An analysis of the fluctuation potential in the modified Poisson-Boltzmann theory for restricted primitive model electrolytes

An approximate analytical solution to the fluctuation potential problem in the modified Poisson-Boltzmann theory of electrolyte solutions in the restricted primitive model is presented. The solution is valid for all inter-ionic distances, including contact values. The fluctuation potential solution is implemented in the theory to describe the structure of the electrolyte in terms of the radial distribution functions, and to calculate some aspects of thermodynamics, viz., configurational reduced energies, and osmotic coefficients. The calculations have been made for symmetric valence 1:1 systems at the physical parameters of ionic diameter $4.25 \times 10^{-10}$ m, relative permittivity 78.5, absolute temperature 298 K, and molar concentrations 0.1038, 0.425, 1.00, and 1.968. Radial distribution functions are compared with the corresponding results from the symmetric Poisson-Boltzmann, and the conventional and modified Poisson-Boltzmann theories. Comparisons have also been done for the contact values of the radial distributions, reduced configurational energies, and osmotic coefficients as functions of electrolyte concentration. Some Monte Carlo simulation data from the literature are also included in the assessment of the thermodynamic predictions. Results show a very good agreement with the Monte Carlo results and some improvement for osmotic coefficients and radial distribution functions contact values relative to these theories. The reduced energy curve shows excellent agreement with Monte Carlo data for molarities up to 1 mol/dm$^{3}$.Comment: 16 pages, 8 figures, 3 table

Polarization and Aharonov-Bohm oscillations in quantum-ring magnetoexcitons

We study interaction and radial polarization effects on the the absorption spectrum of neutral bound magnetoexcitons confined in quantum-ring structures. We show that the size and orientation of the exciton's dipole moment, as well as the interaction screening, play important roles in the Aharonov-Bohm oscillations. In particular, the excitonic absorption peaks display A-B oscillations both in position and amplitude for weak electron-hole interaction and large radial polarization. The presence of impurity scattering induces anticrossings in the exciton spectrum, leading to a modulation in the absorption strength. These properties could be used in experimental investigations of the effect in semiconductor quantum-ring structures.Comment: Updated version, 6 pages, 4 figures. To appear in Phys. Rev.

Impurity-enhanced Aharonov-Bohm effect in neutral quantum-ring magnetoexcitons

We study the role of impurity scattering on the photoluminescence (PL) emission of polarized magnetoexcitons. We consider systems where both the electron and hole are confined on a ring structure (quantum rings) as well as on a type-II quantum dot. Despite their neutral character, excitons exhibit strong modulation of energy and oscillator strength in the presence of magnetic fields. Scattering impurities enhance the PL intensity on otherwise "dark" magnetic field windows and non-zero PL emission appears for a wide magnetic field range even at zero temperature. For higher temperatures, impurity-induced anticrossings on the excitonic spectrum lead to unexpected peaks and valleys on the PL intensity as function of magnetic field. Such behavior is absent on ideal systems and can account for prominent features in recent experimental results.Comment: 7 pages, 7 figures, RevTe

Delocalization of Wannier-Stark ladders by phonons: tunneling and stretched polarons

We study the coherent dynamics of a Holstein polaron in strong electric fields. A detailed analytical and numerical analysis shows that even for small hopping constant and weak electron-phonon interaction, polaron states can become delocalized if a resonance condition develops between the original Wannier-Stark states and the phonon modes, yielding both tunneling and `stretched' polarons. The unusual stretched polarons are characterized by a phonon cloud that {\em trails} the electron, instead of accompanying it. In general, our novel approach allows us to show that the polaron spectrum has a complex nearly-fractal structure, due to the coherent coupling between states in the Cayley tree which describes the relevant Hilbert space. The eigenstates of a finite ladder are analyzed in terms of the observable tunneling and optical properties of the system.Comment: 7 pages, 4 figure

El taller de lectura de cuentos infantiles para el fortalecimiento de la comprensi?n lectora en los estudiantes de grado tercero en el colegio distrital estrella de sur jornada ma?ana

129 p. Recurso Electr?nicoEl proyecto de investigaci?n ?El taller de cuentos infantiles para el fortalecimiento de la comprensi?n lectora en el grado tercero en la I.E.D. Estrella del Sur en la jornada ma?ana?, ubicada en la localidad de Ciudad Bol?var Bogot?, result? del proceso de investigaci?n acci?n participaci?n que ten?a como objetivo general fortalecer el nivel de comprensi?n lectora en los estudiantes de grado tercero de la I.E.D. Estrella del Sur a partir del taller de lectura de cuentos infantiles que le permitan desempe?arse de forma eficaz en las diferentes lecturas que emprenda ya sea en el aula de clase o fuera de ella. En un primer momento se realiz? un diagn?stico para identificar el nivel de comprensi?n lectora en el que se encontraban los estudiantes, a partir de este se procedi? a realizar la sensibilizaci?n utilizando actividades que focalizaron el inter?s de los ni?os y ni?as para desarrollar la propuesta de intervenci?n, teniendo como base los siete niveles de comprensi?n lectora propuestos por el profesor Danilo S?nchez Lih?n. Durante el desarrollo de esta intervenci?n se utilizaron diferentes estrategias y metodolog?as que permitieron que los estudiantes participaran con agrado e inter?s en cada una de las sesiones abordadas, arrojando con ello resultados positivos para la muestra intervenida. Se realiz? el an?lisis cuantitativo cualificando los resultados obtenidos en la intervenci?n realizada utilizando r?bricas y representaci?n gr?fica, los cuales permitieron plantear unas conclusiones y una proyecci?n para la instituci?n intervenida.The researches project "The workshop?s stories to strengthen reading comprehension in the third grade on I. E. D. Southern Star in the day tomorrow, " located in the town of Ciudad Bol?var Bogot?, was the action research process participation overall objective was to strengthen the level of reading comprehension in the third grade students of FDI Southern Star Workshop from reading children's stories to be able to perform effectively in the different readings undertake either in the classroom or outside it. At first diagnosis was performed to identify the level of reading comprehension in which students were, from this we proceeded to conduct awareness activities using focused the interest of children to develop the proposed intervention, based the seven levels of reading comprehension proposed by Professor Danilo Sanchez Lihon. During the development of this intervention different strategies and methodologies that allowed the students to participate with pleasure and interest in each of the sessions addressed, thereby yielding positive results for the operated specimen were used. Keywords: reading, research, workshop, children's stories, levels of understandin

Contribution of picoplankton to the total particulate organic carbon concentration in the eastern South Pacific

International audienceProchlorococcus, Synechococcus, picophytoeukaryotes and bacterioplankton abundances and contributions to the total particulate organic carbon concentration, derived from the total particle beam attenuation coefficient (cp), were determined across the eastern South Pacific between the Marquesas Islands and the coast of Chile. All flow cytometrically derived abundances decreased towards the hyper-oligotrophic centre of the gyre and were highest at the coast, except for Prochlorococcus, which was not detected under eutrophic conditions. Temperature and nutrient availability appeared important in modulating picophytoplankton abundance, according to the prevailing trophic conditions. Although the non-vegetal particles tended to dominate the cp signal everywhere along the transect (50 to 83%), this dominance seemed to weaken from oligo- to eutrophic conditions, the contributions by vegetal and non-vegetal particles being about equal under mature upwelling conditions. Spatial variability in the vegetal compartment was more important than the non-vegetal one in shaping the water column particle beam attenuation coefficient. Spatial variability in picophytoplankton biomass could be traced by changes in both total chlorophyll a (i.e. mono + divinyl chlorophyll a) concentration and cp. Finally, picophytoeukaryotes contributed ~38% on average to the total integrated phytoplankton carbon biomass or vegetal attenuation signal along the transect, as determined by size measurements (i.e. equivalent spherical diameter) on cells sorted by flow cytometry and optical theory. Although there are some uncertainties associated with these estimates, the new approach used in this work further supports the idea that picophytoeukaryotes play a dominant role in carbon cycling in the upper open ocean, even under hyper-oligotrophic conditions