951 research outputs found
Tight-binding molecular-dynamics studies of defects and disorder in covalently-bonded materials
Tight-binding (TB) molecular dynamics (MD) has emerged as a powerful method
for investigating the atomic-scale structure of materials --- in particular the
interplay between structural and electronic properties --- bridging the gap
between empirical methods which, while fast and efficient, lack
transferability, and ab initio approaches which, because of excessive
computational workload, suffer from limitations in size and run times. In this
short review article, we examine several recent applications of TBMD in the
area of defects in covalently-bonded semiconductors and the amorphous phases of
these materials.Comment: Invited review article for Comput. Mater. Sci. (38 pages incl. 18
fig.
Island morphology and adatom self-diffusion on Pt(111)
The results of a density-functional-theory study of the formation energies of
(100)- and (111)-faceted steps on the Pt(111) surface, as well as of the
barrier for diffusion of an adatom on the flat surface, are presented. The step
formation energies are found to be in a ratio of 0.88 in favour of the
(111)-faceted step, in excellent agreement with experiment; the equilibrium
shape of islands should therefore clearly be non-hexagonal. The origin of the
difference between the two steps is discussed in terms of the release of stress
at the surface through relaxation. For the diffusion barrier, we also find
relaxation to be important, leading to a 20% decrease of its energy. The value
we obtain, 0.33 eV, however remains higher than available experimental data;
possible reasons for this discrepancy are discussed. We find the ratio of step
formation energies and the diffusion barrier to be the same whether using the
local-density approximation or the generalized-gradient approximation for the
exchange-and-correlation energy.Comment: Submitted to Physical Review B; 11 postscript pages including 4
figures; this and related publications available from web sites at
http://www.centrcn.umontreal.ca/~lewis and
http://www.fhi-berlin.mpg.de/th/th.htm
Amorphous silicon under mechanical shear deformations: shear velocity and temperature effects
Mechanical shear deformations lead, in some cases, to effects similar to
those resulting from ion irradiation. Here we characterize the effects of shear
velocity and temperature on amorphous silicon (\aSi) modelled using classical
molecular dynamics simulations based on the empirical Environment Dependent
Inter-atomic Potential (EDIP). With increasing shear velocity at low
temperature, we find a systematic increase in the internal strain leading to
the rapid appearance of structural defects (5-fold coordinated atoms). The
impacts of externally applied strain can be almost fully compensated by
increasing the temperature, allowing the system to respond more rapidly to the
deformation. In particular, we find opposite power-law relations between the
temperature and the shear velocity and the deformation energy. The spatial
distribution of defects is also found to strongly depend on temperature and
strain velocity. For low temperature or high shear velocity, defects are
concentrated in a few atomic layers near the center of the cell while, with
increasing temperature or decreasing shear velocity, they spread slowly
throughout the full simulation cell. This complex behavior can be related to
the structure of the energy landscape and the existence of a continuous
energy-barrier distribution.Comment: 10 pages, 17 figure
Structural properties of silicon dioxide thin films densified by medium-energy particles
Classical molecular-dynamics simulations have been carried out to investigate
densification mechanisms in silicon dioxide thin films deposited on an
amorphous silica surface, according to a simplified ion-beam assisted
deposition (IBAD) scenario. We compare the structures resulting from the
deposition of near-thermal (1 eV) SiO particles to those obtained with
increasing fraction of 30 eV SiO particles. Our results show that there
is an energy interval - between 12 and 15 eV per condensing SiO unit on
average - for which the growth leads to a dense, low-stress amorphous
structure, in satisfactory agreement with the results of low-energy ion-beam
experiments. We also find that the crossover between low- and high-density
films is associated with a tensile to compressive stress transition, and a
simultaneous healing of structural defects of the {\em a-}SiO network,
namely three- and four-fold rings. It is observed, finally, that densification
proceeds through significant changes at intermediate length scales (4--10 \AA),
leaving essentially unchanged the ``building blocks'' of the network, viz. the
Si(O) tetrahedra. This latter result is in qualitative agreement
with the mechanism proposed to explain the irreversible densification of
amorphous silica recovered from high pressures ( 15--20 GPa).Comment: 12 pages including 10 postscript figures; submitted to Phys. Rev. B;
related publications can be found on web site
http://www.centrcn.umontreal.ca/~lewi
Isochoric, isobaric and ultrafast conductivities of aluminum, lithium and carbon in the warm dense matter (WDM) regime
We study the conductivities of (i) the equilibrium isochoric state
(), (ii) the equilibrium isobaric state (),
and also the (iii) non-equilibrium ultrafast matter (UFM) state () with the ion temperature less than the the electron temperature
. Aluminum, lithium and carbon are considered, being increasingly complex
warm dense matter (WDM) systems, with carbon having transient covalent bonds.
First-principles calculations, i.e., neutral-pseudoatom (NPA) calculations and
density-functional theory (DFT) with molecular-dynamics (MD) simulations, are
compared where possible with experimental data to characterize and . The NPA are
closest to the available experimental data when compared to results from
DFT+MD, where simulations of about 64-125 atoms are typically used. The
published conductivities for Li are reviewed and the value at a temperature of
4.5 eV is examined using supporting X-ray Thomson scattering calculations. A
physical picture of the variations of with temperature and density
applicable to these materials is given. The insensitivity of to
below 10 eV for carbon, compared to Al and Li, is clarified.Comment: 10 figure
Two-temperature pair potentials and phonon spectra for simple metals in the warm dense matter regime
We develop ion-ion pair potentials for Al, Na and K for densities and
temperatures relevant to the warm-dense-matter (WDM) regime. Furthermore, we
emphasize non-equilibrium states where the ion temperature differs from
the electron temperature . This work focuses mainly on ultra-fast
laser-metal interactions where the energy of the laser is almost exclusively
transferred to the electron sub-system over femtosecond time scales. This
results in a two-temperature system with and with the ions still at
the initial room temperature . First-principles calculations, such as
density functional theory (DFT) or quantum Monte Carlo, are as yet not fully
feasible for WDM conditions due to lack of finite- features, e.g.
pseudopotentials, and extensive CPU time requirements. Simpler methods are
needed to study these highly complex systems. We propose to use two-temperature
pair potentials constructed from linear-response theory
using the non-linear electron density obtained from finite-
DFT with a single ion immersed in the appropriate electron fluid. We compute
equilibrium phonon spectra at which are found to be in very good
agreement with experiments. This gives credibility to our non-equilibrium
phonon dispersion relations which are important in determining thermophysical
properties, stability, energy-relaxation mechanisms and transport coefficients.Comment: International Conf. on Strongly-Coupled Coulombo Systems (SCCS) 201
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