Linear Theory of Electron-Plasma Waves at Arbitrary Collisionality

The dynamics of electron-plasma waves are described at arbitrary collisionality by considering the full Coulomb collision operator. The description is based on a Hermite-Laguerre decomposition of the velocity dependence of the electron distribution function. The damping rate, frequency, and eigenmode spectrum of electron-plasma waves are found as functions of the collision frequency and wavelength. A comparison is made between the collisionless Landau damping limit, the Lenard-Bernstein and Dougherty collision operators, and the electron-ion collision operator, finding large deviations in the damping rates and eigenmode spectra. A purely damped entropy mode, characteristic of a plasma where pitch-angle scattering effects are dominant with respect to collisionless effects, is shown to emerge numerically, and its dispersion relation is analytically derived. It is shown that such a mode is absent when simplified collision operators are used, and that like-particle collisions strongly influence the damping rate of the entropy mode.Comment: 23 pages, 10 figures, accepted for publication on Journal of Plasma Physic

Revealing hidden symmetries and gauge invariance of the massive Carroll-Field-Jackiw model

In this paper we have analyzed the improved version of the Gauge Unfixing (GU) formalism of the massive Carroll-Field-Jackiw model, which breaks both the Lorentz and gauge invariances, to disclose hidden symmetries to obtain gauge invariance, the key stone of the Standard Model. In this process, as usual, we have converted this second-class system into a first-class one and we have obtained two gauge invariant models. We have verified that the Poisson brackets involving the gauge invariant variables, obtained through the GU formalism, coincide with the Dirac brackets between the original second-class variables of the phase space. Finally, we have obtained two gauge invariant Lagrangians where one of them represents the Stueckelberg form.Comment: revised version. To appear in Europhysics Letter

Realistic Exact Solution for the Exterior Field of a Rotating Neutron Star

A new six-parametric, axisymmetric and asymptotically flat exact solution of Einstein-Maxwell field equations having reflection symmetry is presented. It has arbitrary physical parameters of mass, angular momentum, mass--quadrupole moment, current octupole moment, electric charge and magnetic dipole, so it can represent the exterior field of a rotating, deformed, magnetized and charged object; some properties of the closed-form analytic solution such as its multipolar structure, electromagnetic fields and singularities are also presented. In the vacuum case, this analytic solution is matched to some numerical interior solutions representing neutron stars, calculated by Berti & Stergioulas (Mon. Not. Roy. Astron. Soc. 350, 1416 (2004)), imposing that the multipole moments be the same. As an independent test of accuracy of the solution to describe exterior fields of neutron stars, we present an extensive comparison of the radii of innermost stable circular orbits (ISCOs) obtained from Berti & Stergioulas numerical solutions, Kerr solution (Phys. Rev. Lett. 11, 237 (1963)), Hartle & Thorne solution (Ap. J. 153, 807, (1968)), an analytic series expansion derived by Shibata & Sasaki (Phys. Rev. D. 58 104011 (1998)) and, our exact solution. We found that radii of ISCOs from our solution fits better than others with realistic numerical interior solutions.Comment: 13 pages, 13 figures, LaTeX documen

Molecular dynamics study of nitrobenzene and 2-nitrophenyloctyl ether saturated with water

In this paper, results obtained from molecular dynamics (MD) simulations on the water-saturated organic liquids nitrobenzene (NB) and 2-nitrophenyloctyl ether (NPOE) are presented. Both solvents are extremely important in many areas of chemistry, and they are seen as the most promising successors of n-octanol for ion transfer studies at the liquid-liquid interface. Thermodynamic (mass density, enthalpy of vaporization), structural (radial distribution functions) and dynamic (diffusion coefficients) properties of both liquids have been calculated and compared with those obtained from MD simulations of the corresponding pure (water-free) organic liquids. The relatively polar nature of NB allows for an efficient incorporation of water into the organic liquid, minimizing the impact on the solvent structure and dynamics. NPOE exhibits added hydrophobicity due to the presence of along aliphatic chain, and this has an effect on its water solvation properties. Steric effects caused by these chains, together with the much slower dynamic response of the solvent, substantially hinder the motion of water and prevent interactions with other water molecules. The different behaviour of these solvents towards water provides a good means for studying the properties of these liquids at the interface with water

Description of an incompressible variable density approach for the description of supercritical fluids flows

The simplification of the incompressible but variable density jet is based on the visualization of data similarity. Given the initial encouraging results obtained in the past, we extend, in the present paper, this hypothesis to a broader range of conditions to ascertain its applicability and its role as an alternative to the more commonly fully-compressible formulations encountered in the literature. Transcritical and supercritical injection conditions are considered for nitrogen, which works as a surrogate for the oxygen-hydrogen mixture combination, characteristic of liquid rocket propulsion. A close agreement is found between experiments and numerical results in terms of axial profiles and jet spreading rates.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio

Admission testing for higher education:A multi-cohort study on the validity of high-fidelity curriculum-sampling tests

<div><p>We investigated the validity of curriculum-sampling tests for admission to higher education in two studies. Curriculum-sampling tests mimic representative parts of an academic program to predict future academic achievement. In the first study, we investigated the predictive validity of a curriculum-sampling test for first year academic achievement across three cohorts of undergraduate psychology applicants and for academic achievement after three years in one cohort. We also studied the relationship between the test scores and enrollment decisions. In the second study, we examined the cognitive and noncognitive construct saturation of curriculum-sampling tests in a sample of psychology students. The curriculum-sampling tests showed high predictive validity for first year and third year academic achievement, mostly comparable to the predictive validity of high school GPA. In addition, curriculum-sampling test scores showed incremental validity over high school GPA. Applicants who scored low on the curriculum-sampling tests decided not to enroll in the program more often, indicating that curriculum-sampling admission tests may also promote self-selection. Contrary to expectations, the curriculum-sampling tests scores did not show any relationships with cognitive ability, but there were some indications for noncognitive saturation, mostly for perceived test competence. So, curriculum-sampling tests can serve as efficient admission tests that yield high predictive validity. Furthermore, when self-selection or student-program fit are major objectives of admission procedures, curriculum-sampling test may be preferred over or may be used in addition to high school GPA.</p></div

Conducta y percepcion del consumidor dentro de una industria respecto de complaining (quejas), en la ciudad de Talca

200 p.umidor dentro de la industria de la ciudad de Talca , donde estos manifiesten una mayor cantidad de quejas (Complaining). Se examinarán factores que inciden en el estado de insatisfacción, como se comportan los diferentes tipos de consumidores ante una queja, como expresan su insatisfacción, como afecta esta cuestión a las empresas del sector bajo estudio, si existen diferencias demográficas (Sexo, Edad, Educación y Nivel de ingreso), en las personas que manifiestan o no su queja. La industria elegida fue las tienda por departamentos, en estas caen empresas como : Falabella, Tricot, Corona, Urmax, Shopping Group y Germani. Para elegir esta industria se realizó una investigación en Sernac, con los datos que ellos poseen. La investigación esta dividida en dos etapas : Primera etapa : Estudio cualitativo, esta etapa estas divididas en sub-etapas : 1) Revisión Bibliográfica, 2) investigación exploratoria y 3) Entrevistas en profundidad realizadas a personal experto en temas de derechos del consumidor. La sub-etapa que destaca de esta primera etapa es el estudio exploratorio. Las necesidades de información de la investigación exploratoria se definen en los siguientes puntos : 1) Indagar sobre características demográficas tales como : Sexo, estado civil, nivel ocupacional y nivel de remuneración. 2) Indagar acerca de las variables que les produjo el manifestar una queja de la empresa que pertenece al sector industrial de estudio. 3) Indagar sobre los procedimientos que realizo el consumidor al realizar su queja. 4) Indagar sobre la percepción del consumidor sobre la empresa después de manifestar su queja. 5) Indagar las consecuencias que provoca para la empresa la solución o no de su queja. La población objetivo de estudio para la investigación exploratoria correspondieron a personas que manifestaron una queja en SERNAC por alguna tienda por departamentos durante los meses de Enero a Mayo de 1998, el universo total corresponde a 69 personas, se obtuvo una muestra de 40 personas utilizando un nivel de confianza del 95% y un error muestral del 10% Segunda etapa : Estudio cuantitativo, de estas destacan las sub. etapas: investigación Descriptiva Cualitativa Las necesidades de información de esta etapa son : 1) Determinar los factores que inciden en el estado de insatisfacción, que dan a lugar a una queja, por parte de los consumidores hacia el sector industrial en estudio.2) Determinar las diferentes formas en que los consumidores expresan su estado de insatisfacción respecto al sector industrial en estudio en la ciudad de Talca. 3) Determinar si existen diferencias demográficas significativas entre las personas que expresan su insatisfacción (se quejan), y aquellas que no lo hacen en la ciudad de Talca. 4) Determinar la percepción del consumidor que manifestó su respectiva quej

Ab initio study of proper topological ferroelectricity in layered perovskite La2Ti2O7

We present a first-principles investigation of ferroelectricity in layered perovskite oxide La2Ti2O7 (LTO), one of the compounds with highest Curie temperature known (1770 K). Our calculations reveal that LTO's ferroelectric transition results from the condensation of two soft modes that have the same symmetry and are strongly coupled anharmonically. Further, the leading instability mode essentially consists of rotations of the oxygen octahedra that are the basic building block of the perovskite structure; remarkably, because of the particular topology of the lattice, such O6 rotations give raise to a spontaneous polarization in LTO. The effects discussed thus constitute an example of how nano-structuring -- provided here by the natural layering of LTO -- makes it possible to obtain a significant polar character in structural distortions that are typically non-polar. We discuss the implications of our findings as regards the design of novel multifunctional materials. Indeed, the observed proper ferroelectricity driven by O6 rotations provides the ideal conditions to obtain strong magnetoelectric effects.Comment: 13 pages with 4 figures embedded. More information at http://www.icmab.es/dmmis/leem/jorg

Chaos and stability in a two-parameter family of convex billiard tables

We study, by numerical simulations and semi-rigorous arguments, a two-parameter family of convex, two-dimensional billiard tables, generalizing the one-parameter class of oval billiards of Benettin--Strelcyn [Phys. Rev. A 17, 773 (1978)]. We observe interesting dynamical phenomena when the billiard tables are continuously deformed from the integrable circular billiard to different versions of completely-chaotic stadia. In particular, we conjecture that a new class of ergodic billiard tables is obtained in certain regions of the two-dimensional parameter space, when the billiards are close to skewed stadia. We provide heuristic arguments supporting this conjecture, and give numerical confirmation using the powerful method of Lyapunov-weighted dynamics.Comment: 19 pages, 13 figures. Submitted for publication. Supplementary video available at http://sistemas.fciencias.unam.mx/~dsanders