Field-theoretical renormalization group for a flat two-dimensional Fermi surface

We implement an explicit two-loop calculation of the coupling functions and the self-energy of interacting fermions with a two-dimensional flat Fermi surface in the framework of the field theoretical renormalization group (RG) approach. Throughout the calculation both the Fermi surface and the Fermi velocity are assumed to be fixed and unaffected by interactions. We show that in two dimensions, in a weak coupling regime, there is no significant change in the RG flow compared to the well-known one-loop results available in the literature. However, if we extrapolate the flow to a moderate coupling regime there are interesting new features associated with an anisotropic suppression of the quasiparticle weight Z along the Fermi surface, and the vanishing of the renormalized coupling functions for several choices of the external momenta.Comment: 16 pages and 22 figure

A canonical transformation and the tunneling probability for the birth of an asymptotically DeSitter universe with dust

In the present work, we study the quantum cosmology description of closed Friedmann-Robertson-Walker models in the presence of a positive cosmological constant and a generic perfect fluid. We work in the Schutz's variational formalism. If one uses the scale factor and its canonically conjugated momentum as the phase space variables that describe the geometrical sector of these models, one obtains Wheeler-DeWitt equations with operator ordering ambiguities. In order to avoid those ambiguities and simplify the quantum treatment of the models, we introduce new phase space variables. We explicitly demonstrate that the transformation leading from the old set of variables to the new one is canonical. In order to show that the above canonical transformations simplify the quantum treatment of those models, we consider a particular model where the perfect fluid is dust. We solve the Wheeler-DeWitt equation numerically using the Crank-Nicholson scheme and determine the time evolution of the initial wave function. Finally, we compare the results for the present model with the ones for another model where the only difference is the presence of a radiative perfect fluid, instead of dust.Comment: Revtex4, 18 pages, 2 EPS figure

O potencial da pecuária de corte nas regiões tropicais e subtropicais da América Latina.

Vantagens e limitações oferecidas pelos trópicos à produção de bovinos de corte. Estágio de desenvolvimento da pecuária bovina de corte. Mercado consumidor de carne. Potencial do aumento de produção de carne bovina na América Latina.bitstream/item/104684/1/Potencial-da-pecuaria-de-corte.pd

The cyclonic dryer: a numerical and experimental analysis of the influence of geometry on average particle residence time

Particle residence time is an extremely important variable in a cyclonic dryer project. With the goal of obtaining a device that conduces to a long particle residence time, the influence of cyclone dimensions on particle residence time and on the type of flow is discussed here. The research was based on a device with a very high ratio of cyclone diameter to other dimensions. Simulations were developed with computational fluid dynamics techniques by the use of the commercial code CFX 4.4® of AEA Technology. Particles were treated individually. The fit of a turbulence model was also discussed. Theoretical and experimental results showed that the dimensions of the conical part of the cyclone had a very important influence on flow and consequently on particle residence time. The influences of volumetric concentration and particle diameter on particle residence time were also observed.103112Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Levantamento da flora espontânea na cultura do feijão-caupi sob manejo de capoeira triturada no município de Santa Luzia do Paruá - MA.

Os efeitos diferenciados dos sistemas de preparo do solo sobre as plantas espontâneas podem modificar a composição botânica da comunidade. Este estudo teve como objetivo avaliar a composição florística da comunidade espontânea do feijão-caupi (Vigna unguiculata) submetida ao sistema de plantio direto com manejo de capoeira triturada. A pesquisa foi realizada no município de Santa Luzia do Paruá ? MA em área de produtor rural. A cultivar de feijão-caupi utilizada foi BRS Guariba. O levantamento fitossociológico da comunidade espontânea foi realizado aos 30 e 60 dias após a semeadura (DAS) do feijão-caupi com o lançamento ao acaso de retângulos de 0,5 m x 0,3 m, nas entrelinhas da cultura. A cada avaliação, as espécies foram coletadas, identificadas, contadas, secadas em estufa e determinados os índices fitossociológicos: densidade, freqüência, dominância relativa e índice de valor importância. As famílias mais representativas em número de espécies foram Cyperaceae (6), Poaceae (6) e Malvaceae (3). Constatou-se que as espécies com maior índice de valor de importância foram Emilia coccinea e Digitaria. horinzontalis na primeira e segunda avaliações, respectivamente.Resumo expandido