17,162 research outputs found
Initial correlations in nonequilibrium Falicov-Kimball model
The Keldysh boundary problem in a nonequilibrium Falicov-Kimball model in
infinite dimensions is studied within the truncated and self-consistent
perturbation theories, and the dynamical mean-field theory. Within the model
the system is started in equilibrium, and later a uniform electric field is
turned on. The Kadanoff-Baym-Wagner equations for the nonequilibrium Green
functions are derived, and numerically solved. The contributions of initial
correlations are studied by monitoring the system evolution. It is found that
the initial correlations are essential for establishing full electron
correlations of the system and independent on the starting time of preparing
the system in equilibrium. By examining the contributions of the initial
correlations to the electric current and the double occupation, we find that
the contributions are small in relation to the total value of those physical
quantities when the interaction is weak, and significantly increase when the
interaction is strong. The neglect of initial correlations may cause artifacts
in the nonequilibrium properties of the system, especially in the strong
interaction case
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
Chemical pre-processing of cluster galaxies over the past 10 billion years in the IllustrisTNG simulations
We use the IllustrisTNG simulations to investigate the evolution of the
mass-metallicity relation (MZR) for star-forming cluster galaxies as a function
of the formation history of their cluster host. The simulations predict an
enhancement in the gas-phase metallicities of star-forming cluster galaxies
(10^9< M_star<10^10 M_sun) at z<1.0 in comparisons to field galaxies. This is
qualitatively consistent with observations. We find that the metallicity
enhancement of cluster galaxies appears prior to their infall into the central
cluster potential, indicating for the first time a systematic "chemical
pre-processing" signature for {\it infalling} cluster galaxies. Namely,
galaxies which will fall into a cluster by z=0 show a ~0.05 dex enhancement in
the MZR compared to field galaxies at z<0.5. Based on the inflow rate of gas
into cluster galaxies and its metallicity, we identify that the accretion of
pre-enriched gas is the key driver of the chemical evolution of such galaxies,
particularly in the stellar mass range (10^9< M_star<10^10 M_sun). We see
signatures of an environmental dependence of the ambient/inflowing gas
metallicity which extends well outside the nominal virial radius of clusters.
Our results motivate future observations looking for pre-enrichment signatures
in dense environments.Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letter
Number Fluctuation in an interacting trapped gas in one and two dimensions
It is well-known that the number fluctuation in the grand canonical ensemble,
which is directly proportional to the compressibility, diverges for an ideal
bose gas as T -> 0. We show that this divergence is removed when the atoms
interact in one dimension through an inverse square two-body interaction. In
two dimensions, similar results are obtained using a self-consistent
Thomas-Fermi (TF) model for a repulsive zero-range interaction. Both models may
be mapped on to a system of non-interacting particles obeying the Haldane-Wu
exclusion statistics. We also calculate the number fluctuation from the ground
state of the gas in these interacting models, and compare the grand canonical
results with those obtained from the canonical ensemble.Comment: 11 pages, 1 appendix, 3 figures. Submitted to J. Phys. B: Atomic,
Molecular & Optica
Supporting User-Defined Functions on Uncertain Data
Uncertain data management has become crucial in many sensing and scientific applications. As user-defined functions (UDFs) become widely used in these applications, an important task is to capture result uncertainty for queries that evaluate UDFs on uncertain data. In this work, we provide a general framework for supporting UDFs on uncertain data. Specifically, we propose a learning approach based on Gaussian processes (GPs) to compute approximate output distributions of a UDF when evaluated on uncertain input, with guaranteed error bounds. We also devise an online algorithm to compute such output distributions, which employs a suite of optimizations to improve accuracy and performance. Our evaluation using both real-world and synthetic functions shows that our proposed GP approach can outperform the state-of-the-art sampling approach with up to two orders of magnitude improvement for a variety of UDFs. 1
Polarization of Broad Absorption Line QSOs I. A Spectropolarimetric Atlas
We present a spectropolarimetric survey of 36 broad absorption line
quasi-stellar objects (BAL QSOs). The continuum, absorption trough, and
emission line polarization of BAL QSOs yield clues about their structure. We
confirm that BAL QSOs are in general more highly polarized than non-BAL QSOs,
consistent with a more equatorial viewing direction for the former than the
latter. We have identified two new highly-polarized QSOs in our sample
(1232+1325 and 1333+2840). The polarization rises weakly to the blue in most
objects, perhaps due to scattering and absorption by dust particles. We find
that a polarization increase in the BAL troughs is a general property of
polarized BAL QSOs, indicating an excess of scattered light relative to direct
light, and consistent with the unification of BAL QSOs and non-BAL QSOs. We
have also discovered evidence of resonantly scattered photons in the red wing
of the C IV broad emission lines of a few objects. In most cases, the broad
emission lines have lower polarization and a different position angle than the
continuum. The polarization characteristics of low-ionization BAL QSOs are
similar to those of high-ionization BAL QSOs, suggesting a similar BAL wind
geometry.Comment: 39 pages, 6 figures (20 .gif files), accepted for publication in The
Astrophysical Journal Supplement
Renewed diversification is associated with new ecological opportunity in the N eotropical turtle ants
Ecological opportunity, defined as access to new resources free from competitors, is thought to be a catalyst for the process of adaptive radiation. Much of what we know about ecological opportunity, and the larger process of adaptive radiation, is derived from vertebrate diversification on islands. Here, we examine lineage diversification in the turtle ants ( C ephalotes ), a speciesârich group of ants that has diversified throughout the N eotropics. We show that crown group turtle ants originated during the E ocene (around 46 mya), coincident with global warming and the origin of many other clades. We also show a marked lineageâwide slowdown in diversification rates in the M iocene. Contrasting this overall pattern, a species group associated with the young and seasonally harsh C hacoan biogeographic region underwent a recent burst of diversification. Subsequent analyses also indicated that there is significant phylogenetic clustering within the C hacoan region and that speciation rates are highest there. Together, these findings suggest that recent ecological opportunity, from successful colonization of novel habitat, may have facilitated renewed turtle ant diversification. Our findings highlight a central role of ecological opportunity within a successful continental radiation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/102632/1/jeb12300-sup-0001-AppendixFigS1-S4-TableS1-S4.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/102632/2/jeb12300.pd
Exploring the Flame Chemistry of C5Tetrahydrofuranic Biofuels: Tetrahydrofurfuryl Alcohol and 2-Methyltetrahydrofuran
Recently, the combustion chemistry of tetrahydrofurfuryl alcohol (THFA), a potential biofuel, was investigated in a stoichiometric 20 mol % THFA/methane co-fueled premixed flame at 5.3 kPa by our group (Tran, L.-S.; Carstensen, H.-H.; Foo, K. K.; Lamoureux, N.; Gosselin, S.; Gasnot, L.; El-Bakali, A.; Desgroux, P. Experimental and modeling study of the high-temperature combustion chemistry of tetrahydrofurfuryl alcohol. Proc. Combust. Inst. 2021, 38, 631-640, 10.1016/j.proci.2020.07.057). With regard to this, we continue to further explore the combustion chemistry of this biofuel to understand the influence of THFA-doping amounts on the flame chemistry of its mixture with methane and the impact of the alcohol function of THFA on the product spectrum compared to its non-alcoholic fuel counterpart, i.e., 2-methyltetrahydrofuran (MTHF). To accomplish the above said objective, a methane flame, a 10% THFA/methane flame, and a 20% MTHF/methane flame were additionally analyzed at similar conditions using gas chromatography for quantitative species detection and NO laser-induced fluorescence thermometry. More than 40 species (reactants, CO, CO2, H2O, H2, and about 14 hydrocarbons as well as 26 oxygenated intermediates up to 5 carbon atoms) were quantified for each doped biofuel flame. The product distributions and consumption pathways of THFA are similar for the 10 and 20% THFA-doped flames. The maximum yields of most products increase linearly with the amount of doped THFA. However, some species do not follow this trend, indicating interaction chemistry between methane and THFA, which is found to be mainly caused by the reaction of the methyl radical. The difference in the chemical structure in THFA and MTHF has no notable impact on the mole fractions of CO, CO2, H2O, and H2, but significant differences exist for the yields of intermediate species. The doped THFA flame produces more aldehydes, alcohols, and ethers but forms clearly less ketones and hydrocarbons. A slightly upgraded version of our previous kinetic model reproduces most experimental data well and is able to explain the observed differences in intermediate production. © 2021 American Chemical Society
The role of tank-treading motions in the transverse migration of a spheroidal vesicle in a shear flow
The behavior of a spheroidal vesicle, in a plane shear flow bounded from one
side by a wall, is analysed when the distance from the wall is much larger than
the spheroid radius. It is found that tank treading motions produce a
transverse drift away from the wall, proportional to the spheroid eccentricity
and the inverse square of the distance from the wall. This drift is independent
of inertia, and is completely determined by the characteristics of the vesicle
membrane. The relative strength of the contribution to drift from tank-treading
motions and from the presence of inertial corrections, is discussed.Comment: 16 pages, 1 figure, Latex. To appear on J. Phys. A (Math. Gen.
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