Domain Translation Research Articles

Interface crystallography and stability in epitaxial metal (NiAl, CoAl)/III-V Semiconductor heterostructures

The stability and crystallography of AlAs/NiAl/ and AlAs/CoAl interfaces in AlAs/NiAl, CoAl/ AlAs heterostructures grown by molecular beam epitaxy (MBE) are described. The thermal stability of an interface between a transition metal aluminide (TM-Al) film and an AlAs substrate is a result of the bulk thermodynamic stability of these materials in a closed system. The TM-Al film surface, however, is not stable in the arsenic vapor ambient of an MBE chamber. Excessive exposure to arsenic results first in the depletion of aluminum from the TM-Al film to form AlAs followed by the reaction of the TM with arsenic to form a TM arsenide. These reactions can be prevented or controlled by growing the AlAs overlayer under aluminum-rich conditions. The defect structures of the upper and lower interfaces also differ. Since the TM-Al phase with the CsCl structure has higher symmetry than AlAs with the zinc blende structure, a single-variant epitaxial film of TM-Al can be grown on {100} AlAs. The lower symmetry of the AlAs, however, dictates that an AlAs layer grown on the {100} surface of a TM-Al film will contain translation domain boundaries and inversion domain boundaries. The constraints imposed by the large differences in chemistry and structure present challenging obstacles to the realization of high quality epitaxial metal/compound semi-conductor heterostructures of arbitrary complexity.

Observations of domain boundaries and steps on the Rh(111)-c(2✓3 × 4) rect (C 6H 6 + CO) surface by scanning tunneling microscopy

Scanning tunneling microscope (STM) images of the c(2 √3 × 4) rect layer of benzene and carbon monoxide coadsorbed on the Rh(111) surface are shown. Both rotational and translational domain boundaries have been observed. Domain boundaries often appear near isolated defects and atomic steps of the substrate.

Scanning tunneling microscopy study of low-temperature epitaxial growth of silicon on Si(111)-(7×7)

Scanning tunneling microscopy is used to investigate nucleation and growth phenomena in the molecular-beam epitaxial (MBE) growth of silicon on Si(111)-(7×7) from the submonolayer range up to a few monolayers. At room temperature small amorphous clusters form which grow in locally ordered arrays on the (7×7) lattice. Deposition at a higher substrate temperature produces triangular islands of epitaxial silicon which have preferred step propagation in the [1̄1̄2] direction. Preferred nucleation of Si islands is found to occur along boundaries between (7×7) superstructure translational domains of the substrate. The preferred nucleation which arises from defects in the epilayer accounts for the formation of a second epitaxial layer long before the first layer is completed. A variety of metastable reconstructions which differ from (7×7) are also found in the epitaxial islands and are discussed.

Scanning tunneling microscopy observations of benzene molecules on the Rh(111)-(3 x 3) (C6H6+2CO) surface.

Scanning tunneling microscopy (STM) images of the (3×3) superlattice of benzene and carbon monoxide coadsorbed on the Rh(111) surface reveal a well-ordered array of ringlike features associated with individual benzene molecules, while CO is not resolved. Images further show translational domain boundaries, step-edge structures, and evidence for surface diffusion. The origin of the STM image contrast of these molecules and implications for STM imaging of other molecular adsorbates are briefly discussed.

Towards an analyzer (parser) in a machine translation system based on ideas from expert systems

GETA (Groupe d'études pour la traduction automatique) is a research team working basically in the domain of machine translation. GETA's software system, ARIANE‐78, has been tested over various pairs of relatively unrelated languages. Being a product of the late seventies, the system misses out on some of the artificial intelligence technology, particularly that of the eighties. Nevertheless, GETA carries out some research related to artificial intelligence within a general effort to bring improvements to the current system. This paper reports on an effort to embed such work within the framework of an entirely new system based on ideas from expert systems, significantly departing from the methodology of the current system (and that of other currently implemented machine translation systems). The proposed architecture aims for total modularity and flexibility and some degree of intelligence.

SUPERCONDUCTIVITY AND CRYSTAL STRUCTURE OF LaBa2−xCaxCu3O7

The samples with nominal composition LaBa2−xCaxCu3O7 (x=0, 0.5, 1.0, 1.5, 2.0) have been prepared by solid state reactions and studied by x-ray diffraction method and transmission electron microscopy. The substitution of Ba by Ca results in the decrease of the cell volume, increase of the structural orthorhombic distortion and appearance of the c/3 translation domain. Of the five samples studied only those with x=0, 1.0, and 1.5 are superconductors. The x=0 sample shows onset temperature 90 K, the zero resistance temperature 45 K and a weak diamagnetic effect. The x=1.5 sample shows zero resistance at 72 K and sustantial diamagnetic effect. The relation between the crystal structure and superconductivity is discussed.

Nascent polypeptide chains emerge from the exit domain of the large ribosomal subunit: immune mapping of the nascent chain.

The site of the nascent polypeptide chain as it leaves the ribosome has been localized on the "exit domain" of the Escherichia coli ribosome by using IgG antibodies directed against the enzyme beta-galactosidase (EC 3.2.1.23). Thus, a functional site has been mapped on intact 70S ribosomes. The exit site is on the large subunit, approximately 70 A from the interface between subunits and nearly 150 A from the central protuberance, the likely site of peptide transfer. It is adjacent to the region corresponding to the rough endoplasmic membrane binding region of the eukaryotic ribosome but distant from ribosomal components participating in mRNA recognition and polypeptide elongation (i.e., distant from the "translational domain"). These results, together with the protease protection experiments of others, provide evidence that the nascent protein chain probably passes through the ribosome in an unfolded, fully extended conformation.

Magnetic structure and lattice deformation in UO 2

The magnetic phase transition in UO 2 is studied by means of a group theoretical analysis and the admitted symmetry groups in the low temperature phase are determined. With the help of the neutron diffraction data of Faber and Lander a three-arm magnetic and crystallographic structure with two types of translational domains is found and a new interpretation of the experiment of Faber and Lander is given.

Translational regulation: identification of the site on bacteriophage T4 rIIB mRNA recognized by the regA gene function.

The bacteriophage T4 gene regA encodes a protein that diminishes the expression of many unlinked early T4 genes. Previous work demonstrated that regA-mediated repression occurs after transcription. We report here on the identification of the target site on one regA-sensitive mRNA, the message encoding the phage T4 rIIB protein. The target for regA-mediated action overlaps the translational initiation domain of the rIIB messenger. The regA protein may be a repressor that operates translationally on a significant and interesting set of early phage T4 mRNAs.

Quantitative island size determination in the chemisorbed layer W(110) p(2×1)— O: II. Theoretical models

Mathematical models are presented for the quantitative determination, using low-energy electron diffraction, of finite-size effects in ordered overlayers of submonolayer coverage. Models are described that apply for ordered structures at low coverages, near saturation coverage for structures that have only translational antiphase domains, for structures that also have rotational antiphase domains, and for overlayers on a stepped surface. The models are illustrated with sample calculations using diffraction parameters appropriate to W(110)p(2 × 1)—O.

Lattice imaging of the B19 ordering transformation and interfacial structures in Mg 3Cd

The later stages of B19 ordering in Mg 3Cd have been investigated by direct lattice imaging. Unit cell high steps (5.3 Å) have been clearly resolved in the following interfaces: the order-disorder interface; translational antiphase boundaries and rotational domain boundaries. These observations are thought to indicate a ledge mechanism for ordering and domain growth. Lattice imaging and corresponding optical diffraction have also been used for phase identification and determination of unit cell dimensions. In addition, the characteristic domain structure of the B19 phase has been studied in detail, and the unusual multiple domain junctions are explained in terms of the orthorhombic lattice distortion upon transformation. Considerably more information has been discovered by lattice imaging than is possible by conventional electron microscopy.

Equilibrium structure of epitaxial layers

The equilibrium structure of an epitaxial layer on a flat crystal substrate is considered theoretically. Epitaxial stresses as well as the elastic and free energies of the layer are calculated as function of the substrate crystallographic orientation. It is shown that as the layer thickness increases there is the “phase” transition from the state of the best microscopic fit between the layer and substrate lattices (the microdistortion is absent) to the state of the best macroscopic fit (the elastic energy is a minimum). When the layer thickness increases the epitaxial stress relief due to the formation of elastic domain becomes preferable. In particular cases domains may be twins and the layer is a polysynthetic twin. The formation of the first- and second-order polysynthetic twins is discussed. With the increase in the thickness to some critical value the relaxation by twin domains formation becomes less favourable than by the translation domains formation. The correlation between the translation domains and misfit dislocations is under consideration. [Russian Text Ignored]

Analysis of ordering in Cu 3Au utilizing lattice imaging techniques

The resolution of lattice fringes in ordered and partially ordered Cu 3Au has been calculated using the dynamical theory of electron diffraction, taking into consideration the effects of such variables as foil thickness and orientation, objective lens defocus, degree of order and, for partially ordered alloys, the position of small ordered domains in the foil. The experimental observations were found to be in excellent agreement with these calculations. Lattice defects in ordered Cu 3Au have been studied using this technique. Thus, the domain wall thickness at translational antiphase domain boundaries has been shown to be one or two atomic diameters and the antiphase boundary separating the pair of fundamental lattice dislocations which constitute a superdislocation has been revealed. A clear distinction exists between the lattice image contrast of predominantly ordered and disordered regions and this has been utilized to study partial long-range order and short-range order. Short-range order in this system is characterised by a dispersion of small highly ordered domains, ~15 Å dia., in a predominantly disordered matrix.

Observation des parois d'antiphase et réAsolution de la structure ordonnéAe péAriodique (3,8 Å) dans l'alliage Ni2Cr

Abstract OBSERVATION OF ANTIPHASE DOMAINS AND RESOLUTION OF THE PERIODIC ANTIPHASE STRUCTURE (3.8 Å)I THE Ni2Cr ALLOY By suitable annealing of a Ni-32.60 at. % Cr alloy we could grow ordered domains several thousand Angströms in diameter. The size of the domains is sufficient for the antiphase boundaries between translation domains to be observed and for the periodic ordered structure (3.8 Å) of a single Ni2Cr domain to be resolved by the interference of two super lattice electron beams.

Field-ion studies of microstructures and transformations in long-range-ordered alloys

The FIM has been shown in recent years to be a useful tool in the study of microstructures of ordered alloys. Although most of the studies to data have been of a qualitative nature they have shown that it is possible to obtain detailed information about rotational and translational domain boundaries, order-disorder interfaces and local variations in the degree of order. This type of information can be particularly useful in the study of order-disorder transformations.

Behind the Scenes of the Pharmaceutical Center

Current approaches to defining and classifying psychotic disorders are compromised by substantive heterogeneity within, blurred boundaries between, as well as overlaps across the various disorders in outcome, treatment response, emerging evidence regarding pathophysiology and presumed etiology.We herein review the evolution, current status and the constraints posed by classic symptom-based diagnostic approaches. We compare the continuing constructs that underlie the current classification of psychoses, and contrast those to evolving new thinking in other areas of medicine.An important limitation in current psychiatric nosology may stem from the fact that symptom-based diagnoses do not “carve nature at its joints”; while symptom-based classifications have improved our reliability, they may lack validity. Next steps in developing a more valid scientific nosology for psychoses include a) agnostic deconstruction of disease dimensions, identifying disease markers and endophenotypes; b) mapping such markers across translational domains from behaviors to molecules, c) reclustering cross-cutting bio-behavioral data using modern phenotypic and biometric approaches, and finally d) validating such entities using etio-pathology, outcome and treatment-response measures.The proposed steps of deconstruction and “bottom-up” disease definition, as elsewhere in medicine, may well provide a better foundation for developing a nosology for psychotic disorders that may have better utility in predicting outcome, treatment response and etiology, and identifying novel treatment approaches.