GTD analysis of airborne antennas radiating in the presence of lossy dielectric layers

The patterns of monopole or aperture antennas mounted on a perfectly conducting convex surface radiating in the presence of a dielectric or metal plate are computed. The geometrical theory of diffraction is used to analyze the radiating system and extended here to include diffraction by flat dielectric slabs. Modified edge diffraction coefficients valid for wedges whose walls are lossy or lossless thin dielectric or perfectly conducting plates are developed. The width of the dielectric plates cannot exceed a quarter of a wavelength in free space, and the interior angle of the wedge is assumed to be close to 0 deg or 180 deg. Systematic methods for computing the individual components of the total high frequency field are discussed. The accuracy of the solutions is demonstrated by comparisons with measured results, where a 2 lambda by 4 lambda prolate spheroid is used as the convex surface. A jump or kink appears in the calculated pattern when higher order terms that are important are not included in the final solution. The most immediate application of the results presented here is in the modelling of structures such as aircraft which are composed of nonmetallic parts that play a significant role in the pattern

Quantum Hall states under conditions of vanishing Zeeman energy

We report on magneto-transport measurements of a two-dimensional electron gas confined in a Cd$_{0.997}$Mn$_{0.003}$Te quantum well structure under conditions of vanishing Zeeman energy. The electron Zeeman energy has been tuned via the $s-d$ exchange interaction in order to probe different quantum Hall states associated with metallic and insulating phases. We have observed that reducing Zeeman energy to zero does not necessary imply the disappearing of quantum Hall states, i.e. a closing of the spin gap. The spin gap value under vanishing Zeeman energy conditions is shown to be dependent on the filling factor. Numerical simulations support a qualitative description of the experimental data presented in terms of a crossing or an avoided-crossing of spin split Landau levels with same orbital quantum number $N$

Collective character of spin excitations in a system of Mn2+^{2+} spins coupled to a two-dimensional electron gas

We have studied the low energy spin excitations in n-type CdMnTe based dilute magnetic semiconductor quantum wells. For magnetic fields for which the energies for the excitation of free carriers and Mn spins are almost identical an anomalously large Knight shift is observed. Our findings suggests the existence of a magnetic field induced ferromagnetic order in these structures, which is in agreement with recent theoretical predictions [J. K{\"o}nig and A. H. MacDonald, submitted Phys. Rev. Lett. (2002)]Comment: 4 figure

Structural response of concrete-filled elliptical steel hollow sections under eccentric compression

The purpose of this research is to examine the behaviour of elliptical concrete-filled steel tubular stub columns under a combination of axial force and bending moment. Most of the research carried out to date involving concrete-filled steel sections has focussed on circular and rectangular tubes, with each shape exhibiting distinct behaviour. The degree of concrete confinement provided by the hollow section wall has been studied under pure compression but remains ambiguous for combined compressive and bending loads, with no current design provision for this loading combination. To explore the structural behaviour, laboratory tests were carried out using eight stub columns of two different tube wall thicknesses and applying axial compression under various eccentricities. Moment-rotation relationships were produced for each specimen to establish the influence of cross-section dimension and axis of bending on overall response. Full 3D finite element models were developed, comparing the effect of different material constitutive models, until good agreement was found. Finally, analytical interaction curves were generated assuming plastic behaviour and compared with the experimental and finite element results. Ground work provided from these tests paves the way for the development of future design guidelines on the member level

Enhancement of the spin-gap in fully occupied two-dimensional Landau levels

Polarization-resolved magneto-luminescence, together with simultaneous magneto-transport measurements, have been performed on a two-dimensional electron gas (2DEG) confined in CdTe quantum well in order to determine the spin-splitting of fully occupied electronic Landau levels, as a function of the magnetic field (arbitrary Landau level filling factors) and temperature. The spin splitting, extracted from the energy separation of the \sigma+ and \sigma- transitions, is composed of the ordinary Zeeman term and a many-body contribution which is shown to be driven by the spin-polarization of the 2DEG. It is argued that both these contributions result in a simple, rigid shift of Landau level ladders with opposite spins.Comment: 4 pages, 3 figure

Spontaneous polarisation of the neutral interface for valence asymmetric coulombic systems

In this paper, we discuss the phenomenon of a spontaneous polarisation of a neutral hard planar interface for valence asymmetric coulombic systems. Within a field theoretical description, we account for the existence of non trivial charge density and electric potential profiles. The analysis of the phenomenon shows that the effect is related to combinatorics in relation with the existence of the two independent species cations and anions. This simple and basic feature is related to the quantum mechanical properties of the system. The theoretical results are compared with numerical simulations data and are shown to be in very good agreement, which a fortiori justifies our physical interpretation.Comment: 12 pages, 11 figure

Land administration in Ecuador; Current situation and opportunities with adoption of fit-for-purpose land administration approach

The aim of this paper is to explore current land administration situation in Ecuador and identify opportunities for fit-for-purpose (FFP) land administration approach that could improve the land administration functions for the country and its citizens. In this paper, initially literature about land administration, guidelines to improve and assessment frameworks for land administration are presented. The FFP land administration basic concept with three frameworks which are: spatial, legal and institutional frameworks are reviewed. In addition, a fieldwork for collecting data about the status of land administration in Ecuador is performed. Results from the fieldwork in Ecuador are observed in reflection of the basic concept of FFP land administration. Here, positive developments and areas for improvement are identified. Finally recommendations based on the outcome of this paper are presented

Stability of trions in strongly spin-polarized two-dimensional electron gases

Low-temperature magneto-photoluminescence studies of negatively charged excitons (X- trions) are reported for n-type modulation-doped ZnSe/Zn(Cd,Mn)Se quantum wells over a wide range of Fermi energy and spin-splitting. The magnetic composition is chosen such that these magnetic two-dimensional electron gases (2DEGs) are highly spin-polarized even at low magnetic fields, throughout the entire range of electron densities studied (5e10 to 6.5e11 cm^-2). This spin polarization has a pronounced effect on the formation and energy of X-, with the striking result that the trion ionization energy (the energy separating X- from the neutral exciton) follows the temperature- and magnetic field-tunable Fermi energy. The large Zeeman energy destabilizes X- at the nu=1 quantum limit, beyond which a new PL peak appears and persists to 60 Tesla, suggesting the formation of spin-triplet charged excitons.Comment: 5 pages (RevTex), 4 embedded EPS figs. Submitted to PRB-R

Medical students learning styles in Latin American and Spanish universities: relation with geographical and curricular contexts

Objetivo. Determinar si los estilos de aprendizaje (EA) de los estudiantes de medicina se correlacionan con el contexto geográfico, con el contexto curricular o con el nivel de la carrera. Sujetos y métodos. El estudio se realizó en 490 estudiantes de las Escuelas de Medicina de las Universidades de Chile (Santiago, Chile), Nacional de Cuyo (Mendoza, Argentina), San Francisco Xavier (Sucre, Bolivia), Zaragoza y País Vasco (España). Se aplicó el cuestionario Honey-Alonso, que valora la preferencia por cada uno de cuatro EA: activo, reflexivo, teórico y pragmático. También se evaluó el EA de acuerdo al modelo de Kolb. Resultados. Al relacionar el EA con el contexto geográfico se observó que mientras los estudiantes de universidades españolas muestran un estilo preferentemente asimilador, siguiendo la denominación de Kolb, para Chile fue el acomodador y para Bolivia los estudiantes se distribuyen entre los estilos asimilador y divergente. Al comparar la distribución de los EA durante el tercer curso de medicina en dos facultades que poseen diferente currículo, no se observaron diferencias significativas. Los EA en una Facultad de Medicina con un currículo basado en asignaturas (Chile) no mostraron diferencias en los tres cursos del estudio (1.o, 3.o y 5.o), siendo preferentes los estilos reflexivo y teórico. Conclusiones. El estudio permitió establecer diferencias significativas entre los estilos de aprendizaje de los estudiantes de Medicina en relación con el contexto geográfico, más que con los diferentes currículos, o a lo largo de los distintos cursos de la carrera.Aim. To establish a correlation between medical student learning styles (LS) and the geographical context, the curricular context and different academic levels. Subjects and methods. The study was performed in 490 undergraduate students from Medical Schools of the Universities of Chile (Santiago, Chile), Nacional de Cuyo (Mendoza, Argentina), San Francisco Xavier (Sucre, Bolivia), Zaragoza and País Vasco (Spain). The instrument used was the Honey-Alonso learning style questionnaire that assesses the student preference for one of four LS: active, reflexive, theoretic and pragmatic. In addition, LS according to the Kolb inventory were also assessed. Results. Using the Kolb inventory, significant differences were found when the LS were correlated with the geographical context. While Spanish students showed a high preference for the assimilator style of learning, Chilean students resulted to be mainly accommodators, and Bolivian students were both assimilators and divergent. Comparing the LS distribution during the third course in two universities with different curricula (problem and lecture based learning), there were no significant differences. LS of medical students from a Medical School with a lecture based curriculum (University of Chile) were not significantly different during the first, the third and the fifth level of their undergraduate students. They showed a significant preference for reflexive and theoretic styles of learning. Conclusions. The present study allowed demonstrating that significant differences among the styles of learning of medical students correlated with the geographical context more than with the different curricula, or along the different courses of the career.Fil: Diaz Veliz, G.. Universidad de Chile; ChileFil: Mora, S.. Universidad Nacional de Cuyo. Facultad de Ciencias Médicas; ArgentinaFil: Lafuente Sanchez, J. V.. Universidad del País Vasco; EspañaFil: Gargiulo, Pascual Angel. Universidad Nacional de Cuyo. Facultad de Ciencias Médicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Bianchi, R.. Universidad Nacional de Cuyo. Facultad de Ciencias Médicas; ArgentinaFil: Teran, C.. Universidad Andina Simón Bolívar; BoliviaFil: Gorena, D.. Universidad Andina Simón Bolívar; BoliviaFil: Arce, J.. Universidad San Francisco Xavier; BoliviaFil: Escanero Marcen, J. F.. Universidad de Zaragoza; Españ