482 research outputs found
Transport properties of 2D graphene containing structural defects
We propose an extensive report on the simulation of electronic transport in
2D graphene in presence of structural defects. Amongst the large variety of
such defects in sp carbon-based materials, we focus on the Stone-Wales
defect and on two divacancy-type reconstructed defects. First, based on ab
initio calculations, a tight-binding model is derived to describe the
electronic structure of these defects. Then, semiclassical transport properties
including the elastic mean free paths, mobilities and conductivities are
computed using an order-N real-space Kubo-Greenwood method. A plateau of
minimum conductivity () is progressively
observed as the density of defects increases. This saturation of the decay of
conductivity to is associated with defect-dependent
resonant energies. Finally, localization phenomena are captured beyond the
semiclassical regime. An Anderson transition is predicted with localization
lengths of the order of tens of nanometers for defect densities around 1%.Comment: 17 pages, 17 figures, submitted to Phys. Rev.
Higgs mass in Noncommutative Geometry
In the noncommutative geometry approach to the standard model, an extra
scalar field - initially suggested by particle physicist to stabilize the
electroweak vacuum - makes the computation of the Higgs mass compatible with
the 126 GeV experimental value. We give a brief account on how to generate this
field from the Majorana mass of the neutrino, following the principles of
noncommutative geometry.Comment: Proceedings of the Corfou Workshop on noncommutative field theory and
gravity, september 201
Two-Dimensional Graphene with Structural Defects: Elastic Mean Free Path, Minimum Conductivity, and Anderson Transition
4 páginas, 4 figuras.-- PACS numbers: 73.23. b, 72.15.Rn, 73.43.Qt.-- et al.Quantum transport properties of disordered graphene with structural defects (Stone-Wales and divacancies) are investigated using a realistic π-π* tight-binding model elaborated from ab initio calculations. Mean free paths and semiclassical conductivities are then computed as a function of the nature and density of defects (using an order-N real-space Kubo-Greenwood method). By increasing the defect density, the decay of the semiclassical conductivities is predicted to saturate to a minimum value of 4e2/πh over a large range (plateau) of carrier density (>0.5×1014 cm-2). Additionally, strong contributions of quantum interferences suggest that the Anderson localization regime could be experimentally measurable for a defect density as low as 1%.J.-C. C. and A. L. acknowledge financial support from
the FNRS of Belgium. Parts of this work are connected to
the Belgian Program on Interuniversity Attraction Poles
(PAI6), to the NanoHymo ARC, to the ETSF e-I3 project
(Grant No. 211956), and to the NANOSIM-GRAPHENE
Project No. ANR-09-NANO-016-01.Peer reviewe
A survey of spectral models of gravity coupled to matter
This is a survey of the historical development of the Spectral Standard Model
and beyond, starting with the ground breaking paper of Alain Connes in 1988
where he observed that there is a link between Higgs fields and finite
noncommutative spaces. We present the important contributions that helped in
the search and identification of the noncommutative space that characterizes
the fine structure of space-time. The nature and properties of the
noncommutative space are arrived at by independent routes and show the
uniqueness of the Spectral Standard Model at low energies and the Pati-Salam
unification model at high energies.Comment: An appendix is added to include scalar potential analysis for a
Pati-Salam model. 58 Page
Metal-free electrocatalytic hydrogen oxidation using frustrated Lewis pairs and carbon-based Lewis acids
Whilst hydrogen is a potentially clean fuel for energy storage and utilisation technologies, its conversion to electricity comes at a high energetic cost. This demands the use of rare and expensive precious metal electrocatalysts. Electrochemical-frustrated Lewis pairs offer a metal-free, CO tolerant pathway to the electrocatalysis of hydrogen oxidation. They function by combining the hydrogen-activating ability of frustrated Lewis pairs (FLPs) with electrochemical oxidation of the resultant hydride. Here we present an electrochemical–FLP approach that utilises two different Lewis acids – a carbon-based N-methylacridinium cation that possesses excellent electrochemical attributes, and a borane that exhibits fast hydrogen cleavage kinetics and functions as a “hydride shuttle”. This synergistic interaction provides a system that is electrocatalytic with respect to the carbon-based Lewis acid, decreases the required potential for hydrogen oxidation by 1 V, and can be recycled multiple times
Chimpanzees behave prosocially in a group-specific manner
Funding: EJCvL was funded by a Postdoctoral Fellowship awarded by the Research Foundation Flanders (FWO) and an ERC-Synergy Grant (no. 609819) awarded to JC.Chimpanzees act cooperatively in the wild, but whether they afford benefits to others, and whether their tendency to act prosocially varies across communities, is unclear. Here, we show that chimpanzees from neighboring communities provide valuable resources to group members at personal cost, and that the magnitude of their prosocial behavior is group specific. Provided with a resource-donation experiment allowing free (partner) choice, we observed an increase in prosocial acts across the study period in most of the chimpanzees. When group members could profit (test condition), chimpanzees provided resources more frequently and for longer durations than when their acts produced inaccessible resources (control condition). Strikingly, chimpanzees’ prosocial behavior was group specific, with more socially tolerant groups acting more prosocially. We conclude that chimpanzees may purposely behave prosocially toward group members, and that the notion of group-specific sociality in nonhuman animals should crucially inform discussions on the evolution of prosocial behavior.Publisher PDFPeer reviewe
An Electrochemical Study of Frustrated Lewis Pairs: A Metal-free Route to Hydrogen Oxidation
[Image: see text] Frustrated Lewis pairs have found many applications in the heterolytic activation of H(2) and subsequent hydrogenation of small molecules through delivery of the resulting proton and hydride equivalents. Herein, we describe how H(2) can be preactivated using classical frustrated Lewis pair chemistry and combined with in situ nonaqueous electrochemical oxidation of the resulting borohydride. Our approach allows hydrogen to be cleanly converted into two protons and two electrons in situ, and reduces the potential (the required energetic driving force) for nonaqueous H(2) oxidation by 610 mV (117.7 kJ mol(–1)). This significant energy reduction opens routes to the development of nonaqueous hydrogen energy technology
Number partitioning as random energy model
Number partitioning is a classical problem from combinatorial optimisation.
In physical terms it corresponds to a long range anti-ferromagnetic Ising spin
glass. It has been rigorously proven that the low lying energies of number
partitioning behave like uncorrelated random variables. We claim that
neighbouring energy levels are uncorrelated almost everywhere on the energy
axis, and that energetically adjacent configurations are uncorrelated, too.
Apparently there is no relation between geometry (configuration) and energy
that could be exploited by an optimization algorithm. This ``local random
energy'' picture of number partitioning is corroborated by numerical
simulations and heuristic arguments.Comment: 8+2 pages, 9 figures, PDF onl
The Holst Action by the Spectral Action Principle
We investigate the Holst action for closed Riemannian 4-manifolds with
orthogonal connections. For connections whose torsion has zero Cartan type
component we show that the Holst action can be recovered from the heat
asymptotics for the natural Dirac operator acting on left-handed spinor fields.Comment: We correct a sign mistake in Proposition 2.3. As a consequence the
main result (Theorem 3.4) becomes more natura
Are migrants more productive than stayers? Some evidence for a set of highly productive academic economists
This paper compares the average productivity of migrants (who work in a country different from their country of origin) and stayers (whose entire academic career takes place in their country of origin) in a set of 2,530 highly productive economists that work in 2007 in a selection of the top 81 Economics departments worldwide. The main findings are the following two. Firstly, productivity comparisons between migrants and stayers depend on the cohort and the type of department where individuals work in 2007. For example, in the top U.S. departments, foreigners are more productive than stayers only among older individuals; in the bottom U.S. departments, foreigners are more productive than stayers for both cohorts, while in the other countries with at least one department in the sample the productivity of foreigners and stayers is indistinguishable for both cohorts. Secondly, when we restrict our attention to an elite consisting of economists with above average productivity, all productivity differences between migrants and stayers in the U.S. vanish. These results are very robust. However, our ability to interpret these correlations is severely limited by the absence of information on the decision to migrate.Albarrán acknowledges
financial support from the Spanish MEC through grants ECO2009-11165 and ECO2011-29751, and
Carrasco and Ruiz-Castillo through grants No. ECO2012-31358 and ECO2014-55953-P, respectively, as
well as grant MDM 2014-0431 to their Departamento de Economía
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