6,357 research outputs found
Benchmark problems for continuum radiative transfer. High optical depths, anisotropic scattering, and polarisation
Solving the continuum radiative transfer equation in high opacity media
requires sophisticated numerical tools. In order to test the reliability of
such tools, we present a benchmark of radiative transfer codes in a 2D disc
configuration. We test the accuracy of seven independently developed radiative
transfer codes by comparing the temperature structures, spectral energy
distributions, scattered light images, and linear polarisation maps that each
model predicts for a variety of disc opacities and viewing angles. The test
cases have been chosen to be numerically challenging, with midplane optical
depths up 10^6, a sharp density transition at the inner edge and complex
scattering matrices. We also review recent progress in the implementation of
the Monte Carlo method that allow an efficient solution to these kinds of
problems and discuss the advantages and limitations of Monte Carlo codes
compared to those of discrete ordinate codes. For each of the test cases, the
predicted results from the radiative transfer codes are within good agreement.
The results indicate that these codes can be confidently used to interpret
present and future observations of protoplanetary discs.Comment: 15 pages, 10 figures, accepted for publication in A&
The finite-temperature Monte Carlo method and its application to superfluid helium clusters
We review the use of the path integral Monte Carlo (PIMC) methodology to the
study of finite-size quantum clusters, with particular emphasis on recent
applications to pure and impurity-doped He clusters. We describe the principles
of PIMC, the use of the multilevel Metropolis method for sampling particle
permutations, and the methods used to accurately incorporate anisotropic
molecule-helium interactions into the path integral scheme. Applications to
spectroscopic studies of embedded atoms and molecules are summarized, with
discussion of the new concepts of local and nanoscale superfluidity that have
been generated by recent PIMC studies of the impurity-doped He clusters.Comment: P. Huang, Y. Kwon, and K. B. Whaley, in "Quantum Fluids in
Confinement", Vol. 4 of "Advances in Quantum Many-Body Theories", edited by
E. Krotscheck and J. Navarro (World Scientific, Singapore, 2002), in pres
Kinetics of electron-positron pair plasmas using an adaptive Monte Carlo method
A new algorithm for implementing the adaptive Monte Carlo method is given. It
is used to solve the relativistic Boltzmann equations that describe the time
evolution of a nonequilibrium electron-positron pair plasma containing
high-energy photons and pairs. The collision kernels for the photons as well as
pairs are constructed for Compton scattering, pair annihilation and creation,
bremsstrahlung, and Bhabha & Moller scattering. For a homogeneous and isotropic
plasma, analytical equilibrium solutions are obtained in terms of the initial
conditions. For two non-equilibrium models, the time evolution of the photon
and pair spectra is determined using the new method. The asymptotic numerical
solutions are found to be in a good agreement with the analytical equilibrium
states. Astrophysical applications of this scheme are discussed.Comment: 43 pages, 7 postscript figures, to appear in the Astrophysical
Journa
Criticality in coupled quantum spin-chains with competing ladder-like and two-dimensional couplings
Motivated by the geometry of spins in the material CaCuO, we study a
two-layer, spin-half Heisenberg model, with nearest-neighbor exchange couplings
J and \alpha*J along the two axes in the plane and a coupling J_\perp
perpendicular to the planes. We study these class of models using the
Stochastic Series Expansion (SSE) Quantum Monte Carlo simulations at finite
temperatures and series expansion methods at T=0. The critical value of the
interlayer coupling, J_\perp^c, separating the N{\'e}el ordered and disordered
ground states, is found to follow very closely a square root dependence on
. Both T=0 and finite-temperature properties of the model are
presented.Comment: 9 pages, 11 figs., 1 tabl
Analysis of Sample Correlations for Monte Carlo Rendering
Modern physically based rendering techniques critically depend on approximating integrals of high dimensional functions representing radiant light energy. Monte Carlo based integrators are the choice for complex scenes and effects. These integrators work by sampling the integrand at sample point locations. The distribution of these sample points determines convergence rates and noise in the final renderings. The characteristics of such distributions can be uniquely represented in terms of correlations of sampling point locations. Hence, it is essential to study these correlations to understand and adapt sample distributions for low error in integral approximation. In this work, we aim at providing a comprehensive and accessible overview of the techniques developed over the last decades to analyze such correlations, relate them to error in integrators, and understand when and how to use existing sampling algorithms for effective rendering workflows.publishe
Is the Yb2Ti2O7 pyrochlore a quantum spin ice?
We use numerical linked cluster (NLC) expansions to compute the specific
heat, C(T), and entropy, S(T), of a quantum spin ice model of Yb2Ti2O7 using
anisotropic exchange interactions recently determined from inelastic neutron
scattering measurements and find good agreement with experimental calorimetric
data. In the perturbative weak quantum regime, this model has a ferrimagnetic
ordered ground state, with two peaks in C(T): a Schottky anomaly signalling the
paramagnetic to spin ice crossover followed at lower temperature by a sharp
peak accompanying a first order phase transition to the ferrimagnetic state. We
suggest that the two C(T) features observed in Yb2Ti2O7 are associated with the
same physics. Spin excitations in this regime consist of weakly confined
spinon-antispinon pairs. We suggest that conventional ground state with exotic
quantum dynamics will prove a prevalent characteristic of many real quantum
spin ice materials.Comment: 8 pages (two-column), 9 figure
End-to-end Sampling Patterns
Sample patterns have many uses in Computer Graphics, ranging from procedural
object placement over Monte Carlo image synthesis to non-photorealistic
depiction. Their properties such as discrepancy, spectra, anisotropy, or
progressiveness have been analyzed extensively. However, designing methods to
produce sampling patterns with certain properties can require substantial
hand-crafting effort, both in coding, mathematical derivation and compute time.
In particular, there is no systematic way to derive the best sampling algorithm
for a specific end-task.
Tackling this issue, we suggest another level of abstraction: a toolkit to
end-to-end optimize over all sampling methods to find the one producing
user-prescribed properties such as discrepancy or a spectrum that best fit the
end-task. A user simply implements the forward losses and the sampling method
is found automatically -- without coding or mathematical derivation -- by
making use of back-propagation abilities of modern deep learning frameworks.
While this optimization takes long, at deployment time the sampling method is
quick to execute as iterated unstructured non-linear filtering using radial
basis functions (RBFs) to represent high-dimensional kernels. Several important
previous methods are special cases of this approach, which we compare to
previous work and demonstrate its usefulness in several typical Computer
Graphics applications. Finally, we propose sampling patterns with properties
not shown before, such as high-dimensional blue noise with projective
properties
Computational spectroscopy of helium-solvated molecules: effective inertia, from small He clusters toward the nano-droplet regime
Accurate computer simulations of the rotational dynamics of linear molecules
solvated in He clusters indicate that the large-size (nano-droplet) regime is
attained quickly for light rotors (HCN, CO) and slowly for heavy ones (OCS,
NO, CO), thus challenging previously reported results. Those results
spurred the view that the different behavior of light rotors with respect to
heavy ones - including a smaller reduction of inertia upon solvation of the
former - would result from the lack of adiabatic following of the He density
upon molecular rotation. We have performed computer experiments in which the
rotational dynamics of OCS and HCN molecules was simulated using a fictitious
inertia appropriate to the other molecule. These experiments indicate that the
approach to the nano-droplet regime, as well as the reduction of the molecular
inertia upon solvation, is determined by the anistropy of the potential, more
than by the molecular weight. Our findings are in agreement with recent
infrared and/or microwave experimental data which, however, are not yet totally
conclusive by themselves.Comment: 11 pages, 13 figure
Power spectrum of the maxBCG sample: detection of acoustic oscillations using galaxy clusters
We use the direct Fourier method to calculate the redshift-space power
spectrum of the maxBCG cluster catalog -- currently by far the largest existing
galaxy cluster sample. The total number of clusters used in our analysis is
12,616. After accounting for the radial smearing effect caused by photometric
redshift errors and also introducing a simple treatment for the nonlinear
effects, we show that currently favored low matter density "concordance" LCDM
cosmology provides a very good fit to the estimated power. Thanks to the large
volume (~0.4 h^{-3}Gpc^{3}), high clustering amplitude (linear effective bias
parameter b_{eff} ~3x(0.85/sigma_8)), and sufficiently high sampling density
(~3x10^{-5} h^{3}Mpc^{-3}) the recovered power spectrum has high enough signal
to noise to allow us to find evidence (~2 sigma CL) for the baryonic acoustic
oscillations (BAO). In case the clusters are additionally weighted by their
richness the resulting power spectrum has slightly higher large-scale amplitude
and smaller damping on small scales. As a result the confidence level for the
BAO detection is somewhat increased: ~2.5 sigma. The ability to detect BAO with
relatively small number of clusters is encouraging in the light of several
proposed large cluster surveys.Comment: MNRAS accepted, extended analysis of arXiv:0705.1843, 15 page
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