267 research outputs found
Cooperative motion and growing length scales in supercooled confined liquids
Using molecular dynamics simulations we investigate the relaxation dynamics
of a supercooled liquid close to a rough as well as close to a smooth wall. For
the former situation the relaxation times increase strongly with decreasing
distance from the wall whereas in the second case they strongly decrease. We
use this dependence to extract various dynamical length scales and show that
they grow with decreasing temperature. By calculating the frequency dependent
average susceptibility of such confined systems we show that the experimental
interpretation of such data is very difficult.Comment: 7 pages of Latex, 3 figure
Cryptographic Aspects of Real Hyperelliptic Curves
In this paper, we give an overview of cryptographic applications using real hyperelliptic curves. We review previously proposed cryptographic protocols, and discuss the infrastructure of a real hyperelliptic curve, the mathematical structure underlying all these protocols. We then describe recent improvements to infrastructure arithmetic, including explicit formulas for divisor arithmetic in genus 2; and advances in solving the infrastructure discrete logarithm problem, whose presumed intractability is the basis of security for the related cryptographic protocols
Exact results for the optical absorption of strongly correlated electrons in a half-filled Peierls-distorted chain
In this second of three articles on the optical absorption of electrons in a
half-filled Peierls-distorted chain we present exact results for strongly
correlated tight-binding electrons. In the limit of a strong on-site
interaction we map the Hubbard model onto the Harris-Lange model which can
be solved exactly in one dimension in terms of spinless fermions for the charge
excitations. The exact solution allows for an interpretation of the charge
dynamics in terms of parallel Hubbard bands with a free-electron dispersion of
band-width , separated by the Hubbard interaction . The spin degrees of
freedom enter the expressions for the optical absorption only via a momentum
dependent but static ground state expectation value. The remaining spin problem
can be traced out exactly since the eigenstates of the Harris-Lange model are
spin-degenerate. This corresponds to the Hubbard model at temperatures large
compared to the spin exchange energy. Explicit results are given for the
optical absorption in the presence of a lattice distortion and a
nearest-neighbor interaction . We find that the optical absorption for
is dominated by a peak at and broad but weak absorption bands for . For an appreciable nearest-neighbor interaction, ,
almost all spectral weight is transferred to Simpson's exciton band which is
eventually Peierls-split.Comment: 50 pages REVTEX 3.0, 6 postscript figures; hardcopy versions before
May 96 are obsolete; accepted for publication in The Philosophical Magazine
Slow dynamics of a confined supercooled binary mixture II: Q space analysis
We report the analysis in the wavevector space of the density correlator of a
Lennard Jones binary mixture confined in a disordered matrix of soft spheres
upon supercooling. In spite of the strong confining medium the behavior of the
mixture is consistent with the Mode Coupling Theory predictions for bulk
supercooled liquids. The relaxation times extracted from the fit of the density
correlator to the stretched exponential function follow a unique power law
behavior as a function of wavevector and temperature. The von Schweidler
scaling properties are valid for an extended wavevector range around the peak
of the structure factor. The parameters extracted in the present work are
compared with the bulk values obtained in literature.Comment: 8 pages with 8 figures. RevTeX. Accepted for publication in Phys.
Rev.
A mixed effect model for bivariate meta-analysis of diagnostic test accuracy studies using a copula representation of the random effects distribution
Diagnostic test accuracy studies typically report the number of true positives, false positives, true negatives and false negatives. There usually exists a negative association between the number of true positives and true negatives, because studies that adopt less stringent criterion for declaring a test positive invoke higher sensitivities and lower specificities. A generalized linear mixed model (GLMM) is currently recommended to synthesize diagnostic test accuracy studies. We propose a copula mixed model for bivariate meta-analysis of diagnostic test accuracy studies. Our general model includes the GLMM as a special case and can also operate on the original scale of sensitivity and specificity. Summary receiver operating characteristic curves are deduced for the proposed model through quantile regression techniques and different characterizations of the bivariate random effects distribution. Our general methodology is demonstrated with an extensive simulation study and illustrated by re-analysing the data of two published meta-analyses. Our study suggests that there can be an improvement on GLMM in fit to data and makes the argument for moving to copula random effects models. Our modelling framework is implemented in the package CopulaREMADA within the open source statistical environment R
Theory of Electric Field-Induced Photoluminescence Quenching in Disordered Molecular Solids
The dynamics of excitons in disordered molecular solids is studied
theoretically, taking into account migration between different sites,
recombination, and dissociation into free charge carriers in the presence of an
electric field. The theory is applied to interpret the results of electric
field-induced photoluminescence (PL) quenching experiments on molecularly doped
polymers by Deussen et al. [Chem. Phys. 207, 147 (1996)]. Using an
intermolecular dissociation mechanism, the dependence of the PL quenching on
the electric field strength and the dopant concentration, and the time
evolution of the transient PL quenching can be well described. The results
constitute additional proof of the distinct exciton dissociation mechanisms in
conjugated polymer blends and molecularly doped polymers.Comment: 4 pages RevTeX, 3 Postscript figure
Frequency dependent specific heat of viscous silica
We apply the Mori-Zwanzig projection operator formalism to obtain an
expression for the frequency dependent specific heat c(z) of a liquid. By using
an exact transformation formula due to Lebowitz et al., we derive a relation
between c(z) and K(t), the autocorrelation function of temperature fluctuations
in the microcanonical ensemble. This connection thus allows to determine c(z)
from computer simulations in equilibrium, i.e. without an external
perturbation. By considering the generalization of K(t) to finite wave-vectors,
we derive an expression to determine the thermal conductivity \lambda from such
simulations. We present the results of extensive computer simulations in which
we use the derived relations to determine c(z) over eight decades in frequency,
as well as \lambda. The system investigated is a simple but realistic model for
amorphous silica. We find that at high frequencies the real part of c(z) has
the value of an ideal gas. c'(\omega) increases quickly at those frequencies
which correspond to the vibrational excitations of the system. At low
temperatures c'(\omega) shows a second step. The frequency at which this step
is observed is comparable to the one at which the \alpha-relaxation peak is
observed in the intermediate scattering function. Also the temperature
dependence of the location of this second step is the same as the one of the
peak, thus showing that these quantities are intimately connected to
each other. From c'(\omega) we estimate the temperature dependence of the
vibrational and configurational part of the specific heat. We find that the
static value of c(z) as well as \lambda are in good agreement with experimental
data.Comment: 27 pages of Latex, 8 figure
Molecular Dynamics Simulations
A tutorial introduction to the technique of Molecular Dynamics (MD) is given,
and some characteristic examples of applications are described. The purpose and
scope of these simulations and the relation to other simulation methods is
discussed, and the basic MD algorithms are described. The sampling of intensive
variables (temperature T, pressure p) in runs carried out in the microcanonical
(NVE) ensemble (N= particle number, V = volume, E = energy) is discussed, as
well as the realization of other ensembles (e.g. the NVT ensemble). For a
typical application example, molten SiO2, the estimation of various transport
coefficients (self-diffusion constants, viscosity, thermal conductivity) is
discussed. As an example of Non-Equilibrium Molecular Dynamics (NEMD), a study
of a glass-forming polymer melt under shear is mentioned.Comment: 38 pages, 11 figures, to appear in J. Phys.: Condens. Matte
Разработка технологии сборки-сварки системы отопления административно-бытового корпуса
Актуальность работы: в данной выпускной квалификационной работе производится разработка и сравнение технологий: сварка стальных труб, сварка полипропиленовых труб и пайка медных труб при монтаже системы отопления административно-бытового корпуса. Объектом исследования является процесс изготовления системы отопления административно-бытового корпуса. Цели и задачи исследования: в результате данной работы следует выбрать технологию монтажа с наибольшей степенью надежности получаемого изделия и оптимальной себестоимостью производства.The relevance of the work: in this prom qualification development work and comparison technology: steel pipe welding, welding of polypropylene pipes and solder copper pipe when installing the heating system of the administrative Corps. The object of research is the process of manufacturing heating systems the administrative Corps. The goals and objectives of the study: As a result of this work, you must select the mounting technology with the highest degree of reliability of the resulting products and optimal cost price of production
Anomalous transport of a tracer on percolating clusters
We investigate the dynamics of a single tracer exploring a course of fixed
obstacles in the vicinity of the percolation transition for particles confined
to the infinite cluster. The mean-square displacement displays anomalous
transport, which extends to infinite times precisely at the critical obstacle
density. The slowing down of the diffusion coefficient exhibits power-law
behavior for densities close to the critical point and we show that the
mean-square displacement fulfills a scaling hypothesis. Furthermore, we
calculate the dynamic conductivity as response to an alternating electric
field. Last, we discuss the non-gaussian parameter as an indicator for
heterogeneous dynamics
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