296 research outputs found
An Accelerated Multiboson Algorithm for Coulomb Gases with Dynamical Dielectric Effects
A recent reformulation [1] of the problem of Coulomb gases in the presence of
a dynamical dielectric medium showed that finite temperature simulations of
such systems can be accomplished on the basis of completely local Hamiltonians
on a spatial lattice by including additional bosonic fields. For large systems,
the Monte Carlo algorithm proposed in Ref. [1] becomes inefficient due to a low
acceptance rate for particle moves in a fixed background multiboson field. We
show here how this problem can be circumvented by use of a coupled
particle-multiboson update procedure that improves acceptance rates on large
lattices by orders of magnitude. The method is tested on a one-component plasma
with neutral dielectric particles for a variety of system sizes.Comment: 13 pages, 2 figures, fixed typos, added reference
Local Simulation Algorithms for Coulomb Gases with Dynamical Dielectric Effects
We discuss the application of the local lattice technique of Maggs and
Rossetto to problems that involve the motion of objects with different
dielectric constants than the background. In these systems the simulation
method produces a spurious interaction force which causes the particles to move
in an unphysical manner. We show that this term can be removed using a variant
of a method known from high-energy physics simulations, the multiboson method,
and demonstrate the effectiveness of this corrective method on a system of
neutral particles. We then apply our method to a one-component plasma to show
the effect of the spurious interaction term on a charged system.Comment: 13 pages, 4 figure
High-precision Monte Carlo study of directed percolation in (d+1) dimensions
We present a Monte Carlo study of the bond and site directed (oriented)
percolation models in dimensions on simple-cubic and
body-centered-cubic lattices, with . A dimensionless ratio is
defined, and an analysis of its finite-size scaling produces improved estimates
of percolation thresholds. We also report improved estimates for the standard
critical exponents. In addition, we study the probability distributions of the
number of wet sites and radius of gyration, for .Comment: 11 pages, 21 figure
The wave-vector power spectrum of the local tunnelling density of states: ripples in a d-wave sea
A weak scattering potential imposed on a layer of a cuprate
superconductor modulates the local density of states . In recently
reported experimental studies scanning-tunneling maps of have
been Fourier transformed to obtain a wave-vector power spectrum. Here, for the
case of a weak scattering potential, we discuss the structure of this power
spectrum and its relationship to the quasi-particle spectrum and the structure
factor of the scattering potential. Examples of quasi-particle interferences in
normal metals and - and d-wave superconductors are discussed.Comment: 22 pages, 21 figures; enlarged discussion of the d-wave response, to
be published in Physical Review
Heat Conduction and Entropy Production in a One-Dimensional Hard-Particle Gas
We present large scale simulations for a one-dimensional chain of hard-point
particles with alternating masses. We correct several claims in the recent
literature based on much smaller simulations. Both for boundary conditions with
two heat baths at different temperatures at both ends and from heat current
autocorrelations in equilibrium we find heat conductivities kappa to diverge
with the number N of particles. These depended very strongly on the mass
ratios, and extrapolation to N -> infty resp. t -> infty is difficult due to
very large finite-size and finite-time corrections. Nevertheless, our data seem
compatible with a universal power law kappa ~ N^alpha with alpha approx 0.33.
This suggests a relation to the Kardar-Parisi-Zhang model. We finally show that
the hard-point gas with periodic boundary conditions is not chaotic in the
usual sense and discuss why the system, when kept out of equilibrium, leads
nevertheless to energy dissipation and entropy production.Comment: 4 pages (incl. 5 figures), RevTe
Quantity and quality of empathic responding by autistic and non-autistic adolescent girls and boys
Empathy evokes support for the person in distress, and thus strengthening social cohesion. The question is to what extent
empathic reactions can be observed in autistic adolescents and autistic girls in particular, since there is evidence that
they have better social skills than boys, which might hinder their recognition as autistic. We examined 193 adolescents
(autistic/non-autistic boys/girls) during an in vivo task in which the experimenter hurt herself. In line with our predictions,
no group or gender differences appeared related to their attention for the event; yet autistic girls and boys showed
less visible emotional arousal, indicative of less affective empathy. Autistic girls and boys reacted by comforting the
experimenter equally often as their non-autistic peers, but autistic boys seemed to address the problem more often than
any other group; while girls (autistic and non-autistic) more often addressed the emotion of the person in need. Our
findings highlight that empathic behaviour – to some extent – seems similar between autistic and non-autistic boys and
girls. However, differences exist, in terms of expressed emotional arousal and gender-specific comforting styles. Autistic
girls’ higher levels of emotion-focused comforting could be explained by well-developed social skills, camouflaging, or
emotional investment in relationships with others
Phase Transition in the Aldous-Shields Model of Growing Trees
We study analytically the late time statistics of the number of particles in
a growing tree model introduced by Aldous and Shields. In this model, a cluster
grows in continuous time on a binary Cayley tree, starting from the root, by
absorbing new particles at the empty perimeter sites at a rate proportional to
c^{-l} where c is a positive parameter and l is the distance of the perimeter
site from the root. For c=1, this model corresponds to random binary search
trees and for c=2 it corresponds to digital search trees in computer science.
By introducing a backward Fokker-Planck approach, we calculate the mean and the
variance of the number of particles at large times and show that the variance
undergoes a `phase transition' at a critical value c=sqrt{2}. While for
c>sqrt{2} the variance is proportional to the mean and the distribution is
normal, for c<sqrt{2} the variance is anomalously large and the distribution is
non-Gaussian due to the appearance of extreme fluctuations. The model is
generalized to one where growth occurs on a tree with branches and, in this
more general case, we show that the critical point occurs at c=sqrt{m}.Comment: Latex 17 pages, 6 figure
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