Bose-Glass behaviour in Bi_{2}Sr_{2}Ca_{1-x}Y_{x}Cu_{2}O_{8} crystals with columnar defects: experimental evidence for variable-range hopping

We report on vortex transport in Bi_{2}Sr_{2}Ca_{1-x}Y_{x}Cu_{2}O_{8} crystals irradiated at different doses of heavy ions. We show evidence of a flux-creep resistivity typical of a variable-range vortex hopping mechanism as predicted by Nelson and Vinokur.Comment: 5 pages LaTeX2e (uses elsart.cls), 1 Encapsulated PostScript figur

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

Nonlinear Low Noise Particle-in-cell Simulations of Electron Temperature Gradient Driven Turbulence

In this Letter, it is shown that global, nonlinear, particle-in-cell (PIC) simulations of electron temperature driven turbulence recover the same level of transport as flux-tube codes when the level of statistical noise, associated with the PIC discretization, is sufficiently small. An efficient measure of the signal-to-noise ratio, applicable to every PIC code, is introduced. This diagnostic provides a direct measure of the quality of PIC simulations and allows for the validation of analytical estimates of the numerical noise. Global simulations for values of rho(*)(e)< 1/450 (normalized electron gyroradius) show no evidence of a gyro-Bohm scaling. (c) 2007 American Institute of Physics