Screw dislocation in zirconium: An ab initio study

Plasticity in zirconium is controlled by 1/3 screw dislocations gliding in the prism planes of the hexagonal close-packed structure. This prismatic and not basal glide is observed for a given set of transition metals like zirconium and is known to be related to the number of valence electrons in the d band. We use ab initio calculations based on the density functional theory to study the core structure of screw dislocations in zirconium. Dislocations are found to dissociate in the prism plane in two partial dislocations, each with a pure screw character. Ab initio calculations also show that the dissociation in the basal plane is unstable. We calculate then the Peierls barrier for a screw dislocation gliding in the prism plane and obtain a small barrier. The Peierls stress deduced from this barrier is lower than 21 MPa, which is in agreement with experimental data. The ability of an empirical potential relying on the embedded atom method (EAM) to model dislocations in zirconium is also tested against these ab initio calculations

DFT Study of Planar Boron Sheets: A New Template for Hydrogen Storage

We study the hydrogen storage properties of planar boron sheets and compare them to those of graphene. The binding of molecular hydrogen to the boron sheet (0.05 eV) is stronger than that to graphene. We find that dispersion of alkali metal (AM = Li, Na, and K) atoms onto the boron sheet markedly increases hydrogen binding energies and storage capacities. The unique structure of the boron sheet presents a template for creating a stable lattice of strongly bonded metal atoms with a large nearest neighbor distance. In contrast, AM atoms dispersed on graphene tend to cluster to form a bulk metal. In particular the boron-Li system is found to be a good candidate for hydrogen storage purposes. In the fully loaded case this compound can contain up to 10.7 wt. % molecular hydrogen with an average binding energy of 0.15 eV/H2.Comment: 19 pages, 7 figures, and 3 table

Biophysical Correlates of Cognition Among Depressed and Nondepressed Type 2 Diabetic Patients

OBJECTIVE—Caudate magnetization transfer (MT) ratios have indicated an abnormality in the macromolecular protein pool of diabetic patients. This study examined the relationship between MT ratios of the caudate and cognitive performance

Energy landscape of relaxed amorphous silicon

We analyze the structure of the energy landscape of a well-relaxed 1000-atom model of amorphous silicon using the activation-relaxation technique (ART nouveau). Generating more than 40,000 events starting from a single minimum, we find that activated mechanisms are local in nature, that they are distributed uniformly throughout the model and that the activation energy is limited by the cost of breaking one bond, independently of the complexity of the mechanism. The overall shape of the activation-energy-barrier distribution is also insensitive to the exact details of the configuration, indicating that well-relaxed configurations see essentially the same environment. These results underscore the localized nature of relaxation in this material.Comment: 8 pages, 12 figure

"Narrow" Graphene Nanoribbons Made Easier by Partial Hydrogenation

It is a challenge to synthesize graphene nanoribbons (GNRs) with narrow widths and smooth edges in large scale. Our first principles study on the hydrogenation of GNRs shows that the hydrogenation starts from the edges of GNRs and proceeds gradually toward the middle of the GNRs so as to maximize the number of carbon-carbon $\pi$-$\pi$ bonds. Furthermore, the partially hydrogenated wide GNRs have similar electronic and magnetic properties as those of narrow GNRs. Therefore, it is not necessary to directly produce narrow GNRs for realistic applications because partial hydrogenation could make wide GNRs "narrower"

Cerebral asymmetries in 12-week-old C57Bl/6J mice measured by magnetic resonance imaging

Asymmetries of multiple components of the rodent cerebrum have been described at various levels of organization. Yet, despite its ubiquitous nature, many confusing and sometimes contradictory reports regarding structural asymmetries in the rodent brain have been published. There is a need, therefore, for a whole-brain imaging analysis technique for asymmetry studies that is both accurate, reproducible and robust. To this end, a comprehensive three-dimensional examination of differences in brain structure in an inbred mouse strain was undertaken. The goal of this study was thus to use high-resolution magnetic resonance imaging to assess structural asymmetries in the adult C57Bl/6J mouse brain. Fixed brain T2-weighted images of 20 male C57Bl/6J mice were acquired on a 7T scanner at 32 μm isotropic resolution. We used voxel-based analyses to examine structural asymmetries throughout the whole mouse brain. The striatum, medial-posterior regions of the thalamus, and motor, sensorimotor, and visual cortex were found to be asymmetrical. The most significant asymmetry was found in the hippocampus and, specifically, the dentate gyrus. In each case, the left region was larger than the right. No other regions of the mouse brain showed structural asymmetry. The results in the dentate gyrus were confirmed using stereology, revealing a correlation of r = 0.61 between magnetic resonance and stereological measures. Hippocampal, along with cortical asymmetry, has been discussed repeatedly in the literature, yet a clear pattern of directionality, until this point, has not been described. The findings of asymmetry in the striatum and absence of asymmetry in the rest of the brain are novel and show the advantage of using the whole-brain three-dimensional techniques developed herein for assessing asymmetry

Molecular Dynamics Studies of Dislocations in CdTe Crystals from a New Bond Order Potential

Cd1-xZnxTe (CZT) crystals are the leading semiconductors for radiation detection, but their application is limited by the high cost of detector-grade materials. High crystal costs primarily result from property non-uniformity that causes low manufacturing yield. While tremendous efforts have been made in the past to reduce Te inclusions / precipitates in CZT, this has not resulted in an anticipated improvement in material property uniformity. Moreover, it is recognized that in addition to Te particles, dislocation cells can also cause electric field perturbation and the associated property non-uniformity. Further improvement of the material, therefore, requires that dislocations in CZT crystals be understood and controlled. Here we use a recently developed CZT bond order potential to perform representative molecular dynamics simulations to study configurations, energies, and mobilities of 29 different types of possible dislocations in CdTe (i.e., x = 1) crystals. An efficient method to derive activation free energies and activation volumes of thermally activated dislocation motion will be explored. Our focus gives insight into understanding important dislocations in the material, and gives guidance toward experimental efforts for improving dislocation network structures in CZT crystals

O(N) methods in electronic structure calculations

Linear scaling methods, or O(N) methods, have computational and memory requirements which scale linearly with the number of atoms in the system, N, in contrast to standard approaches which scale with the cube of the number of atoms. These methods, which rely on the short-ranged nature of electronic structure, will allow accurate, ab initio simulations of systems of unprecedented size. The theory behind the locality of electronic structure is described and related to physical properties of systems to be modelled, along with a survey of recent developments in real-space methods which are important for efficient use of high performance computers. The linear scaling methods proposed to date can be divided into seven different areas, and the applicability, efficiency and advantages of the methods proposed in these areas is then discussed. The applications of linear scaling methods, as well as the implementations available as computer programs, are considered. Finally, the prospects for and the challenges facing linear scaling methods are discussed.Comment: 85 pages, 15 figures, 488 references. Resubmitted to Rep. Prog. Phys (small changes

Effect of molecular and electronic structure on the light harvesting properties of dye sensitizers

The systematic trends in structural and electronic properties of perylene diimide (PDI) derived dye molecules have been investigated by DFT calculations based on projector augmented wave (PAW) method including gradient corrected exchange-correlation effects. TDDFT calculations have been performed to study the visible absorbance activity of these complexes. The effect of different ligands and halogen atoms attached to PDI were studied to characterize the light harvesting properties. The atomic size and electronegativity of the halogen were observed to alter the relaxed molecular geometries which in turn influenced the electronic behavior of the dye molecules. Ground state molecular structure of isolated dye molecules studied in this work depends on both the halogen atom and the carboxylic acid groups. DFT calculations revealed that the carboxylic acid ligands did not play an important role in changing the HOMO-LUMO gap of the sensitizer. However, they serve as anchor between the PDI and substrate titania surface of the solar cell or photocatalyst. A commercially available dye-sensitizer, ruthenium bipyridine (RuBpy), was also studied for electronic and structural properties in order to make a comparison with PDI derivatives for light harvesting properties. Results of this work suggest that fluorinated, chlorinated, brominated, and iyodinated PDI compounds can be useful as sensitizers in solar cells and in artificial photosynthesis.Comment: Single pdf file, 14 pages with 7 figures and 4 table