228 research outputs found
Acoustic modes in metallic nanoparticles: atomistic versus elasticity modeling
The validity of the linear elasticity theory is examined at the nanometer
scale by investigating the vibrational properties of silver and gold
nanoparticles whose diameters range from about 1.5 to 4 nm. Comparing the
vibration modes calculated by elasticity theory and atomistic simulation based
on the Embedded Atom Method, we first show that the anisotropy of the stiffness
tensor in elastic calculation is essential to ensure a good agreement between
elastic and atomistic models. Second, we illustrate the reduction of the number
of vibration modes due to the diminution of the number of atoms when reducing
the nanoparticles size. Finally, we exhibit a breakdown of the
frequency-spectra scaling of the vibration modes and attribute it to surface
effects. Some critical sizes under which such effects are expected, depending
on the material and the considered vibration modes are given.Comment: Accepted to Phys. Rev.
Unidimensional model of the ad-atom diffusion on a substrate submitted to a standing acoustic wave II. Solutions of the ad-atom motion equation
The ad-atom dynamic equation, a Langevin type equation is analyzed and solved
using some non-linear analytical and numerical tools. We noticeably show that
the effect of the surface acoustic wave is to induce an effective potential
that governs the diffusion of the ad-atom: the minima of this effective
potential correspond to the preferential sites in which the ad-atom spends more
time. The strength of this effective potential is compared to the destructuring
role of the thermal diffusion and to the crystalline potential induced by the
substrate
The inverted pendulum, interface phonons and optic Tamm states
The propagation of waves in periodic media is related to the parametric
oscillators. We transpose the possibility that a parametric pendulum oscillates
in the vicinity of its unstable equilibrium positions to the case of waves in
lossless unidimensional periodic media. This concept formally applies to any
kind of wave. We apply and develop it to the case of phonons in realizable
structures and evidence new classes of phonons. Discussing the case of
electromagnetic waves, we show that our concept is related to optic Tamm states
one but extends it to periodic Optic Tamm state.Comment: Submitted to Phys. Re
Kinetics of shape equilibration for two-dimensional islands
We study the relaxation to equilibrium of two dimensional islands containing
up to 20000 atoms by Kinetic Monte Carlo simulations. We find that the commonly
assumed relaxation mechanism - curvature-driven relaxation via atom diffusion -
cannot explain the results obtained at low temperatures, where the island edges
consist in large facets. Specifically, our simulations show that the exponent
characterizing the dependence of the equilibration time on the island size is
different at high and low temperatures, in contradiction with the above cited
assumptions. Instead, we propose that - at low temperatures - the relaxation is
limited by the nucleation of new atomic rows on the large facets : this allows
us to explain both the activation energy and the island size dependence of the
equilibration time.Comment: 9 pages, revte
Surface Loving and Surface Avoiding modes
We theoretically study the propagation of sound waves in GaAs/AlAs
superlattices focussing on periodic modes in the vicinity of the band gaps.
Based on analytical and numerical calculations, we show that these modes are
the product of a quickly oscillating function times a slowly varying envelope
function. We carefully study the phase of the envelope function compared to the
surface of a semi-infinite superlattice. Especially, the dephasing of the
superlattice compared to its surface is a key parameter. We exhibit two kind of
modes: Surface Avoiding and Surface Loving Modes whose envelope functions have
their minima and respectively maxima in the vicinity of the surface. We finally
consider the observability of such modes. While Surface avoiding modes have
experimentally been observed (Phys. Rev. Lett. 97, 1224301 (2006)), we show
that Surface Loving Modes are likely to be observable and we discuss the
achievement of such experiments. The proposed approach could be easily
transposed to other types of wave propagation in unidimensional semi-infinite
periodic structures as photonic Bragg mirror.Comment: 12 pages, 9 figure
Shear-coupling migration of grain boundaries in UFG Al
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Low temperature shape relaxation of 2-d islands by edge diffusion
We present a precise microscopic description of the limiting step for low
temperature shape relaxation of two dimensional islands in which activated
diffusion of particles along the boundary is the only mechanism of transport
allowed. In particular, we are able to explain why the system is driven
irreversibly towards equilibrium. Based on this description, we present a
scheme for calculating the duration of the limiting step at each stage of the
relaxation process. Finally, we calculate numerically the total relaxation time
as predicted by our results and compare it with simulations of the relaxation
process.Comment: 11 pages, 5 figures, to appear in Phys. Rev.
Stable unidimensional arrays of coherent strained islands
We investigate the equilibrium properties of arrays of coherent strained
islands in heteroepitaxial thin films of bidimensional materials. The model we
use takes into account only three essential ingredients : surface energies,
elastic energies of the film and of the substrate and interaction energies
between islands via the substrate. Using numerical simulations for a simple
Lennard-Jones solid, we can assess the validity of the analytical expressions
used to describe each of these contributions. A simple analytical expression is
obtained for the total energy of the system. Minimizing this energy, we show
that arrays of coherent islands can exist as stable configurations. Even in
this simple approach, the quantitative results turn out to be very sensitive to
some details of the surface energy.Comment: 24 pages, 7 figures. to be published in Surface Scienc
DiagnĂłstico de las nuevas tecnologĂas empleadas para el diseño de mezclas asfálticas densas en caliente MDC-2
El presente trabajo pretende brindar alternativas de modificaciĂłn de las Mezclas Asfálticas Densas en Caliente, empleadas para la pavimentaciĂłn de las vĂas en Colombia, mecanismos que actualmente generan un impacto ambiental negativo debido a la utilizaciĂłn de los materiales pĂ©treos, los cuales debido a su ubicaciĂłn no cumplen con las especificaciones tĂ©cnicas o son de difĂcil acceso en algunas zonas de nuestro paĂs. Es por ello que estudios realizados han demostrado que la fabricaciĂłn de mezclas con asfalto convencional no han sido suficientes para soportar la acciĂłn del tránsito y el clima, por lo tanto se ha recomendado emplear modificadores o aditivos en las mezclas, con el fin de mejorar las caracterĂsticas o propiedades geolĂłgicas tanto del cemento asfáltico como de las mezclas asfálticas, asĂ como emplear desechos de materiales que generan un alto impacto en el ambiente
Shear-coupled migration of grain boundaries: the key missing link in the mechanical behavior of small-grained metals?
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