7,039 research outputs found
Dynamical Evolution of an Unstable Gravastar with Zero Mass
Using the conventional gravastar model, that is, an object constituted by two
components where one of them is a massive infinitely thin shell and the other
one is a de Sitter interior spacetime, we physically interpret a solution
characterized by a zero Schwarzschild mass. No stable gravastar is formed and
it collapses without forming an event horizon, originating what we call a
massive non-gravitational object. The most surprise here is that the collapse
occurs with an exterior de Sitter vacuum spacetime. This creates an object
which does not interact gravitationally with an outside test particle and it
may evolve to a point-like topological defect.Comment: 8 pages, 10 figures, to appear in Astrophysics and Space Scienc
Dressing a Naked Singularity: an Example
Considering the evolution of a perfect fluid with self-similarity of the
second kind, we have found that an initial naked singularity can be trapped by
an event horizon due to collapsing matter. The fluid moves along time-like
geodesics with a self-similar parameter . Since the metric
obtained is not asymptotically flat, we match the spacetime of the fluid with a
Schwarzschild spacetime. All the energy conditions are fulfilled until the
naked singularity.Comment: 14 pages, 1 figure. This version corrects an error in the calculus of
the pressure and in the conclusion
A Geometric Representation for the Proca Model
The Proca model is quantized in an open-path dependent representation that
generalizes the Loop Representation of gauge theories. The starting point is a
gauge invariant Lagrangian that reduces to the Proca Lagrangian when certain
gauge is selected.Comment: 10 pages, Late
Tuning the electrical conductivity of nanotube-encapsulated metallocene wires
We analyze a new family of carbon nanotube-based molecular wires, formed by
encapsulating metallocene molecules inside the nanotubes. Our simulations, that
are based on a combination of non-equilibrium Green function techniques and
density functional theory, indicate that these wires can be engineered to
exhibit desirable magnetotransport effects for use in spintronics devices. The
proposed structures should also be resilient to room-temperature fluctuations,
and are expected to have a high yield.Comment: 4 pages, 6 figures. Accepted in Physical Review Letter
Heat Capacity Measurements in Pulsed Magnetic Fields
The new NHMFL 60T quasi-continuous magnet produces a flat-top field for a
period of 100 ms at 60 Tesla, and for longer time at lower fields, e.g. 0.5 sec
at 35 Tesla. We have developed for the first time the capability to measure
heat capacity at very high magnetic fields in the NHMFL 60 T quasi-continuous
magnet at LANL, using a probe built out of various plastic materials. The field
plateau allows us to utilize a heat-pulse method to obtain heat capacity data.
Proof-of-principle heat capacity experiments were performed on a variety of
correlated electron systems. Both magnet performance characteristics and
physical properties of various materials studied hold out a promise of wide
application of this new tool.Comment: 6 pages, 3 figures, World Scientific Pub. Co., to be publishe
Provenance does matter: links between winter trophic segregation and the migratory origins of European robins
Amongst migratory species, it is common to find individuals from different populations or geographical origins sharing staging or wintering areas. Given their differing life histories, ecological theory would predict that the different groups of individuals should exhibit some level of niche segregation. This has rarely been investigated because of the difficulty in assigning migrating individuals to breeding areas. Here, we start by documenting a broad geographical gradient of hydrogen isotopes (δ2H) in robin Erithacus rubecula feathers across Europe. We then use δ2H, as well as wing-tip shape, as surrogates for broad migratory origin of birds wintering in Iberia, to investigate the ecological segregation of populations. Wintering robins of different sexes, ages and body sizes are known to segregate between habitats in Iberia. This has been attributed to the despotic exclusion of inferior competitors from the best patches by dominant individuals. We find no segregation between habitats in relation to δ2H in feathers, or to wing-tip shape, which suggests that no major asymmetries in competitive ability exist between migrant robins of different origins. Trophic level (inferred from nitrogen isotopes in blood) correlated both with δ2H in feathers and with wing-tip shape, showing that individuals from different geographic origins display a degree of ecological segregation in shared winter quarters. Isotopic mixing models indicate that wintering birds originating from more northerly populations consume more invertebrates. Our multi-scale study suggests that trophic-niche segregation may result from specializations (arising in the population-specific breeding areas) that are transported by the migrants into the shared wintering grounds
High Temperature Thermopower in La_{2/3}Ca_{1/3}MnO_3 Films: Evidence for Polaronic Transport
Thermoelectric power, electrical resistivity and magnetization experiments,
performed in the paramagnetic phase of La_{2/3}Ca_{1/3}MnO_3, provide evidence
for polaron-dominated conduction in CMR materials. At high temperatures, a
large, nearly field-independent difference between the activation energies for
resistivity (rho) and thermopower (S), a characteristic of Holstein Polarons,
is observed, and ln(rho) ceases to scale with the magnetization. On approaching
T_c, both energies become field-dependent, indicating that the polarons are
magnetically polarized. Below T_c, the thermopower follows a law S(H) prop.
1/rho (H) as in non saturated ferromagnetic metals.Comment: 10 pages, 5 .gif figures. Phys. Rev B (in press
Strong light-matter interaction in systems described by a modified Dirac equation
The bulk states of some materials, such as topological insulators, are described by a
modified Dirac equation. Such systems may have trivial and non-trivial phases. In this paper,
we show that in the non-trivial phase a strong light-matter interaction exists in a two-dimensional
system, which leads to an optical conductivity at least one order of magnitude larger than that of
graphene.JES's work contract is financed in the framework of the Program of Recruitment of Post Doctoral Researchers for the Portuguese Scientific and Technological System, with the Operational Program Human Potential (POPH) of the QREN, participated by the European Social Fund (ESF) and national funds of the Portuguese Ministry of Education and Science (MEC). The authors acknowledge support provided to the current research project by FEDER through the COMPETE Program and by FCT in the framework of the Strategic Project PEST-C/FIS/UI607/2011
Gravitational Collapse of Self-Similar and Shear-free Fluid with Heat Flow
A class of solutions to Einstein field equations is studied, which represents
gravitational collapse of thick spherical shells made of self-similar and
shear-free fluid with heat flow. It is shown that such shells satisfy all the
energy conditions, and the corresponding collapse always forms naked
singularities.Comment: 34 pages, 9 figures, late
Controle da podridão cinzenta da maçã por produtos naturais biologicamente ativos
Indexación: Web of Science; ScieloBiorend SC (chitosan), BC-1000 EC (grapefruit extract plus bioflavonoids) and ECO-100 SC (bioflavonoids plus organic acids, citric phytoalexins, fatty acids, glycerides and sugars), respectively, suppressed grey rot of apple caused by B. cinerea by 80.1%, 79.0% and 76.5% when used as post-harvest treatments under controlled conditions. When applied as combined pre- and post-harvest treatments Biorend SC inhibited fruit rot by 49.9 %, while BC-1000 EC and ECO-100 SC were ineffective. None of the products inhibited fruit rot when applied as pre-harvest treatments under controlled conditions or as post-harvest treatments under commercial conditions. The algal polysaccharide ulvan used in post-harvest treatments suppressed grey rot by 56.0% under controlled conditions, but had no inhibitory effect on combined pre- and post-harvest treatments. The inability of products to activate defense mechanisms (chitinase and peroxidase) of fruits was consistent with the unsuccessful control of rot by pre-harvest treatment. The results suggest that the natural products used have potential for use in integrated management of Botrytis rot when applied after harvest.Biorend SC (quitosana), BC-1000 EC (extrato de toranja mais flavonóides), e ECO-100 SC (bioflavonóides mais ácidos orgânicos, fitoalexinas cítricas, ácidos graxos glicerídeos e açúcares) inibiram em 80,1%, 79,0% e 76,5%, respectivamente, a podridão causada por Botrytis cinerea quando utilizados no tratamento pós-colheita de frutos de maçã sob condições controladas.Tratamento combinado de Biorend SC, com aplicação tanto em pré como no pós-colheita, proporcionou 49,9% de inibição da podridão, enquanto BC-1000 e ECO-100 EC não foram efetivos. Nenhum desses produtos inibiu a podridão cinzenta, quando utilizados em tratamento de pré-colheita em condições controladas ou em tratamento de pós-colheita em condições comerciais. O polissacarídeo algal ulvana, utilizado nos tratamentos de pós-colheita, reduziu em 56% a podridão cinzenta das maçãs em condições controladas, mas não teve efeito inibitório nos tratamentos combinados de pré e pós-colheita. A incapacidade dos produtos em ativar mecanismos de defesa (quitinases e peroxidases) nos frutos, após o tratamento em pré-colheita, foi consistente com a falta de controle da podridão nesse tipo de ensaio. Pelos resultados, sugere-se que os produtos naturais utilizados apresentam potencial para a utilização no manejo integrado da podridão de Botrytis quando aplicados em pós-colheita.http://ref.scielo.org/kcxb9
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