April 2010

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24 Reads

Published by Sociedad Cubana de Física

April 2010

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24 Reads

We use linear response theory in order to compute the light absorption
spectrum, in the terahertz band, of a polariton system composed by excitons in
a quantum dot very strongly coupled to the lowest photon mode of a thin
micropillar. In a thermalized (Bose condensed) system at low temperatures, the
spectral function shows a peak (GDR) associated to a 1s - 2p exciton
transition, enhanced by polariton effects. On the other hand, in a
non-equilibrium system absorption is peaked at low energies. Thus, a
measurement of terahertz absorption could give an indication of the degree of
thermalization in the polariton system.

January 2014

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114 Reads

We make a preliminary assessment on the habitability of potential rocky
exoplanets around Alpha Centauri B. We use several indexes: the Earth
Similarity Index, a mathematical model for photosynthesis, and a biological
productivity model. Considering the atmospheres of the exoplanets similar to
current Earth's atmosphere, we find consistent predictions of both the Earth
Similarity Index and the biological productivity model. The mathematical model
for photosynthesis clearly failed because does not consider the temperature
explicitly. For the case of Alpha Centauri B, several simulation runs give 11
planets in the habitable zone. Applying to them above mentioned indexes, we
select the five exoplanets more prone for photosynthetic life; showing that two
of them in principle have better conditions than Earth for this kind of life.

August 2012

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585 Reads

The study of vortex flows in the vicinity of multiple solid obstacles is of
considerable theoretical interest and practical importance. In particular, the
case of flows past a circular cylinder placed above a plane wall has attracted
a lot of attention recently. In this case, a stationary vortex is formed in
front of the cylinder, in contradistinction to the usual case without the plane
where a vortex pair is observed behind the cylinder. In the present work, we
apply modern complex analysis techniques to obtain the complex potential for
the problem of one point-vortex placed in a uniform stream past a circular
cylinder above a plane. A typical streamline pattern is also shown.

January 2014

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139 Reads

En este trabajo se aplica un modelo matemático de fotosíntesis para estimar la habitabilidad de un planeta hipotético orbitando Próxima del Centauro dentro de la zona de habitabilidad. Los resultados sugieren viabilidad apreciable para la productividad biológica, si los organismos presentes han evolucionado hasta desarrollar la capacidad de utilizar luz infrarroja para la fotosíntesis. We apply a mathematical model for photosynthesis to quantitatively assess the habitability of a hypothetical planet orbiting Proxima Centauri, inside the so called habitability zone. Results suggest significant viability for primary biological productivity, provided living organisms have evolved to reach the ability of using infrared light for photosynthesis.

May 2013

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2,771 Reads

This year we are celebrating 101 years since the discovery of cosmic rays.
They are whizzing all around the Universe, and they occur at very different
energies, including the highest particle energies that exist. However, theory
predicts an abrupt suppression (a "cutoff") above a specific huge energy. This
is difficult to verify, the measurements are controversial, but it provides a
unique opportunity to probe established concepts of physics - like Lorentz
Invariance - under extreme conditions. If the observations will ultimately
contradict this "cutoff", this could require a fundamental pillar of physics to
be revised.

October 2014

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261 Reads

The results of a previous work, where it was considered the effect of hole
doping on a simple model of the CuO2 planes in La2CuO4, are extended. The
parameters are adjusted with the objective to fix the known values of the gap
of 2 eV for this material and its dielectric constant of 21. We find again
indications of a "hidden" phase transition beneath the superconductor dome. The
transition is a second order one, and is associated with an energetic
coincidence of a ground insulator state (AFA)with an excited paramagnetic state
showing a pseudogap (PPG), at a critical point of the hole concentration around
xc=0.2. The evolution as a function of doping of the band structures and the
Fermi surface of the system in the phases AFA and PPG, is shown. In the zone of
low doping, the holes begin to occupy the states located at the mid of the
sides of the Brillouin zone, that in the AFA states have the strongest
antiferromagnetic character. Around the critical doping the results show that
in both phases, the Fermi surfaces and the energy spectrum of the filled
electronic states tend to coincide.

November 2012

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222 Reads

The result of 2-dimensional Gaussian lattice fit to a speckle intensity
pattern based on a linear model that includes nearest-neighbor interactions is
presented. We also include a Monte Carlo simulation of the same spatial speckle
pattern that takes the nearest-neighbor interactions into account. These
nearest-neighbor interactions lead to a spatial variance structure on the
lattice. The resulting spatial pattern fluctuates in value from point to point
in a manner characteristic of a stationary stochastic process. The value at a
lattice point in the simulation is interpreted as an inten-sity level and the
difference in values in neighboring cells produces a fluctuating intensity
pattern on the lattice. Changing the size of the mesh changes the relative size
of the speckles. Increasing the mesh size tends to average out the intensity in
the direction of the mean of the stationary process.

December 2005

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163 Reads

This article is concerned with a mathematical tool, the Associated Transfer Matrix T, which proves useful in the study of a wide class of physical problems involving multilayer heterostructures. General properties of linear, second order differential matrix Sturm Liouville operators are discussed as a basis for establishing general properties of T, which is also generally related to the Green function G. Some identities satisfied by T are derived, which prove useful in practice to monitor the numerical quality of computational processes.

August 2012

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47 Reads

A multicanonical formalism is applied to the problem of statistical
equilibrium in a complex system with a hierarchy of dynamical structures. At
the small scales the system is in quasi-equilibrium and follows a
Maxwell-Boltzmann distribution with a slowly fluctuating temperature. The
probability distribution for the temperature is determined using Bayesian
analysis and it is then used to average the Maxwell-Boltzmann distribution. The
resulting energy distribution law is written in terms of generalized
hypergeometric functions, which display power-law tails.

January 2013

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61 Reads

The behavior of dielectric properties for the system Ba 1-3x/2Laxx/2(Ti0.95 Zr 0.05)O3 obtained by the conventional ceramic method was studied. The permittivity and dielectric losses were measured dynamically from 77 up to 500 K using the Thermoelectric Analysis (ATE), noticing that the fusion of the permittivity peaks reveals the possible existence of a phase mixture. Low lanthanum impurification percents can induce a relaxor behavior, a fact rarely approached by the literature on BZT ceramics. The resulting compounds (0.001 ≤ x ≤ 0.010) exhibit marked decreases in the values of Curie temperature as well as increases of the dielectric constant.

June 2012

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26 Reads

[(K 0.5Na 0.5) 0.94Li 0.06] 0.97La 0.01(Nb 0.9Ta 0.1)O 3 lead-free ceramics have been prepared by the traditional ceramic method. The calcined powders were encapsulated and sintered in their own atmosphere. Our results reveal that Li +, La 3+ and Ta 5+ diffuse into the (K 0.5Na 0.5)NbO 3 lattice to form a solid solution with a perovskite structure. X-ray diffraction analysis showed a mixture of phases with a predominant orthorombic structure. The dielectric properties plotted vs temperature show a normal phase transition at 399oC. The piezoelectric behavior is characterized by the values of k p= 0.34, Q m=137 and ε max=1906, allowing us to conclude that a substitute for PZT ceramics is within reach.

January 2015

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113 Reads

The magnetic behavior of the alloy (CuInTe2)1-x(TaTe)x with x = 0.25 had been studied using SQUID techniques. Measurements of DC magnetic susceptibility using the ZFC-FC protocol and variation of magnetization as a function of the applied magnetic field and temperature were performed. It was found the presence of magnetic nanoclusters with a mean diameter of 10 nm and a coercitive field of 0.1 KOe at T = 5 K.

January 2014

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91 Reads

January 2013

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256 Reads

Empleando el Método de Trayectorias Cuasiclásicas, estudiamos la recombinación de moléculas de nitrógeno en una superficie de W(110). La eficiencia de la reacción de recombinación Eley-Rideal (ER) es comparada con la reportada en la referencia [Quintas et al. Rev. Cub. Fis. 27, 244 (2010)], donde dicha reacción es analizada sobre una superficie de W(100). Como ha sido observado en otros sistemas, la reactividad ER aumenta a medida que la interacción entre el átomo inicialmente adsorbido y el substrato disminuye, siendo sobre la superficie de W(110) muy superior a la reportada sobre W(100), especialmente a bajas energías de colisióón. Quasiclassical trajectories simulations are performed to study the recombiantion of Nitrogen molecules over W(110) surface.The efficiency of Eley-Rideal (ER) recombination reaction is compared with that reported on reference [Quintas et al. Rev.Cub. Fis. 27, 244 (2010)], where the ER reaction is analyzed over W(100) surface. As it has been already observed in other systems, ER reactivity increases as the interaction among the initially adsorbed atom and the substrate decreases. Over W(110) surface ER recombination probability is greater than the observed over W(100), specially at low collision energies.

December 2015

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587 Reads

The most abundant particles in the Universe are photons and neutrinos. Both
types of particles are whirling around everywhere, since the early Universe.
Hence the neutrinos are all around us, and permanently pass through our planet
and our bodies, but we do not notice: they are extremely elusive. They were
suggested as a theoretical hypothesis in 1930, and discovered experimentally in
1956. Ever since their properties keep on surprising us; for instance, they are
key players in the violation of parity symmetry. In the Standard Model of
particle physics they appear in three types, known as "flavors", and since
1998/9 we know that they keep on transmuting among these flavors. This
"neutrino oscillation" implies that they are massive, contrary to the previous
picture, with far-reaching consequences. This discovery was awarded the Physics
Nobel Prize 2015.

December 2016

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15 Reads

Essential topics of two phenomena related with the 2016 Nobel Prize in Physics are described: the quantum Hall effect and topological insulators. In particular: i) the importance of topology to explain the stability of the Klitzing constant obtained from the quantum Hall effect; ii) fundamental properties of topological insulators and iii) some aspects of band structure topology that allow us to identify topological insulator candidates.

January 2017

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17 Reads

The first direct detections of gravitational waves have opened new ways to investigate the universe. The achievement took years of refinement of ingenious experimental and theoretical techniques resulting in a new generation of observatories that promise novel ways of seeing the signals coming from violent cosmic events. These observatories are still under development and have not yet reached their optimum sensitivity, but already show very promising results. We discuss the results of these important findings as well as experimental and theoretical aspects of gravitational waves resulting in the Physics Noble Prize of 2017.

January 2015

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64 Reads

In this paper we compare the results obtained from the magnaetization as function of applied magnetic field normalized curve of a powder sample with those related to the transport flux-trapping curve measured in a pellet sample of the same material compacted at low pressure
(< 300 MPa) before the last sintering. A close relationship between both processes, penetration and trapping of the magnetic flux, has been observed even when these occur in powder and pellet samples, respectively.

July 2018

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34 Reads

Recently, extrinsic faulting has been discussed within the framework of computational mechanics allowing to derive expressions for the statistical complexity, entropy density and excess entropy as a function of faulting probability. In this contribution the analysis is extended to consider the combined presence of two planar faults type within the random faulting model. Extrinsic+intrinsic faults are considered. The-machine description of the faulting dynamics is presented. Entropic magnitudes are derived as well as expressions for the hexagonality and the probability of consecutive symbols in the Hägg coding. The analysis continues the study started with individual faulting types under the computational mechanics approach.

December 2015

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18 Reads

Many species of ants forage by building up two files: an outbound one moving
from the nest to the foraging area, and a nestbound one, returning from it to
the nest. Those files are eventually submitted to different threats. If the
danger is concentrated at one point of the file, one might expect that ants
returning to the nest will pass danger information to their nestmates moving in
the opposite direction towards the danger area. In this paper, we construct
simple cellular automata models for foraging ants submitted to localized
abduction, were danger information is transmitted using different protocols,
including the possibility of no transmission. The parameters we have used in
the simulations have been estimated from actual experiments under natural
conditions. So, it would be easy to test our information-transmission hypothese
in real experiments. Preliminary experimental results published elsewhere
suggest that the behavior of foraging ants of the species Atta insularis is
best described using the hypothesis of no transmission of danger information.

September 2016

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139 Reads

Understanding the penetration dynamics of intruders in granular beds is relevant not only for fundamental physics, but also for geophysical processes and construction on sediments or granular soils in areas potentially affected by earthquakes. In this work, we use Lock-in accelerometry to study the penetration of intruders into quasi-2D granular matter fluidized by lateral shaking. We observed that there are two well-defined stages in the penetration dynamics as the intruder sinks into the granular material.

June 2018

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28 Reads

January 2015

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52 Reads

We propose a system of equations to describe the development of the air rise caused by the water vapor condensation latent heat in a vertical cylinder of 10 km of height. The calculus results show a very fast development of the rising flow and its stabilization due to the friction of the droplets given by the water vapor condensation in the air.

July 2017

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587 Reads

The trajectories described by insects of different species in confined regions have been studied in depth. The main limitation found is the effective area covered while the insect is been tracked. Our work describes different image processing algorithms designed to capture the trajectory of an insect in a sequence of frames, and it focuses on the ones that don't depend on keeping fixed the position of the camera. Finally we propose a method for tracking insects in unconfined regions using a combination of some of the algorithms presented and a mobile camera system.

January 2011

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17 Reads

At the solar radiation and daylighting Spanish station of the International Daylight Measurement Program (IDMP) located at the Technical School of Architecture in Madrid (40° 25' N, 3° 41' W) we have studied the luminous efficacy of global, diffuse and direct solar radiation on a horizontal surface for all sky conditions (clear, overcast and intermediate). The experimental data used are mean hourly values of global and diffuse illuminances and irradiances measured on a horizontal surface during 15 months. We have used these data to develop the empirical models for illuminance, irradiance and luminous efficacy of global, diffuse and direct solar radiation, for all sky conditions as a function of the solar altitude multiplicaton sign in box at every 2,5° interval. The main result is verify that is possible to develop a luminous efficacy model from illuminance and irradiance models developed previously, that is very useful to estimate daylighting levels in building.