1,775 research outputs found
Typical length scales in conducting disorderless networks
We take advantage of a recently established equivalence, between the
intermittent dynamics of a deterministic nonlinear map and the scattering
matrix properties of a disorderless double Cayley tree lattice of connectivity
, to obtain general electronic transport expressions and expand our
knowledge of the scattering properties at the mobility edge. From this we
provide a physical interpretation of the generalized localization length.Comment: 12 pages, 3 figure
Poynting's theorem for planes waves at an interface: a scattering matrix approach
We apply the Poynting theorem to the scattering of monochromatic
electromagnetic planes waves with normal incidence to the interface of two
different media. We write this energy conservation theorem to introduce a
natural definition of the scattering matrix S. For the dielectric-dielectric
interface the balance equation lead us to the energy flux conservation which
express one of the properties of S: it is a unitary matrix. For the
dielectric-conductor interface the scattering matrix is no longer unitary due
to the presence of losses at the conductor. However, the dissipative term
appearing in the Poynting theorem can be interpreted as a single absorbing mode
at the conductor such that a whole S, satisfying flux conservation and
containing this absorbing mode, can be defined. This is a simplest version of a
model introduced in the current literature to describe losses in more complex
systems.Comment: 5 pages, 3 figures, submitted to Am. J. Phy
Chaotic scattering with direct processes: A generalization of Poisson's kernel for non-unitary scattering matrices
The problem of chaotic scattering in presence of direct processes or prompt
responses is mapped via a transformation to the case of scattering in absence
of such processes for non-unitary scattering matrices, \tilde S. In the absence
of prompt responses, \tilde S is uniformly distributed according to its
invariant measure in the space of \tilde S matrices with zero average, < \tilde
S > =0. In the presence of direct processes, the distribution of \tilde S is
non-uniform and it is characterized by the average (\neq 0). In
contrast to the case of unitary matrices S, where the invariant measures of S
for chaotic scattering with and without direct processes are related through
the well known Poisson kernel, here we show that for non-unitary scattering
matrices the invariant measures are related by the Poisson kernel squared. Our
results are relevant to situations where flux conservation is not satisfied.
For example, transport experiments in chaotic systems, where gains or losses
are present, like microwave chaotic cavities or graphs, and acoustic or elastic
resonators.Comment: Added two appendices and references. Corrected typo
Caracterización genética de ovinos criollos colombianos
ABSTRACT: indigenous breeds are important for poor farmers because of their natural selection against harsh environments and adaptation to regional conditions. However, inbreeding of indigenous sheep populations has increased in Colombia due to indiscriminate cross-breeding with foreign animals and lack of reproductive controls, with subsequent loss in productivity, which poses a great risk for the conservation of valuable genes. Objective: to determine the genetic diversity in Colombian indigenous sheep by using a panel of 10 microsatellite molecular markers. Methods: blood samples from 362 individuals from 43 farms in 11 Colombian provinces were genotyped and analyzed for a panel of 10 microsatellite markers. Results: a total of 134 alleles were found (13.4 alleles/locus on average) with a range of observed and expected heterozygosity of 0.428 to 0.831 and 0.615 to 0.855, respectively, and 0.742 polymorphic information content (PIC). The average Wright F-statistics (FIS ) of the breeds was 0.107, suggesting moderate levels of inbreeding. Colombian sheep showed a low level of genetic differentiation among breeds (FST = 0.054) and STRUCTURE analysis showed complex patterns of admixture in the breeds. Conclusion: overall, Colombian sheep have high genetic variability, which is very important for future conservation programs and genetic improvement.RESUMEN: las razas animales autóctonas son importantes para los agricultores de escasos recursos a causa de su selección natural contra el duro ambiente y su adaptación a condiciones regionales. Sin embargo en Colombia, debido al cruce indiscriminado con razas foráneas y a la falta de control de la reproducción, ha aumentado la consanguinidad en las poblaciones de ovinos criollos y por lo tanto la pérdida en la productividad, lo que supone un gran riesgo para la conservación de genes valiosos. Objetivo: determinar la diversidad genética en razas criollas de ovinos colombianos utilizando análisis por microsatélites. Métodos: se visitaron 43 granjas localizadas en 11 departamentos del paÃs, en las cuales se tomaron muestras de sangre a 362 individuos. Las muestras fueron genotipadas y analizadas para un panel de 10 marcadores microsatélites. Resultados: un total de 134 alelos fueron encontrados (13,4 alelos/locus en promedio), con un rango de heterocigocidad observada y esperada de 0,428 a 0,831 y 0,615 a 0,855, respectivamente, y un contenido de información polimórfica (PIC) promedio de 0,742. El Wright F-statistics (FIS) promedio de las razas evaluadas fue 0,107, lo cual sugiere que las razas tienen niveles moderados de consanguinidad. Las ovejas colombianas presentaron un bajo grado de diferenciación genética entre las distintas razas (FST = 0,054) y el análisis de STRUCTURE mostro complejos patrones de mezcla en las razas estudiadas. Conclusión: en términos generales, las ovejas colombianas presentan una alta variabilidad genética lo cual es muy importante para futuros programas de conservacion y mejoramiento genetico
Aportación de la densitometrÃa ósea en las artroplastias de rodilla
El presente trabajo consiste en una revisión clÃnica y radiológica de 84 artroplastias
modulares de rodilla de las cuales, en 19 se ha estudiado la masa ósea periprotésica
(7 cementadas y 12 sin cementar) antes, al mes y a los seis meses de la intervención mediante
un densitómetro de doble fotón. Aparte, se ha probado la correlación directa entre la indicación
peroperatoria de no cementar y una mayor masa ósea en todas las zonas periprotésicas;
por otro lado, se ha observado una diferente distribución de la densidad mineral ósea en la
tibia tras la intervención, según se haya utilizado o no cemento. Estos hallazgos confirman el
estudio densitométrico previo a la intervención como un método complementario útil en la
indicación de la cementación de la artroplastia total de rodilla.A clinical and radiological revision of 84 modular knee arthroplasties is
presented. Periprosthesis bone mineral content was studied in 19 prostheses (7 cemented
and 12 uncemented) before, surgery one and 6 months after the intervention using a densitometer
of double energy. A direct correlation between the implantation of a non-cemented
prosthesis and the growth of the bone mass in all the periprosthesis areas was found, existing
a different distribution of the mineral bone density of the tibia after the intervention
depending on the use of cement. These findings show that the densitometric study previous
to an intervention as a useful complementary method in the indication of cemented total
knee arthroplasty
The use of cosmic muons in detecting heterogeneities in large volumes
The muon intensity attenuation method to detect heterogeneities in large
matter volumes is analyzed. Approximate analytical expressions to estimate the
collection time and the signal to noise ratio, are proposed and validated by
Monte Carlo simulations. Important parameters, including point spread function
and coordinate reconstruction uncertainty are also estimated using Monte Carlo
simulations.Comment: 8 pages, 11 figures, submetted to NIM
Direct detection of galaxy stellar halos : NGC 3957 as a test case
We present a direct detection of the stellar halo of the edge-on S0 galaxy
NGC 3957, using ultra-deep VLT/VIMOS V and R images. This is achieved with a
sky subtraction strategy based on infrared techniques. These observations allow
us to reach unprecedented high signal-to-noise ratios up to 15 kpc away from
the galaxy center, rendering photon-noise negligible. The 1 sigma detection
limits are R = 30.6 mag/arcsec^2 and V = 31.4 mag/arcsec^2. We conduct a
thorough analysis of the possible sources of systematic errors that could
affect the data: flat-fielding, differences in CCD responses, scaling of the
sky background, the extended halo itself, and PSF wings. We conclude that the
V-R colour of the NGC 3957 halo, calculated between 5 and 8 kpc above the disc
plane where the systematic errors are modest, is consistent with an old and
preferentially metal-poor normal stellar population, like that revealed in
nearby galaxy halos from studies of their resolved stellar content. We do not
find support for the extremely red colours found in earlier studies of diffuse
halo emission, which we suggest might have been due to residual systematic
errors.Comment: Accepted for publication in A&A - "language edited
Spectroscopy of Bright QUEST RR Lyrae Stars: Velocity Substructures toward Virgo
Using a sample of 43 bright (V<16.1, distance <13 kpc) RR Lyrae stars (RRLS)
from the QUEST survey with spectroscopic radial velocities and metallicities,
we find that several separate halo substructures contribute to the Virgo
overdensity (VOD). While there is little evidence for halo substructure in the
spatial distribution of these stars, their distribution in radial velocity
reveals two moving groups. These results are reinforced when the sample is
combined with a sample of blue horizontal branch stars that were identified in
the SDSS, and the combined sample provides evidence for one additional moving
group. These groups correspond to peaks in the radial velocity distribution of
a sample of F type main-sequence stars that was recently observed in the same
directon by SEGUE, although in one case the RRLS and F star groups may not lie
at the same distance. One of the new substructures has a very narrow range in
metallicity, which is more consistent with it being the debris from a destroyed
globular cluster than from a dwarf galaxy. A small concentration of stars have
radial velocities that are similar to the Virgo Stellar Stream (VSS) that was
identified previously in a fainter sample of RRLS. Our results suggest that
this feature extends to distances as short as ~12 kpc from its previous
detection at ~19 kpc. None of the new groups and only one star in the sample
have velocities that are consistent with membership in the leading tidal stream
from the Sagittarius Dwarf Spheroidal Galaxy, which some authors have suggested
is the origin of the VOD.Comment: Accepted for publication in the A
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