164 research outputs found
Electronic transport through ballistic chaotic cavities: reflection symmetry, direct processes, and symmetry breaking
We extend previous studies on transport through ballistic chaotic cavities
with spatial left-right (LR) reflection symmetry to include the presence of
direct processes. We first analyze fully LR-symmetric systems in the presence
of direct processes and compare the distribution w(T) of the transmission
coefficient T with that for an asymmetric cavity with the same "optical" S
matrix. We then study the problem of "external mixing" of the symmetry caused
by an asymmetric coupling of the cavity to the outside. We first consider the
case where symmetry breaking arises because two symmetrically positioned
waveguides are coupled to the cavity by means of asymmetric tunnel barriers.
Although this system is asymmetric with respect to the LR operation, it has a
striking memory of the symmetry of the cavity it was constructed from.
Secondly, we break LR symmetry in the absence of direct proceses by
asymmetrically positioning the two waveguides and compare the results with
those for the completely asymmetric case.Comment: 15 pages, 8 Postscript figures, submitted to Phys. Rev.
Upper critical field calculations for the high critical temperature superconductors considering inhomogeneities
We perform calculations to obtain the curve of high temperature
superconductors (HTSC). We consider explicitly the fact that the HTSC possess
intrinsic inhomogeneities by taking into account a non uniform charge density
. The transition to a coherent superconducting phase at a critical
temperature corresponds to a percolation threshold among different
superconducting regions, each one characterized by a given .
Within this model we calculate the upper critical field by means of an
average linearized Ginzburg-Landau (GL) equation to take into account the
distribution of local superconducting temperatures . This
approach explains some of the anomalies associated with and why
several properties like the Meissner and Nernst effects are detected at
temperatures much higher than .Comment: Latex text, add reference
Noise Filtering Strategies of Adaptive Signaling Networks: The Case of E. Coli Chemotaxis
Two distinct mechanisms for filtering noise in an input signal are identified
in a class of adaptive sensory networks. We find that the high frequency noise
is filtered by the output degradation process through time-averaging; while the
low frequency noise is damped by adaptation through negative feedback. Both
filtering processes themselves introduce intrinsic noises, which are found to
be unfiltered and can thus amount to a significant internal noise floor even
without signaling. These results are applied to E. coli chemotaxis. We show
unambiguously that the molecular mechanism for the Berg-Purcell time-averaging
scheme is the dephosphorylation of the response regulator CheY-P, not the
receptor adaptation process as previously suggested. The high frequency noise
due to the stochastic ligand binding-unbinding events and the random ligand
molecule diffusion is averaged by the CheY-P dephosphorylation process to a
negligible level in E.coli. We identify a previously unstudied noise source
caused by the random motion of the cell in a ligand gradient. We show that this
random walk induced signal noise has a divergent low frequency component, which
is only rendered finite by the receptor adaptation process. For gradients
within the E. coli sensing range, this dominant external noise can be
comparable to the significant intrinsic noise in the system. The dependence of
the response and its fluctuations on the key time scales of the system are
studied systematically. We show that the chemotaxis pathway may have evolved to
optimize gradient sensing, strong response, and noise control in different time
scalesComment: 15 pages, 4 figure
Amyotrophic lateral sclerosis: one or multiple causes?
The Amyotrophic lateral sclerosis (ALS) is the most common form of motor neuron disease in the adulthood, and it is characterized by rapid and progressive compromise of the upper and lower motor neurons. The majority of the cases of ALS are classified as sporadic and, until now, a specific cause for these cases still is unknown. To present the different hypotheses on the etiology of ALS. It was carried out a search in the databases: Bireme, Scielo and Pubmed, in the period of 1987 to 2011, using the following keywords: Amyotrophic lateral sclerosis, motor neuron disease, etiology, causes and epidemiology and its similar in Portuguese and Spanish. It did not have consensus as regards the etiology of ALS. Researches demonstrates evidences as regards intoxication by heavy metals, environmental and occupational causes, genetic mutations (superoxide dismutase 1), certain viral infections and the accomplishment of vigorous physical activity for the development of the disease. There is still no consensus regarding the involved factors in the etiology of ALS. In this way, new research about these etiologies are necessary, for a better approach of the patients, promoting preventive programs for the disease and improving the quality of life of the patients
Dermatophylosis in Santa Inês sheep from Distrito Federal
Relataram-se quatro casos de dermatofilose em ovinos da raça Santa Inês, no período de um ano. Microscopicamente observaram-se filamentos na forma de "trilho de bonde" e zoósporos nos quatro casos. A tentativa do isolamento do microrganismo foi realizada por meio do método de Haalstra e em apenas um caso obteve-se sucesso, observando-se colônias de aparência lisa, formato circular, cor amarelada e hemolítica em ágar sangue. O exame direto com coloração de Gram mostrou-se um método bastante eficiente na confirmação da presença do microrganismo afetando a epiderme em razão da morfologia típica do agente.Four cases of dermatophylosis were reported in Santa Inês sheep in a study period of one year. Microscopically, septate filaments and coccoid forms zoospores were observed. Attempts to isolate the microorganisms were accomplished using Haastra's method and it was successful in only one case. Dermatophilus congolensis samples have grown on blood agar, colonies where hemolytic, small, round and pigmentation vary from yellow to orange. The gram staining method was efficient to confirm the presence of the microorganism affecting the epidermis due to typical morphology of the agent
Astrobiologically Interesting Stars within 10 parsecs of the Sun
The existence of life based on carbon chemistry and water oceans relies upon
planetary properties, chiefly climate stability, and stellar properties, such
as mass, age, metallicity and Galactic orbits. The latter can be well
constrained with present knowledge. We present a detailed, up-to-date
compilation of the atmospheric parameters, chemical composition, multiplicity
and degree of chromospheric activity for the astrobiologically interesting
solar-type stars within 10 parsecs of the Sun. We determine their state of
evolution, masses, ages and space velocities, and produce an optimized list of
candidates that merit serious scientific consideration by the future
space-based interferometry probes aimed at directly detecting Earth-sized
extrasolar planets and seeking spectroscopic infrared biomarkers as evidence of
photosynthetic life. The initially selected stars number 33 solar-type within
the population of 182 stars (excluding late M-dwarfs) closer than 10 pc. A
comprehensive and detailed data compilation for these objects is still
essentially lacking: a considerable amount of recent data has so far gone
unexplored in this context. We present 13 objects as the nearest "biostars",
after eliminating multiple stars, young, chromospherically active, hard X-ray
emitting stars, and low metallicity objects. Three of these "biostars", HD
1581, 109358 and 115617, closely reproduce most of the solar properties and are
considered as premier targets. We show that approximately 7% of the nearby
stars are optimally interesting targets for exobiology.Comment: 36 pages, recommended for publication in Astrobiolog
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