3,712 research outputs found
Surface Waves and Forced Oscillations in QHE Planar Samples
Dispersion relations and polarizations for surface waves in infinite planar
samples in the QHE regime are explicitly determined in the small wavevector
limit in which the dielectric tensor can be considered as local. The wavelength
and frequency regions of applicability of the results extends to the infrared
region for typical experimental conditions. Then, standard samples with
millimetric sizes seem to be able to support such excitations. Forced
oscillations are also determined which should be generated in the 2DEG by
external electromagnetic sources. They show an almost frequency independent
wavevelength which decreases with the magnetic field. A qualitative model based
in these solutions is also presented to describe a recently found new class of
resonances appearing near the edge of a 2DEG in the QHE regime.Comment: latex file, 18 pages, 3 figures, spelling correcte
From Scattering in Black Hole Backgrounds to Higher-Spin Amplitudes: Part I
The scattering of massless waves of helicity in Schwarzschild and Kerr backgrounds is revisited in the long-wavelenght regime. Using a novel description of such backgrounds in terms of gravitating massive particles, we compute classical wave scattering in terms of QFT amplitudes in flat space, to all orders in spin. The results are Newman-Penrose amplitudes which are in direct correspondence with solutions of the Regge-Wheeler/Teukolsky equation. By introducing a precise prescription for the point-particle limit, in Part I of this work we show how both agree for at finite values of the scattering angle and arbitrary spin orientation. Associated classical observables such as the scattering cross sections, wave polarizations and time delay are studied at all orders in spin. The effect of the black hole spin on the polarization and helicity of the waves is found in agreement with previous analysis at linear order in spin. In the particular limit of small scattering angle, we argue that wave scattering admits a universal, point-particle description determined by the eikonal approximation. We show how our results recover the scattering eikonal phase with spin up to second post-Minkowskian order, and match it to the effective action of null geodesics in a Kerr background. Using this correspondence we derive classical observables such as polar and equatorial scattering angles. This study serves as a preceding analysis to Part II, where the Gravitational Wave () case will be studied in detail
Evolution of an elliptical bubble in an accelerating extensional flow
Mathematical models that describe the dynamical behavior of a thin gas bubble embedded in a glass fiber during a fiber drawing process have been discussed and analyzed.
The starting point for the mathematical modeling was the equations presented in [1] for a glass fiber with a hole undergoing extensional flow. These equations were reconsidered here with the additional reduction that the hole, i.e. the gas bubble, was thin as compared to the radius of the fiber and of finite extent. The primary model considered was one in which the mass of the gas inside the bubble was fixed. This fixed-mass model involved equations for the axial velocity and fiber radius, and equations for the radius of the bubble and the gas pressure inside the bubble. The model equations assumed that the temperature of the furnace of the drawing tower was known.
The governing equations of the bubble are hyperbolic and predict that the bubble cannot extend beyond the limiting characteristics specified by the ends of the initial bubble shape. An analysis of pinch-off was performed, and it was found that pinch-off can occur, depending on the parameters of the model, due to surface tension when the bubble radius is small.
In order to determine the evolution of a bubble, a numerical method of solution was presented. The method was used to study the evolution of two different initial bubble shapes, one convex and the other non-convex. Both initial bubble shapes had fore-aft symmetry, and it was found that the bubbles stretched and elongated severely during the drawing process. For the convex shape, fore-aft symmetry was lost in the middle of the drawing process, but the symmetry was re-gained by the end of the drawing tower. A small amount of pinch-off was observed at each end for this case, so that the final bubble length was slightly shorter than its theoretical maximum length. For the non-convex initial shape, pinch-off occurred in the middle of the bubble resulting in two bubbles by the end of the fiber draw.
The two bubbles had different final pressures and did not have fore-aft symmetry.
An extension of the fixed-mass model was considered in which the gas in the bubble was allowed to diffuse into the surrounding glass. The governing equations for this leaky-mass model were developed and manipulated into a form suitable for a numerical treatment
A note about the ground state of the hydrogen molecular ion
Three simple parametric trial functions for the molecular ion are presented. Each of them provides subsequently the
most accurate approximation for the Born-Oppenheimer ground state energy among
several-parametric trial functions. These trial functions are chosen following
a criterion of physical adequacy and includes the electronic correlation in the
exponential form , where is a variational
parameter. The Born-Oppenheimer energy is found to be \,a.u., respectively, for optimal equilateral triangular
configuration of protons with the equilibrium interproton distance
\,a.u. The variational energy agrees in three significant digits (s.d.)
with most accurate results available at present as well as for major
expectation values.Comment: 12 pages, 1 figure, 3 table
Linking Economic Complexity, Institutions and Income Inequality
A country's mix of products predicts its subsequent pattern of
diversification and economic growth. But does this product mix also predict
income inequality? Here we combine methods from econometrics, network science,
and economic complexity to show that countries exporting complex products (as
measured by the Economic Complexity Index) have lower levels of income
inequality than countries exporting simpler products. Using multivariate
regression analysis, we show that economic complexity is a significant and
negative predictor of income inequality and that this relationship is robust to
controlling for aggregate measures of income, institutions, export
concentration, and human capital. Moreover, we introduce a measure that
associates a product to a level of income inequality equal to the average GINI
of the countries exporting that product (weighted by the share the product
represents in that country's export basket). We use this measure together with
the network of related products (or product space) to illustrate how the
development of new products is associated with changes in income inequality.
These findings show that economic complexity captures information about an
economy's level of development that is relevant to the ways an economy
generates and distributes its income. Moreover, these findings suggest that a
country's productive structure may limit its range of income inequality.
Finally, we make our results available through an online resource that allows
for its users to visualize the structural transformation of over 150 countries
and their associated changes in income inequality between 1963 and 2008
The sustainable transformation of business events: Sociodemographic variables as determinants of attitudes toward sustainable academic conferences
Purpose – This study aimed to assess whether sociodemographic variables explain
significant differences in attitudes towards transforming academic conferences into more
sustainable events.
Design/methodology/approach – An analytical model of participants' attitudes towards
sustainable conferences based on literature review as well as the theories of reasoned action
and planned behavior was developed and applied to a sample of 532 surveyed individuals
from 68 countries who regularly attended academic conferences in the last five years prior
to 2020. The results were refined using statistical and computational techniques to achieve
more empirically robust conclusions.
Findings – Results reveal that sociodemographic variables such as attendees' gender and
age explain differences in attitudes. Women and older adults have stronger pro-environmental attitudes regarding event sustainability. On the other hand, attitudes towards
more sustainable academic conferences are quite strong and positive overall. More
sustainable events' venues, catering, conference materials, and accommodations strongly
influence attendees' attitudes towards more sustainable conferences. The strength of
attitudes was weaker towards transportation.
Originality/value – To our best knowledge, this research is the first to assess whether
sociodemographic variables explain significant differences in attitudes towards the
sustainable transformation of academic conferences.info:eu-repo/semantics/publishedVersio
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