6,575 research outputs found
Does the Impact of Oportunidades Program Increases in Highly Competitive Regions?
Evidence on Oportunidades, a successful anti-poverty program in Mexico, has suggested that changes to the current grant structure may induce considerable improvements to its effectiveness. Moreover, there are proposals addressing the importance of regional, observable and unobservable characteristics, regarding its implementation. It is employed competitiveness level outcomes to investigate if this social policy has heterogeneous performance in different regions of intervention. For this purpose, a Difference-in-Difference model is applied to estimate short and mid-term impacts on enrolment rates. Results indicate that the general competitiveness effect is positive but not robust, given the considerable level of aggregation of the data used, whereas if it is distinguised Oportunidades treatment by selected competitiveness outcomes, states with highly efficient government institutions, middle competitive economic sectors and middle inclusive, educated and healthy individuals, present a larger program impact on enrolment rates. It is confirmed the significant improvements to program effectiveness and the impact of the competitiveness variables when it is considered only a sample of older children.Social policy effectiveness, competitiveness outcomes, school enrolment rates, regional effects, difference-in-difference (DID) model
Intrinsic leakage of the Josephson flux qubit and breakdown of the two-level approximation for strong driving
Solid state devices for quantum bit computation (qubits) are not perfect
isolated two-level systems, since additional higher energy levels always exist.
One example is the Josephson flux qubit, which consists on a mesoscopic SQUID
loop with three Josephson junctions operated at or near a magnetic flux of half
quantum. We study intrinsic leakage effects, i.e., direct transitions from the
allowed qubit states to higher excited states of the system during the
application of pulses for quantum computation operations. The system is started
in the ground state and rf- magnetic field pulses are applied at the qubit
resonant frequency with pulse intensity . A perturbative calculation of
the average leakage for small is performed for this case, obtaining that
the leakage is quadratic in , and that it depends mainly on the matrix
elements of the supercurrent. Numerical simulations of the time dependent
Schr\"odinger equation corresponding to the full Hamiltonian of this device
were also performed. From the simulations we obtain the value of above
which the two-level approximation breaks down, and we estimate the maximum Rabi
frequency that can be achieved. We study the leakage as a function of the ratio
among the Josephson energies of the junctions of the device, obtaining
the best value for minimum leakage (). The effects of flux
noise on the leakage are also discussed.Comment: Final improved version. Some figures have changed with new results
added. To be published in Phys. Rev.
Filogeografía: aplicaciones en taxonomía y conservación
La filogeografía se define como la disciplina que estudia los principios y procesos que gobiernan la distribución geográfica de los linajes genealógicos. Dos de las áreas de estudio donde se utilizan aproximaciones filogeográficas cada vez con mayor frecuencia son la taxonomía y la conservación. En esta revisión presentamos primero un resumen general sobre filogeografía y posteriormente discutimos cómo se han llevado al cabo estudios de taxonomía y conservación empleando aproximaciones filogeográficas, enfatizando sobre todo las limitaciones que deben considerarse. Incluimos ejemplos relevantes de estudios con animales que permitirán a los lectores conocer el sentido y alcance de dichas aplicaciones y diseñar adecuadamente estudios con estos objetivos.
Palabras clave: ADN, Biogeografía, Estructura genética, Genealogía.Phylogeography is defined as the discipline that studies the principles and processes that determine the geographical distribution of genealogical lineages. Two of the study areas where phylogeographic approaches are used more and more frequently are taxonomy and conservation. In this review we first present a general description of phylogeography and then discuss how research in taxonomy and conservation has been addressed when using phylogeographic approaches, emphasising in particular the limitations that need to be considered. We include relevant examples of studies with animals in order to help readers acquire the sense and scope of such applications and select the appropriate study design to meet these objectives.
Key words: DNA, Biogeography, Genetic structure, Genealogy.La filogeografía se define como la disciplina que estudia los principios y procesos que gobiernan la distribución geográfica de los linajes genealógicos. Dos de las áreas de estudio donde se utilizan aproximaciones filogeográficas cada vez con mayor frecuencia son la taxonomía y la conservación. En esta revisión presentamos primero un resumen general sobre filogeografía y posteriormente discutimos cómo se han llevado al cabo estudios de taxonomía y conservación empleando aproximaciones filogeográficas, enfatizando sobre todo las limitaciones que deben considerarse. Incluimos ejemplos relevantes de estudios con animales que permitirán a los lectores conocer el sentido y alcance de dichas aplicaciones y diseñar adecuadamente estudios con estos objetivos.
Palabras clave: ADN, Biogeografía, Estructura genética, Genealogía
Spin-dependent THz oscillator based on hybrid graphene superlattices
We theoretically study the occurrence of Bloch oscillations in biased hybrid
graphene systems with spin-dependent superlattices. The spin-dependent
potential is realized by a set of ferromagnetic insulator strips deposited on
top of a gapped graphene nanoribbon, which induce a proximity exchange
splitting of the electronic states in the graphene monolayer. We numerically
solve the Dirac equation and study Bloch oscillations in the lowest conduction
band of the spin-dependent superlattice. While the Bloch frequency is the same
for both spins, we find the Bloch amplitude to be spin dependent. This
difference results in a spin-polarized ac electric current in the THz range.Comment: 4 pages, 6 figure
Fluorescence decay in aperiodic Frenkel lattices
We study motion and capture of excitons in self-similar linear systems in
which interstitial traps are arranged according to an aperiodic sequence,
focusing our attention on Fibonacci and Thue-Morse systems as canonical
examples. The decay of the fluorescence intensity following a broadband pulse
excitation is evaluated by solving the microscopic equations of motion of the
Frenkel exciton problem. We find that the average decay is exponential and
depends only on the concentration of traps and the trapping rate. In addition,
we observe small-amplitude oscillations coming from the coupling between the
low-lying mode and a few high-lying modes through the topology of the lattice.
These oscillations are characteristic of each particular arrangement of traps
and they are directly related to the Fourier transform of the underlying
lattice. Our predictions can be then used to determine experimentally the
ordering of traps.Comment: REVTeX 3.0 + 3PostScript Figures + epsf.sty (uuencoded). To appear in
Physical Review
Frenkel Excitons in Random Systems With Correlated Gaussian Disorder
Optical absorption spectra of Frenkel excitons in random one-dimensional
systems are presented. Two models of inhomogeneous broadening, arising from a
Gaussian distribution of on-site energies, are considered. In one case the
on-site energies are uncorrelated variables whereas in the second model the
on-site energies are pairwise correlated (dimers). We observe a red shift and a
broadening of the absorption line on increasing the width of the Gaussian
distribution. In the two cases we find that the shift is the same, within our
numerical accuracy, whereas the broadening is larger when dimers are
introduced. The increase of the width of the Gaussian distribution leads to
larger differences between uncorrelated and correlated disordered models. We
suggest that this higher broadening is due to stronger scattering effects from
dimers.Comment: 9 pages, REVTeX 3.0, 3 ps figures. To appear in Physical Review
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