2,702 research outputs found
Evolution of the electronic structure of Be clusters
Using a modified symbiotic genetic algorithm approach and many-body interatomic potential derived from first principles, we have calculated equilibrium geometries and binding energies of the ground-state and low-lying isomers of Be clusters containing up to 41 atoms. Molecular-dynamics study was also carried out to study the frequency of occurrence of the various geometrical isomers as these clusters are annealed during the simulation process. For a selected group of these clusters, higher-energy isomers were more often found than their ground-statestructures due to large catchment areas. The accuracy of the above ground-stategeometries and their corresponding binding energies were verified by carrying out separate ab initio calculations based on molecular-orbital approach and density-functional theory with generalized gradient approximation for exchange and correlation. The atomic orbitals were represented by a Gaussian 6-311G** basis, and the geometry optimization was carried out using the GAUSSIAN 98 code without any symmetry constraint. While the ground-stategeometries and their corresponding binding energies obtained from ab initio calculations do not differ much from those obtained using the molecular-dynamics approach, the relative stability of the clusters and the energy gap between the highest occupied and the lowest unoccupied molecular orbitals show significant differences. The energy gaps, calculated using the density-functional theory, show distinct shell closure effects, namely, sharp drops in their values for Be clusters containing 2, 8, 20, 34, and 40 electrons. While these features may suggest that small Be clusters behave free-electron-like and, hence, are metallic, the evolution of the structure, binding energies, coordination numbers, and nearest-neighbor distances do not show any sign of convergence towards the bulk value. We also conclude that molecular-dynamics simulation based on many-body interatomic potentials may not always give the correct picture of the evolution of the structure and energetics of clusters although they may serve as a useful tool for obtaining starting geometries by efficiently searching a large part of the phase space
Mesoscale magnetism at the grain boundaries in colossal magnetoresistive films
We report the discovery of mesoscale regions with distinctive magnetic
properties in epitaxial LaSrMnO films which exhibit
tunneling-like magnetoresistance across grain boundaries. By using
temperature-dependent magnetic force microscopy we observe that the mesoscale
regions are formed near the grain boundaries and have a different Curie
temperature (up to 20 K {\it higher}) than the grain interiors. Our images
provide direct evidence for previous speculations that the grain boundaries in
thin films are not magnetically and electronically sharp interfaces. The size
of the mesoscale regions varies with temperature and nature of the underlying
defect.Comment: 4 pages of text, 4 figure
Large Area Roller Embossing of Multilayered Ceramic Green Composites
In this paper, we will report our achievements in developing large area
patterning of multilayered ceramic green composites using roller embossing. The
aim of our research is to pattern large area ceramic green composites using a
modified roller laminating apparatus, which is compatible with screen printing
machines, for integration of embossing and screen printing. The instrumentation
of our roller embossing apparatus, as shown in Figure1, consists of roller 1
and rollers 2. Roller 1 is heated up to the desired embossing temperature ;
roller 2 is, however, kept at room temperature. The mould is a nickel template
manufactured by plating nickel-based micro patterns (height : 50 m) on a
nickel film (thickness : 70 m) ; the substrate for the roller embossing is
a multilayered Heraeus Heralock HL 2000 ceramic green composite. Comparing with
the conventional simultaneous embossing, the advantages of roller embossing
include : (1) low embossing force ; (2) easiness of demoulding ; (3) localized
area in contact with heater ; and etc. We have demonstrated the capability of
large area roller embossing with a panel size of 150mmx 150mm on the mentioned
substrate. We have explored and confirmed the impact of parameters (feed speed,
temperature of roller and applied pressure) to the pattern quality of roller
embossing. Furthermore, under the optimized process parameters, we
characterized the variations of pattern dimension over the panel area, and
calculated a scaling factor in order to make the panel compatible with other
processes. Figure 2 shows the embossed patterns on a 150mmx 150mm green ceramic
panel.Comment: Submitted on behalf of EDA Publishing Association
(http://irevues.inist.fr/handle/2042/16838
Topological dilaton black holes
In four-dimensional spacetime, when the two-sphere of black hole event
horizons is replaced by a two-dimensional hypersurface with zero or negative
constant curvature, the black hole is referred to as a topological black hole.
In this paper we present some exact topological black hole solutions in the
Einstein-Maxwell-dilaton theory with a Liouville-type dilaton potential.Comment: 8 pages, Revtex, no figure
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