5,898 research outputs found
A Study of Meteoroid Impact Phenomena
Process of crater formation resulting from impact of hypervelocity projectile - meteoroid impac
Improving impact resistance of ceramic materials by energy absorbing surface layers
Energy absorbing surface layers were used to improve the impact resistance of silicon nitride and silicon carbide ceramics. Low elastic modulus materials were used. In some cases, the low elastic modulus was achieved using materials that form localized microcracks as a result of thermal expansion anisotropy, thermal expansion differences between phases, or phase transformations. In other cases, semi-vitreous or vitreous materials were used. Substantial improvements in impact resistance were observed at room and elevated temperatures
An Altitude Chamber for the Study and Calibration of Aeronautical Instruments
The design and construction of an altitude chamber, in which both pressure and temperature can be varied independently, was carried out by the NACA at the Langley Memorial Aeronautical Laboratory for the purpose of studying the effects of temperature and pressure on aeronautical research instruments. Temperatures from +20c to -50c are obtained by the expansion of CO2from standard containers. The chamber can be used for the calibration of research instruments under altitude conditions simulating those up to 45,000 feet. Results obtained with this chamber have a direct application in the design and calibration of instruments used in free flight research
Diagrammatic theory of the Anderson impurity model with finite Coulomb interaction
We have developed a self-consistent conserving pseudo particle approximation
for the Anderson impurity model with finite Coulomb interaction, derivable from
a Luttinger Ward functional. It contains an infinite series of skeleton
diagrams built out of fully renormalized Green's functions. The choice of
diagrams is motivated by the Schrieffer Wolff transformation which shows that
singly and doubly occupied states should appear in all bare diagrams
symmetrically. Our numerical results for are in excellent agreement with
the exact values known from the Bethe ansatz solution. The low energy physics
of non-Fermi liquid Anderson impurity systems is correctly described while the
present approximation fails to describe Fermi liquid systems, since some
important coherent spin flip and charge transfer processes are not yet
included. It is believed that CTMA (Conserving T-matrix approximation) diagrams
will recover also Fermi liquid behavior for Anderson models with finite Coulomb
interaction as they do for infinite Coulomb interaction.Comment: 4 pages, 2 figures, presented at the NATO Advanced Research Workshop
on "Size Dependent MAgnetic Scattering", Pecs, Hungary, May 28 - June 1, 200
Sharing the Burden of Collective Security in the European Union. Research Note
This article compares European Union (EU) burden-sharing in security governance distinguishing between assurance, prevention, protection, and compellence policies. We employ joint-product models and examine the variation in the level of publicness, the asymmetry of the distribution of costs and benefits, and aggregation technologies in each policy domain. Joint-product models predict equal burden sharing for protection and assurance because of their respective weakest-link and summation aggregation technologies with symmetric costs. Prevention is also characterized by the technology of summation, but asymmetry of costs implies uneven burden-sharing. Uneven burden-sharing is predicted for compellence because it has the largest asymmetry of costs and a best-shot aggregation technology. Evaluating burden-sharing relative to a country?s ability to contribute, Kendall tau-tests examine the rank-correlation between security burden and the capacity of EU member states. These tests show that the smaller EU members disproportionately shoulder the costs of assurance and protection; wealthier EU members carry a somewhat disproportionate burden in the provision of prevention, and larger EU members in the provision of compellence. When analyzing contributions relative to expected benefits, asymmetric marginal costs can largely explain uneven burden-sharing. The main conclusion is that the aggregated burden of collective security governance in the EU is shared quite evenly
Quantum criticality in the pseudogap Bose-Fermi Anderson and Kondo models: Interplay between fermion- and boson-induced Kondo destruction
We address the phenomenon of critical Kondo destruction in pseudogap
Bose-Fermi Anderson and Kondo quantum impurity models. These models describe a
localized level coupled both to a fermionic bath having a density of states
that vanishes like |\epsilon|^r at the Fermi energy (\epsilon=0) and, via one
component of the impurity spin, to a bosonic bath having a sub-Ohmic spectral
density proportional to |\omega|^s. Each bath is capable by itself of
suppressing the Kondo effect at a continuous quantum phase transition. We study
the interplay between these two mechanisms for Kondo destruction using
continuous-time quantum Monte Carlo for the pseudogap Bose-Fermi Anderson model
with 0<r<1/2 and 1/2<s<1, and applying the numerical renormalization-group to
the corresponding Kondo model. At particle-hole symmetry, the models exhibit a
quantum critical point between a Kondo (fermionic strong-coupling) phase and a
localized (Kondo-destroyed) phase. The two solution methods, which are in good
agreement in their domain of overlap, provide access to the many-body spectrum,
as well as to correlation functions including, in particular, the
single-particle Green's function and the static and dynamical local spin
susceptibilities. The quantum-critical regime exhibits the hyperscaling of
critical exponents and \omega/T scaling in the dynamics that characterize an
interacting critical point. The (r,s) plane can be divided into three regions:
one each in which the calculated critical properties are dominated by the
bosonic bath alone or by the fermionic bath alone, and between these two
regions, a third in which the bosonic bath governs the critical spin response
but both baths influence the renormalization-group flow near the quantum
critical point.Comment: 16 pages, 16 figures. Replaced with published version, added
discussion of particle hole asymmetr
Universal out-of-equilibrium Transport in Kondo-correlated quantum dots: Renormalized dual Fermions on the Keldysh contour
The nonlinear conductance of semiconductor heterostructures and single
molecule devices exhibiting Kondo physics has recently attracted attention. We
address the observed sample dependence of the measured steady state transport
coefficients by considering additional electronic contributions in the
effective low-energy model underlying these experiments that are absent in
particle-hole symmetric setups. A novel version of the superperturbation theory
of Hafermann et al. in terms of dual fermions is developed, which correctly
captures the low-temperature behavior. We compare our results with the measured
transport coefficients.Comment: 5 pages, 2 figure
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