604 research outputs found
Shear-band arrest and stress overshoots during inhomogeneous flow in a metallic glass
At the transition from a static to a dynamic deformation regime of a shear band in bulk metallic glasses, stress transients in terms of overshoots are observed. We interpret this phenomenon with a repeated shear-melting transition and are able to access a characteristic time for a liquidlike to solidlike transition in the shear band as a function of temperature, enabling us to understand why shear bands arrest during inhomogenous serrated flow in bulk metallic glasses
A portable platform for accelerated PIC codes and its application to GPUs using OpenACC
We present a portable platform, called PIC_ENGINE, for accelerating
Particle-In-Cell (PIC) codes on heterogeneous many-core architectures such as
Graphic Processing Units (GPUs). The aim of this development is efficient
simulations on future exascale systems by allowing different parallelization
strategies depending on the application problem and the specific architecture.
To this end, this platform contains the basic steps of the PIC algorithm and
has been designed as a test bed for different algorithmic options and data
structures. Among the architectures that this engine can explore, particular
attention is given here to systems equipped with GPUs. The study demonstrates
that our portable PIC implementation based on the OpenACC programming model can
achieve performance closely matching theoretical predictions. Using the Cray
XC30 system, Piz Daint, at the Swiss National Supercomputing Centre (CSCS), we
show that PIC_ENGINE running on an NVIDIA Kepler K20X GPU can outperform the
one on an Intel Sandybridge 8-core CPU by a factor of 3.4
Low energy physical properties of high-Tc superconducting Cu oxides: A comparison between the resonating valence bond and experiments
In a recent review by Anderson and coworkers\cite{Vanilla}, it was pointed
out that an early resonating valence bond (RVB) theory is able to explain a
number of unusual properties of high temperature superconducting (SC)
Cu-oxides. Here we extend previous calculations \cite{anderson87,FC
Zhang,Randeria} to study more systematically low energy physical properties of
the plain vanilla d-wave RVB state, and to compare results with the available
experiments. We use a renormalized mean field theory combined with variational
Monte Carlo and power Lanczos methods to study the RVB state of an extended
model in a square lattice with parameters suitable for the hole doped
Cu-oxides. The physical observable quantities we study include the specific
heat, the linear residual thermal conductivity, the in-plane magnetic
penetration depth, the quasiparticle energy at the antinode , the
superconducting energy gap, the quasiparticle spectra and the Drude weight. The
traits of nodes (including , the Fermi velocity and the velocity
along Fermi surface ), as well as the SC order parameter are also
studied. Comparisons of the theory and the experiments in cuprates show an
overall qualitative agreement, especially on their doping dependences.Comment: 12 pages, 14 figures, 1 tabl
The in-plane electrodynamics of the superconductivity in Bi2Sr2CaCu2O8+d: energy scales and spectral weight distribution
The in-plane infrared and visible (3 meV-3 eV) reflectivity of
Bi2Sr2CaCu2O8+d (Bi-2212) thin films is measured between 300 K and 10 K for
different doping levels with unprecedented accuracy. The optical conductivity
is derived through an accurate fitting procedure. We study the transfer of
spectral weight from finite energy into the superfluid as the system becomes
superconducting. In the over-doped regime, the superfluid develops at the
expense of states lying below 60 meV, a conventional energy of the order of a
few times the superconducting gap. In the underdoped regime, spectral weight is
removed from up to 2 eV, far beyond any conventional scale. The intraband
spectral weight change between the normal and superconducting state, if
analyzed in terms of a change of kinetic energy is ~1 meV. Compared to the
condensation energy, this figure addresses the issue of a kinetic energy driven
mechanism.Comment: 13 pages with 9 figures include
Linearized model Fokker-Planck collision operators for gyrokinetic simulations. II. Numerical implementation and tests
A set of key properties for an ideal dissipation scheme in gyrokinetic
simulations is proposed, and implementation of a model collision operator
satisfying these properties is described. This operator is based on the exact
linearized test-particle collision operator, with approximations to the
field-particle terms that preserve conservation laws and an H-Theorem. It
includes energy diffusion, pitch-angle scattering, and finite Larmor radius
effects corresponding to classical (real-space) diffusion. The numerical
implementation in the continuum gyrokinetic code GS2 is fully implicit and
guarantees exact satisfaction of conservation properties. Numerical results are
presented showing that the correct physics is captured over the entire range of
collisionalities, from the collisionless to the strongly collisional regimes,
without recourse to artificial dissipation.Comment: 13 pages, 8 figures, submitted to Physics of Plasmas; typos fixe
Flux- and gradient-driven global gyrokinetic simulation of tokamak turbulence
The Eulerian gyrokinetic turbulence code GENE has recently been extended to a full torus code. Moreover, it now provides Krook-type sources for gradient-driven simulations where the profiles are maintained on average as well as localized heat sources for a flux-driven type of operation. Careful verification studies and benchmarks are performed successfully. This setup is applied to address three related transport issues concerning nonlocal effects. First, it is confirmed that in gradient-driven simulations, the local limit can be reproduced-provided that finite aspect ratio effects in the geometry are treated carefully. In this context, it also becomes clear that the profile widths (not the device width) may constitute a more appropriate measure for finite-size effects. Second, the nature and role of heat flux avalanches are discussed in the framework of both local and global, flux-and gradient-driven simulations. Third, simulations dedicated to discharges with electron internal barriers are addressed. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3567484
ORB5: a global electromagnetic gyrokinetic code using the PIC approach in toroidal geometry
This paper presents the current state of the global gyrokinetic code ORB5 as
an update of the previous reference [Jolliet et al., Comp. Phys. Commun. 177
409 (2007)]. The ORB5 code solves the electromagnetic Vlasov-Maxwell system of
equations using a PIC scheme and also includes collisions and strong flows. The
code assumes multiple gyrokinetic ion species at all wavelengths for the
polarization density and drift-kinetic electrons. Variants of the physical
model can be selected for electrons such as assuming an adiabatic response or a
``hybrid'' model in which passing electrons are assumed adiabatic and trapped
electrons are drift-kinetic. A Fourier filter as well as various control
variates and noise reduction techniques enable simulations with good
signal-to-noise ratios at a limited numerical cost. They are completed with
different momentum and zonal flow-conserving heat sources allowing for
temperature-gradient and flux-driven simulations. The code, which runs on both
CPUs and GPUs, is well benchmarked against other similar codes and analytical
predictions, and shows good scalability up to thousands of nodes
Remote sensing and geographic information systems: charting Sin Nombre virus infections in deer mice.
We tested environmental data from remote sensing and geographic information system maps as indicators of Sin Nombre virus (SNV) infections in deer mouse (Peromyscus maniculatus) populations in the Walker River Basin, Nevada and California. We determined by serologic testing the presence of SNV infections in deer mice from 144 field sites. We used remote sensing and geographic information systems data to characterize the vegetation type and density, elevation, slope, and hydrologic features of each site. The data retroactively predicted infection status of deer mice with up to 80% accuracy. If models of SNV temporal dynamics can be integrated with baseline spatial models, human risk for infection may be assessed with reasonable accuracy
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