39,426 research outputs found
Isotropic properties of the photonic band gap in quasicrystals with low-index contrast
We report on the formation and development of the photonic band gap in
two-dimensional 8-, 10- and 12-fold symmetry quasicrystalline lattices of low
index contrast. Finite size structures made of dielectric cylindrical rods were
studied and measured in the microwave region, and their properties compared
with a conventional hexagonal crystal. Band gap characteristics were
investigated by changing the direction of propagation of the incident beam
inside the crystal. Various angles of incidence from 0 \degree to 30\degree
were used in order to investigate the isotropic nature of the band gap. The
arbitrarily high rotational symmetry of aperiodically ordered structures could
be practically exploited to manufacture isotropic band gap materials, which are
perfectly suitable for hosting waveguides or cavities.Comment: 16 pages, 7 figures, submitted to PR
Incoherent pion photoproduction on the deuteron with polarization observables I: Formal expressions
Formal expressions are developed for the general five-fold differential cross
section of incoherent -photoproduction on the deuteron including beam and
target polarization. The polarization observables of the cross section are
described by various beam, target and beam-target asymmetries for polarized
photons and/or polarized deuterons. They are given as bilinear hermitean forms
in the reaction matrix elements divided by the unpolarized cross section. In
addition, the corresponding observables for the semi-exclusive reaction are also given.Comment: 11 pages, 1 figur
Trends in Outcomes for Young People with Disabilities: Are We Making Progress?
This paper uses the Current Population Survey (CPS) from 1981-2000 to compare long term trends in socio-economic outcomes for youth (aged 15-21) and young adults (aged 22-29) with work limitations to those for youth and young adults without work limitations. We focus on the years 1988 and 1999: years that roughly correspond to the peaks of successive business cycles. We find that prevalence of work limitations declined for males but increased for females, mostly accounted for by growth for African American females. Despite a substantial reduction in the educational attainment gap between young adults with and without disabilities, gaps in employment, earnings, dependency on public programs and poverty widened substantially. These trends could be due to factors that determine whether individuals report themselves to be work-limited, factors that affect individual outcomes regardless of self-reported work limitation status, or both sets of factors
The LHC di-photon excess and Gauge Coupling Unification in Extra Heterotic-String Derived Models
The di-photon excess observed at the LHC can be explained as a Standard Model
singlet that is produced and decays by heavy vector-like colour triplets and
electroweak doublets in one-loop diagrams. The characteristics of the required
spectrum are well motivated in heterotic-string constructions that allow for a
light . Anomaly cancellation of the symmetry
requires the existence of the Standard Model singlet and vector-like states in
the vicinity of the breaking scale. In this paper we show
that the agreement with the gauge coupling data at one-loop is identical to the
case of the Minimal Supersymmetric Standard Model, owing to cancellations
between the additional states. We further show that effects arising from heavy
thresholds may push the supersymmetric spectrum beyond the reach of the LHC,
while maintaining the agreement with the gauge coupling data. We show that the
string inspired model can indeed account for the observed signal and discuss
the feasibility of obtaining viable scalar mass spectrum.Comment: 26 pages. 11 figures. Published versio
Relativistic theory of the double photoionization of helium-like atoms
A fully relativistic calculation of the double photoionization of helium-like
atoms is presented. The approach is based on the partial-wave representation of
the Dirac continuum states and accounts for the retardation in the
electron-electron interaction as well as the higher-order multipoles of the
absorbed photon. The electron-electron interaction is taken into account to the
leading order of perturbation theory. The relativistic effects are shown to
become prominent already for the medium-Z ions, changing the shape and the
asymptotic behaviour of the photon energy dependence of the ratio of the
double-to-single photoionization cross section
Measured and predicted pressure distributions on the AFTI/F-111 mission adaptive wing
Flight tests have been conducted using an F-111 aircraft modified with a mission adaptive wing (MAW). The MAW has variable-camber leading and trailing edge surfaces that can change the wing camber in flight, while preserving smooth upper surface contours. This paper contains wing surface pressure measurements obtained during flight tests at Dryden Flight Research Facility of NASA Ames Research Center. Upper and lower surface steady pressure distributions were measured along four streamwise rows of static pressure orifices on the right wing for a leading-edge sweep angle of 26 deg. The airplane, wing, instrumentation, and test conditions are discussed. Steady pressure results are presented for selected wing camber deflections flown at subsonic Mach numbers up to 0.90 and an angle-of-attack range of 5 to 12 deg. The Reynolds number was 26 million, based on the mean aerodynamic chord. The MAW flight data are compared to MAW wind tunnel data, transonic aircraft technology (TACT) flight data, and predicted pressure distributions. The results provide a unique database for a smooth, variable-camber, advanced supercritical wing
Fuzzy Modeling and Parallel Distributed Compensation for Aircraft Flight Control from Simulated Flight Data
A method is described that combines fuzzy system identification techniques with Parallel Distributed Compensation (PDC) to develop nonlinear control methods for aircraft using minimal a priori knowledge, as part of NASAs Learn-to-Fly initiative. A fuzzy model was generated with simulated flight data, and consisted of a weighted average of multiple linear time invariant state-space cells having parameters estimated using the equation-error approach and a least-squares estimator. A compensator was designed for each subsystem using Linear Matrix Inequalities (LMI) to guarantee closed-loop stability and performance requirements. This approach is demonstrated using simulated flight data to automatically develop a fuzzy model and design control laws for a simplified longitudinal approximation of the F-16 nonlinear flight dynamics simulation. Results include a comparison of flight data with the estimated fuzzy models and simulations that illustrate the feasibility and utility of the combined fuzzy modeling and control approach
Characteristic frequency of the magnetic radiation of spinor condensates
The magnetic radiation of the fully-condensed states of Na condensates
have been studied. A narrow characteristic spectral line with a wave length
proportional to N^(-2/5) (N is the number of particles) and with a probability
of transition proportional to N^(17/5) emitted (absorbed) by the condensate was
found. It implies that short wave radiation with a huge probability of
transition can be obtained if numerous atoms are trapped. A new technique
developed by the authors, namely, the analytical forms of the fractional
parentage coefficients, was used to calculate analytically the matrix elements
between the total spin-states.Comment: 7 pages, 5 figure
The Operator form of 3H (3He) and its Spin Structure
An operator form of the 3N bound state is proposed. It consists of eight
operators formed out of scalar products in relative momentum and spin vectors,
which are applied on a pure 3N spin 1/2 state. Each of the operators is
associated with a scalar function depending only on the magnitudes of the two
relative momenta and the angle between them. The connection between the
standard partial wave decomposition of the 3N bound state and the operator form
is established, and the decomposition of these scalar function in terms of
partial wave components and analytically known auxiliary functions is given.
That newly established operator form of the 3N bound state exhibits the
dominant angular and spin dependence analytically. The scalar functions are
tabulated and can be downloaded. As an application the spin dependent nucleon
momentum distribution in a polarized 3N bound state is calculated to illustrate
the use of the new form of the 3N bound state.Comment: 21 pages, 1 table, 8 figures, revtex
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