6,143 research outputs found
Ku-Band rendezvous radar performance computer simulation model
All work performed on the Ku-band rendezvous radar performance computer simulation model program since the release of the preliminary final report is summarized. Developments on the program fall into three distinct categories: (1) modifications to the existing Ku-band radar tracking performance computer model; (2) the addition of a highly accurate, nonrealtime search and acquisition performance computer model to the total software package developed on this program; and (3) development of radar cross section (RCS) computation models for three additional satellites. All changes in the tracking model involved improvements in the automatic gain control (AGC) and the radar signal strength (RSS) computer models. Although the search and acquisition computer models were developed under the auspices of the Hughes Aircraft Company Ku-Band Integrated Radar and Communications Subsystem program office, they have been supplied to NASA as part of the Ku-band radar performance comuter model package. Their purpose is to predict Ku-band acquisition performance for specific satellite targets on specific missions. The RCS models were developed for three satellites: the Long Duration Exposure Facility (LDEF) spacecraft, the Solar Maximum Mission (SMM) spacecraft, and the Space Telescopes
Density-matrix approach for an interacting polariton system
Using the Lindblad approach we develop a general formalism for theoretical
description of a spatially inhomogeneous bosonic system with dissipation
provided by the interaction of bosons with a phonon bath. We apply our results
to model the dynamics of an interacting one-dimensional polariton system in
real space and time, analyzing in detail the role of polariton-polariton and
polariton-phonon interactions.Comment: 7 pages + 3 figures. arXiv admin note: text overlap with
arXiv:1103.1336 by other author
Bistability phenomena in one-dimensional polariton wires
We investigate the phenomena of bistability and domain wall propagation in
polaritonic systems with dissipation provided by the interaction with
incoherent phonon bath. The results on the temperature dependence of the
polariton bistability behavior and polariton neuron switching are presented.Comment: 6 pages + 4 figures. Continuation of the work published in Phys. Rev.
B 83, 165316 (2011
Phonon-mediated Josephson oscillations in excitonic and polaritonic condensates
We analyze theoretically the role of the exciton-phonon interactions in
phenomena related to the Josephson effect between two spatially separated
exciton and exciton-polariton condensates. We consider the role of the
dephasing introduced by phonons in such phenomena as Josephson tunneling,
self-trapping and spontaneous polarization separation. In the regime of cw
pumping we find a remarkable bistability effect arising from exciton- exciton
interactions as well as regimes of self- sustained regular and chaotic
oscillations.Comment: 10 pages, Submitted to Phys. Rev.
Excitation properties of the divacancy in 4H-SiC
We investigate the quenching of the photoluminescence (PL) from the divacancy
defect in 4H-SiC consisting of a nearest-neighbour silicon and carbon
vacancies. The quenching occurs only when the PL is excited below certain
photon energies (thresholds), which differ for the four different inequivalent
divacancy configurations in 4H-SiC. Refined theoretical ab initio calculation
for the charge-transfer levels of the divacancy show very good agreement
between the position of the (0/-) level with respect to the conduction band for
each divacancy configurations and the corresponding experimentally observed
threshold, allowing us to associate the PL decay with conversion of the
divacancy from neutral to negative charge state due to capture of electrons
photoionized from other defects (traps) by the excitation. Electron
paramagnetic resonance measurements are conducted in dark and under excitation
similar to that used in the PL experiments and shed light on the possible
origin of traps in the different samples. A simple model built on this concept
agrees well with the experimentally-observed decay curves.Comment: 28 pages, 6 figure
Territoriality of giant otter groups in an area with seasonal flooding.
Territoriality carries costs and benefits, which are commonly affected by the spatial and temporal abundance and predictability of food, and by intruder pressure. Giant otters (Pteronura brasiliensis) live in groups that defend territories along river channels during the dry season using chemical signals, loud vocalizations and agonistic encounters. However, little is known about the territoriality of giant otters during the rainy season, when groups leave their dry season territories and follow fish dispersing into flooded areas. The objective of this study was to analyze long-term territoriality of giant otter groups in a seasonal environment. The linear extensions of the territories of 10 giant otter groups were determined based on locations of active dens, latrines and scent marks in each season. Some groups overlapped the limits of neighboring territories. The total territory extent of giant otters was correlated with group size in both seasons. The extent of exclusive territories of giant otter groups was negatively related to the number of adults present in adjacent groups. Territory fidelity ranged from 0 to 100%between seasons. Some groupsmaintained their territory for long periods, which demanded constant effort in marking and re-establishing their territories during the wet season. These results indicate that the defense capacity of groups had an important role in the maintenance of giant otter territories across seasons, which may also affect the reproductive success of alpha pairs
Efeito da caça no movimento e na área de uso dos jacarés, Pantanal Sul.
Na década de 80, a caça ilegal predominava no Pantanal, principalmente na área de rios, onde muitos jacarés foram mortos e suas carcaças encontradas em acampamentos dentro dos capões de mata. Caçadores mataram sete machos marcados que saíram da área de lagos para os rios. Na área de rios, as áreas usadas pelos cinco machos em locais não-sujeitos à caça. similares das usadas, pelos outros cinco machos em locais não -sujeitos à caça. O movimento de indivíduos de área não caçadas poe ser especialmente importante para o recrutamento de indivíduos à população em áreas caçadas, em longo prazo.bitstream/item/37293/1/BP61.pd
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