7,599 research outputs found
Parametric ordering of complex systems
Cellular automata (CA) dynamics are ordered in terms of two global
parameters, computable {\sl a priori} from the description of rules. While one
of them (activity) has been used before, the second one is new; it estimates
the average sensitivity of rules to small configurational changes. For two
well-known families of rules, the Wolfram complexity Classes cluster
satisfactorily. The observed simultaneous occurrence of sharp and smooth
transitions from ordered to disordered dynamics in CA can be explained with the
two-parameter diagram
Comparison of Dissipative Particle Dynamics and Langevin thermostats for out-of-equilibrium simulations of polymeric systems
In this work we compare and characterize the behavior of Langevin and
Dissipative Particle Dynamics (DPD) thermostats in a broad range of
non-equilibrium simulations of polymeric systems. Polymer brushes in relative
sliding motion, polymeric liquids in Poiseuille and Couette flows, and
brush-melt interfaces are used as model systems to analyze the efficiency and
limitations of different Langevin and DPD thermostat implementations. Widely
used coarse-grained bead-spring models under good and poor solvent conditions
are employed to assess the effects of the thermostats. We considered
equilibrium, transient, and steady state examples for testing the ability of
the thermostats to maintain constant temperature and to reproduce the
underlying physical phenomena in non-equilibrium situations. The common
practice of switching-off the Langevin thermostat in the flow direction is also
critically revisited. The efficiency of different weight functions for the DPD
thermostat is quantitatively analyzed as a function of the solvent quality and
the non-equilibrium situation.Comment: 12 pages, introduction improved, references added, to appear in Phys.
Rev.
Multiple magneto-phonon resonances in graphene
Our low-temperature magneto-Raman scattering measurements performed on
graphene-like locations on the surface of bulk graphite reveal a new series of
magneto-phonon resonances involving both K-point and Gamma-point phonons. In
particular, we observe for the first time the resonant splitting of three
crossing excitation branches. We give a detailed theoretical analysis of these
new resonances. Our results highlight the role of combined excitations and the
importance of multi-phonon processes (from both K and Gamma points) for the
relaxation of hot carriers in graphene.Comment: 20 pages, 11 figure
A micro-magneto-Raman scattering study of graphene on a bulk graphite substrate
We report on a magneto-Raman scattering study of graphene flakes located on
the surface of a bulk graphite substrate. By spatially mapping the Raman
scattering response of the surface of bulk graphite with an applied magnetic
field, we pinpoint specific locations which show the electronic excitation
spectrum of graphene. We present the characteristic Raman scattering signatures
of these specific locations. We show that such flakes can be superimposed with
another flake and still exhibit a graphene-like excitation spectrum.
Two different excitation laser energies (514.5 and 720 nm) are used to
investigate the excitation wavelength dependence of the electronic Raman
scattering signal.Comment: 6 pages, 5 figure
Static and dynamic properties of the interface between a polymer brush and a melt of identical chains
Molecular dynamics simulations of a short-chain polymer melt between two
brush-covered surfaces under shear have been performed. The end-grafted
polymers which constitute the brush have the same chemical properties as the
free chains in the melt and provide a soft deformable substrate. Polymer chains
are described by a coarse-grained bead-spring model with Lennard-Jones
interactions between the beads and a FENE potential between nearest neighbors
along the backbone of the chains. The grafting density of the brush layer
offers a way of controlling the behavior of the surface without altering the
molecular interactions. We perform equilibrium and non-equilibrium Molecular
Dynamics simulations at constant temperature and volume using the Dissipative
Particle Dynamics thermostat. The equilibrium density profiles and the behavior
under shear are studied as well as the interdigitation of the melt into the
brush, the orientation on different length scales (bond vectors, radius of
gyration, and end-to-end vector) of free and grafted chains, and velocity
profiles. The viscosity and slippage at the interface are calculated as
functions of grafting density and shear velocity.Comment: 12 pages, submitted to J Chem Phy
Coupled Fluctuations near Critical Wetting
Recent work on the complete wetting transition has emphasized the role played
by the coupling of fluctuations of the order parameter at the wall and at the
depinning fluid interface. Extending this approach to the wetting transition
itself we predict a novel crossover effect associated with the decoupling of
fluctuations as the temperature is lowered towards the transition temperature
T_W. Using this we are able to reanalyse recent Monte-Carlo simulation studies
and extract a value \omega(T_W)=0.8 at T_W=0.9T_C in very good agreement with
long standing theoretical predictions.Comment: 4 pages, LaTex, 1 postscript figur
Phase separation in fluids exposed to spatially periodic external fields
We consider the liquid-vapor type phase transition for fluids confined within
spatially periodic external fields. For a fluid in d=3 dimensions, the periodic
field induces an additional phase, characterized by large density modulations
along the field direction. At the triple point, all three phases (modulated,
vapor, and liquid) coexist. At temperatures slightly above the triple point and
for low (high) values of the chemical potential, two-phase coexistence between
the modulated phase and the vapor (liquid) is observed. We study this
phenomenon using computer simulations and mean-field theory for the Ising
model. The theory shows that, in order for the modulated phase to arise, the
field wavelength must exceed a threshold value. We also find an extremely low
tension of the interface between the modulated phase and the vapor/liquid
phases. The tension is of the order 10^{-4} kB T per squared lattice spacing,
where kB is the Boltzmann constant, and T the temperature. In order to detect
such low tensions, a new simulation method is proposed. We also consider the
case of d=2 dimensions. The modulated phase then does not survive, leading to a
radically different phase diagram.Comment: 11 pages, 14 figure
Water adsorption on amorphous silica surfaces: A Car-Parrinello simulation study
A combination of classical molecular dynamics (MD) and ab initio
Car-Parrinello molecular dynamics (CPMD) simulations is used to investigate the
adsorption of water on a free amorphous silica surface. From the classical MD
SiO_2 configurations with a free surface are generated which are then used as
starting configurations for the CPMD.We study the reaction of a water molecule
with a two-membered ring at the temperature T=300K. We show that the result of
this reaction is the formation of two silanol groups on the surface. The
activation energy of the reaction is estimated and it is shown that the
reaction is exothermic.Comment: 12 pages, 6 figures, to be published in J. Phys.: Condens. Matte
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