14,354 research outputs found
The role of electron-electron interactions in two-dimensional Dirac fermions
The role of electron-electron interactions on two-dimensional Dirac fermions
remains enigmatic. Using a combination of nonperturbative numerical and
analytical techniques that incorporate both the contact and long-range parts of
the Coulomb interaction, we identify the two previously discussed regimes: a
Gross-Neveu transition to a strongly correlated Mott insulator, and a
semi-metallic state with a logarithmically diverging Fermi velocity accurately
described by the random phase approximation. Most interestingly, experimental
realizations of Dirac fermions span the crossover between these two regimes
providing the physical mechanism that masks this velocity divergence. We
explain several long-standing mysteries including why the observed Fermi
velocity in graphene is consistently about 20 percent larger than the best
values calculated using ab initio and why graphene on different substrates show
different behavior.Comment: 11 pages, 4 figure
Interaction driven metal-insulator transition in strained graphene
The question of whether electron-electron interactions can drive a metal to
insulator transition in graphene under realistic experimental conditions is
addressed. Using three representative methods to calculate the effective
long-range Coulomb interaction between -electrons in graphene and solving
for the ground state using quantum Monte Carlo methods, we argue that without
strain, graphene remains metallic and changing the substrate from SiO to
suspended samples hardly makes any difference. In contrast, applying a rather
large -- but experimentally realistic -- uniform and isotropic strain of about
seems to be a promising route to making graphene an antiferromagnetic
Mott insulator.Comment: Updated version: 6 pages, 3 figure
Urban Cholera transmission hotspots and their implications for Reactive Vaccination: evidence from Bissau city, Guinea Bissau
Use of cholera vaccines in response to epidemics (reactive vaccination) may provide an effective supplement to traditional control measures. In Haiti, reactive vaccination was considered but, until recently, rejected in part due to limited global supply of vaccine. Using Bissau City, Guinea-Bissau as a case study, we explore neighborhood-level transmission dynamics to understand if, with limited vaccine and likely delays, reactive vaccination can significantly change the course of a cholera epidemic
ICT in Portuguese secondary schools: from resistance to innovation
El artÃculo adopta una perspectiva organizacional los modos e intensidades diferentes de implementación de las TecnologÃas de Información y Comunicación (TIC) en doce escuelas secundarias portuguesas. Un ambicioso Plan Tecnológico para la Educación ha generado cambios significativos en las escuelas públicas en los últimos años. Sin embargo, las respuestas de directores, docentes y alumnos en estas escuelas, complementados por los informes de evaluación externa, demuestran que los recursos, estrategias y usos pedagógicos de las TIC varÃan enormemente entre escuelas, desafiando el principio constitucional de la igualdad de oportunidades. La investigación ha identificado tres categorÃas de escuelas con relación a este dominio: innovadoras, ambivalentes y tradicionales. Liderazgo, coordinación tecnológica, identidad de escuela, cooperación entre departamentos y acuerdos con organizaciones del entorno son señalados como factores clave para explicar el alto grado de diversidad. El artÃculo concluye con una perspectiva de las implicaciones de estos resultados, tanto para la investigación como para las polÃticas públicas
Accelerating networks
Evolving out-of-equilibrium networks have been under intense scrutiny
recently. In many real-world settings the number of links added per new node is
not constant but depends on the time at which the node is introduced in the
system. This simple idea gives rise to the concept of accelerating networks,
for which we review an existing definition and -- after finding it somewhat
constrictive -- offer a new definition. The new definition provided here views
network acceleration as a time dependent property of a given system, as opposed
to being a property of the specific algorithm applied to grow the network. The
defnition also covers both unweighted and weighted networks. As time-stamped
network data becomes increasingly available, the proposed measures may be
easily carried out on empirical datasets. As a simple case study we apply the
concepts to study the evolution of three different instances of Wikipedia,
namely, those in English, German, and Japanese, and find that the networks
undergo different acceleration regimes in their evolution.Comment: 12 pages, 8 figure
Social-Cognitive Theories to Explain Physical Activity
Despite the widely recognized health benefits of physical activity, participation in regular physical activity falls far short of the Global Action Plan on Physical Activity 2018–2030. More than half of the population does not reach current recommendations, and around one-fourth is not sufficiently active at all. Understanding social-cognitive theories may aid in the creation of interventions to improve long-term physical activity maintenance. The current work covers theory and research on physical activity patterns. It specifically provides an overview of contemporary conceptualizations of motivational and cognitive theories, as well as reviews recent perspectives on how physical activity can be adopted and maintained. Key questions, such as whether physical activity can be executed purely based on cognitive functions, are also addressed. Furthermore, this review identifies possible and effective intervention strategies to promote physical activity.info:eu-repo/semantics/publishedVersio
Recomendações para o controle de plantas daninhas na cultura da soja.
bitstream/item/54012/1/32.pd
Generalization of Dirac Non-Linear Electrodynamics, and Spinning Charged Particles
In this note we generalized the Dirac non-linear electrodynamics, by
introducing two potentials (namely, the vector potential A and the
pseudo-vector potential gamma^5 B of the electromagnetic theory with charges
and magnetic monopoles) and by imposing the pseudoscalar part of the product
omega.omega* to be zero, with omega = A + gamma^5 B. We show that the field
equations of such a theory possess a soliton-like solution which can represent
a priori a "charged particle", since it is endowed with a Coulomb field plus
the field of a magnetic dipole. The rest energy of the soliton is finite, and
the angular momentum stored in its electromagnetic field can be identified
--for suitable choices of the parameters-- with the spin of the charged
particle. Thus this approach seems to yield a classical model for the charged
(spinning) particle, which does not meet the problems met by earlier attempts
in the same direction.Comment: standard LaTeX file; 16 pages; it is a corrected version of a paper
appeared in Found. Phys. (issue in honour of A.O.Barut) 23 (1993) 46
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