1,980 research outputs found
Quantum shutter approach to tunneling time scales with wave packets
The quantum shutter approach to tunneling time scales (G. Garc\'{\i
}a-Calder\'{o}n and A. Rubio, Phys. Rev. A \textbf{55}, 3361 (1997)), which
uses a cutoff plane wave as the initial condition, is extended in such a way
that a certain type of wave packet can be used as the initial condition. An
analytical expression for the time evolved wave function is derived. The
time-domain resonance, the peaked structure of the probability density (as the
function of time) at the exit of the barrier, originally found with the cutoff
plane wave initial condition, is studied with the wave packet initial
conditions. It is found that the time-domain resonance is not very sensitive to
the width of the packet when the transmission process is in the tunneling
regime.Comment: 6 page
From Informal Housing to Housing at Risk, Preliminary Diagnosis of Some Neighborhoods in Quito
In the second half of the twentieth century in Ecuador, collective housing projects undertaken by the state began to emerge, while informal settlements consolidated on the outskirts of the city. The way in which these neighborhoods were consolidated has had an impact on the quality of life of their inhabitants. By allowing them to identify natural, social, and anthropic risks, they have accentuated and conditioned the safety of users, such as hillside location, context, materiality, accessibility, mobility, infrastructure, etc. The research is based on a comparative study of risks in three neighborhoods of Quito, to understand that both the participation of the people and the cooperation of the regulating entity are important for giving rise to housing projects. Based on the method proposed by Livingston and supported by the literature and bibliographic review, we seek to identify the risks that are generated or emphasized in the different stages of housing, highlighting the importance of building an integrated city based on the inhabitants and their way of inhabiting the space. Finally, we seek to understand that in the creation of housing projects, the appropriation of space makes it possible to turn a neighborhood into a community that reflects the diverse way of living of the users.
Keywords: citizen participation, social housing, informal settlements, Quito.
Resumen
En la segunda mitad del siglo XX en Ecuador empezaron a surgir proyectos de vivienda colectiva emprendidos por el estado central, en paralelo en la periferia de la ciudad se consolidaron asentamientos informales. La forma en la que se consolidaron estos barrios ha tenido un impacto en la calidad de vida de sus habitantes; esto ha permitido identificar riesgos naturales, sociales y antrópicos, los mismos que se han acentuado y condicionan la seguridad de los usuarios, tales como ubicación sobre laderas, contexto, materialidad, accesibilidad, movilidad, infraestructura, etc. La investigación se basa en un estudio comparativo sobre riesgos presentes en tres barrios de Quito, con el fin de entender que tanto la participación de las personas como la cooperación de la entidad reguladora, son importantes al momento de dar lugar a proyectos de vivienda. Partiendo del método propuesto por Livingston y apoyado en la revisión literaria y bibliográfica, se busca identificar los riesgos que se generan o enfatizan en las diferentes etapas de la vivienda, resaltando la importancia de construir una ciudad integrada que tiene como base a los habitantes y su forma de habitar el espacio. Finalmente, se busca entender que en la creación de proyectos habitacionales la apropiación del espacio permite convertir a un barrio en una comunidad que refleja la forma diversa de habitar de los usuarios.
Palabras Clave: Participación Ciudadana, Vivienda Social, Asentamientos Informales, Quito
Dynamical description of the buildup process in resonant tunneling: Evidence of exponential and non-exponential contributions
The buildup process of the probability density inside the quantum well of a
double-barrier resonant structure is studied by considering the analytic
solution of the time dependent Schr\"{o}dinger equation with the initial
condition of a cutoff plane wave. For one level systems at resonance condition
we show that the buildup of the probability density obeys a simple charging up
law, where is the
stationary wave function and the transient time constant is exactly
two lifetimes. We illustrate that the above formula holds both for symmetrical
and asymmetrical potential profiles with typical parameters, and even for
incidence at different resonance energies. Theoretical evidence of a crossover
to non-exponential buildup is also discussed.Comment: 4 pages, 2 figure
A proto brown dwarf candidate in Taurus
Aims. We search for brown dwarfs at the Class 0/I evolutionary stage, or proto brown dwarfs.
Methods. We present a multi wavelength study, ranging from optical at 0.8 μm to radio wavelengths at 6 cm, of a cool, very faint, and red multiple object, SSTB213 J041757, detected by Spitzer toward the Barnard 213 dark cloud, in Taurus.
Results. The SED of SSTB213 J041757 displays a clear excess at long wavelengths resembling that of a Class I object. The mid-IR source has two possible counterparts, A and B, in the near-IR and optical images, and the 350 μm observations detect clear extended emission, presumably from an envelope around the two sources. The position of A & B in the (Ic− J) versus (J − [3.6]) colour-colour diagram is consistent with them being Galactic sources and not extragalactic contaminants. A proper-motion study confirms this result for A, while it is inconclusive for B. The temperature and mass of the two possible central objects, according to COND evolutionary models, range between 1550−1750 K and 3−4 M_(Jupiter), and 950−1300 K and 1−2 M_(Jupiter), for A and B, respectively. The integrated SED provides bolometric temperatures and luminosities of 280 K and 0.0034 L_⊙, assuming that the emission at wavelengths > 5 μm is associated with component A, and 150 K and 0.0033 L_⊙, assuming that the emission at wavelengths > 5 μm is associated with component B, which would imply the SSTB213 J041757 object has a luminosity well below the luminosity of other very low luminosity objects discovered up to date.
Conclusions. With these characteristics, SSTB213 J041757 seems to be a promising, and perhaps double, proto brown dwarf candidate
On an inverse problem for anisotropic conductivity in the plane
Let be a bounded domain with smooth
boundary and a smooth anisotropic conductivity on .
Starting from the Dirichlet-to-Neumann operator on
, we give an explicit procedure to find a unique domain
, an isotropic conductivity on and the boundary
values of a quasiconformal diffeomorphism which
transforms into .Comment: 9 pages, no figur
Composite Spin Waves, Quasi-Particles and Low Temperature resistivity in Double Exchange Systems
We make a quantum description of the electron low temperature properties of
double exchange materials. In these systems there is a strong coupling between
the core spin and the carriers spin. This large coupling makes the low energy
spin waves to be a combination of ion and electron density spin waves. We study
the form and dispersion of these composite spin wave excitations. We also
analyze the spin up and down spectral functions of the temperature dependent
quasi-particles of this system. Finally we obtain that the thermally activated
composite spin waves renormalize the carriers effective mass and this gives
rise to a low temperature resistivity scaling as T ^{5/2}.Comment: 4 pages, REVTE
Tunneling dynamics in relativistic and nonrelativistic wave equations
We obtain the solution of a relativistic wave equation and compare it with
the solution of the Schroedinger equation for a source with a sharp onset and
excitation frequencies below cut-off. A scaling of position and time reduces to
a single case all the (below cut-off) nonrelativistic solutions, but no such
simplification holds for the relativistic equation, so that qualitatively
different ``shallow'' and ``deep'' tunneling regimes may be identified
relativistically. The nonrelativistic forerunner at a position beyond the
penetration length of the asymptotic stationary wave does not tunnel;
nevertheless, it arrives at the traversal (semiclassical or
B\"uttiker-Landauer) time "tau". The corresponding relativistic forerunner is
more complex: it oscillates due to the interference between two saddle point
contributions, and may be characterized by two times for the arrival of the
maxima of lower and upper envelops. There is in addition an earlier
relativistic forerunner, right after the causal front, which does tunnel.
Within the penetration length, tunneling is more robust for the precursors of
the relativistic equation
Mid-Infrared Variability of protostars in IC 1396A
We have used Spitzer/IRAC to conduct a photometric monitoring program of the
IC1396A dark globule in order to study the mid-IR (3.6 - 8 micron) variability
of the heavily embedded Young Stellar Objects (YSOs) present in that area. We
obtained light curves covering a 14 day timespan with a twice daily cadence for
69 YSOs, and continuous light curves with approximately 12 second cadence over
7 hours for 38 YSOs. Typical accuracies for our relative photometry were 1-2%
for the long timespan data and a few mmag, corresponding to less than 0.5%, for
the 7 hour continuous "staring-mode" data. More than half of the YSOs showed
detectable variability, with amplitudes from ~0.05 mag to ~0.2 mag. About
thirty percent of the YSOs showed quasi-sinusoidal light curve shapes with
apparent periods from 5-12 days and light curve amplitudes approximately
independent of wavelength over the IRAC bandpasses. We have constructed models
which simulate the time dependent spectral energy distributions of Class I and
I I YSOs in order to attempt to explain these light curves. Based on these
models, the apparently periodic light curves are best explained by YSO models
where one or two high latitude photospheric spots heat the inner wall of the
circumstellar disk, and where we view the disk at fairly large inclination
angle. Disk inhomogeneities, such as increasing the height where the accretion
funnel flows to the stellar hotspot, enhances the light curve modulations. The
other YSOs in our sample show a range of light curve shapes, some of which are
probably due to varying accretion rate or disk shadowing events. One star,
IC1396A-47, shows a 3.5 hour periodic light curve; this object may be a PMS
Delta Scuti star
A search for pre- and proto-brown dwarfs in the dark cloud Barnard 30 with ALMA
In this work we present ALMA continuum observations at 880 m of 30
sub-mm cores previously identified with APEX/LABOCA at 870m in the Barnard
30 cloud. The main goal is to characterize the youngest and lowest mass
population in the cloud. As a result, we report the detection of five (out of
30) spatially unresolved sources with ALMA, with estimated masses between 0.9
and 67 M. From these five sources, only two show gas emission. The
analysis of multi-wavelength photometry from these two objects, namely B30-LB14
and B30-LB19, is consistent with one Class II- and one Class I low-mass stellar
object, respectively. The gas emission is consistent with a rotating disk in
the case of B30-LB14, and with an oblate rotating envelope with infall
signatures in the case of LB19. The remaining three ALMA detections do not have
infrared counterparts and can be classified as either deeply embedded objects
or as starless cores if B30 members. In the former case, two of them (LB08 and
LB31) show internal luminosity upper limits consistent with Very Low Luminosity
objects, while we do not have enough information for LB10. In the starless core
scenario, and taking into account the estimated masses from ALMA and the
APEX/LABOCA cores, we estimate final masses for the central objects in the
substellar domain, so they could be classified as pre-BD core candidates.Comment: Published in A&
Haematological and Histological Bone Marrow Findings in Experimental Classical Swine Fever
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