757 research outputs found
Application of dynamical systems theory to the high angle of attack dynamics of the F-14
A lens-coupled scintillation counter in cryogenic environment
In this work we present an elegant solution for a scintillation counter to be
integrated into a cryogenic system. Its distinguishing feature is the absence
of a continuous light guide coupling the scintillation and the photodetector
parts, operating at cryogenic and room temperatures respectively. The prototype
detector consists of a plastic scintillator with glued-in wavelength-shifting
fiber located inside a cryostat, a Geiger-mode Avalanche Photodiode (G-APD)
outside the cryostat, and a lens system guiding the scintillation light
re-emitted by the fiber to the G-APD through optical windows in the cryostat
shields. With a 0.8mm diameter multiclad fiber and a 1mm active area G-APD the
coupling efficiency of the "lens light guide" is about 50%. A reliable
performance of the detector down to 3K is demonstrated.Comment: 14 pages, 11 figure
Quantum States of Topologically Massive Electrodynamics and Gravity
The free quantum states of topologically massive electrodynamics and gravity
in 2+1 dimensions, are explicitly found. It is shown that in both theories the
states are described by infrared-regular polarization tensors containing a
regularization phase which depends on the spin. This is done by explicitly
realizing the quantum algebra on a functional Hilbert space and by finding the
Wightman function to define the scalar product on such a Hilbert space. The
physical properties of the states are analyzed defining creation and
annihilation operators.
For both theories, a canonical and covariant quantization procedure is
developed. The higher order derivatives in the gravitational lagrangian are
treated by means of a preliminary Dirac procedure.
The closure of the Poincar\'e algebra is guaranteed by the
infrared-finiteness of the states which is related to the spin of the
excitations through the regularization phase. Such a phase may have interesting
physical consequences.Comment: 21 page, latex, no figure
Direct mode summation for the Casimir energy of a solid ball
The Casimir energy of a solid ball placed in an infinite medium is calculated
by a direct frequency summation using the contour integration. It is assumed
that the permittivity and permeability of the ball and medium satisfy the
condition . Upon deriving the general
expression for the Casimir energy, a dilute compact ball is considered
. In this case the
calculations are carried out which are of the first order in and take
account of the five terms in the Debye expansion of the Bessel functions
involved. The implication of the obtained results to the attempts of explaining
the sonoluminescence via the Casimir effect is shortly discussed.Comment: REVTeX, 7 pages, no figures and tables, treatment of a dilute
dielectric ball is revised, new references are adde
Localization in Strongly Chaotic Systems
We show that, in the semiclassical limit and whenever the elements of the
Hamiltonian matrix are random enough, the eigenvectors of strongly chaotic
time-independent systems in ordered bases can on average be exponentially
localized across the energy shell and decay faster than exponentially outside
the energy shell. Typically however, matrix elements are strongly correlated
leading to deviations from such behavior.Comment: RevTeX, 5 pages + 3 postscript figures, submitted to Phys. Rev. Let
Global solutions of a free boundary problem for selfgravitating scalar fields
The weak cosmic censorship hypothesis can be understood as a statement that
there exists a global Cauchy evolution of a selfgravitating system outside an
event horizon. The resulting Cauchy problem has a free null-like inner
boundary. We study a selfgravitating spherically symmetric nonlinear scalar
field. We show the global existence of a spacetime with a null inner boundary
that initially is located outside the Schwarzschild radius or, more generally,
outside an apparent horizon. The global existence of a patch of a spacetime
that is exterior to an event horizon is obtained as a limiting case.Comment: 31 pages, revtex, to appear in the Classical and Quantum Gravit
On the Resolution of the Time-Like Singularities in Reissner-Nordstrom and Negative-Mass Schwarzschild
Certain time-like singularities are shown to be resolved already in classical
General Relativity once one passes from particle probes to scalar waves. The
time evolution can be defined uniquely and some general conditions for that are
formulated. The Reissner-Nordstrom singularity allows for communication through
the singularity and can be termed "beam splitter" since the transmission
probability of a suitably prepared high energy wave packet is 25%. The high
frequency dependence of the cross section is w^{-4/3}. However, smooth
geometries arbitrarily close to the singular one require a finite amount of
negative energy matter. The negative-mass Schwarzschild has a qualitatively
different resolution interpreted to be fully reflecting. These 4d results are
similar to the 2d black hole and are generalized to an arbitrary dimension d>4.Comment: 47 pages, 5 figures. v2: See end of introduction for an important
note adde
Essays on farm household credit constraint, productivity and consumption inequality in Malawi
Credit has proven to be a necessary tool for economic development affecting positively the welfare of households and individuals. However, one major area in which rural households lack is access to financial markets including credit. The studies included in this thesis contribute to the access to credit literature and the credit constraint/unconstraint impact on some welfare outcomes. The first empirical study examined farm households' access to credit in rural Malawi. Unlike previous empirical studies, particular attention is given to discouraged borrowers who are mostly ignored in such studies. Using the 2010/2011 household survey data from Malawi the study determines the demographic and socio-economic characteristics that distinguish farm households who need credit, who are the discouraged borrowers and who are rejected applicants. A three-step sequential estimation model following a trivariate probit model with double sample selection was adopted. The findings revealed that there were over 7 times more discouraged borrowers than denied applicants. Women were more likely to be discouraged from applying for credit but, if they applied, they were more likely to be successful in obtaining credit than males. This shows that when examining farm households' access to credit discouraged borrowers should be given special consideration.
Capturing discouraged borrowers as also credit constrained, the second empirical study employed a switching model to estimate the impact of credit constraint status on farm productivity for each credit constraint regime. The study further compared the expected production under actual and counterfactual conditions for a household being credit constrained or unconstrained. The findings suggest that a household that is constrained is less productive than a randomly selected household from the sample would but that for the unconstrained household is inconclusive, however, the counterfactual arguments as seen from the analysis shows that being credit unconstrained was beneficial to the increase in productivity.
Studies have shown that undeveloped financial markets have been a major contributing factor increasing inequality, especially in developing countries. The third empirical study examined the impact of household credit constraint on the consumption inequality of rural households in Malawi. Factors that explain the within and between credit constrained and unconstrained status of consumption inequality were examined. The General Entropy (GE) Index and the Regression-Based Inequality Decomposition Methods, Field's (2003) and Blinder-Oaxaca Decomposition were employed. The findings show that inequality was more prominent within the groups than between them. Also, the size of households and the value of assets were the major contributors to the within-group inequalities for credit constrained and unconstrained households. Further, only the endowment component was important in explaining the consumption inequality gap between the credit constrained and unconstrained households. Adjusting the level of endowments of constrained households to that of the unconstrained households increased their welfare by 15.7 percent
Adenoviral gene transfer of PLD1-D4 enhances insulin sensitivity in mice by disrupting phospholipase D1 interaction with PED/PEA-15.
Over-expression of phosphoprotein enriched in diabetes/phosphoprotein enriched in astrocytes (PED/PEA-15) causes insulin resistance by interacting with the D4 domain of phospholipase D1 (PLD1). Indeed, the disruption of this association restores insulin sensitivity in cultured cells over-expressing PED/PEA-15. Whether the displacement of PLD1 from PED/PEA-15 improves insulin sensitivity in vivo has not been explored yet. In this work we show that treatment with a recombinant adenoviral vector containing the human D4 cDNA (Ad-D4) restores normal glucose homeostasis in transgenic mice overexpressing PED/PEA-15 (Tg ped/pea-15) by improving both insulin sensitivity and secretion. In skeletal muscle of these mice, D4 over-expression inhibited PED/PEA-15-PLD1 interaction, decreased Protein Kinase C alpha activation and restored insulin induced Protein Kinase C zeta activation, leading to amelioration of insulin-dependent glucose uptake. Interestingly, Ad-D4 administration improved insulin sensitivity also in high-fat diet treated obese C57Bl/6 mice. We conclude that PED/PEA-15-PLD1 interaction may represent a novel target for interventions aiming at improving glucose tolerance
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