Computing Nearly Singular Solutions Using Pseudo-Spectral Methods

In this paper, we investigate the performance of pseudo-spectral methods in computing nearly singular solutions of fluid dynamics equations. We consider two different ways of removing the aliasing errors in a pseudo-spectral method. The first one is the traditional 2/3 dealiasing rule. The second one is a high (36th) order Fourier smoothing which keeps a significant portion of the Fourier modes beyond the 2/3 cut-off point in the Fourier spectrum for the 2/3 dealiasing method. Both the 1D Burgers equation and the 3D incompressible Euler equations are considered. We demonstrate that the pseudo-spectral method with the high order Fourier smoothing gives a much better performance than the pseudo-spectral method with the 2/3 dealiasing rule. Moreover, we show that the high order Fourier smoothing method captures about $12 \sim 15$ more effective Fourier modes in each dimension than the 2/3 dealiasing method. For the 3D Euler equations, the gain in the effective Fourier codes for the high order Fourier smoothing method can be as large as 20% over the 2/3 dealiasing method. Another interesting observation is that the error produced by the high order Fourier smoothing method is highly localized near the region where the solution is most singular, while the 2/3 dealiasing method tends to produce oscillations in the entire domain. The high order Fourier smoothing method is also found be very stable dynamically. No high frequency instability has been observed.Comment: 26 pages, 23 figure

Classification-driven search for effective sm partitioning in multitasking GPUs

Graphics processing units (GPUs) feature an increasing number of streaming multiprocessors (SMs) with each successive generation. At the same time, GPUs are increasingly widely adopted in cloud services and data centers to accelerate general-purpose workloads. Running multiple applications on a GPU in such environments requires effective multitasking support. Spatial multitasking in which independent applications co-execute on different sets of SMs is a promising solution to share GPU resources. Unfortunately, how to effectively partition SMs is an open problem. In this paper, we observe that compared to widely-used even partitioning, dynamic SM partitioning based on the characteristics of the co-executing applications can significantly improve performance and power efficiency. Unfortunately, finding an effective SM partition is challenging because the number of possible combinations increases exponentially with the number of SMs and co-executing applications. Through offline analysis, we find that first classifying workloads, and then searching an effective SM partition based on the workload characteristics can significantly reduce the search space, making dynamic SM partitioning tractable. Based on these insights, we propose Classification-Driven search (CD-search) for low-overhead dynamic SM partitioning in multitasking GPUs. CD-search first classifies workloads using a novel off-SM bandwidth model, after which it enters the performance mode or power mode depending on the workload's characteristics. Both modes follow a specific search strategy to quickly determine the optimum SM partition. Our evaluation shows that CD-search improves system throughput by 10.4% on average (and up to 62.9%) over even partitioning for workloads that are classified for the performance mode. For workloads classified for the power mode, CD-search reduces power consumption by 25% on average (and up to 41.2%). CD-search incurs limited runtime overhead

First observation of spin-helical Dirac fermions and topological phases in undoped and doped Bi2Te3 demonstrated by spin-ARPES spectroscopy

Electron systems that possess light-like dispersion relations or the conical Dirac spectrum, such as graphene and bismuth, have recently been shown to harbor unusual collective states in high magnetic fields. Such states are possible because their light-like electrons come in spin pairs that are chiral,which means that their direction of propagation is tied to a quantity called pseudospin that describes their location in the crystal lattice. An emerging direction in quantum materials research is the manipulation of atomic spin-orbit coupling to simulate the effect of a spin dependent magnetic field,in attempt to realize novel spin phases of matter. This effect has been proposed to realize systems consisting of unpaired Dirac cones that are helical, meaning their direction of propagation is tied to the electron spin itself, which are forbidden to exist in graphene or bismuth. The experimental existence of topological order can not be determined without spin-resolved measurements. Here we report a spin-and angle-resolved photoemission study of the hexagonal surface of the Bi2Te3 and Bi{2-x}MnxTe3 series, which is found to exhibit a single helical Dirac cone that is fully spin-polarized. Our observations of a gap in the bulk spin-degenerate band and a spin-resolved surface Dirac node close to the chemical potential show that the low energy dynamics of Bi2Te3 is dominated by the unpaired spin-helical Dirac modes. Our spin-texture measurements prove the existence of a rare topological phase in this materials class for the first time, and suggest its suitability for novel 2D Dirac spin device applications beyond the chiral variety or traditional graphene.Comment: 13 pages, 4 figure

Topological Control: Systematic control of topological insulator Dirac fermion density on the surface of Bi2Te3

Three dimensional (3D) topological insulators are quantum materials with a spin-orbit induced bulk insulating gap that exhibit quantum-Hall-like phenomena in the absence of applied magnetic fields. The proposed applications of topological insulators in device geometries rely on the ability to tune the chemical potential on their surfaces in the vicinity of the Dirac node. Here, we demonstrate a suite of surface control methods based on a combination of photo-doping and molecular-doping to systematically tune the Dirac fermion density on the topological (111) surface of Bi2Te3. Their efficacy is demonstrated via direct electronic structure topology measurements using high resolution angle-resolved photoemission spectroscopy (ARPES). These results open up new opportunities for probing topological behavior of Dirac electrons on the Bi2Te3 surface. At least one of the methods demonstrated here can be successfully applied to other topological insulators such as the Bi{1-x}Sb{x}, Sb2Te3 and Bi2Se3 which will be shown elsewhere. More importantly, our methods of topological surface state manipulation demonstrated here are highly suitable for future spectroscopic studies of topological phenomena which will complement the transport results gained from the traditional electrical gating techniques.Comment: 4 Figures, 12 page

The UNC-Wisconsin rhesus macaque neurodevelopment database: A structural MRI and DTI database of early postnatal development

Rhesus macaques are commonly used as a translational animal model in neuroimaging and neurodevelopmental research. In this report, we present longitudinal data from both structural and diffusion MRI images generated on a cohort of 34 typically developing monkeys from 2 weeks to 36 months of age. All images have been manually skull stripped and are being made freely available via an online repository for use by the research community

Variability and multi-periodic oscillations in the X-ray light curve of the classical nova V4743 Sgr

The classical nova V4743 Sgr was observed with XMM-Newton for about 10 hours on April 4 2003, 6.5 months after optical maximum. At this time, this nova had become the brightest supersoft X-ray source ever observed. We present the results of a time series analysis performed on the X-ray light curve obtained in this observation, and in a previous shorter observation done with Chandra 16 days earlier. Intense variability, with amplitude as large as 40% of the total flux, was observed both times. Similarities can be found between the two observations in the structure of the variations. Most of the variability is well represented as a combination of oscillations at a set of discrete frequencies lower than 1.7 mHz. At least five frequencies are constant over the 16 day time interval between the two observations. We suggest that a periods in the power spectrum of both light curves at the frequency of 0.75 mHz and its first harmonic are related to the spin period of the white dwarf in the system, and that other observed frequencies are signatures of nonradial white dwarf pulsations. A possible signal with a 24000 sec period is also found in the XMM-Newton light curve: a cycle and a half are clearly identified. This period is consistent with the 24278 s periodicity discovered in the optical light curve of the source and thought to be the orbital period of the nova binary system.Comment: In press in Monthly Notices of the Royal Astronomical Societ

Discrete kink dynamics in hydrogen-bonded chains I: The one-component model

We study topological solitary waves (kinks and antikinks) in a nonlinear one-dimensional Klein-Gordon chain with the on-site potential of a double-Morse type. This chain is used to describe the collective proton dynamics in quasi-one-dimensional networks of hydrogen bonds, where the on-site potential plays role of the proton potential in the hydrogen bond. The system supports a rich variety of stationary kink solutions with different symmetry properties. We study the stability and bifurcation structure of all these stationary kink states. An exactly solvable model with a piecewise ``parabola-constant'' approximation of the double-Morse potential is suggested and studied analytically. The dependence of the Peierls-Nabarro potential on the system parameters is studied. Discrete travelling-wave solutions of a narrow permanent profile are shown to exist, depending on the anharmonicity of the Morse potential and the cooperativity of the hydrogen bond (the coupling constant of the interaction between nearest-neighbor protons).Comment: 12 pages, 20 figure