5,633 research outputs found
SHARP: A Spatially Higher-order, Relativistic Particle-in-Cell Code
Numerical heating in particle-in-cell (PIC) codes currently precludes the
accurate simulation of cold, relativistic plasma over long periods, severely
limiting their applications in astrophysical environments. We present a
spatially higher-order accurate relativistic PIC algorithm in one spatial
dimension, which conserves charge and momentum exactly. We utilize the
smoothness implied by the usage of higher-order interpolation functions to
achieve a spatially higher-order accurate algorithm (up to fifth order). We
validate our algorithm against several test problems -- thermal stability of
stationary plasma, stability of linear plasma waves, and two-stream instability
in the relativistic and non-relativistic regimes. Comparing our simulations to
exact solutions of the dispersion relations, we demonstrate that SHARP can
quantitatively reproduce important kinetic features of the linear regime. Our
simulations have a superior ability to control energy non-conservation and
avoid numerical heating in comparison to common second-order schemes. We
provide a natural definition for convergence of a general PIC algorithm: the
complement of physical modes captured by the simulation, i.e., those that lie
above the Poisson noise, must grow commensurately with the resolution. This
implies that it is necessary to simultaneously increase the number of particles
per cell and decrease the cell size. We demonstrate that traditional ways for
testing for convergence fail, leading to plateauing of the energy error. This
new PIC code enables us to faithfully study the long-term evolution of plasma
problems that require absolute control of the energy and momentum conservation.Comment: 26 pages, 19 figures, discussion about performance is added,
published in Ap
Spectroscopic Evidence for Multiple Order Parameter Components in the Heavy Fermion Superconductor CeCoIn_5
Point-contact spectroscopy was performed on single crystals of the
heavy-fermion superconductor CeCoIn_5 between 150 mK and 2.5 K. A pulsed
measurement technique ensured minimal Joule heating over a wide voltage range.
The spectra show Andreev-reflection characteristics with multiple structures
which depend on junction impedance. Spectral analysis using the generalized
Blonder-Tinkham-Klapwijk formalism for d-wave pairing revealed two coexisting
order parameter components, with amplitudes Delta_1 = 0.95 +/- 0.15 meV and
Delta_2 = 2.4 +/- 0.3 meV, which evolve differently with temperature. Our
observations indicate a highly unconventional pairing mechanism, possibly
involving multiple bands.Comment: 4 pages, 3 figure
Multi-dimensional numerical simulations of type Ia supernova explosions
The major role type Ia supernovae play in many fields of astrophysics and in
particular in cosmological distance determinations calls for self-consistent
models of these events. Since their mechanism is believed to crucially depend
on phenomena that are inherently three-dimensional, self-consistent numerical
models of type Ia supernovae must be multi-dimensional. This field has recently
seen a rapid development, which is reviewed in this article. The different
modeling approaches are discussed and as an illustration a particular explosion
model -- the deflagration model -- in a specific numerical implementation is
presented in greater detail. On this exemplary case, the procedure of
validating the model on the basis of comparison with observations is discussed
as well as its application to study questions arising from type Ia supernova
cosmology.Comment: 30 pages, 7 figures (Fig. 6 with reduced resolution
Surface Extraction from Neural Unsigned Distance Fields
We propose a method, named DualMesh-UDF, to extract a surface from unsigned
distance functions (UDFs), encoded by neural networks, or neural UDFs. Neural
UDFs are becoming increasingly popular for surface representation because of
their versatility in presenting surfaces with arbitrary topologies, as opposed
to the signed distance function that is limited to representing a closed
surface. However, the applications of neural UDFs are hindered by the notorious
difficulty in extracting the target surfaces they represent. Recent methods for
surface extraction from a neural UDF suffer from significant geometric errors
or topological artifacts due to two main difficulties: (1) A UDF does not
exhibit sign changes; and (2) A neural UDF typically has substantial
approximation errors. DualMesh-UDF addresses these two difficulties.
Specifically, given a neural UDF encoding a target surface to be
recovered, we first estimate the tangent planes of at a set of sample
points close to . Next, we organize these sample points into local
clusters, and for each local cluster, solve a linear least squares problem to
determine a final surface point. These surface points are then connected to
create the output mesh surface, which approximates the target surface. The
robust estimation of the tangent planes of the target surface and the
subsequent minimization problem constitute our core strategy, which contributes
to the favorable performance of DualMesh-UDF over other competing methods. To
efficiently implement this strategy, we employ an adaptive Octree. Within this
framework, we estimate the location of a surface point in each of the octree
cells identified as containing part of the target surface. Extensive
experiments show that our method outperforms existing methods in terms of
surface reconstruction quality while maintaining comparable computational
efficiency.Comment: ICCV 202
Controlling the energy of defects and interfaces in the amplitude expansion of the phase-field crystal model
One of the major difficulties in employing phase field crystal (PFC) modeling
and the associated amplitude (APFC) formulation is the ability to tune model
parameters to match experimental quantities. In this work we address the
problem of tuning the defect core and interface energies in the APFC
formulation. We show that the addition of a single term to the free energy
functional can be used to increase the solid-liquid interface and defect
energies in a well-controlled fashion, without any major change to other
features. The influence of the newly added term is explored in two-dimensional
triangular and honeycomb structures as well as bcc and fcc lattices in three
dimensions. In addition, a finite element method (FEM) is developed for the
model that incorporates a mesh refinement scheme. The combination of the FEM
and mesh refinement to simulate amplitude expansion with a new energy term
provides a method of controlling microscopic features such as defect and
interface energies while simultaneously delivering a coarse-grained examination
of the system.Comment: 14 pages, 9 figure
Nonlinear diffusion & thermo-electric coupling in a two-variable model of cardiac action potential
This work reports the results of the theoretical investigation of nonlinear
dynamics and spiral wave breakup in a generalized two-variable model of cardiac
action potential accounting for thermo-electric coupling and diffusion
nonlinearities. As customary in excitable media, the common Q10 and Moore
factors are used to describe thermo-electric feedback in a 10-degrees range.
Motivated by the porous nature of the cardiac tissue, in this study we also
propose a nonlinear Fickian flux formulated by Taylor expanding the voltage
dependent diffusion coefficient up to quadratic terms. A fine tuning of the
diffusive parameters is performed a priori to match the conduction velocity of
the equivalent cable model. The resulting combined effects are then studied by
numerically simulating different stimulation protocols on a one-dimensional
cable. Model features are compared in terms of action potential morphology,
restitution curves, frequency spectra and spatio-temporal phase differences.
Two-dimensional long-run simulations are finally performed to characterize
spiral breakup during sustained fibrillation at different thermal states.
Temperature and nonlinear diffusion effects are found to impact the
repolarization phase of the action potential wave with non-monotone patterns
and to increase the propensity of arrhythmogenesis
Assembly Bias and Splashback in Galaxy Clusters
We use publicly available data for the Millennium Simulation to explore the
implications of the recent detection of assembly bias and splashback signatures
in a large sample of galaxy clusters. These were identified in the SDSS/DR8
photometric data by the redMaPPer algorithm and split into high- and
low-concentration subsamples based on the projected positions of cluster
members. We use simplified versions of these procedures to build cluster
samples of similar size from the simulation data. These match the observed
samples quite well and show similar assembly bias and splashback signals.
Previous theoretical work has found the logarithmic slope of halo density
profiles to have a well-defined minimum whose depth decreases and whose radius
increases with halo concentration. Projected profiles for the observed and
simulated cluster samples show trends with concentration which are opposite to
these predictions. In addition, for high-concentration clusters the minimum
slope occurs at significantly smaller radius than predicted. We show that these
discrepancies all reflect confusion between splashback features and features
imposed on the profiles by the cluster identification and concentration
estimation procedures. The strong apparent assembly bias is not reflected in
the three-dimensional distribution of matter around clusters. Rather it is a
consequence of the preferential contamination of low-concentration clusters by
foreground or background groups.Comment: 17 pages, 16 figures, 3 tables, accepted versio
Elastic and inelastic diffraction of fast atoms,\linebreak Debye-Waller factor and M\"{o}ssbauer-Lamb-Dicke regime
The diffraction of fast atoms at crystal surfaces is ideal for a detailed
investigation of the surface electronic density. However, instead of sharp
diffraction spots, most experiments show elongated streaks characteristic of
inelastic diffraction. This paper describes these inelastic profiles in terms
of individual inelastic collisions with surface atoms taking place along the
projectile trajectory and leading to vibrational excitation of the local Debye
oscillator. A quasi-elastic regime where only one inelastic event contributes
is identified as well as a mixed quantum-classical regime were several
inelastic collision are involved. These regimes describe a smooth evolution of
the scattering profiles from sharp spots to elongated streaks merging
progressively into the classical diffusion regime
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