15,439 research outputs found
Adiabatic and Non-Adiabatic Contributions to the Free Energy from the Electron-Phonon Interaction for Na, K, Al, and Pb
We calculate the adiabatic contributions to the free energy due to the
electron--phonon interaction at intermediate temperatures, for the elemental metals Na, K, Al, and Pb. Using our
previously published results for the nonadiabatic contributions we show that
the adiabatic contribution, which is proportional to at low
temperatures and goes as at high temperatures, dominates the
nonadiabatic contribution for temperatures above a cross--over temperature,
, which is between 0.5 and 0.8 , where is the melting
temperature of the metal. The nonadiabatic contribution falls as for
temperatures roughly above the average phonon frequency.Comment: Updated versio
An \emph{ab initio} method for locating characteristic potential energy minima of liquids
It is possible in principle to probe the many--atom potential surface using
density functional theory (DFT). This will allow us to apply DFT to the
Hamiltonian formulation of atomic motion in monatomic liquids [\textit{Phys.
Rev. E} {\bf 56}, 4179 (1997)]. For a monatomic system, analysis of the
potential surface is facilitated by the random and symmetric classification of
potential energy valleys. Since the random valleys are numerically dominant and
uniform in their macroscopic potential properties, only a few quenches are
necessary to establish these properties. Here we describe an efficient
technique for doing this. Quenches are done from easily generated "stochastic"
configurations, in which the nuclei are distributed uniformly within a
constraint limiting the closeness of approach. For metallic Na with atomic pair
potential interactions, it is shown that quenches from stochastic
configurations and quenches from equilibrium liquid Molecular Dynamics (MD)
configurations produce statistically identical distributions of the structural
potential energy. Again for metallic Na, it is shown that DFT quenches from
stochastic configurations provide the parameters which calibrate the
Hamiltonian. A statistical mechanical analysis shows how the underlying
potential properties can be extracted from the distributions found in quenches
from stochastic configurations
Scaling Analysis and Evolution Equation of the North Atlantic Oscillation Index Fluctuations
The North Atlantic Oscillation (NAO) monthly index is studied from 1825 till
2002 in order to identify the scaling ranges of its fluctuations upon different
delay times and to find out whether or not it can be regarded as a Markov
process. A Hurst rescaled range analysis and a detrended fluctuation analysis
both indicate the existence of weakly persistent long range time correlations
for the whole scaling range and time span hereby studied. Such correlations are
similar to Brownian fluctuations. The Fokker-Planck equation is derived and
Kramers-Moyal coefficients estimated from the data. They are interpreted in
terms of a drift and a diffusion coefficient as in fluid mechanics. All partial
distribution functions of the NAO monthly index fluctuations have a form close
to a Gaussian, for all time lags, in agreement with the findings of the scaling
analyses. This indicates the lack of predictive power of the present NAO
monthly index. Yet there are some deviations for large (and thus rare) events.
Whence suggestions for other measurements are made if some improved
predictability of the weather/climate in the North Atlantic is of interest. The
subsequent Langevin equation of the NAO signal fluctuations is explicitly
written in terms of the diffusion and drift parameters, and a characteristic
time scale for these is given in appendix.Comment: 6 figures, 54 refs., 16 pages; submitted to Int. J. Mod. Phys. C:
Comput. Phy
Output and price level effects of monetary uncertainty in a matching model
output;prices;uncertainty;monetary economics
Millimeter wave satellite concepts, volume 1
The identification of technologies necessary for development of millimeter spectrum communication satellites was examined from a system point of view. Development of methodology based on the technical requirements of potential services that might be assigned to millimeter wave bands for identifying the viable and appropriate technologies for future NASA millimeter research and development programs, and testing of this methodology with selected user applications and services were the goals of the program. The entire communications network, both ground and space subsystems was studied. Cost, weight, and performance models for the subsystems, conceptual design for point-to-point and broadcast communications satellites, and analytic relationships between subsystem parameters and an overall link performance are discussed along with baseline conceptual systems, sensitivity studies, model adjustment analyses, identification of critical technologies and their risks, and brief research and development program scenarios for the technologies judged to be moderate or extensive risks. Identification of technologies for millimeter satellite communication systems, and assessment of the relative risks of these technologies, was accomplished through subsystem modeling and link optimization for both point-to-point and broadcast applications
Stirring Unmagnetized Plasma
A new concept for spinning unmagnetized plasma is demonstrated
experimentally. Plasma is confined by an axisymmetric multi-cusp magnetic field
and biased cathodes are used to drive currents and impart a torque in the
magnetized edge. Measurements show that flow viscously couples momentum from
the magnetized edge (where the plasma viscosity is small) into the unmagnetized
core (where the viscosity is large) and that the core rotates as a solid body.
To be effective, collisional viscosity must overcome the ion-neutral drag due
to charge exchange collisions
Investigating the Impact of Stakeholder Management on the Implementation of a Public Access Project: The Case of Smart Cape
This paper reports on a study that uses the Stakeholder Management Theory to analyse the implementation of a pilot phase of public access project, the Smart Cape Access Initiative, a Cape Town City Council e-government initiative. Data for the study was gathered through in-depth interviews with individuals who were involved, influenced and were affected by the implementation of the pilot project. The study identified the major stakeholders of the project and assessed their importance and influence on the project. Numerous interactions between the stakeholders were identified. It was further noted that no formal stakeholder management was undertaken at the identification and planning stages of the project lifecycle. In addition, results showed that there were missed opportunities for appropriate stakeholder management throughout the project. This study offers insights into agencies involved in planning and running public access projects
Coarse-graining microscopic strains in a harmonic, two-dimensional solid and its implications for elasticity: non-local susceptibilities and non-affine noise
In soft matter systems the local displacement field can be accessed directly
by video microscopy enabling one to compute local strain fields and hence the
elastic moduli using a coarse-graining procedure. We study this process for a
simple triangular lattice of particles connected by harmonic springs in
two-dimensions. Coarse-graining local strains obtained from particle
configurations in a Monte Carlo simulation generates non-trivial, non-local
strain correlations (susceptibilities), which may be understood within a
generalized, Landau type elastic Hamiltonian containing up to quartic terms in
strain gradients (K. Franzrahe et al., Phys. Rev. E 78, 026106 (2008)). In
order to demonstrate the versatility of the analysis of these correlations and
to make our calculations directly relevant for experiments on colloidal solids,
we systematically study various parameters such as the choice of statistical
ensemble, presence of external pressure and boundary conditions. We show that
special care needs to be taken for an accurate application of our results to
actual experiments, where the analyzed area is embedded within a larger system,
to which it is mechanically coupled. Apart from the smooth, affine strain
fields, the coarse-graining procedure also gives rise to a noise field made up
of non-affine displacements. Several properties of this noise field may be
rationalized for the harmonic solid using a simple "cell model" calculation.
Furthermore the scaling behavior of the probability distribution of the noise
field is studied and a master curve is obtained.Comment: 16 pages, 12 figure
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