grown to different thicknesses (10, 25, 50 and 100 nm) and analyzed for biaxial texture evolution using a highly surface sensitive reflection high-energy electron diffraction (RHEED) pole figure technique. The initial polycrystalline film begins to show the inception of biaxial texture with a fiber background between 10 and 25 nm. Biaxial texture development is eventually completed between 50 and 100 nm thicknesses of the film. The out-of-plane crystallographic direction is [002] and the in-plane texture is selected so as to obtain maximum capture area. In a comparison with 100 nm thick inclined tungsten nanorods deposited at 85 degrees without substrate rotation, it is found that the selection of in-plane texture does not maintain maximum in-plane capture area. This anomalous behavior is observed when the [002] texture axis is tilted approximately 17 degrees from the substrate normal in the direction towards the glancing incident flux.

Large neutral barrier at grain boundaries in chalcopyrite thin films

The electronic structure of grain boundaries in polycrystalline Cu(In,Ga)Se2 thin films and their role on solar cell device efficiency is currently under intense investigation. A neutral barrier of about 0.5 eV has been suggested as the reason for the benign behavior of grain boundaries in chalcopyrites. Previous experimental investigations have in fact shown a neutral barrier but only a few 10 meV high, which cannot be expected to have a significant influence on the solar cell efficiency. Here we show that a full investigation of the electrical behavior of charged and neutral grain boundaries shows the existence of an additional narrow neutral barrier, several 100 meV high, which is tunneled through by the majority carriers but is sufficiently high to explain the benign behavior of the grain boundaries.

The formation of vertically aligned biaxial tungsten nanorods using a novel shadowing growth technique

Biaxially textured tungsten nanorods (A15 crystal structure) have been grown by oblique angle DC magnetron sputtering using a novel rotation mode called 'two-step rotation'. In this mode, the substrate is given a fast rotation through 180 degrees at 90 rpm and this is followed by a rest period of 30 s. These nanorods are vertically aligned and have a [100] texture normal to the substrate along with preferential in-plane texture as shown by x-ray pole figure analysis. In contrast, the tungsten nanorods obtained without substrate rotation are slanted at an angle of approximately 45 degrees and have a [100] texture tilted 16 degrees with respect to the substrate normal. The flux is incident from two diametrically opposite points on the sample at an oblique angle, averaging out the growth into vertical columns that retain the in-plane texture. Scanning electron microscopy shows that the tungsten nanorods have a mixture of {211} and {421} crystal habits; these planes are both minimum surface energy planes for a cubic A15 crystal structure.

Effect of a compressive uniaxial strain on the critical current density of grain boundaries in superconducting YBa2Cu3O7-delta films

The mechanism by which grain boundaries impede current flow in high-temperature superconductors has resisted explanation for over two decades. We provide evidence that the strain fields around grain boundary dislocations in YBa2Cu3O7-delta thin films substantially suppress the local critical current density Jc. The removal of strain from the superconducting grain boundary channels by the application of compressive strain causes a remarkable increase in Jc. Contrary to previous understanding, the strain-free Jc of the grain boundary channels is comparable to the intrinsic Jc of the grains themselves.

First-principles calculations of electronic states and self-doping effects at a 45 degrees grain boundary in the high temperature YBa2Cu3O7 superconductor

The charge redistribution at grain boundaries determines the applicability of high-T_{c} superconductors in electronic devices because the transport across the grains can be hindered considerably. We investigate the local charge transfer and the modification of the electronic states in the vicinity of the grain-grain interface by ab initio calculations for a (normal-state) 45 degrees -tilted [001] grain boundary in YBa2Cu3O7. Our results explain the suppressed interface transport and the influence of grain boundary doping in a quantitative manner, in accordance with the experimental situation. The charge redistribution is found to be strongly inhomogeneous, which has a substantial effect on transport properties since it gives rise to a self-doping of 0.10+/-0.02 holes per Cu atom.

Atomic-resolution spectroscopic imaging: past, present and future

This review examines the development of atomically resolved electron energy loss spectroscopy from the first demonstration of plane-by-plane compositional profiling, through column-by-column spectroscopy to full two-dimensional and potentially three-dimensional spectroscopic imaging. Examples will be presented to highlight the increasing analytical sensitivity and image contrast obtained through each generation of aberration correction, moving towards the ultimate goal of mapping electronic structure inside materials with atomic resolution.

Investigating the optical properties of dislocations by scanning transmission electron microscopy

The scanning transmission electron microscope (STEM) allows collection of a number of simultaneous signals, such as cathodoluminescence (CL), transmitted electron intensity and spectroscopic information from individual localized defects. This review traces the development of CL and atomic resolution imaging from their early inception through to the possibilities that exist today for achieving a true atomic-scale understanding of the optical properties of individual dislocations cores. This review is dedicated to Professor David Holt, a pioneer in this field.

Statistical physics of grain-boundary engineering

Percolation theory is now standard in the analysis of polycrystalline materials where the grain boundaries can be divided into two distinct classes, namely "good" boundaries that have favorable properties and "bad" boundaries that seriously degrade the material performance. Grain-boundary engineering (GBE) strives to improve material behavior by engineering the volume fraction c and arrangement of good grain boundaries. Two key percolative processes in GBE materials are the onset of percolation of a strongly connected aggregate of grains, and the onset of a connected path of weak grain boundaries. Using realistic polycrystalline microstructures, we find that in two dimensions the threshold for strong aggregate percolation c(SAP) and the threshold for weak boundary percolation c(WBP) are equivalent and have the value c(SAP) = c(WBP) =0.38 (1) , which is slightly higher than the threshold found for regular hexagonal grain structures, c(RH) =2 sin (pi/18) =0.347... . In three dimensions strong aggregate percolation and weak boundary percolation occur at different locations and we find c(SAP) =0.12 (3) and c(WBP) =0.77 (3) . The critical current in high T(c) materials and the cohesive energy in structural systems are related to the critical manifold problem in statistical physics. We develop a theory of critical manifolds in GBE materials, which has three distinct regimes: (i) low concentrations, where random manifold theory applies, (ii) critical concentrations where percolative scaling theory applies, and (iii) high concentrations, c> c(SAP) , where the theory of periodic elastic media applies. Regime (iii) is perhaps most important practically and is characterized by a critical length L(c) , which is the size of cleavage regions on the critical manifold. In the limit of high contrast epsilon-->0 , we find that in two dimensions L(c) proportional, gc/ (1-c) , while in three dimensions L(c) proportional, g exp [ b(0) c/ (1-c) ] / [c (1-c) ](1/2) , where g is the average grain size, epsilon is the ratio of the bonding energy of the weak boundaries to that of the strong boundaries, and b(0) is a constant which is of order 1. Many of the properties of GBE materials can be related to L(c) , which diverges algebraically on approach to c=1 in two dimensions, but diverges exponentially in that limit in three dimensions. We emphasize that GBE percolation processes and critical manifold behavior are very different in two dimensions as compared to three dimensions. For this reason, the use of two dimensional models to understand the behavior of bulk GBE materials can be misleading.

X-ray microdiffraction study of growth modes and crystallographic tilts in oxide films on metal substrates

The crystallographic texture of thin-film coatings plays an essential role in determining such diverse materials properties as wear resistance, recording density in magnetic media and electrical transport in superconductors. Typically, X-ray pole figures provide a macroscopically averaged description of texture, and electron backscattering provides spatially resolved surface measurements. In this study, we have used focused, polychromatic synchrotron X-ray microbeams to penetrate multilayer materials and simultaneously characterize the local structure, orientation and strain tensor of different heteroepitaxial layers with submicrometre resolution. Grain-by-grain microstructural studies of cerium oxide films grown on textured nickel foils reveal two distinct kinetic growth regimes on vicinal surfaces: ledge growth at elevated temperatures and island growth at lower temperatures. In addition, a combinatorial approach reveals that crystallographic tilting associated with these complex interfaces is qualitatively described by a simple geometrical model applicable to brittle films on ductile substrates. The sensitivity of conducting percolation paths to tilt-induced texture improvement is demonstrated.

Critical current of YBa(2)Cu(3)O(7-delta) low-angle grain boundaries

Transport critical current measurements have been performed on 5 degrees [001]-tilt thin film YBa(2)Cu(3)O(7-delta) single grain boundaries with the magnetic field rotated in the plane of the film, phi. The variation of the critical current has been determined as a function of the angle between the magnetic field and the grain boundary plane. In applied fields above 1 T the critical current j(c) is found to be strongly suppressed only when the magnetic field is within an angle phi(k) of the grain boundary. Outside this angular range the behavior of the artificial grain boundary is dominated by the critical current of the grains. We show that the phi dependence of j(c) in the suppressed region is well described by a flux cutting model.

Experimental test for subdominant superconducting phases with complex order parameters in cuprate grain boundary junctions

We propose and implement a direct experimental test for subdominant superconducting phases with broken time-reversal symmetry in d-wave superconductors. The critical current of 45 degrees -asymmetric grain boundary junctions is shown to be extremely sensitive to the predicted onset of a complex order parameter at (110) surfaces and near magnetic impurities. Measurements in pure Ni-doped YBa2Cu3O7-x junctions indicate that the symmetry at the surface is consistent with pure d-wave at all temperatures, putting limits on the magnitude and chiral domain structure of any subdominant symmetry component.

High angle [001] twist boundaries in platelet colonies of BiSrCaCuO (2223) superconductor

The microstructure of BiPbSrCaCuO (2223) ceramic was studied by polarised light microscopy and the etch-pits technique. The high-angle twist grain boundaries in the platelet colonies were confirmed to be [001] coincidence type twisted boundaries or rotation twin boundaries with the (001) twinning plane.