15,866 research outputs found
On the dust environment of Main-Belt Comet 313P/Gibbs
We present observations carried out using the 10.4 m Gran Telescopio Canarias
and an interpretative model of the dust environment of activated asteroid
313P/Gibbs. We discuss three different models relating to different values of
the dust parameters, i.e, dust loss rate, maximum and minimum sizes of
particles, power index of the size distribution, and emission pattern. The best
model corresponds with an isotropic emission of particles which started on
August 1st. The size of grains were in the range of m, with
velocities for 100 m particles between ms, with a dust
production rate in the range of kgs. The dust tails'
brightness and morphology are best interpreted in terms of a model of sustained
and low dust emission driven by water-ice sublimation, spanning since 2014
August 1st, and triggered by a short impulsive event. This event produced an
emission of small particles of about 0.1 m with velocities of 4
ms. From our model we deduce that the activity of this Main-Belt
Comet continued for, at least, four months, since activation.Comment: 14 pages, 4 figures, 2 tables. Accepted for publication in The
Astrophysical Journa
Dust loss from activated asteroid P/2015 X6
We present observations and dust tail models of activated asteroid P/2015 X6
from deep imaging data acquired at the 10.4m Gran Telescopio Canarias (GTC)
from mid-December 2015 to late January 2016. The results of the modeling
indicate that the asteroid has undergone a sustained dust loss over a two-month
or longer period. The dust parameters, derived from multidimensional fits of
the available images, are compatible with either ice sublimation or rotational
instability processes. An impulsive event, as it could be associated to an
impact with another body, is less likely. A power-law distribution of
particles, with minimum and maximum radius of 1 m and 1 cm, and power
index of --3.3 is found to be consistent with the observations. Depending on
the ejection velocity model adopted, the particle velocities are found in the
0.3 to 10 m s range. The activation time was between 18-26 days before
discovery. The total ejected mass from that time to the most recent observation
is in the range 5-910 kg. No dust features giving indication of
past activity earlier than the activation time have been observed.Comment: Accepted by ApJ, May 15th 201
The role of excitons and trions on electron spin polarization in quantum wells
We have studied the time evolution of the electron spin polarization under
continuous photoexcitation in remotely n-doped semiconductor quantum wells. The
doped region allows us to get the necessary excess of free electrons to form
trions. We have considered electron resonant photoexcitation at free, exciton
and trion electron energy levels. Also, we have studied the relative effect of
photoexcitation energy density and doping concentration. In order to obtain the
two-dimensional density evolution of the different species, we have performed
dynamic calculations through the matrix density formalism. Our results indicate
that photoexcitation of free electron level leads to a higher spin
polarization. Also, we have found that increasing the photoexcitation energy or
diminishing the doping enhances spin polarization.Comment: 30 pages, 11 figures, 1 tabl
Domain-wall profile in the presence of anisotropic exchange interactions: Effective on-site anisotropy
Starting from a D-dimensional XXZ ferromagnetic Heisenberg model in an
hypercubic lattice, it is demonstrated that the anisotropy in the exchange
coupling constant leads to a D-dependent effective on-site anisotropy
interaction often ignored for D>1. As a result the effective width of the wall
depends on the dimensionality of the system. It is shown that the effective
one-dimensional Hamiltonian is not the one-dimensional XXZ version as assumed
in previous theoretical work. We derive a new expression for the wall profile
that generalizes the standard Landau-Lifshitz form. Our results are found to be
in very good agreement with earlier numerical work using the Monte Carlo
method. Preceding theories concerning the domain wall contribution to
magnetoresistance have considered the role of D only through the modification
of the density of states in the electronic band structure. This Brief Report
reveals that the wall profile itself contains an additional D dependence for
the case of anisotropic exchange interactions.Comment: 4 pages; new title and abstract; 1 figure comparing our results with
earlier numerical work; a more general model containing the usual on-site
anisotropy; new remarks and references on the following two topics: (a)
experimental evidence for the existence of spin exchange anisotropy, and (b)
preceding theories concerning the domain wall contribution to
magnetoresistance; to appear in Phys. Rev.
Disrupted asteroid P/2016 G1. II. Follow-up observations from the Hubble Space Telescope
After the early observations of the disrupted asteroid P/2016 G1 with the
10.4m Gran Telescopio Canarias (GTC), and the modeling of the dust ejecta, we
have performed a follow-up observational campaign of this object using the
Hubble Space Telescope (HST) during two epochs (June 28 and July 11, 2016). The
analysis of these HST images with the same model inputs obtained from the GTC
images revealed a good consistency with the predicted evolution from the GTC
images, so that the model is applicable to the whole observational period from
late April to early July 2016. This result confirms that the resulting dust
ejecta was caused by a relatively short-duration event with onset about 350
days before perihelion, and spanning about 30 days (HWHM). For a size
distribution of particles with a geometric albedo of 0.15, having radii limits
of 1 m and 1 cm, and following a power-law with index --3.0, the total
dust mass ejected is 210 kg. As was the case with the GTC
observations, no condensations in the images that could be attributed to a
nucleus or fragments released after the disruption event were found. However,
the higher limiting magnitude reachable with the HST images in comparison with
those from GTC allowed us to impose a more stringent upper limit to the
observed fragments of 30 m.Comment: 10 pages, 2 figures Accepted by Astronomical Journal, Nov. 2, 201
Andreev tunneling through a double quantum-dot system coupled to a ferromagnet and a superconductor: effects of mean field electronic correlations
We study the transport properties of a hybrid nanostructure composed of a
ferromagnet, two quantum dots, and a superconductor connected in series. By
using the non-equilibrium Green's function approach, we have calculated the
electric current, the differential conductance and the transmittance for
energies within the superconductor gap. In this regime, the mechanism of charge
transmission is the Andreev reflection, which allows for a control of the
current through the ferromagnet polarization. We have also included interdot
and intradot interactions, and have analyzed their influence through a mean
field approximation. In the presence of interactions, Coulomb blockade tend to
localized the electrons at the double-dot system, leading to an asymmetric
pattern for the density of states at the dots, and thus reducing the
transmission probability through the device. In particular, for non-zero
polarization, the intradot interaction splits the spin degeneracy, reducing the
maximum value of the current due to different spin-up and spin-down densities
of states. Negative differential conductance (NDC) appears for some regions of
the voltage bias, as a result of the interplay of the Andreev scattering with
electronic correlations. By applying a gate voltage at the dots, one can tune
the effect, changing the voltage region where this novel phenomenon appears.
This mechanism to control the current may be of importance in technological
applications.Comment: 12 pages, 11 figure
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