9,757 research outputs found
A variance-minimization scheme for optimizing Jastrow factors
We describe a new scheme for optimizing many-electron trial wave functions by
minimizing the unreweighted variance of the energy using stochastic integration
and correlated-sampling techniques. The scheme is restricted to parameters that
are linear in the exponent of a Jastrow correlation factor, which are the most
important parameters in the wave functions we use. The scheme is highly
efficient and allows us to investigate the parameter space more closely than
has been possible before. We search for multiple minima of the variance in the
parameter space and compare the wave functions obtained using reweighted and
unreweighted variance minimization.Comment: 19 pages; 12 figure
Global persistence exponent of the double-exchange model
We obtained the global persistence exponent for a continuous spin
model on the simple cubic lattice with double-exchange interaction by using two
different methods. First, we estimated the exponent by following the
time evolution of probability that the order parameter of the model does
not change its sign up to time . Afterwards,
that exponent was estimated through the scaling collapse of the universal
function for different lattice sizes. Our results for
both approaches are in very good agreement each other.Comment: 4 pages, 3 figures, and 3 tables. To appear in Physical Review
Probing the Primordial Power Spectrum with Cluster Number Counts
We investigate how well galaxy cluster number counts can constrain the
primordial power spectrum. Measurements of the primary anisotropies in the
cosmic microwave background (CMB) may be limited, by the presence of
foregrounds from secondary sources, to probing the primordial power spectrum at
wave numbers less than about 0.30 h Mpc^{-1}. We break up the primordial power
spectrum into a number of nodes and interpolate linearly between each node.
This allows us to show that cluster number counts could then extend the
constraints on the form of the primordial power spectrum up to wave numbers of
about 0.45 h Mpc^{-1}. We estimate combinations of constraints from PLANCK and
SPT primary CMB and their respective SZ surveys. We find that their
constraining ability is limited by uncertainties in the mass scaling relations.
We also estimate the constraint from clusters detected from a SNAP like
gravitational lensing survey. As there is an unambiguous and simple
relationship between the filtered shear of the lensing survey and the cluster
mass, it may be possible to obtain much tighter constraints on the primordial
power spectrum in this case.Comment: Clarifications added and a few minor corrections made. Matches
version to appear in PR
Reaction Brownian Dynamics and the effect of spatial fluctuations on the gain of a push-pull network
Brownian Dynamics algorithms are widely used for simulating soft-matter and
biochemical systems. In recent times, their application has been extended to
the simulation of coarse-grained models of cellular networks in simple
organisms. In these models, components move by diffusion, and can react with
one another upon contact. However, when reactions are incorporated into a
Brownian Dynamics algorithm, attention must be paid to avoid violations of the
detailed-balance rule, and therefore introducing systematic errors in the
simulation. We present a Brownian Dynamics algorithm for reaction-diffusion
systems that rigorously obeys detailed balance for equilibrium reactions. By
comparing the simulation results to exact analytical results for a bimolecular
reaction, we show that the algorithm correctly reproduces both equilibrium and
dynamical quantities. We apply our scheme to a ``push-pull'' network in which
two antagonistic enzymes covalently modify a substrate. Our results highlight
that the diffusive behaviour of the reacting species can reduce the gain of the
response curve of this network.Comment: 25 pages, 7 figures, submitted to Journal of Chemical Physic
Structural signatures of the unjamming transition at zero temperature
We study the pair correlation function for zero-temperature,
disordered, soft-sphere packings just above the onset of jamming. We find
distinct signatures of the transition in both the first and split second peaks
of this function. As the transition is approached from the jammed side (at
higher packing fraction) the first peak diverges and narrows on the small-
side to a delta-function. On the high- side of this peak, decays as a
power-law. In the split second peak, the two subpeaks are both singular at the
transition, with power-law behavior on their low- sides and step-function
drop-offs on their high- sides. These singularities at the transition are
reminiscent of empirical criteria that have previously been used to distinguish
glassy structures from liquid ones.Comment: 8 pages, 13 figure
A quantum Peierls-Nabarro barrier
Kink dynamics in spatially discrete nonlinear Klein-Gordon systems is
considered. For special choices of the substrate potential, such systems
support continuous translation orbits of static kinks with no (classical)
Peierls-Nabarro barrier. It is shown that these kinks experience, nevertheless,
a lattice-periodic confining potential, due to purely quantum effects anaolgous
to the Casimir effect of quantum field theory. The resulting ``quantum
Peierls-Nabarro potential'' may be calculated in the weak coupling
approximation by a simple and computationally cheap numerical algorithm, which
is applied, for purposes of illustration, to a certain two-parameter family of
substrates.Comment: 13 pages LaTeX, 7 figure
A Solution of the Maxwell-Dirac Equations in 3+1 Dimensions
We investigate a class of localized, stationary, particular numerical
solutions to the Maxwell-Dirac system of classical nonlinear field equations.
The solutions are discrete energy eigenstates bound predominantly by the
self-produced electric field.Comment: 12 pages, revtex, 2 figure
Degeneracy measures for the algebraic classification of numerical spacetimes
We study the issue of algebraic classification of the Weyl curvature tensor,
with a particular focus on numerical relativity simulations. The spacetimes of
interest in this context, binary black hole mergers, and the ringdowns that
follow them, present subtleties in that they are generically, strictly
speaking, Type I, but in many regions approximately, in some sense, Type D. To
provide meaning to any claims of "approximate" Petrov class, one must define a
measure of degeneracy on the space of null rays at a point. We will investigate
such a measure, used recently to argue that certain binary black hole merger
simulations ring down to the Kerr geometry, after hanging up for some time in
Petrov Type II. In particular, we argue that this hangup in Petrov Type II is
an artefact of the particular measure being used, and that a geometrically
better-motivated measure shows a black hole merger produced by our group
settling directly to Petrov Type D.Comment: 14 pages, 7 figures. Version 2 adds two references
Stable resonances and signal propagation in a chaotic network of coupled units
We apply the linear response theory developed in \cite{Ruelle} to analyze how
a periodic signal of weak amplitude, superimposed upon a chaotic background, is
transmitted in a network of non linearly interacting units. We numerically
compute the complex susceptibility and show the existence of specific poles
(stable resonances) corresponding to the response to perturbations transverse
to the attractor. Contrary to the poles of correlation functions they depend on
the pair emitting/receiving units. This dynamic differentiation, induced by non
linearities, exhibits the different ability that units have to transmit a
signal in this network.Comment: 10 pages, 3 figures, to appear in Phys. rev.
Effects of P-wave Annihilation on the Angular Power Spectrum of Extragalactic Gamma-rays from Dark Matter Annihilation
We present a formalism for estimating the angular power spectrum of
extragalactic gamma-rays produced by dark matter annihilating with any general
velocity-dependent cross section. The relevant density and velocity
distribution of dark matter is modeled as an ensemble of smooth, universal,
rigid, disjoint, spherical halos with distribution and universal properties
constrained by simulation data. We apply this formalism to theories of dark
matter with p-wave annihilation, for which the relative-velocity-weighted
annihilation cross section is \sigma v=a+bv^2. We determine that this
significantly increases the gamma-ray power if b/a >> 10^6. The effect of
p-wave annihilation on the angular power spectrum is very similar for the
sample of particle physics models we explored, suggesting that the important
effect for a given b/a is largely determined by the cosmic dark matter
distribution. If the dark matter relic from strong p-wave theories is thermally
produced, the intensities of annihilation gamma-rays are strongly p-wave
suppressed, making them difficult to observe. If an angular power spectrum
consistent with a strong p-wave were to be observed, it would likely indicate
non-thermal production of dark matter in the early Universe.Comment: 20 pages, 3 figure
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