1,649 research outputs found
Cenozoic evolution of Neotethys and implications for the causes of plate motions
Africa-North America-Eurasia plate circuit rotations, combined with Red Sea rotations and new estimates of crustal shortening in Iran define the Cenozoic history of the Neotethyan ocean between Arabia and Eurasia. The new constraints indicate that Arabia-Eurasia convergence has been fairly constant at 2 to 3 cm/yr since 56 Ma with slowing of Africa-Eurasia motion to <1 cm/yr near 25 Ma, coeval with the opening of the Red Sea. Ocean closure occurred no later than 10 Ma, and could have occurred prior to this time only if a large amount of continental lithosphere was subducted, suggesting that slowing of Africa significantly predated the Arabia-Eurasia collision. These kinematics imply that Africa's disconnection with the negative buoyancy of the downgoing slab of lithosphere beneath southern Eurasia slowed its motion. The slow, steady rate of northward subduction since 56 Ma contrasts with strongly variable rates of magma production in the Urumieh-Dokhtar arc, implying magma production rate in continental arcs is not linked to subduction rate
Classical-Wigner Phase Space Approximation to Cumulative Matrix Elements in Coherent Control
The classical limit of the Wigner-Weyl representation is used to approximate
products of bound-continuum matrix elements that are fundamental to many
coherent control computations. The range of utility of the method is quantified
through an examination of model problems, single-channel Na_2 dissociation and
multi-arrangement channel photodissociation of CH_2IBr. Very good agreement
with the exact quantum results is found for a wide range of system parameters.Comment: 17 pages, 12 figures, to appear in Journal of Chemical Physic
Statics and dynamics of a cylindrical droplet under an external body force
We study the rolling and sliding motion of droplets on a corrugated substrate
by Molecular Dynamics simulations. Droplets are driven by an external body
force (gravity) and we investigate the velocity profile and dissipation
mechanisms in the steady state. The cylindrical geometry allows us to consider
a large range of droplet sizes. The velocity of small droplets with a large
contact angle is dominated by the friction at the substrate and the velocity of
the center of mass scales like the square root of the droplet size. For large
droplets or small contact angles, however, viscous dissipation of the flow
inside the volume of the droplet dictates the center of mass velocity that
scales linearly with the size. We derive a simple analytical description
predicting the dependence of the center of mass velocity on droplet size and
the slip length at the substrate. In the limit of vanishing droplet velocity we
quantitatively compare our simulation results to the predictions and good
agreement without adjustable parameters is found.Comment: Submitted to the Journal of Chemical Physic
Dynamics of a Rigid Rod in a Glassy Medium
We present simulations of the motion of a single rigid rod in a disordered
static 2d-array of disk-like obstacles. The rotational, , and
center-of-mass translational, , diffusion constants are calculated
for a wide range of rod length and density of obstacles . It is found
that follows the behavior predicted by kinetic theory for a hard
disk with an effective radius . A dynamic crossover is observed in
for comparable to the typical distance between neighboring
obstacles . Using arguments from kinetic theory and reptation, we
rationalize the scaling laws, dynamic exponents, and prefactors observed for
. In analogy with the enhanced translational diffusion observed in
deeply supercooled liquids, the Stokes-Einstein-Debye relation is violated for
.Comment: 8 pages, 4 figures. Major changes. To be published in Europhysics
Letter
Transport and Helfand moments in the Lennard-Jones fluid. I. Shear viscosity
We propose a new method, the Helfand-moment method, to compute the shear
viscosity by equilibrium molecular dynamics in periodic systems. In this
method, the shear viscosity is written as an Einstein-like relation in terms of
the variance of the so-called Helfand moment. This quantity, is modified in
order to satisfy systems with periodic boundary conditions usually considered
in molecular dynamics. We calculate the shear viscosity in the Lennard-Jones
fluid near the triple point thanks to this new technique. We show that the
results of the Helfand-moment method are in excellent agreement with the
results of the standard Green-Kubo method.Comment: Submitted to the Journal of Chemical Physic
Calculations of canonical averages from the grand canonical ensemble
Grand canonical and canonical ensembles become equivalent in the
thermodynamic limit, but when the system size is finite the results obtained in
the two ensembles deviate from each other. In many important cases, the
canonical ensemble provides an appropriate physical description but it is often
much easier to perform the calculations in the corresponding grand canonical
ensemble. We present a method to compute averages in canonical ensemble based
on calculations of the expectation values in grand canonical ensemble. The
number of particles, which is fixed in the canonical ensemble, is not
necessarily the same as the average number of particles in the grand canonical
ensemble
Quantum transport on small-world networks: A continuous-time quantum walk approach
We consider the quantum mechanical transport of (coherent) excitons on
small-world networks (SWN). The SWN are build from a one-dimensional ring of N
nodes by randomly introducing B additional bonds between them. The exciton
dynamics is modeled by continuous-time quantum walks and we evaluate
numerically the ensemble averaged transition probability to reach any node of
the network from the initially excited one. For sufficiently large B we find
that the quantum mechanical transport through the SWN is, first, very fast,
given that the limiting value of the transition probability is reached very
quickly; second, that the transport does not lead to equipartition, given that
on average the exciton is most likely to be found at the initial node.Comment: 8 pages, 8 figures (high quality figures available upon request
Long wavelength structural anomalies in jammed systems
The structural properties of static, jammed packings of monodisperse spheres
in the vicinity of the jamming transition are investigated using large-scale
computer simulations. At small wavenumber , we argue that the anomalous
behavior in the static structure factor, , is consequential of an
excess of low-frequency, collective excitations seen in the vibrational
spectrum. This anomalous feature becomes more pronounced closest to the jamming
transition, such that at the transition point. We introduce an
appropriate dispersion relation that accounts for these phenomena that leads us
to relate these structural features to characteristic length scales associated
with the low-frequency vibrational modes of these systems. When the particles
are frictional, this anomalous behavior is suppressed providing yet more
evidence that jamming transitions of frictional spheres lie at lower packing
fractions that that for frictionless spheres. These results suggest that the
mechanical properties of jammed and glassy media may therefore be inferred from
measurements of both the static and dynamical structure factors.Comment: 8 pages, 6 figure captions. Completely revised version to appear in
Phys. Rev.
Deterministic reaction models with power-law forces
We study a one-dimensional particles system, in the overdamped limit, where
nearest particles attract with a force inversely proportional to a power of
their distance and coalesce upon encounter. The detailed shape of the
distribution function for the gap between neighbouring particles serves to
discriminate between different laws of attraction. We develop an exact
Fokker-Planck approach for the infinite hierarchy of distribution functions for
multiple adjacent gaps and solve it exactly, at the mean-field level, where
correlations are ignored. The crucial role of correlations and their effect on
the gap distribution function is explored both numerically and analytically.
Finally, we analyse a random input of particles, which results in a stationary
state where the effect of correlations is largely diminished
On Calculation of Thermal Conductivity from Einstein Relation in Equilibrium MD
In equilibrium molecular dynamics, Einstein relation can be used to calculate
the thermal conductivity. This method is equivalent to Green-Kubo relation and
it does not require a derivation of an analytical form for the heat current.
However, it is not commonly used as Green-Kubo relationship. Its wide use is
hindered by the lack of a proper definition for integrated heat current (energy
moment) under periodic boundary conditions. In this paper, we developed an
appropriate definition for integrated heat current to calculate thermal
conductivity of solids under periodic conditions. We applied this method to
solid argon and silicon based systems; compared and contrasted with the
Green-Kubo approach.Comment: We updated this manuscript from second version by changing the title
and abstract. This paper is submitted to J. Chem. Phy
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