15,398 research outputs found
Phonon engineering with superlattices: generalized nanomechanical potentials
Earlier implementations to simulate coherent wave propagation in
one-dimensional potentials using acoustic phonons with gigahertz-terahertz
frequencies were based on coupled nanoacoustic resonators. Here, we generalize
the concept of adiabatic tuning of periodic superlattices for the
implementation of effective one-dimensional potentials giving access to cases
that cannot be realized by previously reported phonon engineering approaches,
in particular the acoustic simulation of electrons and holes in a quantum well
or a double well potential. In addition, the resulting structures are much more
compact and hence experimentally feasible. We demonstrate that potential
landscapes can be tailored with great versatility in these multilayered
devices, apply this general method to the cases of parabolic, Morse and
double-well potentials and study the resulting stationary phonon modes. The
phonon cavities and potentials presented in this work could be probed by
all-optical techniques like pump-probe coherent phonon generation and Brillouin
scattering
CIXL2: A Crossover Operator for Evolutionary Algorithms Based on Population Features
In this paper we propose a crossover operator for evolutionary algorithms
with real values that is based on the statistical theory of population
distributions. The operator is based on the theoretical distribution of the
values of the genes of the best individuals in the population. The proposed
operator takes into account the localization and dispersion features of the
best individuals of the population with the objective that these features would
be inherited by the offspring. Our aim is the optimization of the balance
between exploration and exploitation in the search process. In order to test
the efficiency and robustness of this crossover, we have used a set of
functions to be optimized with regard to different criteria, such as,
multimodality, separability, regularity and epistasis. With this set of
functions we can extract conclusions in function of the problem at hand. We
analyze the results using ANOVA and multiple comparison statistical tests. As
an example of how our crossover can be used to solve artificial intelligence
problems, we have applied the proposed model to the problem of obtaining the
weight of each network in a ensemble of neural networks. The results obtained
are above the performance of standard methods
Rotational properties of the binary and non-binary populations in the Trans-Neptunian belt
We present results for the short-term variability of Binary Trans-Neptunian
Objects (BTNOs). We performed CCD photometric observations using the 3.58 m
Telescopio Nazionale Galileo, the 1.5 m Sierra Nevada Observatory telescope,
and the 1.23 m Centro Astronomico Hispano Aleman telescope at Calar Alto
Observatory. We present results based on five years of observations and report
the short-term variability of six BTNOs. Our sample contains three classical
objects: 2003MW12, or Varda, 2004SB60, or Salacia, and 2002 VT130; one detached
disk object: 2007UK126; and two resonant objects: 2007TY430 and 2000EB173, or
Huya. For each target, possible rotational periods and/or photometric
amplitudes are reported. We also derived some physical properties from their
lightcurves, such as density, primary and secondary sizes, and albedo. We
compiled and analyzed a vast lightcurve database for Trans-Neptunian Objects
(TNOs) including centaurs to determine the lightcurve amplitude and spin
frequency distributions for the binary and non-binary populations. The mean
rotational periods, from the Maxwellian fits to the frequency distributions,
are 8.63+/-0.52 h for the entire sample, 8.37+/-0.58 h for the sample without
the binary population, and 10.11+/-1.19 h for the binary population alone.
Because the centaurs are collisionally more evolved, their rotational periods
might not be so primordial. We computed a mean rotational period, from the
Maxwellian fit, of 8.86+/-0.58 h for the sample without the centaur population,
and of 8.64+/-0.67 h considering a sample without the binary and the centaur
populations. According to this analysis, regular TNOs spin faster than
binaries, which is compatible with the tidal interaction of the binaries.
Finally, we examined possible formation models for several systems studied in
this work and by our team in previous papers.Comment: Accepted for publication in Astronomy and Astrophysics (June 26th,
2014); minor changes with published version; 21 pages, 17 figures, 7 table
A photometric search for active Main Belt asteroids
It is well known that some Main Belt asteroids show comet-like features. A
representative example is the first known Main Belt comet 133P/(7968)
Elst-Pizarro. If the mechanisms causing this activity are too weak to develop
visually evident comae or tails, the objects stay unnoticed. We are presenting
a novel way to search for active asteroids, based on looking for objects with
deviations from their expected brightnesses in a database. Just by using the
MPCAT-OBS Observation Archive we have found five new candidate objects that
possibly show a type of comet-like activity, and the already known Main Belt
comet 133P/(7968) Elst-Pizarro. Four of the new candidates, (315) Constantia,
(1026) Ingrid, (3646) Aduatiques, and (24684) 1990 EU4, show brightness
deviations independent of the object's heliocentric distance, while (35101)
1991 PL16 shows deviations dependent on its heliocentric distance, which could
be an indication of a thermal triggered mechanism. The method could be
implemented in future sky survey programmes to detect outbursts on Main Belt
objects almost simultaneously with their occurrence.Comment: 8 pages, 10 figures. Accepted for publication in A&A on December 20,
201
A comparison of the Bering Sea, Gulf of Alaska, and Aleutian Islands large marine ecosystems through food web modeling / by K. Aydin ... [et al.]
Detailed mass balance food web models were constructed to compare ecosystem characteristics for three Alaska regions: the eastern Bering Sea (EBS), the Gulf of Alaska (GOA), and the Aleutian Islands (AI). This paper documents the methods and data used to construct the models and compares ecosystem structure and indicators across models. The common modeling framework, including biomass pool and fishery definitions, resulted in comparable food webs for the three ecosystems which showed that they all have the same apex predator—the Pacific halibut longline fishery. However, despite the similar methods used to construct the models, the data from each system included in the analysis clearly define differences in food web structure which may be important considerations for fishery management in Alaska ecosystems. The results showed that the EBS ecosystem has a much larger benthic influence in its food web than either the GOA or the AI. Conversely, the AI ecosystem has the strongest pelagic influence in its food web relative to the other two systems. The GOA ecosystem appears balanced between benthic and pelagic pathways, but is notable in having a smaller fisheries catch relative to the other two systems, and a high biomass of fish predators above trophic level (TL) 4, arrowtooth flounder and halibut. The patterns visible in aggregated food webs were confirmed in additional more detailed analyses of biomass and consumption in each ecosystem, using both the single species and whole ecosystem indicators developed here
A generalized Finch-Skea class one static solution
In the present article, we discuss relativistic anisotropic solutions of the
Einstein field equation for the spherically symmetric line element under the
class I condition. To do so we apply the embedding class one technique using
Eisland condition. Within this approach, one arrives at a particular
differential equation that links the two metric components and
. In order to obtain the full space-time description inside the
stellar configuration we ansatz the generalized form of metric component
corresponding to the Finch-Skea solution. Once the space-time geometry
is specified we obtain the complete thermodynamic description i.e. the matter
density , the radial, and tangential pressures and ,
respectively. Graphical analysis shows that the obtained model respects the
physical and mathematical requirements that all ultra-high dense collapsed
structures must obey. The diagram suggests that the solution yields
stiffer EoS as parameter increases. The graph is in agreement with
the concepts of Bejgar et al. \cite{bej} that the mass at is lesser
by few percent (for this solution ) from . This suggests
that the EoSs is without any strong high-density softening due to
hyperonization or phase transition to an exotic state.Comment: 14 figures, Accepted in European Physical Journal
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