4,359 research outputs found
Molecular dynamics simulations of lead clusters
Molecular dynamics simulations of nanometer-sized lead clusters have been
performed using the Lim, Ong and Ercolessi glue potential (Surf. Sci. {\bf
269/270}, 1109 (1992)). The binding energies of clusters forming crystalline
(fcc), decahedron and icosahedron structures are compared, showing that fcc
cuboctahedra are the most energetically favoured of these polyhedral model
structures. However, simulations of the freezing of liquid droplets produced a
characteristic form of ``shaved'' icosahedron, in which atoms are absent at the
edges and apexes of the polyhedron. This arrangement is energetically favoured
for 600-4000 atom clusters. Larger clusters favour crystalline structures.
Indeed, simulated freezing of a 6525-atom liquid droplet produced an imperfect
fcc Wulff particle, containing a number of parallel stacking faults. The
effects of temperature on the preferred structure of crystalline clusters below
the melting point have been considered. The implications of these results for
the interpretation of experimental data is discussed.Comment: 11 pages, 18 figues, new section added and one figure added, other
minor changes for publicatio
Supported magnetic nanoclusters: Softlanding of Pd clusters on a MgO surface
Low-energy deposition of neutral Pd_N clusters (N=2-7 and 13) on a MgO(001)
surface F-center (FC) was studied by spin-density-functional molecular dynamics
simulations. The incident clusters are steered by an attractive "funnel"
created by the FC, resulting in adsorption of the cluster, with one of its
atoms bonded atop of the FC. The deposited Pd_2-Pd_6 clusters retain their
gas-phase structures, while for N>6 surface-commensurate isomers are
energetically more favorable. Adsorbed clusters with N > 3 are found to remain
magnetic at the surface.Comment: 5 pages, 2 figs, Phys.Rev.Lett., accepte
Kidney cell electrophoresis in space flight: Rationale, methods, results and flow cytometry applications
Cultures of human embryonic kidney cells consistently contain an electrophoretically separable subpopulation of cells that produce high levels of urokinase and have an electrophoretic mobility about 85 percent as high as that of the most mobile human embryonic kidney cells. This subpopulation is rich in large epithelioid cells that have relatively little internal structure. When resolution and throughput are adequate, free fluid electrophoresis can be used to isolate a broad band of low mobility cells which also produces high levels of plasminogen activators (PAs). In the course of performing this, it was discovered that all electrophoretic subpopulations of cultured human embryonic kidney cells produce some PAs and that separate subpopulations produce high quantities of different types of PA's. This information and the development of sensitive assays for this project have provided new insights into cell secretion mechanisms related to fibrinolysis. These advances would probably not have been made without the NASA program to explore fundamental questions of free fluid electrophoresis in space
Surface-reconstructed Icosahedral Structures for Lead Clusters
We describe a new family of icosahedral structures for lead clusters. In
general, structures in this family contain a Mackay icosahedral core with a
reconstructed two-shell outer-layer. This family includes the anti-Mackay
icosahedra, which have have a Mackay icosahedral core but with most of the
surface atoms in hexagonal close-packed positions. Using a many-body glue
potential for lead, we identify two icosahedral structures in this family which
have the lowest energies of any known structure in the size range from 900 to
15000 lead atoms. We show that these structures are stabilized by a feature of
the many-body glue part of the interatomic potential.Comment: 9 pages, 8 figure
Krylov-Bogoliubov-Mitropolsky Averaging Used to Construct Effective Hamiltonians in the Theory of Strongly Correlated Electron Systems
We show that the Krylov-Bogoliubov-Mitropolsky averaging in the canonical
formulation can be used as a method for constructing effective Hamiltonians in
the theory of strongly correlated electron systems. As an example, we consider
the transition from the Hamiltonians of the Hubbard and Anderson models to the
respective Hamiltonians of the t-J and Kondo models. This is a very general
method, has several advantages over other methods, and can be used to solve a
wide range of problems in the physics of correlated systems.Comment: 9 page
Investigation of DC-8 nacelle modifications to reduce fan-compressor noise in airport communities. Part 4 - Flight acoustical and performance evaluations, for period May 1967 - October 1969
Flight acoustical and performance evaluations of DC 8 nacelle modifications to reduce fan-compressor noise in airport communitie
New Periodic Variables from the Hipparcos Epoch Photometry
Two selection statistics are used to extract new candidate periodic variables
from the epoch photometry of the Hipparcos catalogue. The primary selection
criterion is a signal to noise ratio. The dependence of this statistic on the
number of observations is calibrated using about 30 000 randomly permuted
Hipparcos datasets. A significance level of 0.1% is used to extract a first
batch of candidate variables. The second criterion requires that the optimal
frequency be unaffected if the data are de-trended by low order polynomials. We
find 2675 new candidate periodic variables, of which the majority (2082) are
from the Hipparcos "unsolved" variables. Potential problems with the
interpretation of the data (e.g. aliasing) are discussed.Comment: 18 pages, 9 figures, accepted for publication in MNRA
Entropic effects on the Size Evolution of Cluster Structure
We show that the vibrational entropy can play a crucial role in determining
the equilibrium structure of clusters by constructing structural phase diagrams
showing how the structure depends upon both size and temperature. These phase
diagrams are obtained for example rare gas and metal clusters.Comment: 5 pages, 3 figure
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