10,818 research outputs found
The Most Detailed Picture Yet of an Embedded High-mass YSO
High-mass star formation is not well understood chiefly because examples are
deeply embedded, relatively distant, and crowded with sources of emission.
Using VLA and VLBA observations of water and SiO maser emission, we have mapped
in detail the structure and proper motion of material 20-500 AU from the
closest high-mass YSO, radio source-I in the Orion KL region. We observe
streams of material driven in a rotating, wide angle, bipolar wind from the
surface of an edge-on accretion disk. The example of source-I provides strong
evidence that high-mass star formation proceeds via accretionComment: typo corrected and word added to abstract 6 pages including 4 B&W
figures. To appear in the Proceeding of IAU Symposium 221, Star Formation at
High Angular Resolution, Editors M. Burton, R. Jayawardhana & T. Bourke,
Astronomical Society of the Pacifi
Water exchange at a hydrated platinum electrode is rare and collective
We use molecular dynamics simulations to study the exchange kinetics of water
molecules at a model metal electrode surface -- exchange between water
molecules in the bulk liquid and water molecules bound to the metal. This
process is a rare event, with a mean residence time of a bound water of about
40 ns for the model we consider. With analysis borrowed from the techniques of
rare-event sampling, we show how this exchange or desorption is controlled by
(1) reorganization of the hydrogen bond network within the adlayer of bound
water molecules, and by (2) interfacial density fluctuations of the bulk liquid
adjacent to the adlayer. We define collective coordinates that describe the
desorption mechanism. Spatial and temporal correlations associated with a
single event extend over nanometers and tens of picoseconds.Comment: 10 pages, 9 figure
Time scale for the onset of Fickian diffusion in supercooled liquids
We propose a quantitative measure of a time scale on which Fickian diffusion
sets in for supercooled liquids and use Brownian Dynamics computer simulations
to determine the temperature dependence of this onset time in a Lennard-Jones
binary mixture. The time for the onset of Fickian diffusion ranges between 6.5
and 31 times the relaxation time (the relaxation time is the
characteristic relaxation time of the incoherent intermediate scattering
function). The onset time increases faster with decreasing temperature than the
relaxation time. Mean squared displacement at the onset time increases
with decreasing temperature
Spin-torque switching: Fokker-Planck rate calculation
We describe a new approach to understanding and calculating magnetization
switching rates and noise in the recently observed phenomenon of "spin-torque
switching". In this phenomenon, which has possible applications to information
storage, a large current passing from a pinned ferromagnetic (FM) layer to a
free FM layer switches the free layer. Our main result is that the spin-torque
effect increases the Arrhenius factor in the switching rate, not
by lowering the barrier , but by raising the effective spin temperature .
To calculate this effect quantitatively, we extend Kramers' 1940 treatment of
reaction rates, deriving and solving a Fokker-Planck equation for the energy
distribution including a current-induced spin torque of the Slonczewski type.
This method can be used to calculate slow switching rates without long-time
simulations; in this Letter we calculate rates for telegraph noise that are in
good qualitative agreement with recent experiments. The method also allows the
calculation of current-induced magnetic noise in CPP (current perpendicular to
plane) spin valve read heads.Comment: 11 pages, 8 figures, 1 appendix Original version in Nature format,
replaced by Phys. Rev. Letters format. No substantive change
Finite-temperature critical point of a glass transition
We generalize the simplest kinetically constrained model of a glass-forming
liquid by softening kinetic constraints, allowing them to be violated with a
small finite rate. We demonstrate that this model supports a first-order
dynamical (space-time) phase transition, similar to those observed with hard
constraints. In addition, we find that the first-order phase boundary in this
softened model ends in a finite-temperature dynamical critical point, which we
expect to be present in natural systems. We discuss links between this critical
point and quantum phase transitions, showing that dynamical phase transitions
in dimensions map to quantum transitions in the same dimension, and hence
to classical thermodynamic phase transitions in dimensions. We make these
links explicit through exact mappings between master operators, transfer
matrices, and Hamiltonians for quantum spin chains.Comment: 10 pages, 5 figure
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The use of the Continuously Regenerating Trap (CRT<sup>TM</sup>) and SCRT<sup>TM</sup> Systems to meet future emissions legislation
The progressive tightening of particulate matter (PM) legislation presents challenges to the engine development and aftertreatment communities. The Continuously Regenerating Trap (CRTTM) has been developed to enable diesel engines to meet the proposed future legislation. This passive filter system combines an oxidation catalyst with a Diesel Particulate Filter (DPF); the filter traps the PM and the oxidation catalyst generates NO2 which combusts the trapped PM at substantially lower temperatures than is possible using oxygen.
This paper outlines the operating principle of the CRTTM, and describes the performance of the system. It has been shown that the very high PM conversions obtained with the CRTTM can enable even Euro 1 engines to meet the PM limits proposed for introduction in Europe in 2005. In addition, the system removes PM across the whole particle size range, including ultrafine particulates. These results will be discussed, as will in-field durability studies which have shown that the system is still capable of converting 90% of PM after very high mileage operation (up to 600,000 km).
In addition to requiring very high PM conversion, the proposed future legislation requires substantial reductions in NOx emissions form heavy duty diesel vehicles. To meet these challenges the SCRTTM has been developed. This combines the CRTTM with SCR (Selective Catalytic Reduction) technology, and enables very high simultaneous conversions of CO, HC, PM and NOx to be achieved. The SCRTTM system is described, and its operating characteristics are discussed. It has been shown that the SCRTTM can potentially meet the legislative limits proposed for introduction in Europe in 2008
Dynamical Exchanges in Facilitated Models of Supercooled liquids
We investigate statistics of dynamical exchange events in coarse--grained
models of supercooled liquids in spatial dimensions , 2, and 3. The
models, based upon the concept of dynamical facilitation, capture generic
features of statistics of exchange times and persistence times. Here,
distributions for both times are related, and calculated for cases of strong
and fragile glass formers over a range of temperatures. Exchange time
distributions are shown to be particularly sensitive to the model parameters
and dimensions, and exhibit more structured and richer behavior than
persistence time distributions. Mean exchange times are shown to be Arrhenius,
regardless of models and spatial dimensions. Specifically, , with being the excitation concentration. Different dynamical
exchange processes are identified and characterized from the underlying
trajectories. We discuss experimental possibilities to test some of our
theoretical findings.Comment: 11 pages, 14 figures, minor corrections made, paper published in
Journal of Chemical Physic
Two-dimensional colloidal fluids exhibiting pattern formation
Fluids with competing short range attraction and long range repulsive
interactions between the particles can exhibit a variety of microphase
separated structures. We develop a lattice-gas (generalised Ising) model and
analyse the phase diagram using Monte Carlo computer simulations and also with
density functional theory (DFT). The DFT predictions for the structures formed
are in good agreement with the results from the simulations, which occur in the
portion of the phase diagram where the theory predicts the uniform fluid to be
linearly unstable. However, the mean-field DFT does not correctly describe the
transitions between the different morphologies, which the simulations show to
be analogous to micelle formation. We determine how the heat capacity varies as
the model parameters are changed. There are peaks in the heat capacity at state
points where the morphology changes occur. We also map the lattice model onto a
continuum DFT that facilitates a simplification of the stability analysis of
the uniform fluid.Comment: 13 pages, 15 figure
Isomorphic classical molecular dynamics model for an excess electron in a supercritical fluid
Ring polymer molecular dynamics (RPMD) is used to directly simulate the
dynamics of an excess electron in a supercritical fluid over a broad range of
densities. The accuracy of the RPMD model is tested against numerically exact
path integral statistics through the use of analytical continuation techniques.
At low fluid densities, the RPMD model substantially underestimates the
contribution of delocalized states to the dynamics of the excess electron.
However, with increasing solvent density, the RPMD model improves, nearly
satisfying analytical continuation constraints at densities approaching those
of typical liquids. In the high density regime, quantum dispersion
substantially decreases the self-diffusion of the solvated electron.
In this regime where the dynamics of the electron is strongly coupled to the
dynamics of the atoms in the fluid, trajectories that can reveal diffusive
motion of the electron are long in comparison to .Comment: 24 pages, 4 figure
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