6,374 research outputs found
Mathematizing C++ concurrency
Shared-memory concurrency in C and C++ is pervasive in systems programming, but has long been poorly defined. This motivated an ongoing shared effort by the standards committees to specify concurrent behaviour in the next versions of both languages. They aim to provide strong guarantees for race-free programs, together with new (but subtle) relaxed-memory atomic primitives for high-performance concurrent code. However, the current draft standards, while the result of careful deliberation, are not yet clear and rigorous definitions, and harbour substantial problems in their details.
In this paper we establish a mathematical (yet readable) semantics for C++ concurrency. We aim to capture the intent of the current (`Final Committee') Draft as closely as possible, but discuss changes that fix many of its problems. We prove that a proposed x86 implementation of the concurrency primitives is correct with respect to the x86-TSO model, and describe our Cppmem tool for exploring the semantics of examples, using code generated from our Isabelle/HOL definitions.
Having already motivated changes to the draft standard, this work will aid discussion of any further changes, provide a correctness condition for compilers, and give a much-needed basis for analysis and verification of concurrent C and C++ programs
Wavefunction extended Lagrangian Born-Oppenheimer molecular dynamics
Extended Lagrangian Born-Oppenheimer molecular dynamics [Niklasson, Phys.
Rev. Lett. 100 123004 (2008)] has been generalized to the propagation of the
electronic wavefunctions. The technique allows highly efficient first
principles molecular dynamics simulations using plane wave pseudopotential
electronic structure methods that are stable and energy conserving also under
incomplete and approximate self-consistency convergence. An implementation of
the method within the planewave basis set is presented and the accuracy and
efficiency is demonstrated both for semi-conductor and metallic materials.Comment: 6 pages, 3 figure
Reactions of Ar+, Ne+, and He+ with SiF4 from thermal energy to 50 eV c.m.
Journal ArticleGuided ion-beam techniques are used to measure the cross sections for reaction of SiF4 with Ar+, Ne+, and He+ from thermal to 50 eV. Charge transfer followed by loss of F atoms are the sole processes observed. All SiFx + (x = 0-4) products are observed, except for SiF4 + from reaction with Ne+ and He+, and Si+ from reaction with Ar+
Energetics and dynamics in the reaction of Si+ with SiF4. Thermochemistry of SiFx and SiFx+ (x=1,2,3)
Journal ArticleThe title reaction is studied using guided ion beam mass spectrometry. Absolute reaction cross sections are measured as a function of kinetic energy from thermal to 40 eV, and three endothermic product channels are observed. The dominant SiF+ + SiF3 channel is only slightly endothermic, while the SiF3+ + SiF and SiF2+ + SiF2 channels have much higher thresholds. The SiF3 + cross section magnitude is about half that of SiF+, while the SiF2+ cross section is an order of magnitude smaller than that of SiF+
Observations of Subarcsecond Bright Dots in the Transition Region above Sunspots with the Interface Region Imaging Spectrograph
Observations with the Interface Region Imaging Spectrograph (IRIS) have
revealed numerous sub-arcsecond bright dots in the transition region above
sunspots. These bright dots are seen in the 1400\AA{} and 1330\AA{} slit-jaw
images. They are clearly present in all sunspots we investigated, mostly in the
penumbrae, but also occasionally in some umbrae and light bridges. The bright
dots in the penumbrae typically appear slightly elongated, with the two
dimensions being 300--600 km and 250--450 km, respectively. The long sides of
these dots are often nearly parallel to the bright filamentary structures in
the penumbrae but sometimes clearly deviate from the radial direction. Their
lifetimes are mostly less than one minute, although some dots last for a few
minutes or even longer. Their intensities are often a few times stronger than
the intensities of the surrounding environment in the slit-jaw images. About
half of the bright dots show apparent movement with speeds of
10--40~km~s in the radial direction. Spectra of a few bright dots
were obtained and the Si~{\sc{iv}}~1402.77\AA{} line profiles in these dots are
significantly broadened. The line intensity can be enhanced by one to two
orders of magnitude. Some relatively bright and long-lasting dots are also
observed in several passbands of the Atmospheric Imaging Assembly onboard the
Solar Dynamics Observatory, and they appear to be located at the bases of
loop-like structures. Many of these bright dots are likely associated with
small-scale energy release events at the transition region footpoints of
magnetic loops.Comment: 5 figures, will appear in ApJ
Vanadium dioxide : A Peierls-Mott insulator stable against disorder
Vanadium dioxide undergoes a first order metal-insulator transition at 340 K.
In this work, we develop and carry out state of the art linear scaling DFT
calculations refined with non-local dynamical mean-field theory. We identify a
complex mechanism, a Peierls-assisted orbital selection Mott instability, which
is responsible for the insulating M phase, and furthermore survives a
moderate degree of disorder.Comment: 5 pages, 4 figures. Supplementary material 8 pages, 4 figures. This
version (v2) matches that accepted for Physical Review Letters on 16th May
201
Inadequate supply of vitamins and DHA in the elderly: Implications for brain aging and Alzheimer-type dementia
AbstractAlzheimer's disease (AD) is the most prevalent, severe, and disabling cause of dementia worldwide. To date, AD therapy is primarily targeted toward palliative treatment of symptoms rather than prevention of disease progression. So far, no pharmacologic interventions have changed the onset or progression of AD and their use is accompanied by side effects. The major obstacle in managing AD and designing therapeutic strategies is the difficulty in retarding neuronal loss in the diseased brain once the pathologic events leading to neuronal death have started. Therefore, a promising alternative strategy is to maintain a healthy neuronal population in the aging brain for as long as possible. One factor evidently important for neuronal health and function is the optimal supply of nutrients necessary for maintaining normal functioning of the brain. Mechanistic studies, epidemiologic analyses, and randomized controlled intervention trials provide insight to the positive effects of docosahexaenoic acid (DHA) and micronutrients such as the vitamin B family, and vitamins E, C, and D, in helping neurons to cope with aging. These nutrients are inexpensive in use, have virtually no side effects when used at recommended doses, are essential for life, have established modes of action, and are broadly accepted by the general public. This review provides some evidence that the use of vitamins and DHA for the aging population in general, and for individuals at risk in particular, is a viable alternative approach to delaying brain aging and for protecting against the onset of AD pathology
Hydropower production and river rehabilitation: A case study on an alpine river
Despite the numerous benefits of hydropower production, this renewable energy source can have serious negative consequences on the environment. For example, dams act as barriers for the longitudinal migration of organisms and transport of particulate matter. Accelerated siltation processes in the receiving river reduce the vertical connectivity between river and groundwater. Hydropeaks, caused by short-term changes in hydropower operation, result in a negative impact on both habitat and organisms, especially during winter months when natural discharge is low and almost constant. In this study, we report the current deficits present in the River Rhone from two different scientific perspectives - fish ecology and hydrology. Potential rehabilitation solutions in synergy with flood protection measures are discussed. We focus on the effects of hydropeaking in relation to longitudinal and vertical dimensions and discuss local river widening as a potential rehabilitation tool. The fish fauna in the Rhone is characterized by a highly unnatural structure (low diversity, impaired age distribution). A high correlation between fish biomass and monotonous morphology (poor cover availability) was established. Tracer hydrology provided further details about the reduced permeability of the riverbank, revealing a high degree of siltation with K values of about 4.7 × 10−6m s−1. Improving the hydrologic situation is therefore essential for the successful rehabilitation of the Rhone River. To this end, hydropeaks in the river reaches must be attenuated. This can be realized by a combination of different hard technical and soft operational measures such as retention reservoirs or slower up and down ramping of turbine
OpenSPIM - an open access platform for light sheet microscopy
Light sheet microscopy promises to revolutionize developmental biology by
enabling live in toto imaging of entire embryos with minimal phototoxicity. We
present detailed instructions for building a compact and customizable Selective
Plane Illumination Microscopy (SPIM) system. The integrated OpenSPIM hardware
and software platform is shared with the scientific community through a public
website, thereby making light sheet microscopy accessible for widespread use
and optimization to various applications.Comment: 7 pages, 3 figures, 6 supplementary videos, submitted to Nature
Methods, associated public website http://openspim.or
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