44,031 research outputs found
Relaxation in a glassy binary mixture: Mode-coupling-like power laws, dynamic heterogeneity and a new non-Gaussian parameter
We examine the relaxation of the Kob-Andersen Lennard-Jones binary mixture
using Brownian dynamics computer simulations. We find that in accordance with
mode-coupling theory the self-diffusion coefficient and the relaxation time
show power-law dependence on temperature. However, different mode-coupling
temperatures and power laws can be obtained from the simulation data depending
on the range of temperatures chosen for the power-law fits. The temperature
that is commonly reported as this system's mode-coupling transition
temperature, in addition to being obtained from a power law fit, is a crossover
temperature at which there is a change in the dynamics from the high
temperature homogeneous, diffusive relaxation to a heterogeneous, hopping-like
motion. The hopping-like motion is evident in the probability distributions of
the logarithm of single-particle displacements: approaching the commonly
reported mode-coupling temperature these distributions start exhibiting two
peaks. Notably, the temperature at which the hopping-like motion appears for
the smaller particles is slightly higher than that at which the hopping-like
motion appears for the larger ones. We define and calculate a new non-Gaussian
parameter whose maximum occurs approximately at the time at which the two peaks
in the probability distribution of the logarithm of displacements are most
evident.Comment: Submitted for publication in Phys. Rev.
Correlated k-distribution method for radiative transfer in climate models: Application to effect of cirrus clouds on climate
A radiative transfer method appropriate for use in simple climate models and three dimensional global climate models was developed. It is fully interactive with climate changes, such as in the temperature-pressure profile, cloud distribution, and atmospheric composition, and it is accurate throughout the troposphere and stratosphere. The vertical inhomogeneity of the atmosphere is accounted for by assuming a correlation of gaseous k-distributions of different pressures and temperatures. Line-by-line calculations are made to demonstrate that The method is remarkably accurate. The method is then used in a one-dimensional radiative-convective climate model to study the effect of cirrus clouds on surface temperature. It is shown that an increase in cirrus cloud cover can cause a significant warming of the troposphere and the Earth's surface, by the mechanism of an enhanced green-house effect. The dependence of this phenomenon on cloud optical thickness, altitude, and latitude is investigated
Process feasibility study in support of silicon material task 1
Initial results for gas thermal conductivity of silicon tetrafluoride and trichlorosilane are reported in respective temperature ranges of 25 to 400 C and 50 to 400 C. For chemical engineering analyses, the preliminary process design for the original silane process of Union Carbide was completed for Cases A and B, Regular and Minimum Process Storage. Included are raw material usage, utility requirements, major process equipment lists, and production labor requirements. Because of the large differences in surge tankage between major unit operations the fixed capital investment varied from 11,138,000 for Cases A and B, respectively. For the silane process the original flowsheet was revised for a more optimum arrangement of major equipment, raw materials and operating conditions. The initial issue of the revised flowsheet (Case C) for the silane process indicated favorable cost benefits over the original scheme
How the Polyakov loop and the regularization affect strangeness and restoration of symmetries at finite T
The effects of the Polyakov loop and of a regularization procedure that
allows the presence of high momentum quark states at finite temperature is
investigated within the Polyakov-loop extended Nambu--Jona-Lasinio model. The
characteristic temperatures, as well as the behavior of observables that signal
deconfinement and restoration of chiral and axial symmetries, are analyzed,
paying special attention to the behavior of strangeness degrees of freedom. We
observe that the cumulative effects of the Polyakov loop and of the
regularization procedure contribute to a better description of the
thermodynamics, as compared with lattice estimations. We find a faster partial
restoration of chiral symmetry and the restoration of the axial symmetry
appears as a natural consequence of the full recovering of the chiral symmetry
that was dynamically broken. These results show the relevance of the effects of
the interplay among the Polyakov loop dynamics, the high momentum quark sates
and the restoration of the chiral and axial symmetries at finite temperature.Comment: Talk given at XIII International Conference on Hadron Spectroscopy
(Hadron 2009), Tallahassee, Florida, USA, 29 Nov - 4 Dec, 200
Scaling behavior in the dynamics of a supercooled Lennard-Jones mixture
We present the results of a large scale molecular dynamics computer
simulation of a binary, supercooled Lennard-Jones fluid. At low temperatures
and intermediate times the time dependence of the intermediate scattering
function is well described by a von Schweidler law. The von Schweidler exponent
is independent of temperature and depends only weakly on the type of
correlator. For long times the correlation functions show a Kohlrausch behavior
with an exponent that is independent of temperature. This dynamical
behavior is in accordance with the mode-coupling theory of supercooled liquids.Comment: 6 pages, RevTex, three postscript figures available on request,
MZ-Physics-10
Are polar liquids less simple?
Strong correlation between equilibrium fluctuations of the potential energy,
U, and the virial, W, is a characteristic of a liquid that implies the presence
of certain dynamic properties, such as density scaling of the relaxation times
and isochronal superpositioning of the relaxation function. In this work we
employ molecular dynamics simulations (mds) on methanol and two variations,
lacking hydrogen bonds and a dipole moment, to assess the connection between
the correlation of U and W and these dynamic properties. We show, in accord
with prior results of others [T.S. Ingebrigtsen, T.B. Schroder, J.C. Dyre,
Phys. Rev. X 2, 011011 (2012).], that simple van der Waals liquids exhibit both
strong correlations and the expected dynamic behavior. However, for polar
liquids this correspondence breaks down - weaker correlation between U and W is
not associated with worse conformance to density scaling or isochronal
superpositioning. The reason for this is that strong correlation between U and
W only requires their proportionality, whereas the expected dynamic behavior
depends primarily on constancy of the proportionality constant for all state
points. For hydrogen-bonded liquids, neither strong correlation nor adherence
to the dynamic properties is observed; however, this nonconformance is not
directly related to the concentration of hydrogen bonds, but rather to the
greater deviation of the intermolecular potential from an inverse power law
(IPL). Only (hypothetical) liquids having interactions governed strictly by an
IPL are perfectly correlating and exhibit the consequent dynamic properties
over all thermodynamic conditions.Comment: 14 pages, 8 figure
Mesonic correlation functions at finite temperature and density in the Nambu-Jona-Lasinio model with a Polyakov loop
We investigate the properties of scalar and pseudo-scalar mesons at finite
temperature and quark chemical potential in the framework of the
Nambu-Jona-Lasinio (NJL) model coupled to the Polyakov loop (PNJL model) with
the aim of taking into account features of both chiral symmetry breaking and
deconfinement. The mesonic correlators are obtained by solving the
Schwinger-Dyson equation in the RPA approximation with the Hartree (mean field)
quark propagator at finite temperature and density. In the phase of broken
chiral symmetry a narrower width for the sigma meson is obtained with respect
to the NJL case; on the other hand, the pion still behaves as a Goldstone
boson. When chiral symmetry is restored, the pion and sigma spectral functions
tend to merge. The Mott temperature for the pion is also computed.Comment: 24 pages, 9 figures, version to appear in Phys. Rev.
- …