6,939 research outputs found
Comparison of FDMA and CDMA for second generation land-mobile satellite communications
Code Division Multiple Access (CDMA) and Frequency Division Multiple Access (FDMA) (both analog and digital) systems capacities are compared on the basis of identical link availabilities and physical propagation models. Parameters are optimized for a bandwidth limited, multibeam environment. For CDMA, the benefits of voice activated carriers, antenna discrimination, polarization reuse, return link power control and multipath suppression are included in the analysis. For FDMA, the advantages of bandwidth efficient modulation/coding combinations, voice activated carriers, polarization reuse, beam placement, and frequency staggering were taken into account
The gravitational path integral and trace of the diffeomorphisms
I give a resolution of the conformal mode divergence in the Euclidean
gravitational path-integral by isolating the trace of the diffeomorphisms and
its contribution to the Faddeev-Popov measure.Comment: 20 pgs
Quantitative FDG-PET/CT predicts local recurrence and survival for squamous cell carcinoma of the anus
Endotaxial Si nanolines in Si(001):H
We present a detailed study of the structural and electronic properties of a
self-assembled silicon nanoline embedded in the H-terminated silicon (001)
surface, known as the Haiku stripe. The nanoline is a perfectly straight and
defect free endotaxial structure of huge aspect ratio; it can grow micrometre
long at a constant width of exactly four Si dimers (1.54nm). Another remarkable
property is its capacity to be exposed to air without suffering any
degradation. The nanoline grows independently of any step edges at tunable
densities, from isolated nanolines to a dense array of nanolines. In addition
to these unique structural characteristics, scanning tunnelling microscopy and
density functional theory reveal a one-dimensional state confined along the
Haiku core. This nanoline is a promising candidate for the long sought after
electronic solid-state one-dimensional model system to explore the fascinating
quantum properties emerging in such reduced dimensionality.Comment: 8 pages, 6 figure
One dimensional Si-in-Si(001) template for single-atom wire growth
Single atom metallic wires of arbitrary length are of immense technological
and scientific interest. We describe a novel silicon-only template enabling the
self-organised growth of isolated micrometer long surface and subsurface
single-atom chains. It consists of a one dimensional, defect-free
reconstruction - the Haiku core, here revealed for the first time in details -
self-assembled on hydrogenated Si(001) terraces, independent of any step edges.
We discuss the potential of this Si-in-Si template as an appealing alternative
to vicinal surfaces for nanoscale patterning.Comment: 3 pages, 2 figure
Seed populations for large solar particle events of cycle 23
Using high-resolution mass spectrometers on board the Advanced Composition Explorer (ACE), we surveyed the event-averaged ~0.1-60 MeV/nuc heavy ion elemental composition in 64 large solar energetic particle (LSEP) events of cycle 23. Our results show the following: (1) The rare isotope ^3He is greatly enhanced over the corona or the solar wind values in 46% of the events. (2) The Fe/O ratio decreases with increasing energy up to ~10 MeV/nuc in ~92% of the events and up to ~60 MeV/nuc in ~64% of the events. (3) Heavy ion abundances from C-Fe exhibit systematic M/g-dependent enhancements that are remarkably similar to those seen in ^3He-rich SEP events and CME-driven interplanetary (IP) shock events. Taken together, these results confirm the role of shocks in energizing particles up to ~60 MeV/nuc in the majority of large SEP events of cycle 23, but also show that the seed population is not
dominated by ions originating from the ambient corona or the thermal solar wind, as previously
believed. Rather, it appears that the source material for CME-associated large SEP events
originates predominantly from a suprathermal population with a heavy ion enrichment pattern
that is organized according to the ion's mass-per-charge ratio. These new results indicate that
current LSEP models must include the routine production of this dynamic suprathermal seed
population as a critical pre-cursor to the CME shock acceleration process
Charge States of Energetic Particles From Corotating Interaction Regions As Constraints On Their Source
Since the discovery of large (factor of greater than 1000) enhancements of singly ionized helium in corotating interaction regions (CIRs), it has become clear that low charge state ions from interstellar neutrals or other heliospheric sources can be preferentially injected into the energetic particle population. The large enhancements in suprathermal and energetic He suggest that low charge state ions may likewise affect the composition of CIR-related heavy ions such as C or Ne. Therefore, a key observational test of the contribution of pickup ions to the energetic particles associated with CIRs is their ionization state. We have used instrumentation on board the low Earth orbiting SAMPEX satellite, along with the
geomagnetic cutoff technique, to investigate the ionization states of CIR heavy ions in the energy range 0.5-1.0 MeV nucleon^(-1). In a sum of 14 CIR events, we find heavy-ion charge states similar to those of solar energetic particles from interplanetary shocks, with upper limits to the contribution of singly ionized particles on the order of a few percent of the total flux. We find that stripping in the interplanetary medium of singly ionized heavy ions energized in the CIR could not account for the observed high
charge states. Thus, the high charge state material we observed must have originated in the bulk solar wind or its suprathermal tail
Large-scale radio continuum properties of 19 Virgo cluster galaxies The influence of tidal interactions, ram pressure stripping, and accreting gas envelopes
Deep scaled array VLA 20 and 6cm observations including polarization of 19
Virgo spirals are presented. This sample contains 6 galaxies with a global
minimum of 20cm polarized emission at the receding side of the galactic disk
and quadrupolar type large-scale magnetic fields. In the new sample no
additional case of a ram-pressure stripped spiral galaxy with an asymmetric
ridge of polarized radio continuum emission was found. In the absence of a
close companion, a truncated HI disk, together with a ridge of polarized radio
continuum emission at the outer edge of the HI disk, is a signpost of ram
pressure stripping. 6 out of the 19 observed galaxies display asymmetric 6cm
polarized emission distributions. Three galaxies belong to tidally interacting
pairs, two galaxies host huge accreting HI envelopes, and one galaxy had a
recent minor merger. Tidal interactions and accreting gas envelopes can lead to
compression and shear motions which enhance the polarized radio continuum
emission. In addition, galaxies with low average star formation rate per unit
area have a low average degree of polarization. Shear or compression motions
can enhance the degree of polarization. The average degree of polarization of
tidally interacting galaxies is generally lower than expected for a given
rotation velocity and star formation activity. This low average degree of
polarization is at least partly due to the absence of polarized emission from
the thin disk. Ram pressure stripping can decrease whereas tidal interactions
most frequently decreases the average degree of polarization of Virgo spiral
galaxies. We found that moderate active ram pressure stripping has no influence
on the spectral index, but enhances the global radio continuum emission with
respect to the FIR emission, while an accreting gas envelope can but not
necessarily enhances the radio continuum emission with respect to the FIR
emission.Comment: 37 pages, 26 figures, accepted for publication in A&
Deuteron-equivalent and phase-equivalent interactions within light nuclei
Background: Phase-equivalent transformations (PETs) are well-known in quantum
scattering and inverse scattering theory. PETs do not affect scattering phase
shifts and bound state energies of two-body system but are conventionally
supposed to modify two-body bound state observables such as the rms radius and
electromagnetic moments. Purpose: In order to preserve all bound state
observables, we propose a new particular case of PETs, a deuteron-equivalent
transformation (DET-PET), which leaves unchanged not only scattering phase
shifts and bound state (deuteron) binding energy but also the bound state wave
function. Methods: The construction of DET-PET is discussed; equations defining
the simplest DET-PETs are derived. We apply these simplest DET-PETs to the
JISP16 interaction and use the transformed interactions in
calculations of H and He binding energies in the No-core Full
Configuration (NCFC) approach based on extrapolations of the No-core Shell
Model (NCSM) basis space results to the infinite basis space. Results: We
demonstrate the DET-PET modification of the scattering wave functions and
study the DET-PET manifestation in the binding energies of H and He
nuclei and their correlation (Tjon line). Conclusions: It is shown that some
DET-PETs generate modifications of the central component while the others
modify the tensor component of the interaction. DET-PETs are able to
modify significantly the scattering wave functions and hence the off-shell
properties of the interaction. DET-PETs give rise to significant changes
in the binding energies of H (in the range of approximately 1.5 MeV) and
He (in the range of more than 9 MeV) and are able to modify the correlation
patterns of binding energies of these nuclei
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