2,668 research outputs found
A feasibility study for advanced technology integration for general aviation
An investigation was conducted to identify candidate technologies and specific developments which offer greatest promise for improving safety, fuel efficiency, performance, and utility of general aviation airplanes. Interviews were conducted with general aviation airframe and systems manufacturers and NASA research centers. The following technologies were evaluated for use in airplane design tradeoff studies conducted during the study: avionics, aerodynamics, configurations, structures, flight controls, and propulsion. Based on industry interviews and design tradeoff studies, several recommendations were made for further high payoff research. The most attractive technologies for use by the general aviation industry appear to be advanced engines, composite materials, natural laminar flow airfoils, and advanced integrated avionics systems. The integration of these technologies in airplane design can yield significant increases in speeds, ranges, and payloads over present aircraft with 40 percent to 50 percent reductions in fuel used
Curved One-Dimensional Wire as a Spin Rotator
We propose a semiconductor structure that can rotate the electron spin
without using ferromagnetic contacts, tunneling barriers, external radiation
etc. The structure consists of a strongly curved one-dimensional ballistic wire
with intrinsic spin-orbit interactions of Rashba type. Our calculations and
analytical formulae show that the proposed device can redistribute the current
densities between the two spin-split modes without backscattering and, thus,
serve as a reflectionless and high-speed spin switcher. Using parameters
relevant for InAs we investigate the projection of current density spin
polarization on the spin-quantization axis as a function of the Rashba
constant, external magnetic field, and radius of the wire's curvature.Comment: 10 pages, 6 figures; replaced with considerably extended versio
AUTORADIOGRAPHIC ANALYSIS OF LYMPHOPOIESIS AND LYMPHOCYTE MIGRATION IN MICE BEARING MULTIPLE THYMUS GRAFTS
Lymphopoiesis was studied in 3-month-old normal C57Bl mice and in 3-month-old C57Bl mice carrying from 12 to 48 C57Bl thymus grafts using tritiated thymidine labeling. Thymus graft lymphopoiesis was found to be identical with that of normal thymus tissue and the presence of thymus grafts was found to have no influence on host thymus lymphopoiesis. No evidence was found that the massive amounts of thymus graft tissue in the mice affected any parameter of host lymph node lymphopoiesis nor was any evidence detected for the migration of thymic lymphocytes from these massive deposits of thymus graft tissue either to host lymph nodes and blood or to other organs in the host animal. It is concluded that the majority of small lymphocytes produced in the thymus and thymus graft tissue do not migrate from these tissues but die locally at the end of their intrathymic life span of 3 to 4 days
Viscous diffusion and photoevaporation of stellar disks
The evolution of a stellar disk under the influence of viscous evolution,
photoevaporation from the central source, and photoevaporation by external
stars is studied. We take the typical parameters of TTSs and the Trapezium
Cluster conditions. The photoionizing flux from the central source is assumed
to arise both from the quiescent star and accretion shocks at the base of
stellar magnetospheric columns, along which material from the disk accretes.
The accretion flux is calculated self-consistently from the accretion mass loss
rate. We find that the disk cannot be entirely removed using only viscous
evolution and photoionization from the disk-star accretion shock. However, when
FUV photoevaporation by external massive stars is included the disk is removed
in 10^6 -10^7yr; and when EUV photoevaporation by external massive stars is
included the disk is removed in 10^5 - 10^6yr.
An intriguing feature of photoevaporation by the central star is the
formation of a gap in the disk at late stages of the disk evolution. As the gap
starts forming, viscous spreading and photoevaporation work in resonance.
There is no gap formation for disks nearby external massive stars because the
outer annuli are quickly removed by the dominant EUV flux. On the other hand,
at larger, more typical distances (d>>0.03pc) from the external stars the flux
is FUV dominated. As a consequence, the disk is efficiently evaporated at two
different locations; forming a gap during the last stages of the disk
evolution.Comment: 27 pages, 11 figures, accepted for publication in Ap
On the induced gauge invariant mass
We derive a general expression for the gauge invariant mass (m_G) for an
Abelian gauge field, as induced by vacuum polarization, in 1+1 dimensions. From
its relation to the chiral anomaly, we show that m_G has to satisfy a certain
quantization condition. This quantization can be, on the other hand, explicitly
verified by using the exact general expression for the gauge invariant mass in
terms of the fermion propagator. This result is applied to some explicit
examples, exploring the possibility of having interesting physical situations
where the value of departs from its canonical value. We also study the
possibility of generalizing the results to the 2+1 dimensional case at finite
temperature, showing that there are indeed situations where a finite and
non-vanishing gauge invariant mass is induced.Comment: 18 pages, Latex, 3 figures (pstex
Quantum transport theory for nanostructures with Rashba spin-orbital interaction
We report on a general theory for analyzing quantum transport through devices
in the Metal-QD-Metal configuration where QD is a quantum dot or the device
scattering region which contains Rashba spin-orbital and electron-electron
interactions. The metal leads may or may not be ferromagnetic, they are assumed
to weakly couple to the QD region. Our theory is formulated by second
quantizing the Rashba spin-orbital interaction in spectral space (instead of
real space), and quantum transport is then analyzed within the Keldysh
nonequilibrium Green's function formalism. The Rashba interaction causes two
main effects to the Hamiltonian: (i) it gives rise to an extra spin-dependent
phase factor in the coupling matrix elements between the leads and the QD; (ii)
it gives rise to an inter-level spin-flip term but forbids any intra-level
spin-flips. Our formalism provides a starting point for analyzing many quantum
transport issues where spin-orbital effects are important. As an example, we
investigate transport properties of a Aharnov-Bohm ring in which a QD having
Rashba spin-orbital and e-e interactions is located in one arm of the ring. A
substantial spin-polarized conductance or current emerges in this device due to
a combined effect of a magnetic flux and the Rashba interaction. The direction
and strength of the spin-polarization are shown to be controllable by both the
magnetic flux and a gate voltage.Comment: 12 pages, 8 figure
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