2,624 research outputs found
Conocimiento local aplicado a la colocación de refugios artificiales en un programa de mejoramiento de la pesquerÃa de langosta en Yucatán, México
Andreev reflection in bosonic condensates
We study the bosonic analog of Andreev reflection at a normal-superfluid
interface where the superfluid is a boson condensate. We model the normal
region as a zone where nonlinear effects can be neglected. Against the
background of a decaying condensate, we identify a novel contribution to the
current of reflected atoms. The group velocity of this Andreev reflected
component differs from that of the normally reflected one. For a
three-dimensional planar or two-dimensional linear interface Andreev reflection
is neither specular nor conjugate.Comment: 5 pages, 3 figures. Text revise
On the kinematics of massive star forming regions: the case of IRAS 17233-3606
Direct observations of accretion disks around high-mass young stellar objects
would help to discriminate between different models of formation of massive
stars. However, given the complexity of massive star forming regions, such
studies are still limited in number. Additionally, there is still no general
consensus on the molecular tracers to be used for such investigations. Because
of its close distance and high luminosity, IRAS 17233-3606 is a potential good
laboratory to search for traces of rotation in the inner gas around the
protostar(s). Therefore, we selected the source for a detailed analysis of its
molecular emission at 230 GHz with the SMA. We systematically investigated the
velocity fields of transitions in the SMA spectra which are not affected by
overlap with other transitions, and searched for coherent velocity gradients to
compare them to the distribution of outflows in the region. Beside CO emission
we also used high-angular H2 images to trace the outflow motions driven by the
IRAS 17233-3606 cluster. We find linear velocity gradients in many transitions
of the same molecular species and in several molecules. We report the first
detection of HNCO in molecular outflows from massive YSOs. We discuss the CH3CN
velocity gradient taking into account various scenarios: rotation, presence of
multiple unresolved sources with different velocities, and outflow(s). Although
other interpretations cannot be ruled out, we propose that the CH3CN emission
might be affected by the outflows of the region. Higher angular observations
are needed to discriminate between the different scenarios. The present
observations, with the possible association of CH3CN with outflows in a few
thousands AU around the YSOs' cluster, (i) question the choice of the tracer to
probe rotating structures, and (ii) show the importance of the use of H2 images
for detailed studies of kinematics.Comment: accepted for publication in A&
Resonant Hawking radiation in Bose-Einstein condensates
We study double-barrier interfaces separating regions of asymptotically
subsonic and supersonic flow of Bose condensed atoms. These setups contain at
least one black hole sonic horizon from which the analog of Hawking radiation
should be generated and emitted against the flow in the subsonic region.
Multiple coherent scattering by the double-barrier structure strongly modulates
the transmission probability of phonons, rendering it very sensitive to their
frequency. As a result, resonant tunneling occurs with high probability within
a few narrow frequency intervals. This gives rise to highly non-thermal spectra
with sharp peaks. We find that these peaks are mostly associated to decaying
resonances and only occasionally to dynamical instabilities. Even at achievable
nonzero temperatures, the radiation peaks can be dominated by the spontaneous
emission, i.e. enhanced zero-point fluctuations, and not, as often in analog
models, by stimulated emission.Comment: 20 pages, 9 figures, revised versio
Chemical-potential standard for atomic Bose-Einstein condensates
When subject to an external time periodic perturbation of frequency , a
Josephson-coupled two-state Bose-Einstein condensate responds with a constant
chemical potential difference , where is Planck's constant
and is an integer. We propose an experimental procedure to produce
ac-driven atomic Josephson devices that may be used to define a standard of
chemical potential. We investigate how to circumvent some of the specific
problems derived from the present lack of advanced atom circuit technology. We
include the effect of dissipation due to quasiparticles, which is essential to
help the system relax towards the exact Shapiro resonance, and set limits to
the range of values which the various physical quantities must have in order to
achieve a stable and accurate chemical potential difference between the
macroscopic condensates.Comment: 13 pages, 4 figure
Reducing vortex density in superconductors using the ratchet effect
A serious obstacle that impedes the application of low and high temperature
superconductor (SC) devices is the presence of trapped flux. Flux lines or
vortices are induced by fields as small as the Earth's magnetic field. Once
present, vortices dissipate energy and generate internal noise, limiting the
operation of numerous superconducting devices. Methods used to overcome this
difficulty include the pinning of vortices by the incorporation of impurities
and defects, the construction of flux dams, slots and holes and magnetic
shields which block the penetration of new flux lines in the bulk of the SC or
reduce the magnetic field in the immediate vicinity of the superconducting
device. Naturally, the most desirable would be to remove the vortices from the
bulk of the SC. There is no known phenomenon, however, that could form the
basis for such a process. Here we show that the application of an ac current to
a SC that is patterned with an asymmetric pinning potential can induce vortex
motion whose direction is determined only by the asymmetry of the pattern. The
mechanism responsible for this phenomenon is the so called ratchet effect, and
its working principle applies to both low and high temperature SCs. As a first
step here we demonstrate that with an appropriate choice of the pinning
potential the ratchet effect can be used to remove vortices from low
temperature SCs in the parameter range required for various applications.Comment: 7 pages, 4 figures, Nature (in press
¿Son los refugios artificiales una opción para mejorar la pesquerÃa de langosta? El caso de la pesquerÃa de Yucatán
Superconducting Fluxon Pumps and Lenses
We study stochastic transport of fluxons in superconductors by alternating
current (AC) rectification. Our simulated system provides a fluxon pump,
"lens", or fluxon "rectifier" because the applied electrical AC is transformed
into a net DC motion of fluxons. Thermal fluctuations and the asymmetry of the
ratchet channel walls induce this "diode" effect, which can have important
applications in devices, like SQUID magnetometers, and for fluxon optics,
including convex and concave fluxon lenses. Certain features are unique to this
novel two-dimensional (2D) geometric pump, and different from the previously
studied 1D ratchets.Comment: Phys. Rev. Lett. 83, in press (1999); 4 pages, 5 .gif figures;
figures also available at http://www-personal.engin.umich.edu/~nori/ratche
La educación en ciencias de la naturaleza en la secundaria vista por universitarios colombianos
Se presentan los resultados de unainvestigación relacionada con la percepción del estudiantado universitario acerca de la contribución de la Educación en Ciencias de la Naturaleza recibida durante la secundariaa la formación en las competencias cientÃficas planteadas por el Ministerio deEducación Nacional de Colombia.Los resultados muestran que un mayor número de estudiantes considera que la educación en ciencias contribuyó al desarrollo de las diversas competencias analizadas. Los factores que más influyeron en esta formación fueron la motivación propia del estudiante, las estrategias de enseñanza y las caracterÃsticas de personalidad de los profesores. Para aquellos estudiantes que consideraron que la educación en ciencias no contribuyó a su formación, los resultados muestran que los factores que más influyeron en sus respuestas fueron las estrategias de enseñanza de los profesores, la motivación propia del estudiante y los contenidos temáticos tratados en los cursos de ciencias
Extremely high velocity gas from the massive YSOs in IRAS 17233-3606
Molecular outflows from high-mass young stellar objects provide an excellent
way to study the star formation process, and investigate if they are scaled-up
versions of their low-mass counterparts. We selected the nearby massive star
forming region IRAS 17233-3606 in order to study the kinematics and physics
along the molecular outflow(s) originating from this source. We observed IRAS
17233-3606 in CO, a typical tracer of gas associated with molecular outflow,
with the Submillimeter Array in the (2-1) transition, and with the APEX
telescope in the higher excitation (6-5) line. Additional infrared H2
observations were performed with the UKIRT telescope. The CO data were analysed
using a LVG approach. Our data resolve the previously detected molecular
outflow in at least three different components, one of them with a high
collimation factor ~4, and characterised by emission at extremely high
velocities (|v-v_{LSR}|>120 km s^{-1}). The estimate of the kinematical outflow
parameters are typical of massive YSOs, and in agreement with the measured
bolometric luminosity of the source. The kinematic ages of the flows are in the
range 10^2-10^3 yr, and therefore point to young objects that still did not
reach the main sequence.Comment: accepted for publication in A&
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