3,241 research outputs found
Structural studies of phosphorus induced dimers on Si(001)
Renewed focus on the P-Si system due to its potential application in quantum
computing and self-directed growth of molecular wires, has led us to study
structural changes induced by P upon placement on Si(001)-. Using
first-principles density functional theory (DFT) based pseudopotential method,
we have performed calculations for P-Si(001) system, starting from an isolated
P atom on the surface, and systematically increasing the coverage up to a full
monolayer. An isolated P atom can favorably be placed on an {\bf M} site
between two atoms of adjacent Si dimers belonging to the same Si dimer row. But
being incorporated in the surface is even more energetically beneficial due to
the participation of the {\bf M} site as a receptor for the ejected Si. Our
calculations show that up to 1/8 monolayer coverage, hetero-dimer structure
resulting from replacement of surface Si atoms with P is energetically
favorable. Recently observed zig-zag features in STM are found to be consistent
with this replacement process. As coverage increases, the hetero-dimers give
way to P-P ortho-dimers on the Si dimer rows. This behavior is similar to that
of Si-Si d-dimers but are to be contrasted with the Al-Al dimers, which are
found between adjacent Si dimers rows and in a para-dimer arrangement. Unlike
Al-Si system P-Si does not show any para to ortho transition. For both systems,
the surface reconstruction is lifted at about one monolayer coverage. These
calculations help us in understanding the experimental data obtained using
scanning tunneling microscope.Comment: To appear in PR
D-Terms, Unification, and the Higgs Mass
We study gauge extensions of the MSSM that contain non-decoupling D-terms,
which contribute to the Higgs boson mass. These models naturally maintain gauge
coupling unification and raise the Higgs mass without fine-tuning. Unification
constrains the structure of the gauge extensions, limiting the Higgs mass in
these models to roughly less than 150 GeV. The D-terms contribute to the Higgs
mass only if the extended gauge symmetry is broken at energies of a few TeV,
leading to new heavy gauge bosons in this mass range.Comment: 30+1 pages, 7 figure
A STUDY OF CONCEPTUAL DATA MODELING IN DATABASE DESIGN: SIMILARITIES AND DIFFERENCES BETWEEN EXPERT AND NOVICE DESIGNERS
This paper explores the similarities and differences between experts and novices engaged in a conceptual data modeling task, a critical part of overall database design, using data gathered in the form of think-aloud protocols. It develops a three-level process model of the subjects\u27 behavior and the differentiated application of this model by experts and novices. The study found that the experts focussed on generating a holistic understanding of the problem before developing the conceptual model. They were able to categorize problem descriptions into standard abstractions. The novices tended to have more errors in their solutions largely due to their inability to map parts of the problem description into appropriate knowledge structures. The study also found that the expert and novice behavior was similar in terms of modeling facets like entities, identifiers, descriptors, and binary and ternary relationships but was different in the modeling of unary relationships and categories. These findings are discussed in relation to the results of previous expert-novice studies in other domains
Dynamic Adiabatic Shear Band Development In A Bimetallic Body Containing A Void
We study the problem of the initiation and subsequent growth of a shear band in a thermally softening viscoplaslic prismatic body of square cross-section and containing two symmetrically placed thin layers of a different viscoplastic material and an elliptical void at the center. The yield stress of the material of the thin layer in a quasistatic simple compression test is taken to be either five times or one-fifth that of the matrix material. The body is deformed in plane strain compression at a nominal strain rate of 5.000 s \u27. These deformations are assumed to be symmetrical about the centroidal axes. It is found that shear bands initiate from the ends of the major axes of the ellipsoidal void and propagate in the direction of the maximum shear stress. These bands are arrested by the strong virtually rigid material of the thin layer but pass through the weaker material of the thin layer rather easily. Other shear bands original from points where the thin layers meet the free boundaries and propagate into the matrix material along the direction ol maximum shearing when the material of the thin layer is stronger but propagate into the thin layer when its material is weaker than the matrix material. Tile band in the weaker material of the thin layer bifurcates into two hands that propagate into the matrix material in the direction of the maximum shearing stress. © 1991
Structure of aluminum atomic chains
First-principles density functional calculations reveal that aluminum can
form planar chains in zigzag and ladder structures. The most stable one has
equilateral triangular geometry with four nearest neighbors; the other stable
zigzag structure has wide bond angle and allows for two nearest neighbors. An
intermediary structure has the ladder geometry and is formed by two strands.
All these planar geometries are, however, more favored energetically than the
linear chain. We found that by going from bulk to a chain the character of
bonding changes and acquires directionality. The conductance of zigzag and
linear chains is 4e^2/h under ideal ballistic conditions.Comment: modified detailed version, one new structure added, 4 figures,
modified figure1, 1 tabl
Para to Ortho transition of metallic dimers on Si(001)
Extensive electronic structure calculations are performed to obtain the
stable geometries of metals like Al, Ga and In on the Si(001) surface at 0.5 ML
and 1 ML coverages. Our results coupled with previous theoretical findings
explain the recent experimental data in a comprehensive fashion. At low
coverages, as shown by previous works, `Para' dimers give the lowest energy
structure. With increasing coverage beyond 0.5 ML, `Ortho' dimers become part
of low energy configurations leading toward a `Para' to `Ortho' transition at 1
ML coverage. For In mixed staggered dimers (`Ortho' and `Para') give the lowest
energy configuration. For Ga, mixed dimers are non-staggered, while for Al
`Para' to `Ortho' transition of dimers is complete. Thus at intermediate
coverages between 0.5 and 1 ML, the `Ortho' and `Para' dimers may coexist on
the surface. Consequently, this may be an explanation of the fact that the
experimental observations can be successfully interpreted using either
orientation. A supported zigzag structure at 0.5 ML, which resembles , does not undergo a dimerization transition, and hence stays
semi-metallic. Also, unlike the soliton formation is ruled out
for this structure.Comment: 8 pages, 6 figure
Studies in Structure Formation in theories with a repulsive long range gravitational force
This article reports on emergence of structures in a class of alternative
theories of gravity. These theories do not have any horizon, flatness, initial
cosmological singularity and (possibly) quantization problems. The model is
characterised by a dynamically induced gravitational constant with a ``wrong''
sign corresponding to repulsive gravitation on the large scale. A non - minimal
coupling of a scalar field in the model can give rise to non - topological
solitons in the theory. This results in domains (gravity - balls) inside which
an effective, canonical, attractive gravitational constant is induced. We
consider simulations of the formation and evolution of such solutions. Starting
with a single gravity - ball, we consider its fragmentation into smaller (lower
mass) balls - evolving by mutual repulsion. After several runs, we have been
able to identify two parameters: the strength of the long range gravitational
constant and the size of the gravity balls, which can be used to generate
appropriate two point correlations of the distribution of these balls.Comment: Normal Tex, 7 pages, 5 figures available from the author
Unusual Higgs or Supersymmetry from Natural Electroweak Symmetry Breaking
This review provides an elementary discussion of electroweak symmetry
breaking in the minimal and the next-to-minimal supersymmetric models with the
focus on the fine-tuning problem -- the tension between natural electroweak
symmetry breaking and the direct search limit on the Higgs boson mass. Two
generic solutions of the fine-tuning problem are discussed in detail: models
with unusual Higgs decays; and models with unusual pattern of soft
supersymmetry breaking parameters.Comment: 23 pages, 6 figures; invited review by MPL
Features of the fluctuation -electromagnetic interaction between a small conducting particle and polarizable medium
For the first time, new important features of the fluctuation electromagnetic
interaction between a small conducting particle and a smooth surface of
polarizable medium (both dielectric and metallic) are worked out. The particle
is characterized by classical electric and magnetic polarizabilities. The
temperature dependence and retardation effects are explicitly taken into
account. The resulting interaction force between a metallic particle and the
surface of metal proves to be determined to great extent by magnetic coupling
and reveals specific dependences on distance, temperature, particle radius and
material properties of contacting materials. Numerical estimations are given in
the case of a Cu particle above a smooth Cu substrate at different particle
radius and temperature of the system.Comment: 13 pages, 5 figure
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