76,012 research outputs found
Mechanism of the fcc-hcp Phase Transformation in Solid Ar
We present an atomistic description of the {\it fcc}--to--{\it hcp}
transformation mechanism in solid argon (Ar) obtained from transition path
sampling molecular dynamics simulation. The phase transition pathways collected
during the sampling for an 8000--particle system reveal three transition types
according to the lattice deformation and relaxation details. In all three
transition types, we see a critical accumulation of defects and uniform growth
of a less ordered transition state, followed by a homogeneous growth of an
ordered phase. Stacking disorder is discussed to describe the transition
process and the cooperative motions of atoms in \{111\} planes. We investigate
the nucleation with larger system. In a system of 18000--particles, the
collective movements of atoms required for this transition are facilitated by
the formation and growth of stacking faults. However the enthalpy barrier is
still far beyond the thermal fluctuation. The high barrier explains previous
experimental observations of the inaccessibility of the bulk transition at low
pressure and its sluggishness even at extremely high pressure. The transition
mechanism in bulk Ar is different from Ar nanoclusters as the orthorhombic
intermediate structure proposed for the latter is not observed in any of our
simulations.Comment: 25 pages, 12 figures, journal supplementary included as appendi
Mobile Formation Coordination and Tracking Control for Multiple Non-holonomic Vehicles
This paper addresses forward motion control for trajectory tracking and
mobile formation coordination for a group of non-holonomic vehicles on SE(2).
Firstly, by constructing an intermediate attitude variable which involves
vehicles' position information and desired attitude, the translational and
rotational control inputs are designed in two stages to solve the trajectory
tracking problem. Secondly, the coordination relationships of relative
positions and headings are explored thoroughly for a group of non-holonomic
vehicles to maintain a mobile formation with rigid body motion constraints. We
prove that, except for the cases of parallel formation and translational
straight line formation, a mobile formation with strict rigid-body motion can
be achieved if and only if the ratios of linear speed to angular speed for each
individual vehicle are constants. Motion properties for mobile formation with
weak rigid-body motion are also demonstrated. Thereafter, based on the proposed
trajectory tracking approach, a distributed mobile formation control law is
designed under a directed tree graph. The performance of the proposed
controllers is validated by both numerical simulations and experiments
Direct Observation of Early-stage Quantum Dot Growth Mechanisms with High-temperature Ab Initio Molecular Dynamics
Colloidal quantum dots (QDs) exhibit highly desirable size- and
shape-dependent properties for applications from electronic devices to imaging.
Indium phosphide QDs have emerged as a primary candidate to replace the more
toxic CdSe QDs, but production of InP QDs with the desired properties lags
behind other QD materials due to a poor understanding of how to tune the growth
process. Using high-temperature ab initio molecular dynamics (AIMD)
simulations, we report the first direct observation of the early stage
intermediates and subsequent formation of an InP cluster from separated indium
and phosphorus precursors. In our simulations, indium agglomeration precedes
formation of In-P bonds. We observe a predominantly intercomplex pathway in
which In-P bonds form between one set of precursor copies while the carboxylate
ligand of a second indium precursor in the agglomerated indium abstracts a
ligand from the phosphorus precursor. This process produces an indium-rich
cluster with structural properties comparable to those in bulk zinc-blende InP
crystals. Minimum energy pathway characterization of the AIMD-sampled reaction
events confirms these observations and identifies that In-carboxylate
dissociation energetics solely determine the barrier along the In-P bond
formation pathway, which is lower for intercomplex (13 kcal/mol) than
intracomplex (21 kcal/mol) mechanisms. The phosphorus precursor chemistry, on
the other hand, controls the thermodynamics of the reaction. Our observations
of the differing roles of precursors in controlling QD formation strongly
suggests that the challenges thus far encountered in InP QD synthesis
optimization may be attributed to an overlooked need for a cooperative tuning
strategy that simultaneously addresses the chemistry of both indium and
phosphorus precursors.Comment: 40 pages, 9 figures, submitted for publicatio
Self-Organized Vortices of Circling Self-Propelled Particles and Curved Active Flagella
Self-propelled point-like particles move along circular trajectories when
their translocation velocity is constant and the angular velocity related to
their orientation vector is also constant. We investigate the collective
behavior of ensembles of such circle swimmers by Brownian dynamics simulations.
If the particles interact via a "velocity-trajectory coordination" rule within
neighboring particles, a self-organized vortex pattern emerges. This vortex
pattern is characterized by its particle-density correlation function ,
the density correlation function of trajectory centers, and an order
parameter representing the degree of the aggregation of the particles.
Here, we systematically vary the system parameters, such as the particle
density and the interaction range, in order to reveal the transition of the
system from a light-vortex-dominated to heavy-vortex-dominated state, where
vortices contain mainly a single and many self-propelled particles,
respectively. We also study a semi-dilute solution of curved,
sinusoidal-beating flagella, as an example of circling self-propelled particles
with explicit propulsion mechanism and excluded-volume interactions. Our
simulation results are compared with previous experimental results for the
vortices in sea-urchin sperm solutions near a wall. The properties of the
vortices in simulations and experiments are found to agree quantitatively.Comment: 14 pages, 15 figure
Formation control of a group of micro aerial vehicles (MAVs)
Coordinated motion of Unmanned Aerial Vehicles (UAVs) has been a growing research interest in the last decade. In this paper we propose a coordination model that makes use of virtual springs and dampers to generate reference trajectories for a group of quadrotors. Virtual forces exerted on each vehicle are produced by using projected distances between the quadrotors. Several coordinated task scenarios are presented and the performance of the proposed method is verified by simulations
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