arXiv
(422.153 recursos)
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2.
Optimum Paths for Systems Subject to Internal Noise - Einchcomb, S. J. B.; McKane, A. J.
We formulate the stochastic dynamics of a particle subject to internal
non-white (coloured) noise in terms of path-integrals. In the simplest case,
where the noise is exponentially correlated, the weak-noise limit is
characterised by optimum paths which are given by third order differential
equations. In contrast to systems subject to white noise or external coloured
noise, the overdamped limit for these systems is singular. We analyse the
origin of this behaviour. The whole formalism is generalised to more general
noise processes and the essential features are shown to be similar to the
exponentially correlated case.
3.
Infrared and Raman Evidence for Dimers and Polymers in Rb1C60 - Martin, Michael C.; Koller, Daniel; Rosenberg, A.; Kendziora, C.; Mihaly, L.
The infrared- and Raman-active vibrational modes of C60 were measured in the
various structural states of Rb1C60. According to earlier studies, Rb1C60 has
an $fcc$ structure at temperatures above ~100C, a linear chain polymer
orthorhombic structure when slowly cooled, and an as yet undetermined structure
when very rapidly cooled ("quenched"). We show that the spectra obtained in the
polymer state are consistent with each C60 molecule having bonds to two
diametrically opposite neighbors. In the quenched state, we find evidence for
further symmetry breaking, implying a lower symmetry structure than the polymer
state. The spectroscopic data of the quenched phase are shown to be consistent
with Rb2(C60)2, a dimerization...
4.
Free Thermal Convection Driven by Nonlocal Effects - Ibsen, Jorge; Soto, Rodrigo; Cordero, Patricio
We report and explain a convective phenomenon observed in molecular dynamics
simulations that cannot be classified either as a hydrodynamics instability nor
as a macroscopically forced convection. Two complementary arguments show that
the velocity field by a thermalizing wall is proportional to the ratio between
the heat flux and the pressure. This prediction is quantitatively corroborated
by our simulations.
5.
Coadsorption of Copper and Sulfate on Au(111) Electrodes: Monte Carlo
Simulation of a Lattice-Gas Model - Zhang, J.; Rikvold, P. A.; Sung, Y. -E.; Wieckowski, A.
We report ground-state calculations and Monte Carlo simulations for a
lattice-gas model of the underpotential deposition of copper on Au(111) in
sulfate-containing electrolytes. In a potential range of approximately
100$\sim$150 mV, this system exhibits a $(\sqrt3\!\times\!\sqrt3)$ mixed phase
with 2/3 monolayer (ML) copper and 1/3 ML sulfate. Our simulation results agree
well with experimental results and with other theoretical work.
6.
Critical point and coexistence curve properties of the Lennard-Jones
fluid: A finite-size scaling study - Wilding, Nigel B.
Monte Carlo simulations within the grand canonical ensemble are used to
explore the liquid-vapour coexistence curve and critical point properties of
the Lennard-Jones fluid. Attention is focused on the joint distribution of
density and energy fluctuations at coexistence. In the vicinity of the critical
point, this distribution is analysed using mixed-field finite-size scaling
techniques aided by histogram reweighting methods. The analysis yields highly
accurate estimates of the critical point parameters, as well as exposing the
size and character of corrections to scaling. In the sub-critical coexistence
region the density distribution is obtained by combining multicanonical
simulations with histogram reweighting techniques. It is demonstrated that this
procedure permits an efficient and...
7.
Liquids with Chiral Bond Order - Kamien, Randall D.
I describe new phases of a chiral liquid crystal with nematic and hexatic
order. I find a conical phase, similar to that of a cholesteric in an applied
magnetic field for Frank elastic constants $K_2>K_3$. I discuss the role of
fluctuations in the context of this phase and the possibility of satisfying the
inequality for sufficiently long polymers. In addition I discuss the
topological constraint relating defects in the bond order field to textures of
the nematic and elucidate its physical meaning. Finally I discuss the analogy
between smectic liquid crystals and chiral hexatics and propose a
defect-riddled ground state, akin to the Renn-Lubensky twist grain boundary
phase of...
8.
Calculation of the Density of States Using Discrete Variable
Representation and Toeplitz Matrices - Eisenberg, Eli; Baram, Asher; Baer, Michael
A direct and exact method for calculating the density of states for systems
with localized potentials is presented. The method is based on explicit
inversion of the operator $E-H$. The operator is written in the discrete
variable representation of the Hamiltonian, and the Toeplitz property of the
asymptotic part of the obtained {\it infinite} matrix is used. Thus, the
problem is reduced to the inversion of a {\it finite} matrix.
9.
Solid-to-solid isostructural transition in the hard sphere/attractive
Yukawa system - Rascon, C.; Mederos, L.; Navascues, G.
A thermodynamically consistent density functional-perturbation theory is used
to study the isostructural solid-to-solid transition which takes place in the
hard sphere/attractive Yukawa system when the Yukawa tail is sufficiently
short-ranged. A comparison with results for the square well potential allows us
to study the effect of the attractive potential form on the solid-solid
transition. Reasonable agreement with simulations is found for the main
transition properties as well as for the phase diagram evolution with the the
range of the attractive potential.
10.
Molecular ordering of precursor films during spreading of tiny liquid
droplets - Haataja, M.; Nieminen, J. A.; Ala-Nissila, T.
In this work we address a novel feature of spreading dynamics of tiny liquid
droplets on solid surfaces, namely the case where the ends of the molecules
feel different interactions to the surface. We consider a simple model of
dimers and short chain--like molecules which cannot form chemical bonds with
the surface. We study the spreading dynamics by Molecular Dynamics techniques.
In particular, we examine the microscopic structure of the time--dependent
precursor film and find that in some cases it can exhibit a high degree of
local order. This order persists even for flexible chains. Our results suggest
the possibility of extracting information about molecular interactions from the
structure...
11.
Imbibition: An Example of Nonconserved Cellular Automaton - Kumar, P. B. Sunil; Jana, Debnarayan
We report an imbibition experiment in 2D random porous media in which height
- height correlation function grows with a nonuniversal exponent. We find the
exponent to depend on evaporation. A cellular automaton model for imbibition
based on first principles is presented. A numerical study of the model gives
results which are consistent with the experiment. The interface is shown to
exhibit self-organised criticality (SOC).
12.
Individual Entanglements in a Simulated Polymer Melt - Ben-Naim, E.; Grest, G. S.; Witten, T. A.; Baljon, A. R. C.
We examine entanglements using monomer contacts between pairs of chains in a
Brownian-dynamics simulation of a polymer melt. A map of contact positions with
respect to the contacting monomer numbers (i,j) shows clustering in small
regions of (i,j) which persists in time, as expected for entanglements. Using
the ``space''-time correlation function of the aforementioned contacts, we show
that a pair of entangled chains exhibits a qualitatively different behavior
than a pair of distant chains when brought together. Quantitatively, about 50%
of the contacts between entangled chains are persistent contacts not present in
independently moving chains. In addition, we account for several observed
scaling properties of the contact correlation function.
13.
Dynamical simulation of transport in one-dimensional quantum wires - Leung, Kevin; Egger, Reinhold; Mak, C. H.
Transport of single-channel spinless interacting fermions (Luttinger liquid)
through a barrier has been studied by numerically exact quantum Monte Carlo
methods. A novel stochastic integration over the real-time paths allows for
direct computation of nonequilibrium conductance and noise properties. We have
examined the low-temperature scaling of the conductance in the crossover region
between a very weak and an almost insulating barrier.
14.
Theory of Chiral Order in Random Copolymers - Selinger, J. V.; Selinger, R. L. B.
Recent experiments have found that polyisocyanates composed of a mixture of
opposite enantiomers follow a chiral ``majority rule:'' the chiral order of the
copolymer, measured by optical activity, is dominated by whichever enantiomer
is in the majority. We explain this majority rule theoretically by mapping the
random copolymer onto the random-field Ising model. Using this model, we
predict the chiral order as a function of enantiomer concentration, in
quantitative agreement with the experiments, and show how the sharpness of the
majority-rule curve can be controlled.
15.
Is surface melting a surface phase transition? - Bastiaansen, Paul J. M.; Knops, Hubert J. F.
Monte Carlo or Molecular Dynamics calculations of surfaces of Lennard-Jones
systems often indicate, apart from a gradual disordering of the surface called
surface melting, the presence of a phase transition at the surface, but cannot
determine the nature of the transition. In the present paper, we provide for a
link between the continuous Lennard-Jones system and a lattice model. We apply
the method for the (001) surface of a Lennard-Jones fcc structure pertaining to
Argon. The corresponding lattice model is a Body Centered Solid on Solid model
with an extended range of interaction, showing in principle rough, flat and
disordered flat phases. We observe that entropy effects considerably...
16.
Corrections to scaling in 2--dimensional polymer statistics - Shannon, S. R.; Choy, T. C.; Fleming, R. J.
Writing $ = AN^{2\nu}(1+BN^{-\Delta_1}+CN^{-1}+ ...)$ for the mean
square end--to--end length $$ of a self--avoiding polymer chain of $N$
links, we have calculated $\Delta_1$ for the two--dimensional {\em continuum}
case from a new {\em finite} perturbation method based on the ground state of
Edwards self consistent solution which predicts the (exact) $\nu=3/4$ exponent.
This calculation yields $\Delta_1=1/2$. A finite size scaling analysis of data
generated for the continuum using a biased sampling Monte Carlo algorithm
supports this value, as does a re--analysis of exact data for two--dimensional
lattices.
17.
Diffusion-annihilation dynamics in one spatial dimension - Santos, Jaime E.; Schutz, Gunter M.; Stinchcombe, Robin B.
We discuss a reaction-diffusion model in one dimension subjected to an
external driving force. Each lattice site may be occupied by at most one
particle. The particles hop with asymmetric rates (the sum of which is one) to
the right or left nearest neighbour site if it is vacant, and annihilate with
rate one if it is occupied.
We compute the long time behaviour of the space dependent average density in
states where the initial density profiles are step functions. We also compute
the exact time dependence of the particle density for uncorrelated random
initial conditions. The representation of the uncorrelated random initial state
and also of the...
18.
Kinetics of fragmentation-annihilation processes - Filipe, Joao A. N.; Rodgers, Geoff J.
We investigate the kinetics of systems in which particles of one species
undergo binary fragmentation and pair annihilation. In the latter, nonlinear
process, fragments react at collision to produce an inert species, causing loss
of mass. We analyse these systems in the reaction-limited regime by solving a
continuous model within the mean-field approximation. The rate of
fragmentation, for a particle of mass $x$ to break into fragments of masses $y$
and $x-y$, has the form $x^{\lambda-1}$ ($\lambda>0$), and the annihilation
rate is constant and independent of the masses of the reactants. We find that
the asymptotic regime is characterized by the annihilation of small-mass
clusters. The results are compared...
19.
Fluid mixtures of parallel hard cubes - Cuesta, Jose A.
The direct correlation function of a fluid mixture of parallel hard cubes is
obtained by using Rosenfeld's fundamental measure approximation. This
approximation is thermodynamically consistent (compressibility and virial
equations of state are equal) and predicts a spinodal instability of the binary
mixture for large-to-small side ratio larger than roughly 10, in qualitative
agreement with simulations on the lattice version of the model. In two
dimensions the system never demix, also in agreement with the simulations.
20.
Short-time rotational diffusion in monodisperse charge-stabilized
colloidal suspensions - Watzlawek, M.; Naegele, G.
We investigate the combined effects of electrostatic interactions and
hydrodynamic interactions on the short-time rotational self-diffusion
coefficient in charge-stabilized suspensions. We calculate this coefficient as
a function of volume fraction for various effective particle charges and
amounts of added electrolyte. The influence of the hydrodynamic interactions on
the rotational diffusion coefficient is less pronounced for charged particles
than for uncharged ones. Salt-free suspensions are weakly influenced by
hydrodynamic interactions. For these strongly correlated systems we obtain a
quadratic volume fraction-dependence of the diffusion coefficient, which is
well explained in terms of an effective hard sphere model.
1.
