13,066 research outputs found
Current injection by coherent one- and two-photon excitation in graphene and its bilayer
Coherent control of optically-injected carrier distributions in single and
bilayer graphene allows the injection of electrical currents. Using a
tight-binding model and Fermi's golden rule, we derive the carrier and
photocurrent densities achieved via interference of the quantum amplitudes for
two-photon absorption at a fundamental frequency, , and one-photon
absorption at the second harmonic, . Strong currents are injected
under co-circular and linear polarizations. In contrast, opposite-circular
polarization yields no net current. For single-layer graphene, the magnitude of
the current is unaffected by the rotation of linear-polarization axes, in
contrast with the bilayer and with conventional semiconductors. The dependence
of the photocurrent on the linear-polarization axes is a clear and measurable
signature of interlayer coupling in AB-stacked multilayer graphene. We also
find that single and bilayer graphene exhibit a strong, distinct
linear-circular dichroism in two-photon absorption.Comment: 9 pages, 8 figure
Modular localization and Wigner particles
We propose a framework for the free field construction of algebras of local
observables which uses as an input the Bisognano-Wichmann relations and a
representation of the Poincare' group on the one-particle Hilbert space. The
abstract real Hilbert subspace version of the Tomita-Takesaki theory enables us
to bypass some limitations of the Wigner formalism by introducing an intrinsic
spacetime localization. Our approach works also for continuous spin
representations to which we associate a net of von Neumann algebras on
spacelike cones with the Reeh-Schlieder property. The positivity of the energy
in the representation turns out to be equivalent to the isotony of the net, in
the spirit of Borchers theorem. Our procedure extends to other spacetimes
homogeneous under a group of geometric transformations as in the case of
conformal symmetries and de Sitter spacetime.Comment: 22 pages, LaTeX. Some errors have been corrected. To appear on Rev.
Math. Phy
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The Neurobiology of Eating Disorders.
Eating disorders are severe psychiatric illnesses with a typical age of onset in adolescence. Brain research in youth and young adults may help us identify specific neurobiology that contributes to onset and maintenance of those disorders. This article provides a state-of-the-art review of our current understanding of the neurobiology of anorexia nervosa and bulimia nervosa. This includes brain structure and function studies to understand food restriction, binge-eating or purging behaviors, cognitive and emotional factors, as well as interoception. Binge-eating disorder and avoidant restrictive food intake disorder are also discussed, but the literature is still very small
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Recent advances in understanding anorexia nervosa.
Anorexia nervosa is a complex psychiatric illness associated with food restriction and high mortality. Recent brain research in adolescents and adults with anorexia nervosa has used larger sample sizes compared with earlier studies and tasks that test specific brain circuits. Those studies have produced more robust results and advanced our knowledge of underlying biological mechanisms that may contribute to the development and maintenance of anorexia nervosa. It is now recognized that malnutrition and dehydration lead to dynamic changes in brain structure across the brain, which normalize with weight restoration. Some structural alterations could be trait factors but require replication. Functional brain imaging and behavioral studies have implicated learning-related brain circuits that may contribute to food restriction in anorexia nervosa. Most notably, those circuits involve striatal, insular, and frontal cortical regions that drive learning from reward and punishment, as well as habit learning. Disturbances in those circuits may lead to a vicious cycle that hampers recovery. Other studies have started to explore the neurobiology of interoception or social interaction and whether the connectivity between brain regions is altered in anorexia nervosa. All together, these studies build upon earlier research that indicated neurotransmitter abnormalities in anorexia nervosa and help us develop models of a distinct neurobiology that underlies anorexia nervosa
How do neural networks see depth in single images?
Deep neural networks have lead to a breakthrough in depth estimation from
single images. Recent work often focuses on the accuracy of the depth map,
where an evaluation on a publicly available test set such as the KITTI vision
benchmark is often the main result of the article. While such an evaluation
shows how well neural networks can estimate depth, it does not show how they do
this. To the best of our knowledge, no work currently exists that analyzes what
these networks have learned.
In this work we take the MonoDepth network by Godard et al. and investigate
what visual cues it exploits for depth estimation. We find that the network
ignores the apparent size of known obstacles in favor of their vertical
position in the image. Using the vertical position requires the camera pose to
be known; however we find that MonoDepth only partially corrects for changes in
camera pitch and roll and that these influence the estimated depth towards
obstacles. We further show that MonoDepth's use of the vertical image position
allows it to estimate the distance towards arbitrary obstacles, even those not
appearing in the training set, but that it requires a strong edge at the ground
contact point of the object to do so. In future work we will investigate
whether these observations also apply to other neural networks for monocular
depth estimation.Comment: Submitte
The Economics of Local Tourist Systems
In this paper we analyse the Local Tourist System (LTS) as a particular case of Marshallian Industrial District. The LTS allows the identification of more effective policy tools for managing tourism. First, through the concept of LTS, the policy maker can take into account the complexity of tourism, characterised by a strong heterogeneity of goods, services and subjects involved; second, LTS helps promote a stronger co-ordination between the public and the private sector, by identifying a homogeneous territory and recognising its importance in tourists' decisions; third, through the LTS the policymaker can analyze the externalities and promotes the idea of collaborating networks in a context of local development. In the LTS framework, the anticommon problem can be analysed and contrasted. As the tourist has to buy different but intertwined goods which compose the holiday package, the failure in one of the markets can lead to the overall failure of the package. A LTS policy has to: i) co-ordinate the price policy of the different firms supplying âsingle componentsâ of the tourist product; ii) fix the price of the whole product; iii) impute a price to each component. We demonstrate that, through price policy co-ordination and under general conditions, the LTS can increase the size of tourism and the firmsâ profits, thereby reaching a more effective and efficient target in tourism policy. The recent introduction of LTS in the Italian legislation can be seen as a positive attempt of improving co-ordination in a complex sector such as tourism.Local tourist systems, Tourism policy
Temperature in complex networks
Various statistical-mechanics approaches to complex networks have been proposed to describe expected topological properties in terms of ensemble averages. Here we extend this formalism by introducing the fundamental concept of graph temperature, controlling the degree of topological optimization of a network. We recover the temperature-dependent version of various important models as particular cases of our approach, and show examples where, remarkably, the onset of a percolation transition, a scale-free degree distribution, correlations and clustering can be understood as natural properties of an optimized (low-temperature) topology. We then apply our formalism to real weighted networks and we compute their temperature, finding that various techniques used to extract information from complex networks are again particular cases of our approach
Inversion Formulas for the Dunkl Intertwining Operator and Its Dual on Spaces of Functions and Distributions
In this paper we prove inversion formulas for the Dunkl intertwining operator
and for its dual and we deduce the expression of the
representing distributions of the inverse operators and
, and we give some applications.Comment: This is a contribution to the Special Issue on Dunkl Operators and
Related Topics, published in SIGMA (Symmetry, Integrability and Geometry:
Methods and Applications) at http://www.emis.de/journals/SIGMA
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