9,143 research outputs found
Pressure suppression of electron correlation in the collapsed tetragonal phase of CaFe2As2: A DFT-DMFT investigation
Recent studies reveal a pressure induced transition from a paramagnetic tetragonal phase (T) to a collapsed tetragonal phase (CT) in CaFe2As2, which was found to be superconducting with pressure at low temperature. We have investigated the effects of electron correlation and a local fluctuating moment in both tetragonal and collapsed tetragonal phases of the paramagnetic CaFe2As2 using self-consistent DFT-DMFT with continuous time quantum Monte Carlo as the impurity solver. From the computed optical conductivity, we find a gain in the optical kinetic energy due to the loss in Hund's rule coupling energy in the CT phase. We find that the transition from T to CT turns CaFe2As2 from a bad metal into a good metal. Computed mass enhancement and local moments also show a significant decrease in the CT phase, which confirms the suppression of the electron correlation in the CT phase of CaFe2As2
Shutters, Boxes, But No Paradoxes: Time Symmetry Puzzles in Quantum Theory
The ``N-Box Experiment'' is a much-discussed thought experiment in quantum
mechanics. It is claimed by some authors that a single particle prepared in a
superposition of N+1 box locations and which is subject to a final
``post-selection'' measurement corresponding to a different superposition can
be said to have occupied ``with certainty'' N boxes during the intervening
time. However, others have argued that under closer inspection, this surprising
claim fails to hold. Aharonov and Vaidman have continued their advocacy of the
claim in question by proposing a variation on the N-box experiment, in which
the boxes are replaced by shutters and the pre- and post-selected particle is
entangled with a photon. These authors argue that the resulting ``N-shutter
experiment'' strengthens their original claim regarding the N-box experiment.
It is argued in this paper that the apparently surprising features of this
variation are no more robust than those of the N-box experiment and that it is
not accurate to say that the particle is ``with certainty'' in all N shutters
at any given time.Comment: Presentation improved; to appear in International Studies in
Philosophy of Scienc
Collision and fusion of counterpropagating micron-sized optical beams in non-uniformly biased photorefractive crystals
We theoretically investigate collision of optical beams travelling in
opposite directions through a centrosymmetric photorefractive crystal biased by
a spatially non-uniform voltage. We analytically predict the fusion of
counterpropagating solitons in conditions in which the applied voltage is
rapidly modulated along the propagation axis, so that self-bending is
suppressed by the "restoring symmetry" mechanism. Moreover, when the applied
voltage is slowly modulated, we predict that the modified self-bending allows
conditions in which the two beams fuse together, forming a curved light-channel
splice.Comment: 12 page
Constraints on the crystalline structure of the inner core: Mechanical instability of BCC iron at high pressure
We examine the energetics of close-packed phases of iron (bcc, fee, hcp) throughout the pressure regime of the earth with elaborate first principles electronic structure calculations. The calculations reproduce the relevant experimental observations including the equation of state to over 300 GPa and the pressure of the bcc to hcp phase transition. The bcc structure is found to be energetically unfavorable and mechanically unstable with respect to a tetragonal strain at high pressure (P>150 GPa). This phase is thus highly unlikely to exist in the earth's inner core as has frequently been proposed
Hybrid Algorithms Based on Integer Programming for the Search of Prioritized Test Data in Software Product Lines
In Software Product Lines (SPLs) it is not possible, in general, to test all products of the family. The number of products denoted by a SPL is very high due to the combinatorial explosion of features. For this reason, some coverage criteria have been proposed which try to test at least all feature interactions without the necessity to test all products, e.g., all pairs of features (pairwise coverage). In addition, it is desirable to first test products composed by a set of priority features. This problem is known as the Prioritized Pairwise Test Data Generation Problem. In this work we propose two hybrid algorithms using Integer Programming (IP) to generate a prioritized test suite. The first one is based on an integer linear formulation and the second one is based on a integer quadratic (nonlinear) formulation. We compare these techniques with two state-of-the-art algorithms, the Parallel Prioritized Genetic Solver (PPGS) and a greedy algorithm called prioritized-ICPL. Our study reveals that our hybrid nonlinear approach is clearly the best in both, solution quality and computation time. Moreover, the nonlinear variant (the fastest one) is 27 and 42 times faster than PPGS in the two groups of instances analyzed in this work.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. Partially funded by the Spanish Ministry of Economy and Competitiveness and FEDER under contract TIN2014-57341-R, the University of Málaga, Andalucía Tech and the Spanish Network TIN2015-71841-REDT (SEBASENet)
What influences the speed of prototyping? An empirical investigation of twenty software startups
It is essential for startups to quickly experiment business ideas by building
tangible prototypes and collecting user feedback on them. As prototyping is an
inevitable part of learning for early stage software startups, how fast
startups can learn depends on how fast they can prototype. Despite of the
importance, there is a lack of research about prototyping in software startups.
In this study, we aimed at understanding what are factors influencing different
types of prototyping activities. We conducted a multiple case study on twenty
European software startups. The results are two folds, firstly we propose a
prototype-centric learning model in early stage software startups. Secondly, we
identify factors occur as barriers but also facilitators for prototyping in
early stage software startups. The factors are grouped into (1) artifacts, (2)
team competence, (3) collaboration, (4) customer and (5) process dimensions. To
speed up a startups progress at the early stage, it is important to incorporate
the learning objective into a well-defined collaborative approach of
prototypingComment: This is the author's version of the work. Copyright owner's version
can be accessed at doi.org/10.1007/978-3-319-57633-6_2, XP2017, Cologne,
German
Characterizing the Natural System: Toward Sustained, Integrated Coastal Ocean Acidification Observing Networks to Facilitate Resource Management and Decision Support
Coastal ocean ecosystems have always served human populations they provide food security, livelihoods, coastal protection, and defense. Ocean acidification is a global threat to these ecosystem services, particularly when other local and regional stressors combine with it to jeopardize coastal health. Monitoring efforts call for a coordinated global approach toward sustained, integrated coastal ocean health observing networks to address the region-specific mix of factors while also adhering to global ocean acidification observing network principles to facilitate comparison among regions for increased utility and understanding. Here, we generalize guidelines for scoping and designing regional coastal ocean acidification observing networks and provide examples of existing efforts. While challenging in the early stages of coordinating the design and prioritizing the implementation Of these observing networks, it is essential to actively engage all of the relevant stakeholder groups from the outset, including private industries, public agencies, regulatory bodies, decision makers, and the general public. The long-term sustainability of these critical observing networks will rely on leveraging of resources and the strength of partnerships across the consortium of stakeholders and those implementing coastal ocean health observing networks
Computing the vertices of tropical polyhedra using directed hypergraphs
We establish a characterization of the vertices of a tropical polyhedron
defined as the intersection of finitely many half-spaces. We show that a point
is a vertex if, and only if, a directed hypergraph, constructed from the
subdifferentials of the active constraints at this point, admits a unique
strongly connected component that is maximal with respect to the reachability
relation (all the other strongly connected components have access to it). This
property can be checked in almost linear-time. This allows us to develop a
tropical analogue of the classical double description method, which computes a
minimal internal representation (in terms of vertices) of a polyhedron defined
externally (by half-spaces or hyperplanes). We provide theoretical worst case
complexity bounds and report extensive experimental tests performed using the
library TPLib, showing that this method outperforms the other existing
approaches.Comment: 29 pages (A4), 10 figures, 1 table; v2: Improved algorithm in section
5 (using directed hypergraphs), detailed appendix; v3: major revision of the
article (adding tropical hyperplanes, alternative method by arrangements,
etc); v4: minor revisio
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