23 research outputs found
Islands, craters, and a moving surface step on a hexagonally reconstructed (100) noble metal surface
Deposition/removal of metal atoms on the hex reconstructed (100) surface of
Au, Pt and Ir should present intriguing aspects, since a new island implies hex
-> square deconstruction of the substrate, and a new crater the square -> hex
reconstruction of the uncovered layer. To obtain a microscopic understanding of
how islands/craters form in these conditions, we have conducted simulations of
island and crater growth on Au(100), whose atomistic behavior, including the
hex reconstruction on top of the square substrate, is well described by mean s
of classical many-body forces. By increasing/decreasing the Au coverage on
Au(100), we find that island/craters will not grow unless they exceed a
critical size of about 8-10 atoms. This value is close to that which explains
the nonlinear coverage dependence observed in molecular adsorption on the
closely related surface Pt (100). This threshold size is rationalized in terms
of a transverse step correlation length, measuring the spatial extent where
reconstruction of a given plane is disturbed by the nearby step.Comment: 11 pages, 5 figures, accepted for publication in Surface Science
(ECOSS-18
Realistic grand canonical Monte Carlo surface simulation: application to Ar(111)
Most realistic, off-lattice surface simulations are done canonically---
conserving particles. For some applications, however, such as studying the
thermal behavior of rare gas solid surfaces, these constitute bad working
conditions. Surface layer occupancies are believed to change with temperature,
particularly at preroughening, and naturally call for a grand canonical
approach, where particle number is controlled by a chemical potential. We
report preliminary results of novel realistic grand canonical Monte Carlo
simulations of the Lennard-Jones (LJ) fcc(111) surface, believed to represent a
quantitative model of e.g. Ar(111). The results are successful and highly
informative for temperatures up to roughly 0.8 T_m, where clear precursor
signals of preroughening are found. At higher temperatures, convergence to
equilibrium is hampered by large fluctuations.Comment: 4 pages, REVTeX, 3 PostScript figure
Surface molecular dynamics simulation with two orthogonal surface steps: how to beat the particle conservation problem
Due to particle conservation, Canonical Molecular Dynamics (MD) simulations
fail in the description of surface phase transitions involving coverage or
lateral density changes. However, a step on the surface can act effectively as
a source or a sink of atoms, in the simulation as well as in real life. A
single surface step can be introduced by suitably modifying planar Periodic
Boundary Conditions (PBC), to accommodate the generally inequivalent stacking
of two adjacent layers. We discuss here how, through the introduction of two
orthogonal surface steps, particle number conservation may no longer represent
a fatal constraint for the study of these surface transitions. As an example,
we apply the method for estimating temperature-induced lateral density increase
of the reconstructed
Au (001) surface; the resulting anisotropic cell change is consistent with
experimental observations. Moreover, we implement this kind of scheme in
conjunction with the variable curvature MD method, recently introduced by our
group.Comment: 9 pages, 5 figures, accepted for publication in Surface Science
(ECOSS-19
Role of Layering Oscillations at Liquid Metal Surfaces in Bulk Recrystallization and Surface Melting
The contrasting melting behavior of different surface orientations in metals
can be explained in terms of a repulsive or attractive effective interaction
between the solid-liquid and the liquid-vapor interface. We show how a crucial
part of this interaction originates from the layering effects near the liquid
metal surface. Its sign depends on the relative tuning of layering oscillations
to the crystal interplanar spacing, thus explaining the orientational
dependence. Molecular dynamics recrystallization simulations of Au surfaces
provide direct and quantitative evidence of this phenomenon.Comment: 10 pages (RevTeX) plus 3 figures (PostScript
Variable Curvature Slab Molecular Dynamics as a Method to Determine Surface Stress
A thin plate or slab, prepared so that opposite faces have different surface
stresses, will bend as a result of the stress difference. We have developed a
classical molecular dynamics (MD) formulation where (similar in spirit to
constant-pressure MD) the curvature of the slab enters as an additional
dynamical degree of freedom. The equations of motion of the atoms have been
modified according to a variable metric, and an additional equation of motion
for the curvature is introduced. We demonstrate the method to Au surfaces, both
clean and covered with Pb adsorbates, using many-body glue potentials.
Applications to stepped surfaces, deconstruction and other surface phenomena
are under study.Comment: 16 pages, 8 figures, REVTeX, submitted to Physical Review
Noncrystalline structures of ultrathin unsupported nanowires
Computer simulations suggest that ultrathin metal wires should develop exotic, non-crystalline stable atomic structures, once their diameter decreases below a critical size of the order of a few atomic spacings. The new structures, whose details depend upon the material and the wire thickness, may be dominated by icosahedral packings. Helical, spiral-structured wires with multi-atom pitches are also predicted. The phenomenon, analogous to the appearance of icosahedral and other non-crystalline shapes in small clusters, can be rationalized in terms of surface energy anisotropy and optimal packing
Molecular dynamics studies of gold: bulk, defects, surfaces and clusters
In recent years, computer simulation methods have provided much insight
into several structural, dynamical and thermal properties of solids
and liquids. Computational methods are particularly well suited to the
study of low symmetry sys.terns (e.g., defects, surfaces, clusters), where
the complexity of analytical treatments may become overwhelming, and
of systems at finite temperature.
The key ingredients in computer simulations are interatomic forces.
The problem we wish to solve can be simply stated as follows: given
a set of N atoms having some positions r 1 \u2022.. rN and linear momenta
p1 .\u2022\u2022 PN, what forces will they experience ? Calculating these forces
ab initio is a very difficult task. Even if we are not interested in the
electronic\ub7 properties of the system, but only in ionic properties (e.g.,
vibrations, equilibrium structures, etc.), we must generally take into
full account the electronic aspect of the problem.
In the Born-Oppenheimer adiabatic approximation [1 J, the forces
can be obtained by considering the nuclei as fixed and searching for
the minimum energy state of the electronic system. This .may be done
using the Hartree-Fock approximation, or in a density functional theory
framework. The force acting on a nucleus is then determined as the
gradient of the total energy respect to a displacement of that nucleus.
After all nuclei have been moved accordingly to the forces computed in
_this way, the whole process may be iterated for the new configuration,
\ub7thus performing a dynamical calculation. This approach, however, is
computationally extremely expensive, and not feasible when the number
of particles is of the order of ten or more...
Lecture notes on Tight-Binding Molecular Dynamics,
and Tight-Binding justification of classical potential