80 research outputs found
Real space first-principles derived semiempirical pseudopotentials applied to tunneling magnetoresistance
In this letter we present a real space density functional theory (DFT)
localized basis set semi-empirical pseudopotential (SEP) approach. The method
is applied to iron and magnesium oxide, where bulk SEP and local spin density
approximation (LSDA) band structure calculations are shown to agree within
approximately 0.1 eV. Subsequently we investigate the qualitative
transferability of bulk derived SEPs to Fe/MgO/Fe tunnel junctions. We find
that the SEP method is particularly well suited to address the tight binding
transferability problem because the transferability error at the interface can
be characterized not only in orbital space (via the interface local density of
states) but also in real space (via the system potential). To achieve a
quantitative parameterization, we introduce the notion of ghost semi-empirical
pseudopotentials extracted from the first-principles calculated Fe/MgO bonding
interface. Such interface corrections are shown to be particularly necessary
for barrier widths in the range of 1 nm, where interface states on opposite
sides of the barrier couple effectively and play a important role in the
transmission characteristics. In general the results underscore the need for
separate tight binding interface and bulk parameter sets when modeling
conduction through thin heterojunctions on the nanoscale.Comment: Submitted to Journal of Applied Physic
A branch-and-cut approach to the crossing number problem
The crossing number of a graph is the minimum number of edge crossings in any drawing of the graph in the plane. Extensive research has produced bounds on the crossing number and exact formulae for special graph classes, yet the crossing numbers of graphs such as K_{11} or K_{9,11} are still unknown. Finding the crossing number is NP-hard for general graphs and no practical algorithm for its computation has been published so far. We present an integer linear programming formulation that is based on a reduction of the general problem to a restricted version of the crossing number problem in which each edge may be crossed at most once. We also present cutting plane generation heuristics and a column generation scheme. As we demonstrate in a computational study, a branch-and-cut algorithm based on these techniques as well as recently published preprocessing algorithms can be used to successfully compute the crossing number for small to medium sized general graphs
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Effect of varying experimental conditions on the viscosity of α-pinene derived secondary organic material
Knowledge of the viscosity of particles containing secondary organic material (SOM) is useful for predicting reaction rates and diffusion in SOM particles. In this study we investigate the viscosity of SOM particles as a function of relative humidity and SOM particle mass concentration, during SOM synthesis. The SOM was generated via the ozonolysis of α-pinene at < 5 % relative humidity (RH). Experiments were carried out using the poke-and-flow technique, which measures the experimental flow time (τexp, flow) of SOM after poking the material with a needle. In the first set of experiments, we show that τexp, flow increased by a factor of 3600 as the RH increased from < 0.5 RH to 50 % RH, for SOM with a production mass concentration of 121 µg m−3. Based on simulations, the viscosities of the particles were between 6 × 105 and 5 × 107 Pa s at < 0.5 % RH and between 3 × 102 and 9 × 103 Pa s at 50 % RH. In the second set of experiments we show that under dry conditions τexp, flow decreased by a factor of 45 as the production mass concentration increased from 121 to 14 000 µg m−3. From simulations of the poke-and-flow experiments, the viscosity of SOM with a production mass concentration of 14 000 µg m−3 was determined to be between 4 × 104 and 1.5 × 106 Pa s compared to between 6 × 105 and 5 × 107 Pa s for SOM with a production mass concentration of 121 µg m−3. The results can be rationalized by a dependence of the chemical composition of SOM on production conditions. These results emphasize the shifting characteristics of SOM, not just with RH and precursor type, but also with the production conditions, and suggest that production mass concentration and the RH at which the viscosity was determined should be considered both when comparing laboratory results and when extrapolating these results to the atmosphere
Atomic matter wave scanner
We report on the experimental realization of an atom optical device, that
allows scanning of an atomic beam. We used a time-modulated evanescent wave
field above a glass surface to diffract a continuous beam of metastable Neon
atoms at grazing incidence. The diffraction angles and efficiencies were
controlled by the frequency and form of modulation, respectively. With an
optimized shape, obtained from a numerical simulation, we were able to transfer
more than 50% of the atoms into the first order beam, which we were able to
move over a range of 8 mrad.Comment: 4 pages, 4 figure
Planar projections of graphs
We introduce and study a new graph representation where vertices are embedded
in three or more dimensions, and in which the edges are drawn on the
projections onto the axis-parallel planes. We show that the complete graph on
vertices has a representation in planes. In 3
dimensions, we show that there exist graphs with edges that can be
projected onto two orthogonal planes, and that this is best possible. Finally,
we obtain bounds in terms of parameters such as geometric thickness and linear
arboricity. Using such a bound, we show that every graph of maximum degree 5
has a plane-projectable representation in 3 dimensions.Comment: Accepted at CALDAM 202
Assessing personality in San Joaquin kit fox in situ: efficacy of field-based experimental methods and implications for conservation management
Utilisation of animal personality has potential benefit for conservation management. Due to logistics of robust behavioural evaluation in situ, the majority of studies on wild animals involve taking animals into captivity for testing, potentially compromising results. Three in situ tests for evaluation of boldness in San Joaquin kit fox (Vulpes macrotis mutica) were developed (ENOT: extended novel object test; RNOT: rapid novel object test; TH: trap/handling test). Each test successfully identified variation in boldness within its target age class(es). The TH test was suitable for use across all age classes. Tests were assessed for in situ suitability and for quantity/quality of data yielded. ENOT was rated as requiring high levels of time, cost and labour with greater likelihood of failure. However, it was rated highly for data quantity/quality. The TH test was rated as requiring little time, labour and cost, but yielding lower quality data. RNOT was rated in the middle. Each test had merit and could be adapted to suit project or species constraints. We recommend field-based evaluation of personality, reducing removal of animals from the wild and facilitating routine incorporation of personality assessment into conservation projects
TULIP 5
International audienceTulip is an information visualization framework dedicated to the analysis and visualization of relational data. Based on more than 16 years of research and development, Tulip is built on a suite of tools and techniques, that can be used to address a large variety of domain-specific problems. With \tulip, we aim to provide Python and/or C++ developers a complete library, supporting the design of interactive information visualization applications for relational data, that can be customized to address a wide range of visualization problems. In its current iteration, \tulip enables the development of algorithms, visual encodings, interaction techniques, data models, and domain-specific visualizations. This development pipeline makes the framework efficient for creating research prototypes as well as developing end-user applications. The recent addition of a complete Python programming layer wraps up Tulip as an ideal tool for fast prototyping and treatment automation, allowing to focus on problem solving, and as a great system for teaching purposes at all education levels
Les Houches 2019 Physics at TeV Colliders: New Physics Working Group Report
This report presents the activities of the `New Physics' working group for the `Physics at TeV Colliders' workshop (Les Houches, France, 10--28 June, 2019). These activities include studies of direct searches for new physics, approaches to exploit published data to constrain new physics, as well as the development of tools to further facilitate these investigations. Benefits of machine learning for both the search for new physics and the interpretation of these searches are also presented
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