6,778 research outputs found
Some boundary effects in quantum field theory
We have constructed a quantum field theory in a finite box, with periodic
boundary conditions, using the hypothesis that particles living in a finite box
are created and/or annihilated by the creation and/or annihilation operators,
respectively, of a quantum harmonic oscillator on a circle. An expression for
the effective coupling constant is obtained showing explicitly its dependence
on the dimension of the box.Comment: 12 pages, Late
Semiclassical description of resonant tunneling
We derive a semiclassical formula for the tunneling current of electrons
trapped in a potential well which can tunnel into and across a wide quantum
well. The calculations idealize an experimental situation where a strong
magnetic field tilted with respect to an electric field is used. The resulting
semiclassical expression is written as the sum over special periodic orbits
which hit both walls of the quantum well and are perpendicular to the first
wall.Comment: LaTeX, 8 page
Produção de raízes finas em cultivos de palma de óleo (Elaeis guineensis) no Nordeste paraense.
O objetivo do trabalho foi avaliar a variabilidade espacial da produção de raízes finas (diâmetro ? 2 mm) em um cultivo convencional de palma de óleo (Elaeis guineensis). O estudo foi realizado no município de Tomé-Açu (PA), em área de agricultor familiar. As amostras foram coletadas entre 0 e 10 cm de profundidade por um cilindro telado (?ingrowth core?) de malha de abertura igual a 2 mm e diâmetro igual a 5 cm. Os locais de coleta avaliados foram: (a) entre plantas, (b) empilhamento e (c) carreador, e em diferentes distâncias a partir do estipe: 0,6; 1,2; 2; e 4 m. Os dados obtidos foram analisados e interpretados em relação à variação pluviométrica média regional. A produção mensal de raízes finas não diferiu nas diferentes distâncias da base da planta, exceto na camada foliar, em que a distância de 0,6 m apresentou uma maior produção de raízes com (média ± erro padrão = 54,20 ± 7,95 g m-2 ano-1, n = 5). A variação intranual da produção de raízes finas foi influenciada pela sazonalidade da precipitaçã
Valuing nested names in the Portuguese olive oil market: An exploratory study
The Portuguese olive oil market had a
remarkable development in recent years. Production is rising steadily is response to a EU program supporting a renewal of olive groves. Moreover there is a proliferation of
national brands and private labels. These are often associated to regional collective labels or to organic production. The aim of our research is to determine how consumers value these nested names or co-brands. We
conducted a pilot survey on a convenience sample of 103 consumers in the Oporto and Lisbon metropolitan areas as
ell in a rural area. Our results reveal some contradictions, for instance while origin is an important purchasing criteria,
few PDO olive oils are recognized. Moreover, only 25% of respondents identify organic olive oils sold in the market and this attribute is one of the last purchasing criteria, but organic olive oils have the highest willingness to pay.
Finally we find that associating a PDO to private labels increases willingness to pay by 33.3%, but doesn’t affect valuation of national brands. While we can’t take definite
conclusions our findings give us interesting cues for future research. Therefore we aim to investigate whether regional identity, alternative usage and health or environmental conscience determine of affect valuation and choices of different olive oils brands and labels
More than comparing with majorities: the importance of alternative comparisons between children from different minority groups
Using a social identity perspective as the guiding framework, this article examines how children of minorities use alternative comparisons to achieve positive distinctiveness of their minority group. We conducted a quasi-experimental study with 60 Portuguese children (Black, White and Gypsy) who were asked for their group preferences and attributions towards their own and other groups. Majority children showed in-group favoritism on both preference and attribution measures, but members of both minorities showed greater preference for majority members as well as for members of the in-group than for members of the other minority group. We concluded that, as predicted by social identity theory, one creative identity management strategy is that minorities emphasize their similarity with the high status majority while downgrading another minority
Angiogenesis in the Normal Adrenal Fetal Cortex and Adrenocortical Tumors
Simple Summary Pharmacological angiogenesis modulation was robustly demonstrated to be a powerful clinical resource in oncotherapy. Adrenocortical carcinomas (ACC) often have a poor prognosis for which therapeutic options are limited. Understanding the mechanisms that regulate adrenocortical angiogenesis both under physiological conditions and in ACC could provide important clues on how these processes could be modulated for clinical purposes. This report summarizes the current knowledge on adrenal cortex angiogenesis regulation in physiological conditions and ACC. Embryonic adrenal angiogenesis is regulated by VEGF and Ang-Tie signaling pathways. VEGF angiogenic pathway was initially considered a promising therapeutic target for improving ACC prognosis. However, every single VEGF pathway-targeting clinical trial in ACC so far conducted yielded disappointing results. In contrast, the potential of Ang-Tie pathway-targeting in ACC is yet to be explored. Therefore, further investigation on the role and efficacy of modulating both Ang-Tie and VEGF pathways in ACC is still an unmet need. Angiogenesis plays an important role in several physiological and pathological processes. Pharmacological angiogenesis modulation has been robustly demonstrated to achieve clinical benefits in several cancers. Adrenocortical carcinomas (ACC) are rare tumors that often have a poor prognosis. In addition, therapeutic options for ACC are limited. Understanding the mechanisms that regulate adrenocortical angiogenesis along the embryonic development and in ACC could provide important clues on how these processes could be pharmacologically modulated for ACC treatment. In this report, we performed an integrative review on adrenal cortex angiogenesis regulation in physiological conditions and ACC. During embryonic development, adrenal angiogenesis is regulated by both VEGF and Ang-Tie signaling pathways. In ACC, early research efforts were focused on VEGF signaling and this pathway was identified as a good prognostic factor and thus a promising therapeutic target. However, every clinical trial so far conducted in ACC using VEGF pathway- targeting drugs, alone or in combination, yielded disappointing results. In contrast, although the Ang-Tie pathway has been pointed out as an important regulator of fetal adrenocortical angiogenesis, its role is yet to be explored in ACC. In the future, further research on the role and efficacy of modulating both Ang-Tie and VEGF pathways in ACC is needed
Spin configurations in hard-soft coupled bilayer systems: from rigid magnet to exchange spring transitions
We investigate equilibrium properties of an exchange-spring magnetic system
constituted of a soft layer (e.g. Fe) of a given thickness on top of a hard
magnetic layer (e.g. FePt). The magnetization profile M(z) as a function of the
atomic position ranging from the bottom of the hard layer to the top of the
soft layer is obtained in two cases with regard to the hard layer: i) in the
case of a rigid interface (the FePt layer is a single layer), the profile is
obtained analytically as the exact solution of a sine-Gordon equation with
Cauchy's boundary conditions. Additional numerical simulations also confirm
this result. Asymptotic expressions of M(z) show a linear behavior near the
bottom and the top of the soft layer. In addition, a critical value of the
number of atomic planes in the soft layer, that is necessary for the onset of
spin deviations, is obtained in terms of the anisotropy and exchange coupling
between the adjacent plane in the soft layer. ii) in the case of a relaxed
interface (the FePt layer is a multilayer), the magnetization profile is
obtained numerically for various Fe and FePt films thicknesses and applied
field.Comment: 10 pages, 9 figures, PRB submitted (12-07-2010
Theory of 2-kicked Quantum Rotors
We examine the quantum dynamics of cold atoms subjected to {\em pairs} of
closely spaced -kicks from standing waves of light, and find behaviour
quite unlike the well-studied quantum kicked rotor (QKR). Recent experiments
[Jones et al, {\em Phys. Rev. Lett. {\bf 93}, 223002 (2004)}] identified a
regime of chaotic, anomalous classical diffusion. We show that the
corresponding quantum phase-space has a cellular structure, arising from a
unitary matrix with oscillating band-width. The corresponding eigenstates are
exponentially localized, but scale with a fractional power, , in contrast to the QKR for which . The
effect of inter-cell (and intra-cell) transport is investigated by studying the
spectral fluctuations with both periodic as well as `open' boundary conditions.Comment: 12 pages with 14 figure
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