In recent years, many efforts have been made to fabricate high-Tc superconductor/normal-metal/ superconductor (SNS) Josephson junctions, but no breakthrough has been made. The major difficulties are the poor reproducibility of the junction parameters due to the structural complexity and defects in the barrier layer. Some junctions, such as the one with Co-doped YBCO barrier, have relatively small spreads in properties. However, the value of their normal state resistance Rn is low, which limits their use for many applications. One approach to solve this “low-Rn” problem is to use materials of high resistivity for the barrier layer. PBCO is an insulator near the metal-insulator transition. Doping with Co drives it further into the insulating regime. The lattice constants of PBCO are close to that of YBCO, even closer when doped with Co. Therefore, Co-PBCO may form a high-resistivity barrier which matches with YBCO very well and hence is a potential candidate barrier material.

(Bi,Pb)2Sr2Ca2Cu3Ox/Ag composite tapes are currently the most promising high-temperature superconductors for large-scale applications. A major issue in further improving the supercurrent-carrying-capability and optimizing fabrication procedures of these tapes is to understand the formation kinetics of the Bi(2223) from precursor powders of Bi(2212), CaO, and CuO. Since the microstructural transformation during the complicated phase evolution is not well known yet, identification of minor phases at different transformation stage is desirable. Here, we report our crystallographic analysis of several frequently observed intermediate phases during the phase evolution.

The heat treatment of the tapes consisted of ramping the temperature to 840°C in air and of holding it for a prescribed period and then oil-quenching from various temperatures.

Grain boundaries (GBs) in laser deposited YB2Cu3O7-δ/MgO(001) thin films have been investigated by high-resolution transmission electron microscopy (TEM) and scanning TEM (STEM). We report both statistics and atomic structure of low-angle and high-angle [001] tilt grain boundaries resulting from almost perfect c-axis textured YBCO films.

Atomic structure of low-angle GBs was analyzed using a dislocation model. These boundaries have been found to be aligned primarily along (100) and (110) interface planes. For (100) boundary plane, the GB consists of a periodic array of [100] dislocations (Fig.l). For (110) boundary plane, the array is also periodic but every [110] dislocation is split by ∼ 1.5 nm into two [100] and [010] dislocations (Fig.2). We have calculated energy of various configurations and shown that the energy of the (110) boundary with dissociated dislocations is comparable to that of (100) boundary, which explains the coexistence of (100) and (110) interface facets along the boundary.

Sometimes oxides are friendly; x-ray or neutron diffraction solves the structure and (at most) simple HREM does a double check. Sometimes they are not. Problems can range from phase purity (identifiable using EDX) to more subtle issues about the true unit cell and/or atom locations. The latter are often particularaly hard for x-ray or neutron to handle when single crystal data is not available and, for instance, the samples are polyphase or textured.

One common problem is that the unit cell is rather larger than originally thought due to a superstructure. While superstructure reflections may be obvious in the microscope due to the very large dynamic range of TED patterns, they may not show in powder patterns. The issue then is to determine the true structure using (often) a combination of HREM and TED plus multislice simulations to confirm the structure.

A key towards understanding the properties of high-temperature superconductors is knowledge about the distribution of the valence electrons and holes. Despite their complicated crystal structure, electronic structure of many cuprates has been calculated. The most successful calculation method is based on the theorem that the ground state energy is a unique functional of the electron density. Thus, experimental techniques that give information about the valence electron density are essential to test the calculations and to gain further insight into these important materials.

To move beyond locating the atoms in the crystal unit cell towards addressing the details of the electron distribution requires in general extremely accurate diffraction experiments. Highly perfect crystals with small unit cells have been approached. It has been known for several decades that in electron diffraction the amplitudes of those reflections at small scattering angles that are present in crystals with large unit cells, are strongly influenced by the valence electron distribution.

Cu2O and CuO are of great interest after the discovery of the high Tc superconductors. The superconductivity is believed to be dependent on charge states in CuO2 layers whose structure is similar to that of CuO. These transition-metal compounds are also important because of the different copper coordination numbers and different copper-oxygen distances, therefore these reflect different electron density distributions and bonding mechanisms. The electronic structure of cuprite has been studied extensively both theoretically and experimentally because of its symmetrical crystallographic structure (space group ) while that of CuO (space group C2/c) which has a monoclinic structure has been studied only recently. Previous experimental and theoretical results show that the valence electrons of copper in cuprite form hybrid states which is proposed by Orgel instead of spherical d10 state as in ionic copper.

A knowledge of grain boundaries (GBs) and interfaces is essential in establishing the relationship between the structural details of the boundaries and the strength of superconducting coupling and critical current densities at the boundaries in textured Bi cuprates. As a step towards such understanding, we studied GB crystallography and lattice mismatch using the constrained Coincidence-Site-Lattice (cCSL) theory. We note that to characterize arbitrary GBs in a complex system such as Bi2212, it would be desirable to find some simple analytical solutions for the cCSL approach.

We report our new concept of a “two-mirrors” model (2M-model), which allow us, in principle, to find sufficient conditions to generate corresponding analytical solutions for CSL (or cCSL), when it is applied to crystals with cubic, hexagonal, or orthorhombic symmetry. The 2M-model is based on the fact that any boundary can be described by rotating one crystal relative to the other along a common axis.

Magnetic vortices in superconductors have become directly and dynamically observable[l] by Lorentz microscopy in our 300 kV field-emission TEM[2]. Since material defects can also be observed simultaneously though the images are defocused, the microscopic flux pinning mechanism can be directly observed. Using this technique, we observed when and how vortices trapped at artificial defects were depinned.

Artificial point defects were produced by the irradiation of a focused Ga ion beam. Electron microscopy revealed that the defect consisted of a pit 300 Å in radius surrounded by entangled dislocations. The depth of a pit increased with the ion dose.

We first investigated the pinning force of individual defect: by observing the depinning of vortices from defects produced with various ion-doses. When a force was exerted on vortices trapped at the defects by changing the magnetic field and increased, we found that the pinning force of defects increased with the ion doses.

Due to the extemely short coherence lengths of the high-Tc superconductors (around 30 Å in the a-b plane), defects such as grain boundaries are obvious barriers to the flow of supercurrent. Within a few months of the discovery of these materials, it was shown how the critical current dropped four orders of magnitude as the grain boundary misorientaion increased from zero to 45°. Even today, there is no quantitative understanding of this behavior. A qualitative understanding is however possible through atomic resolution Z-contrast imaging on YBa2cu3O7-δ and SrTiO3 bicrystal grain boundaries, combined with bond-valence-sum analysis.

The Z-contrast image of a YBa2cu3O7-δ low angle grain boundary in Fig. 1 shows the same kind of reconstructed dislocation cores as seen in SrTiO3, containing reconstructions on both the Cu and Y/Ba sublattices.

Developing an atomic scale study of the structure-property relationships of grain boundaries in high-Tc superconductors is essential to understand their current dissipation mechanism and for incorporating these materials into viable devices. Thin YBa2Cu3O7-δ films have been deposited by pulsed laser deposition (PLD) on SrTiO3 symmetric bicrystals. Transport measurements in a magnetic field have been conducted across the grain boundaries through a wide bridge. The data obtained are consistent with microstructural observation in a VG Microscopes HB603 U and a VG HB501 UX dedicated STEM. Of particular interest in the study of high-Tc materials is the use of EELS, which can highlight the presence of non-superconducting regions through interpretation of the onset positions and finestructure (ELNES) of characteristic core-edge features.

The V(I) curves recorded across a 24° boundary for several magnetic fields (fig. 1 - left) show an onset critical current density followed by a linear differential ohmic behavior which gives a negative intercept.

High temperature superconductivity in cuprates has become a fairly mature field now that more than a decade has passed by since its discovery. While scientific appeal of the field continues, the technological implications have faced harsh realities. Although the transition temperatures are high, the coherence length in oxide superconductors is awfully short and thus inhomegeneity or perturbation at the length scale of a few nanometers proves extremely detrimental to their properties, especially transport. Further, slight changes in the oxygen stoichiometry in cuprates have a profound effect on their superconducting properties and it is often not clear whether the nonstoichiometry at defect sites (e.g. grain boundaries- GBs) is intrinsic or extrinsic.

The anisotropy of electronic structure is well reflected in low loss EELS spectra of these materials. Figure 1 shows low loss EELS spectra from Y124: one taken with momentum transfer confined to the Cu-02 planes, and the other normal to them. These observations are in accord with the fact that conduction is predominantly along the ab-planes, within single crystals.

A cubic millimeter (10-9 m3) of condensed matter contains typically ∼1020 atoms, and a complete solution to the general structure problem requires finding ∼3 ×l020 atom coordinates. For crystals, long-range translational periodicity immensely simplifies the structure elucidation problem to something like a 101-102 problem (further reduced by the symmetry accompanying orientational order), because only the contents of the unit cell (< 100 atoms for most inorganic structures) and the unit cell geometry need be determined. Amorphous is the epithet traditionally bestowed on structures with neither orientation nor translational order, a more precise term for which is topologically disordered [1], A structural average for an amorphous solid can be established with a good many fewer than 1020 atoms, because many atom-atom correlations nevertheless exist in topologically-disordered structures due to bonding, packing and associated topological constraints. In fact, assemblies with less than 200 atoms can usually adequately represent characteristic structural topologies [1].

Covalently bonded materials of the light elements B, C, N, O and Al form an interesting series of structures which are readily prepared by physical vapour deposition methods. The most important structural issues are the degree of positional order (that is, whether the material is amorphous or nano phase crystalline) and for alloys, the degree of site occupancy order (that is, the extent of ‘chemical’ order). We propose that the degree of positional order is dependent on the bond angle stiffness of the covalent bonding and the degree of chemical order is dependent on the energy difference of bonds between like and unlike atoms (see figure 1).

The observation of structure in these materials is conveniently carried out by electron optical techniques in an electron microscope. The techniques of value are (a) Energy Filtered Electron Diffraction, and (b) Electron Energy Loss Spectroscopy (EELS).

Observations made more than ten years ago1 showed that SiC could be made amorphous at cryogenic temperatures by in-situ 300kV electron irradiation. However, high-voltage electron microscope (HVEM) results indicate a threshold voltage of 725 kV for amorphization of SiC at 140 K. In addition, a recent review exposes the considerable uncertainty in the literature regarding displacement energies for SiC. Therefore, further experiments have been performed in a Philips CM30 (LaB6 cathode) with a Gatan double-tilt cooling holder in an attempt to determine the threshold voltage for amorphization at ∼ 140 K. Sintered α-SiC (defected 6H polytype), beam direction B=< 1120 >, and probes containing ∽ 75 nA in ∽0.5 μm, were used. Amorphization occurred in <10 min at 300 kV and after ∽60 min at 180 kV (Fig. 1); visible darkening occurred at lower voltages and doses. Similar behavior occurred for B=[0001]. The critical dose for amorphization was measured as a function of accelerating voltage.

Abstract

Fast electrons are a particularly useful chemical and structural probe for the small sample volumes associated with ion- or fast electron-irradiation-induced amorphization, because of their much stronger interaction with matter than for X-rays or neutrons, and also because they can be readily focused to small probes. Three derivative signals are particularly rich in information: the angular distribution of scattered electrons (which is utilized in both diffraction and imaging studies); the energy loss spectrum of scattered electrons (electron energy loss spectroscopy, or EELS); and the emission spectrum of characteristic X-rays resulting from ionization energy losses (energy dispersive X-ray spectroscopy, or EDXS). We have applied the first two to the study of three amorphized compounds (AIPO4, SiO2, SiC) using MIT's Vacuum Generators HB603 field-emission (FEG) scanning transmission electron microscope (STEM), operating at 250 kV and equipped with a Gatan digital parallel-detection electron energy-loss spectrometer (digiPEELS).

Amorphous materials are devoid of periodic long range order, but at the nearest-neighbor level they possess a high degree of short-range order. In amorphous tetrahedral semiconductors, such as Si and Ge, this short-range order arises because each atom attempts to satisfy four bonds arranged as a regular tetrahedron. It is the rotations about each bond, from the second-nearest-neighbor outwards, that result in the loss of long-range order. It is apparent from modeling of amorphous materials, that there is considerable flexibility as to how rapidly the medium-range-order diminishes. The continuous random network (CRN) is a hypothetical tetrahedral extended structure wherein the atoms possess full four-connected coordination, but have minimal medium-range order. However, real amorphous materials infrequently exhibit true CRN-like topologies. Traditionally, diffraction has been used to study short- and medium-range order in amorphous materials. Assuming kinematical scattering, and that every atom has a similar environment, a radial distribution function (RDF) can be extracted which is sensitive only to the averaged atom pair-correlations out to ∼1 nm.

The electron microscope provides an ideal environment for the structural analysis of small volumes of amorphous and polycrystalline materials by collecting scattering information as a function of energy loss and momentum transfer. The scattering intensity at zero energy loss can be readily processed to a reduced density function G(r), providing information on nearest neighbour distances and bond angles[l]. Figure 1(a) shows the G(r) for glassy carbon, a turbostratic form of graphite. The three nearest neighbours in glassy carbon (labelled 1-3 in figure 1) are at 1.42 Å, 2.44 Å and 3.75 Å respectively. These distances correspond to the first three nearest neighbours in a graphite sheet and are expected in glassy carbon which is know to have good in-plane graphitic order. In figure 1(b) the G(r) of cathodic arc deposited tetrahedral amorphous carbon is shown. This material contains a high fraction of diamond-like bonding[2] and has a 1st nearest neighbour peak at 1.52 Å.

The recent availability of Gatan's imaging filter (GIF) has improved detail in electron energy loss spectroscopy (EELS) owing to the greatly reduced channel-to-channel gain variation. In particular, extended electron loss fine structure (EXELFS) can be rapidly obtained with relatively low beam exposures to the sample. Previous studies using EXELFS (of which there are relatively few) have been limited to rather beam-insensitive samples and have required extensive effort to correct detector gain variation. [1,2] We will discuss here some of our ongoing work on use of EXELFS as a complement to other analytical microscopy techniques to study changes in materials caused by radiation damage (ion and alpha) and transmutation. Using an imaging filter as a spectrometer, we have obtained EXELFS from a rather beam-sensitive alkali aluminosilicate, pollucite (CsAlSi2O6).

The recent discovery of graphite cones [1,2] has raised some interesting questions about the nucleation and growth of curved graphitic structures. Here we report the structural peculiarities of one member of the ensemble, i.e. the flat graphite disks. Unlike graphite flakes seen in carbon soot which are irregularly shaped, the disks in this sample often show regular faceting which might give us additional insight into the growth mechanisms.

The samples were examined in a Hitachi H9000 NAR TEM. Samples were sonicated in high purity methanol and dispersed on a 300 mesh Cu grid coated with a 20 nm thick amorphous carbon layer. Typical disk diameters range from 0.5 to 3.0 μm, with thickness ranging from 10 nm to 50 nm.

A close examination reveals that the edges of the disks are faceted and typically have 12 sides. In most cases, the edges show a long facet alternating with a shorter facet.

Boron-based thin film materials have numerous uses ranging from application as hard and refractory protective coatings to potential employment in wide-band gap semiconductor electronics. Of particular interest are the boron carbides, nitrides, oxides, and phosphides. These compounds exhibit a broad range of structural and bonding variations. The crystalline form of elemental boron is based on an arrangement of 12-atom boron icosahedra positioned at the vertices of a rhombohedral lattice. Related materials, such as B4C, and B12P, also possess structures closely related to this icosahedrally coordinated prototype. However, the structural coordination of the boron carbides and phosphides will vary with composition. For instance, BP possesses a tetrahedrally coordinated zinc-blende structure, and the carbon-rich boron carbides will form in a graphitic structure. Boron nitride has the added complication that for the same composition (BN) multiple bonding and polytypic variations are possible: both soft, graphitic sp2-rich (e.g., hexagonal and rhombohedral BN) and hard, diamond-like sp3-rich phases (e.g., cubic and wurtzitic BN) can be formed.