374 research outputs found
Invariant measures for monotone SPDE's with multiplicative noise term
We study diffusion processes corresponding to infinite dimensional semilinear
stochastic differential equations with local Lipschitz drift term and an
arbitrary Lipschitz diffusion coefficient. We prove tightness and the Feller
property of the solution to show existence of an invariant measure. As an
application we discuss stochastic reaction diffusion equations.Comment: 10 page
A spatio-temporal entropy-based approach for the analysis of cyber attacks (demo paper)
Computer networks are ubiquitous systems growing exponentially with a predicted 50 billion devices connected by 2050. This dramatically increases the potential attack surface of Internet networks. A key issue in cyber defense is to detect, categorize and identify these attacks, the way they are propagated and their potential impacts on the systems affected. The research presented in this paper models cyber attacks at large by considering the Internet as a complex system in which attacks are propagated over a network. We model an attack as a path from a source to a target, and where each attack is categorized according to its intention. We setup an experimental testbed with the concept of honeypot that evaluates the spatiotemporal distribution of these Internet attacks. The preliminary results show a series of patterns in space and time that illustrate the potential of the approach, and how cyber attacks can be categorized according to the concept and measure of entropy
Fast relaxation in a fragile liquid under pressure
The incoherent dynamic structure factor of ortho-terphenyl has been measured
by neutron time-of-flight and backscattering technique in the pressure range
from 0.1 MPa to 240 MPa for temperatures between 301 K and 335 K.
Tagged-particle correlations in the compressed liquid decay in two steps. The
alpha-relaxation lineshape is independent of pressure, and the relaxation time
proportional to viscosity. A kink in the amplitude f_Q(P) reveals the onset of
beta relaxation. The beta-relaxation regime can be described by the
mode-coupling scaling function; amplitudes and time scales allow a consistent
determination of the critical pressure P_c(T). alpha and beta relaxation depend
in the same way on the thermodynamic state; close to the mode-coupling
cross-over, this dependence can be parametrised by an effective coupling Gamma
~ n*T**{-1/4}.Comment: 4 Pages of RevTeX, 4 figures (submitted to Physical Review Letters
Wavenumber dependence of structural alpha relaxation in a molecular liquid
Structural alpha relaxation in liquid orthoterphenyl is studied by means of
coherent neutron time-of-flight and backscattering spectroscopy over a large
temperature range. Not only amplitude and relaxation time but also the spectral
line shape show a significant variation with wavenumber. As expected from mode
coupling theory, these variations are correlated with the static structure
factor. Even far above the melting point, alpha relaxation remains
non-exponential.Comment: 6 pages of LaTeX, 4 figure
Flow equations for cold Bose gases
Wederive flow equations for cold atomic gases with one macroscopically populated energy level. The
generator is chosen such that the ground state decouples from all other states in the system as the
renormalization group flow progresses.Wepropose a self-consistent truncation scheme for the flow
equations at the level of three-body operators and show how they can be used to calculate the ground
state energy of a generalN-body system. Moreover, we provide a general method to estimate the
truncation error in the calculated energies. Finally, we test our scheme by benchmarking to the exactly
solvable Lieb–Liniger model and find good agreement for weak and moderate interaction strengths
Molecular dynamics simulation of the fragile glass former ortho-terphenyl: a flexible molecule model
We present a realistic model of the fragile glass former orthoterphenyl and
the results of extensive molecular dynamics simulations in which we
investigated its basic static and dynamic properties. In this model the
internal molecular interactions between the three rigid phenyl rings are
described by a set of force constants, including harmonic and anharmonic terms;
the interactions among different molecules are described by Lennard-Jones
site-site potentials. Self-diffusion properties are discussed in detail
together with the temperature and momentum dependencies of the
self-intermediate scattering function. The simulation data are compared with
existing experimental results and with the main predictions of the Mode
Coupling Theory.Comment: 20 pages and 28 postscript figure
Sensitivity of european temperature to albedo parameterization in the regional climate model COSMO-CLM linked to extreme land use changes
Previous studies based on observations and models are uncertain about the biophysical impact of af- and deforestation in the northern hemisphere mid-latitude summers, and show either a cooling or warming. The spatial distribution, magnitude and direction are still uncertain. In this study, the effect of three different albedo parameterizations in the regional climate model COSMO-CLM (v5.09) is examined performing idealized experiments at 0.44° horizontal resolution across the EURO-CORDEX domain during 1986–2015. De- and af-forestation simulations are compared to a simulation with no land cover change. Emphasis is put on the impact of changes in radiation and turbulent fluxes. A clear latitudinal pattern is found, which results partly due to the strong land cover conversion from forest- to grassland in the high latitudes and open land to forest conversion in mid-latitudes. Afforestation warms the climate in winter, and strongest in mid-latitudes. Results are indifferent in summer owing to opposing albedo and evapotranspiration effects of comparable size but different sign. Thus, the net effect is small for summer. Depending on the albedo parameterization in the model, the temperature effect can turn from cooling to warming in mid-latitude summers. The summer warming due to deforestation to grassland is up to 3°C higher than due to afforestation. The cooling by grass or warming by forest is in magnitude comparable and small in winter. The strength of the described near-surface temperature changes depends on the magnitude of the individual biophysical changes in the specific background climate conditions of the region. Thus, the albedo parameterization need to account for different vegetation types. Furthermore, we found that, depending on the region, the land cover change effect is more important than the model uncertainty due to albedo parameterization. This is important information for model development
Structural relaxation in a system of dumbbell molecules
The interaction-site-density-fluctuation correlators, the dipole-relaxation
functions, and the mean-squared displacements of a system of symmetric
dumbbells of fused hard spheres are calculated for two representative
elongations of the molecules within the mode-coupling theory for the evolution
of glassy dynamics. For large elongations, universal relaxation laws for states
near the glass transition are valid for parameters and time intervals similar
to the ones found for the hard-sphere system. Rotation-translation coupling
leads to an enlarged crossover interval for the mean-squared displacement of
the constituent atoms between the end of the von Schweidler regime and the
beginning of the diffusion process. For small elongations, the superposition
principle for the reorientational -process is violated for parameters
and time intervals of interest for data analysis, and there is a strong
breaking of the coupling of the -relaxation scale for the diffusion
process with that for representative density fluctuations and for dipole
reorientations.Comment: 15 pages, 14 figures, Phys. Rev. E in pres
Test of the semischematic model for a liquid of linear molecules
We apply to a liquid of linear molecules the semischematic mode-coupling
model, previously introduced to describe the center of mass (COM) slow dynamics
of a network-forming molecular liquid. We compare the theoretical predictions
and numerical results from a molecular dynamics simulation, both for the time
and the wave-vector dependence of the COM density-density correlation function.
We discuss the relationship between the presented analysis and the results from
an approximate solution of the equations from molecular mode-coupling theory
[R. Schilling and T. Scheidsteger, Phys. Rev. E 56 2932 (1997)].Comment: Revtex, 10 pages, 4 figure
Static and Dynamic Properties of a Viscous Silica Melt Molecular Dynamics Computer Simulations
We present the results of a large scale molecular dynamics computer
simulation in which we investigated the static and dynamic properties of a
silica melt in the temperature range in which the viscosity of the system
changes from O(10^-2) Poise to O(10^2) Poise. We show that even at temperatures
as high as 4000 K the structure of this system is very similar to the random
tetrahedral network found in silica at lower temperatures. The temperature
dependence of the concentration of the defects in this network shows an
Arrhenius law. From the partial structure factors we calculate the neutron
scattering function and find that it agrees very well with experimental neutron
scattering data. At low temperatures the temperature dependence of the
diffusion constants shows an Arrhenius law with activation energies which
are in very good agreement with the experimental values. With increasing
temperature we find that this dependence shows a cross-over to one which can be
described well by a power-law, D\propto (T-T_c)^gamma. The critical temperature
T_c is 3330 K and the exponent gamma is close to 2.1. Since we find a similar
cross-over in the viscosity we have evidence that the relaxation dynamics of
the system changes from a flow-like motion of the particles, as described by
the ideal version of mode-coupling theory, to a hopping like motion. We show
that such a change of the transport mechanism is also observed in the product
of the diffusion constant and the life time of a Si-O bond, or the space and
time dependence of the van Hove correlation functions.Comment: 30 pages of Latex, 14 figure
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