3,368 research outputs found
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
CEOS Land Surface Imaging Constellation Mid-Resolution Optical Guidelines
The LSI community of users is large and varied. To reach all these users as well as potential instrument contributors this document has been organized by measurement parameters of interest such as Leaf Area Index and Land Surface Temperature. These measurement parameters and the data presented in this document are drawn from multiple sources, listed at the end of the document, although the two primary ones are "The Space-Based Global Observing System in 2010 (GOS-2010)" that was compiled for the World Meteorological Organization (WMO) by Bizzarro Bizzarri, and the CEOS Missions, Instruments, and Measurements online database (CEOS MIM). For each measurement parameter the following topics will be discussed: (1) measurement description, (2) applications, (3) measurement spectral bands, and (4) example instruments and mission information. The description of each measurement parameter starts with a definition and includes a graphic displaying the relationships to four general land surface imaging user communities: vegetation, water, earth, and geo-hazards, since the LSI community of users is large and varied. The vegetation community uses LSI data to assess factors related to topics such as agriculture, forest management, crop type, chlorophyll, vegetation land cover, and leaf or canopy differences. The water community analyzes snow and lake cover, water properties such as clarity, and body of water delineation. The earth community focuses on minerals, soils, and sediments. The geo-hazards community is designed to address and aid in emergencies such as volcanic eruptions, forest fires, and large-scale damaging weather-related events
Potential energy landscape-based extended van der Waals equation
The inherent structures ({\it IS}) are the local minima of the potential
energy surface or landscape, , of an {\it N} atom system.
Stillinger has given an exact {\it IS} formulation of thermodynamics. Here the
implications for the equation of state are investigated. It is shown that the
van der Waals ({\it vdW}) equation, with density-dependent and
coefficients, holds on the high-temperature plateau of the averaged {\it IS}
energy. However, an additional ``landscape'' contribution to the pressure is
found at lower . The resulting extended {\it vdW} equation, unlike the
original, is capable of yielding a water-like density anomaly, flat isotherms
in the coexistence region {\it vs} {\it vdW} loops, and several other desirable
features. The plateau energy, the width of the distribution of {\it IS}, and
the ``top of the landscape'' temperature are simulated over a broad reduced
density range, , in the Lennard-Jones fluid. Fits to the
data yield an explicit equation of state, which is argued to be useful at high
density; it nevertheless reproduces the known values of and at the
critical point
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
Entropy, Dynamics and Instantaneous Normal Modes in a Random Energy Model
It is shown that the fraction f of imaginary frequency instantaneous normal
modes (INM) may be defined and calculated in a random energy model(REM) of
liquids. The configurational entropy S and the averaged hopping rate among the
states R are also obtained and related to f, with the results R~f and
S=a+b*ln(f). The proportionality between R and f is the basis of existing INM
theories of diffusion, so the REM further confirms their validity. A link to S
opens new avenues for introducing INM into dynamical theories. Liquid 'states'
are usually defined by assigning a configuration to the minimum to which it
will drain, but the REM naturally treats saddle-barriers on the same footing as
minima, which may be a better mapping of the continuum of configurations to
discrete states. Requirements of a detailed REM description of liquids are
discussed
Ballooning Modes in Thin Accretion Disks: Limits for their Excitation
The conditions that limit the possible excitation of ideal MHD axisymmetric
ballooning modes in thin accretion disks are discussed. As shown earlier by
Coppi and Coppi (2001), these modes are well-localized in the vertical
direction but have characteristic oscillatory and non-localized profiles in the
radial direction. A necessary condition for their excitation is that the
magnetic energy be considerably lower than the thermal energy. Even when this
is satisfied, there remains the problem of identifying the possible physical
factors which can make the considered modes radially localized. The general
solution of the normal mode equation describing the modes is given, showing
that it is characterized by a discrete spectrum of eigensolutions. The growth
rates are reduced and have a different scaling relative to those of the
"long-cylinder" modes, commonly known as the Magneto Rotational Instability,
that have been previously studied.Comment: 25 pages, 7 figures. Accepted to Ap
The dynamical structure factor in topologically disordered systems
A computation of the dynamical structure factor of topologically disordered
systems, where the disorder can be described in terms of euclidean random
matrices, is presented. Among others, structural glasses and supercooled
liquids belong to that class of systems. The computation describes their
relevant spectral features in the region of the high frequency sound. The
analytical results are tested with numerical simulations and are found to be in
very good agreement with them. Our results may explain the findings of
inelastic X-ray scattering experiments in various glassy systems.Comment: Version to be published in J. Chem. Phy
Recommended from our members
Ceramics Technology Project database: September 1991 summary report
The piston ring-cylinder liner area of the internal combustion engine must withstand very-high-temperature gradients, highly-corrosive environments, and constant friction. Improving the efficiency in the engine requires ring and cylinder liner materials that can survive this abusive environment and lubricants that resist decomposition at elevated temperatures. Wear and friction tests have been done on many material combinations in environments similar to actual use to find the right materials for the situation. This report covers tribology information produced from 1986 through July 1991 by Battelle columbus Laboratories, Caterpillar Inc., and Cummins Engine Company, Inc. for the Ceramic Technology Project (CTP). All data in this report were taken from the project`s semiannual and bimonthly progress reports and cover base materials, coatings, and lubricants. The data, including test rig descriptions and material characterizations, are stored in the CTP database and are available to all project participants on request. Objective of this report is to make available the test results from these studies, but not to draw conclusions from these data
Mean-atom-trajectory model for the velocity autocorrelation function of monatomic liquids
We present a model for the motion of an average atom in a liquid or
supercooled liquid state and apply it to calculations of the velocity
autocorrelation function and diffusion coefficient . The model
trajectory consists of oscillations at a distribution of frequencies
characteristic of the normal modes of a single potential valley, interspersed
with position- and velocity-conserving transits to similar adjacent valleys.
The resulting predictions for and agree remarkably well with MD
simulations of Na at up to almost three times its melting temperature. Two
independent processes in the model relax velocity autocorrelations: (a)
dephasing due to the presence of many frequency components, which operates at
all temperatures but which produces no diffusion, and (b) the transit process,
which increases with increasing temperature and which produces diffusion.
Because the model provides a single-atom trajectory in real space and time,
including transits, it may be used to calculate all single-atom correlation
functions.Comment: LaTeX, 8 figs. This is an updated version of cond-mat/0002057 and
cond-mat/0002058 combined Minor changes made to coincide with published
versio
Recommended from our members
Ceramic Technology Project database: September 1990 summary report
Data generated within the Ceramic Technology Project (CTP) represent a valuable resource for both research and industry. The CTP database was created to provide easy access to this information in electronic and hardcopy forms by using a computerized database and by issuing periodic hardcopy reports on the database contents. This report is the sixth in a series of semiannual database summaries and covers recent additions to the database, including joined brazed specimen test data. It covers 1 SiC, 34 SiN, 10 whisker-reinforced SiN, 2 zirconia-toughened aluminas, 8 zirconias, and 34 joints
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