258 research outputs found
Underpotential deposition of Cu on Au(111) in sulfate-containing electrolytes: a theoretical and experimental study
We study the underpotential deposition of Cu on single-crystal Au(111)
electrodes in sulfate-containing electrolytes by a combination of computational
statistical-mechanics based lattice-gas modeling and experiments. The
experimental methods are in situ cyclic voltammetry and coulometry and ex situ
Auger electron spectroscopy and low-energy electron diffraction. The
experimentally obtained voltammetric current and charge densities and adsorbate
coverages are compared with the predictions of a two-component lattice-gas
model for the coadsorption of Cu and sulfate. This model includes effective,
lateral interactions out to fourth-nearest neighbors. Using group-theoretical
ground-state calculations and Monte Carlo simulations, we estimate effective
electrovalences and lateral adsorbate--adsorbate interactions so as to obtain
overall agreement with experiments, including both our own and those of other
groups. In agreement with earlier work, we find a mixed R3xR3 phase consisting
of 2/3 monolayer Cu and 1/3 monolayer sulfate at intermediate electrode
potentials, delimited by phase transitions at both higher and lower potentials.
Our approach provides estimates of the effective electrovalences and lateral
interaction energies, which cannot yet be calculated by first-principles
methods.Comment: 36 pages, 14 Postscript figures are in uufiles for
Void Growth in BCC Metals Simulated with Molecular Dynamics using the Finnis-Sinclair Potential
The process of fracture in ductile metals involves the nucleation, growth,
and linking of voids. This process takes place both at the low rates involved
in typical engineering applications and at the high rates associated with
dynamic fracture processes such as spallation. Here we study the growth of a
void in a single crystal at high rates using molecular dynamics (MD) based on
Finnis-Sinclair interatomic potentials for the body-centred cubic (bcc) metals
V, Nb, Mo, Ta, and W. The use of the Finnis-Sinclair potential enables the
study of plasticity associated with void growth at the atomic level at room
temperature and strain rates from 10^9/s down to 10^6/s and systems as large as
128 million atoms. The atomistic systems are observed to undergo a transition
from twinning at the higher end of this range to dislocation flow at the lower
end. We analyze the simulations for the specific mechanisms of plasticity
associated with void growth as dislocation loops are punched out to accommodate
the growing void. We also analyse the process of nucleation and growth of voids
in simulations of nanocrystalline Ta expanding at different strain rates. We
comment on differences in the plasticity associated with void growth in the bcc
metals compared to earlier studies in face-centred cubic (fcc) metals.Comment: 24 pages, 12 figure
Time-dependent energy absorption changes during ultrafast lattice deformation
The ultrafast time-dependence of the energy absorption of covalent solids
upon excitation with femtosecond laser pulses is theoretically analyzed. We use
a microscopic theory to describe laser induced structural changes and their
influence on the electronic properties. We show that from the time evolution of
the energy absorbed by the system important information on the electronic and
atomic structure during ultrafast phase transitions can be gained. Our results
reflect how structural changes affect the capability of the system to absorb
external energy.Comment: 7 pages RevTeX, 8 ps figures, submitted to Journal of Appl. Physic
Simulation of Mechanical Deformation and Tribology of Nano-Thin Amorphous Hydrogenated Carbon (a:Ch) Films Using Molecular Dynamics
Molecular dynamics computer simulations are used to study the effect of substrate temperature on microstructure of deposited amorphous hydrogenated carbon (a:CH) films. A transition from dense diamond- like films to porous graphite-like films is observed between substrate temperatures of 400 and 600 K for a deposition energy of 20 eV. The dense a:CH film grown at 300 K and 20 eV has a hardness ({similar_to}50 GPa) about half that of a pure carbon (a:C) film grown under the same conditions
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Molecular dynamics modeling of ultrathin amorphous carbon films
Amorphous carbon films about 20 mn thick are used by the computer industry as protective coatings on magnetic disks. The structure and function of this family of materials at the atomic level is poorly understood. The growth and properties of a:C and a:CH films 1 to 5 nm thick has been simulated using classical molecular dynamics and a bond-order potential with torsional terms. Studies of quenched melts that verify the ability of this potential to reproduce known features of extended structures of carbon in two and three dimensions are briefly described. In molecular dynamics calculations the incident species were neutral atoms C, or C and H with energies up to 100 eV. The stoichiometry, chemical bonding and distribution functions within the films were analyzed using IBM`s Power Visualization System for different incident gas energies. Microscopic features such as multiple ring structures, including planar graphitic structures, were easily identified. Some preliminary studies of the nanotribology of the a:C films are described, including nano-indentation and sliding in contact with a rigid probe
Molecular Dynamics Simulation of Mechanical Deformation of Ultra-Thin Amorphous Carbon Films
Amorphous carbon films approximately 20nm thick are used throughout the computer industry as protective coatings on magnetic storage disks. The structure and function of this family of materials at the atomic level is poorly understood. Recently. we simulated the growth of a:C and a:CH films 1 to 5 nm thick using Brenner`s bond-order potential model with added torsional energy terms. The microstructure shows a propensity towards graphitic structures at low deposition energy (20eV). In this paper we present simulations of the evolution of this microstructure for the dense 20eV films during a simulated indentation by a hard diamond tip. We also simulate sliding, the tip across the surface to study dynamical processes like friction, energy transport and microstructure evolution during sliding
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Simulating Solidification in Metals at High Pressure: The Drive to Petascale Computing
We investigate solidification in metal systems ranging in size from 64,000 to 524,288,000 atoms on the IBM BlueGene/L computer at LLNL. Using the newly developed ddcMD code, we achieve performance rates as high as 103 TFlops, with a performance of 101.7 TFlop sustained over a 7 hour run on 131,072 cpus. We demonstrate superb strong and weak scaling. Our calculations are significant as they represent the first atomic-scale model of metal solidification to proceed, without finite size effects, from spontaneous nucleation and growth of solid out of the liquid, through the coalescence phase, and into the onset of coarsening. Thus, our simulations represent the first step towards an atomistic model of nucleation and growth that can directly link atomistic to mesoscopic length scales
High pressure diamond-like liquid carbon
We report density-functional based molecular dynamics simulations, that show
that, with increasing pressure, liquid carbon undergoes a gradual
transformation from a liquid with local three-fold coordination to a
'diamond-like' liquid. We demonstrate that this unusual structural change is
well reproduced by an empirical bond order potential with isotropic long range
interactions, supplemented by torsional terms. In contrast, state-of-the-art
short-range bond-order potentials do not reproduce this diamond structure. This
suggests that a correct description of long-range interactions is crucial for a
unified description of the solid and liquid phases of carbon.Comment: 4 pages, 5 figure
Nature of phase transition(s) in striped phase of triangular-lattice Ising antiferromagnet
Different scenarios of the fluctuation-induced disordering of the striped
phase which is formed at low temperatures in the triangular-lattice Ising model
with the antiferromagnetic interaction of nearest and next-to-nearest neighbors
are analyzed and compared. The dominant mechanism of the disordering is related
to the formation of a network of domain walls, which is characterized by an
extensive number of zero modes and has to appear via the first-order phase
transition. In principle, this first-order transition can be preceded by a
continuous one, related to the spontaneous formation of double domain walls and
a partial restoration of the broken symmetry, but the realization of such a
scenario requires the fulfillment of rather special relations between the
coupling constants.Comment: 10 pages, 7 figures, ReVTeX
Structural transitions and nonmonotonic relaxation processes in liquid metals
Structural transitions in melts as well as their dynamics are considered. It
is supposed that liquid represents the solution of relatively stable solid-like
locally favored structures (LFS) in the surrounding of disordered normal-liquid
structures. Within the framework of this approach the step changes of liquid Co
viscosity are considered as liquid-liquid transitions. It is supposed that this
sort of transitions represents the cooperative medium-range bond ordering, and
corresponds to the transition of the "Newtonian fluid" to the "structured
fluid". It is shown that relaxation processes with oscillating-like time
behavior (~) of viscosity are possibly close to
this point
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