8,268 research outputs found
A flexible flight display research system using a ground-based interactive graphics terminal
Requirements and research areas for the air transportation system of the 1980 to 1990's were reviewed briefly to establish the need for a flexible flight display generation research tool. Specific display capabilities required by aeronautical researchers are listed and a conceptual system for providing these capabilities is described. The conceptual system uses a ground-based interactive graphics terminal driven by real-time radar and telemetry data to generate dynamic, experimental flight displays. These displays are scan converted to television format, processed, and transmitted to the cockpits of evaluation aircraft. The attendant advantages of a Flight Display Research System (FDRS) designed to employ this concept are presented. The detailed implementation of an FDRS is described. The basic characteristics of the interactive graphics terminal and supporting display electronic subsystems are presented and the resulting system capability is summarized. Finally, the system status and utilization are reviewed
Structure and diffusion in amorphous aluminium silicate: A molecular dynamics computer simulation
The amorphous aluminium silicate (Al2O3)2(SiO2) [AS2] is investigated by
means of large scale molecular dynamics computer simulations. We consider fully
equilibrated melts in the temperature range 6100K >= T >= 2300K as well as
glass configurations that were obtained from cooling runs from T=2300K to 300K
with a cooling rate of about 10^12K/s. Already at temperatures as high as
4000K, most of the Al and Si atoms are four-fold coordinated by oxygen atoms.
Thus, the structure of AS2 is that of a disordered tetrahedral network. The
packing of AlO4 tetrahedra is very different from that of SiO4 tetrahedra in
that Al is involved with a relatively high probability in small-membered rings
and in triclusters in which an O atom is surrounded by four cations. We find as
typical configurations two-membered rings with two Al atoms in which the shared
O atoms form a tricluster. On larger length scales, the system shows a
microphase separation in which the Al-rich network structure percolates through
the SiO2 network. The latter structure gives rise to a prepeak in the static
structure factor at a wavenumber q=0.5\AA^{-1}. The comparison of experimental
X-ray data with the results from the simulation shows a good agreement for the
structure function. The diffusion dynamics in AS2 is found to be much faster
than in SiO2. We show that the self-diffusion constants for O and Al are very
similar and that they are by a factor of 2-3 larger than the one for Si.Comment: 30 pages of Latex, 13 figure
Amorphous silica between confining walls and under shear: a computer simulation study
Molecular dynamics computer simulations are used to investigate a silica melt
confined between walls at equilibrium and in a steady-state Poisseuille flow.
The walls consist of point particles forming a rigid face-centered cubic
lattice and the interaction of the walls with the melt atoms is modelled such
that the wall particles have only a weak bonding to those in the melt, i.e.
much weaker than the covalent bonding of a Si-O unit. We observe a pronounced
layering of the melt near the walls. This layering, as seen in the total
density profile, has a very irregular character which can be attributed to a
preferred orientational ordering of SiO4 tetrahedra near the wall. On
intermediate length scales, the structure of the melt at the walls can be well
distinguished from that of the bulk by means of the ring size distribution.
Whereas essentially no structural changes occur in the bulk under the influence
of the shear fields considered, strong structural rearrangements in the ring
size distribution are present at the walls as far as there is a slip motion.
For the sheared system, parabolic velocity profiles are found in the bulk
region as expected from hydrodynamics and the values for the shear viscosity as
extracted from those profiles are in good agreement with those obtained in pure
bulk simulations from the appropriate Green-Kubo formula.Comment: 23 pages of Late
Potential Energy Landscape of the Apparent First-Order Phase Transition between Low-Density and High-Density Amorphous Ice
The potential energy landscape (PEL) formalism is a valuable approach within
statistical mechanics for describing supercooled liquids and glasses. Here we
use the PEL formalism and computer simulations to study the pressure-induced
transformations between low-density amorphous ice (LDA) and high-density
amorphous ice (HDA) at different temperatures. We employ the ST2 water model
for which the LDA-HDA transformations are remarkably sharp, similar to what is
observed in experiments, and reminiscent of a first-order phase transition. Our
results are consistent with the view that LDA and HDA configurations are
associated with two distinct regions (megabasins) of the PEL that are separated
by a potential energy barrier. At higher temperature, we find that low-density
liquid (LDL) configurations are located in the same megabasin as LDA, and that
high-density liquid (HDL) configurations are located in the same megabasin as
HDA. We show that the pressure-induced LDL-HDL and LDA-HDA transformations
occur along paths that interconnect these two megabasins, but that the path
followed by the liquid is different than the path followed by the amorphous
solid. At higher pressure, we also study the liquid-to-ice-VII first-order
phase transition, and find that the behavior of the PEL properties across this
transition are qualitatively similar to the changes found during the LDA-HDA
transformation. This similarity supports the interpretation that the LDA-HDA
transformation is a first-order-like phase transition between
out-of-equilibrium states.Comment: 29 pages, 8 figure
Intrinsic and structural isotope effects in Fe-based superconductors
The currently available results of the isotope effect on the superconducting
transition temperature T_c in Fe-based high-temperature superconductors (HTS)
are highly controversial. The values of the Fe isotope effect (Fe-IE) exponent
\alpha_Fe for various families of Fe-based HTS were found to be as well
positive, as negative, or even be exceedingly larger than the BCS value
\alpha_BCS=0.5. Here we demonstrate that the Fe isotope substitution causes
small structural modifications which, in turn, affect T_c. Upon correcting the
isotope effect exponent for these structural effects, an almost unique value of
\alpha~0.35-0.4 is observed for at least three different families of Fe-based
HTS.Comment: 4 pages, 2 figure
Coplanar stripline antenna design for optically detected magnetic resonance on semiconductor quantum dots
We report on the development and testing of a coplanar stripline antenna that
is designed for integration in a magneto-photoluminescence experiment to allow
coherent control of individual electron spins confined in single self-assembled
semiconductor quantum dots. We discuss the design criteria for such a structure
which is multi-functional in the sense that it serves not only as microwave
delivery but also as electrical top gate and shadow mask for the single quantum
dot spectroscopy. We present test measurements on hydrogenated amorphous
silicon, demonstrating electrically detected magnetic resonance using the
in-plane component of the oscillating magnetic field created by the coplanar
stripline antenna necessary due to the particular geometry of the quantum dot
spectroscopy. From reference measurements using a commercial electron spin
resonance setup in combination with finite element calculations simulating the
field distribution in the structure, we obtain an average magnetic field of
~0.2mT at the position where the quantum dots would be integrated into the
device. The corresponding pi-pulse time of ~0.3us fully meets the requirements
set by the high sensitivity optical spin read-out scheme developed for the
quantum dot
Axially open nonradiative structures: an example of single-mode resonator based on the sample holder
The concept of nonradiative dielectric resonator is generalized in order to
include axially open configurations having rotational invariance. The resulting
additional nonradiative conditions are established for the different resonance
modes on the basis of their azimuthal modal index. An approximate chart of the
allowed dielectric and geometrical parameters for the TE011 mode is given. A
practical realization of the proposed device based on commercial fused quartz
tubes is demonstrated at millimeter wavelengths, together with simple
excitation and tuning mechanisms. The observed resonances are characterized in
their basic parameters, as well as in the field distribution by means of a
finite element method. The predictions of the theoretical analysis are well
confirmed, both in the general behaviour and in the expected quality factors.
The resulting device, in which the sample holder acts itself as single-mode
resonating element, combines an extreme ease of realization with
state-of-the-art performances. The general benefits of the proposed open
single-mode resonators are finally discussed.Comment: 18 pages, 10 figure
Granular circulation in a cylindrical pan: simulations of reversing radial and tangential flows
Granular flows due to simultaneous vertical and horizontal excitations of a
flat-bottomed cylindrical pan are investigated using event-driven molecular
dynamics simulations. In agreement with recent experimental results, we observe
a transition from a solid-like state, to a fluidized state in which circulatory
flow occurs simultaneously in the radial and tangential directions. By going
beyond the range of conditions explored experimentally, we find that each of
these circulations reverse their direction as a function of the control
parameters of the motion. We numerically evaluate the dynamical phase diagram
for this system and show, using a simple model, that the solid-fluid transition
can be understood in terms of a critical value of the radial acceleration of
the pan bottom; and that the circulation reversals are controlled by the phase
shift relating the horizontal and vertical components of the vibrations. We
also discuss the crucial role played by the geometry of the boundary
conditions, and point out a relationship of the circulation observed here and
the flows generated in vibratory conveyors.Comment: 10 pages, 8 figure
Interpretation of Nuclear Quadrupole Resonance Spectra in Doped LaCuO
The nuclear quadrupole resonance (NQR) spectrum of strontium doped
LaCuO surprisingly resembles the NQR spectrum of LaCuO doped
with excess oxygen, both spectra being dominated by a main peak and one
principal satellite peak at similar frequencies. Using first-principles cluster
calculations this is investigated here by calculating the electric field
gradient (EFG) at the central copper site of the cluster after replacing a
lanthanum atom in the cluster with a strontium atom or adding an interstitial
oxygen to the cluster. In each case the EFG was increased by approximately 10 %
leading unexpectedly to the explanation that the NQR spectra are only
accidentally similar and the origins are quite different. Additionally the
widths of the peaks in the NQR spectra are explained by the different EFG of
copper centres remote from the impurity. A model, based on holes moving rapidly
across the planar oxygen atoms, is proposed to explain the observed increase in
frequency of both the main and satellite peaks in the NQR spectrum as the
doping concentration is increased
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