2,721 research outputs found
Analysis of high excitation planetary nebulae
Combination of extensive ground-based spectroscopic observation of high excitation planetary with IUE data permit determination not only of improved diagnostics but also better abundances for elements such as C and N that are well represented in the ultraviolet spectra and also C, Ar and metals Na, Ca and K whose lines appear in the wavelength 3200-8100 A region
The Potential Energy Landscape and Mechanisms of Diffusion in Liquids
The mechanism of diffusion in supercooled liquids is investigated from the
potential energy landscape point of view, with emphasis on the crossover from
high- to low-T dynamics. Molecular dynamics simulations with a time dependent
mapping to the associated local mininum or inherent structure (IS) are
performed on unit-density Lennard-Jones (LJ). New dynamical quantities
introduced include r2_{is}(t), the mean-square displacement (MSD) within a
basin of attraction of an IS, R2(t), the MSD of the IS itself, and g_{loc}(t)
the mean waiting time in a cooperative region. At intermediate T, r2_{is}(t)
posesses an interval of linear t-dependence allowing calculation of an
intrabasin diffusion constant D_{is}. Near T_{c} diffusion is intrabasin
dominated with D = D_{is}. Below T_{c} the local waiting time tau_{loc} exceeds
the time, tau_{pl}, needed for the system to explore the basin, indicating the
action of barriers. The distinction between motion among the IS below T_{c} and
saddle, or border dynamics above T_{c} is discussed.Comment: submitted to pr
Inherent-Structure Dynamics and Diffusion in Liquids
The self-diffusion constant D is expressed in terms of transitions among the
local minima of the potential (inherent structure, IS) and their correlations.
The formulae are evaluated and tested against simulation in the supercooled,
unit-density Lennard-Jones liquid. The approximation of uncorrelated
IS-transition (IST) vectors, D_{0}, greatly exceeds D in the upper temperature
range, but merges with simulation at reduced T ~ 0.50. Since uncorrelated IST
are associated with a hopping mechanism, the condition D ~ D_{0} provides a new
way to identify the crossover to hopping. The results suggest that theories of
diffusion in deeply supercooled liquids may be based on weakly correlated IST.Comment: submitted to PR
Configurational entropy of hard spheres
We numerically calculate the configurational entropy S_conf of a binary
mixture of hard spheres, by using a perturbed Hamiltonian method trapping the
system inside a given state, which requires less assumptions than the previous
methods [R.J. Speedy, Mol. Phys. 95, 169 (1998)]. We find that S_conf is a
decreasing function of packing fraction f and extrapolates to zero at the
Kauzmann packing fraction f_K = 0.62, suggesting the possibility of an ideal
glass-transition for hard spheres system. Finally, the Adam-Gibbs relation is
found to hold.Comment: 10 pages, 6 figure
Stochastic Paleoclimatology: Modeling the EPICA Ice Core Climate Records
We analyze and model the stochastic behavior of paleoclimate time series and
assess the implications for the coupling of climate variables during the
Pleistocene glacial cycles. We examine 800 kyr of carbon dioxide, methane,
nitrous oxide, and temperature proxy data from the EPICA Dome-C ice core, which
are characterized by 100~kyr glacial cycles overlain by fluctuations across a
wide range of time scales. We quantify this behavior through multifractal
time-weighted detrended fluctuation analysis, which distinguishes near
red-noise and white-noise behavior below and above the 100~kyr glacial cycle
respectively in all records. This allows us to model each time series as a
one-dimensional periodic non-autonomous stochastic dynamical system, and assess
the stability of physical processes and the fidelity of model-simulated time
series. We extend this approach to a four-variable model with linear coupling
terms, which we interpret in terms of the interrelationships between the time
series. Methane and nitrous oxide are found to have significant destabilizing
influences, while carbon dioxide and temperature have smaller stabilizing
influences. We draw conclusions about causal relationships in glacial
transitions and the climate processes that may have facilitated these
couplings, and highlight opportunities to further develop stochastic modeling
approaches.Comment: 14 pages, 6 figure
Instantaneous Normal Mode analysis of liquid HF
We present an Instantaneous Normal Modes analysis of liquid HF aimed to
clarify the origin of peculiar dynamical properties which are supposed to stem
from the arrangement of molecules in linear hydrogen-bonded network. The
present study shows that this approach is an unique tool for the understanding
of the spectral features revealed in the analysis of both single molecule and
collective quantities. For the system under investigation we demonstrate the
relevance of hydrogen-bonding ``stretching'' and fast librational motion in the
interpretation of these features.Comment: REVTeX, 7 pages, 5 eps figures included. Minor changes in the text
and in a figure. Accepted for publication in Phys. Rev. Let
Recent Deformation in the Bottom Sediments of Western and Southeastern Lake Ontario and its Association with Major Structures and Seismicity
Geophysical surveys, undertaken in the Toronto-Burlington corridor of western Lake Ontario and in the Rochester Basin of southeastern Lake Ontario, revealed the presence of features affecting the young lake-bottom sediments. In the western part of the lake, they include inferred pop-ups in bedrock, and plumose structures, dark linear patterns, and linear belts of circular to elliptical signatures in the modern mud. In southeastern Lake Ontario the glacial and post-glacial sediments display vertical separations of on the order of 10-15 m. Pop-ups are tectonically-induced structures. The features in the modern mud commonly parallel the orientation of P-stresses measured in Paleozoic rocks nearby and, along with the pop-ups, are spatially related to an aeromagnetic lineament. Furthermore, all of these features occur within a seismically active belt. The vertical displacements of the layered glacial and post-glacial sediments, within the Rochester Basin, are located along the southern margin of the postulated WSW extension of the seismically active St. Lawrence rift system and are interpreted to be due to faulting. The geologically young age of the sediments affected by the various deformational features, along with the characteristics of the features themselves, suggest that the lake-bottom sediments surveyed in this study may have recorded the effects of neotectonic processes.Des levés géophysiques effectués dans le corridor Toronto-Burlington, dans l'ouest du lac Ontario, a révélé la présence d'éléments qui altèrent les jeunes sédiments lacustres du fond. Dans la partie ouest du lac, dans la roche en place, il s'agit de structures de soulèvement (pop-ups) et, dans les boues récentes, de structures plumeuses, de réseaux de traits sombres et de zones linéaires de tracés circulaires à elliptiques. Dans la partie sud-est du lac Ontario, il y a dans les sédiments glaciaires et postglaciaires des rejets verticaux de l'ordre de 10 à 15 m. Les structures de soulèvement sont d'origine tectonique. Les formes dans les boues récentes, généralement parallèles à l'orientation des contraintes de compression mesurées dans les roches paléozoïques voisines, sont spatialement reliés, comme les structures de soulèvement, à un linéament aéromagnétique. De plus, toutes ces formes se trouvent dans une zone sismique active. Les rejets verticaux dans les sédiments glaciaires et postglaciaires stratifiés, à l'intérieur du basssin de Rochester, sont localisés le long de la bordure sud du prolongement présumé WSW du système actif du rift du Saint-Laurent et sont probablement attribuables à la formation de failles. La jeunesse des sédiments altérés par les déformations et les caractéristiques des déformations mêmes laissent croire que ces sédiments ont probablement enregistré les effets de processus néotectoniques.Geophysikalische Vermessungen, die im Toronto Burlington-Korridor des westlichen Ontariosees und im Rochester-Becken des sùdôstlichen Ontariosees durchgefùhrt wurden, deckten die Anwesenheit von Elementen auf, welche auf die jungen Seegrundsedimente einwirken. Im westlichen Teil des Sees bestehen sie aus Hebungen im anste-henden Gestein und im modernen Schlamm aus federartigen Strukturen, dunklen linearen Mustern und linearen Gùrteln mit kreisformigen bis ellipsenfôrmigen Umrissen. Im sùdôstlichen Ontariosee gibt es in den glazialen und postglazialen Sedimenten verti-kale Verwùrfe der GrôRenordnung von 10-15 m. Die Hebungen sind tektonischen Ursprungs. Die Formen im modernen Schlamm liegen im allgemeinen paralell zu der Orientierung der in den benachbarten palàozoischen Felsen gemessenen P-Stresse und sind zusammen mit den Hebungen ràumlich mit einem aeromagnetischen Lineament verbunden. AuRerdem treten aile dièse Formen innerhalb eines seismisch aktiven Gùrtels auf. Die vertikalen Verstellungen der geschichteten glazialen und postglazialen Sedimente innerhalb des Rochester-Beckens werden entlang des sùdlichen Rands der angenommenen WSW-Verlàngerung des seismisch aktiven Sankt-Lorenz-Spaltensystems lokalisiert. Das geologisch relativ junge Alter der durch die verschiedenen Verformungen Iàf3t vermu-ten, daB die in dieser Studie gemessenen Seegrundsedimente moglicherweise die Wirkungen neotektonischer Prozesse aufgezeichnet haben
Heterogeneous Dynamics, Marginal Stability and Soft Modes in Hard Sphere Glasses
In a recent publication we established an analogy between the free energy of
a hard sphere system and the energy of an elastic network [1]. This result
enables one to study the free energy landscape of hard spheres, in particular
to define normal modes. In this Letter we use these tools to analyze the
activated transitions between meta-bassins, both in the aging regime deep in
the glass phase and near the glass transition. We observe numerically that
structural relaxation occurs mostly along a very small number of
nearly-unstable extended modes. This number decays for denser packing and is
significantly lowered as the system undergoes the glass transition. This
observation supports that structural relaxation and marginal modes share common
properties. In particular theoretical results [2, 3] show that these modes
extend at least on some length scale where
corresponds to the maximum packing fraction, i.e. the jamming
transition. This prediction is consistent with very recent numerical
observations of sheared systems near the jamming threshold [4], where a similar
exponent is found, and with the commonly observed growth of the rearranging
regions with compression near the glass transition.Comment: 6 pages, improved versio
Saddles in the energy landscape probed by supercooled liquids
We numerically investigate the supercooled dynamics of two simple model
liquids exploiting the partition of the multi-dimension configuration space in
basins of attraction of the stationary points (inherent saddles) of the
potential energy surface. We find that the inherent saddles order and potential
energy are well defined functions of the temperature T. Moreover, decreasing T,
the saddle order vanishes at the same temperature (T_MCT) where the inverse
diffusivity appears to diverge as a power law. This allows a topological
interpretation of T_MCT: it marks the transition from a dynamics between basins
of saddles (T>T_MCT) to a dynamics between basins of minima (T<T_MCT).Comment: 4 pages, 3 figures, to be published on PR
Instantaneous Normal Mode Analysis of Supercooled Water
We use the instantaneous normal mode approach to provide a description of the
local curvature of the potential energy surface of a model for water. We focus
on the region of the phase diagram in which the dynamics may be described by
the mode-coupling theory. We find, surprisingly, that the diffusion constant
depends mainly on the fraction of directions in configuration space connecting
different local minima, supporting the conjecture that the dynamics are
controlled by the geometric properties of configuration space. Furthermore, we
find an unexpected relation between the number of basins accessed in
equilibrium and the connectivity between them.Comment: 5 pages, 4 figure
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