HRTEM assessment of Wurtzite and Zinc-Blende phases in GaAs nanowires for optoelectronic devices

Màster en Nanociència i Nanotecnologia. Curs 2007-2008. Directors: Francesca Peiró i Martínez and Jordi Arbiol i Cobos

Spin-orbit effects in GaAs quantum wells: Interplay between Rashba, Dresselhaus, and Zeeman interactions

The interplay between Rashba, Dresselhaus, and Zeeman interactions in a quantum well submitted to an external magnetic field is studied by means of an accurate analytical solution of the Hamiltonian, including electron-electron interactions in a sum-rule approach. This solution allows us to discuss the influence of the spin-orbit coupling on some relevant quantities that have been measured in inelastic light scattering and electron-spin resonance experiments on quantum wells. In particular, we have evaluated the spin-orbit contribution to the spin splitting of the Landau levels and to the splitting of charge- and spin-density excitations. We also discuss how the spin-orbit effects change if the applied magnetic field is tilted with respect to the direction perpendicular to the quantum well.

Spin-orbit effects in GaAs quantum wells: Interplay between Rashba, Dresselhaus, and Zeeman interactions

The interplay between Rashba, Dresselhaus, and Zeeman interactions in a quantum well submitted to an external magnetic field is studied by means of an accurate analytical solution of the Hamiltonian, including electron-electron interactions in a sum-rule approach. This solution allows us to discuss the influence of the spin-orbit coupling on some relevant quantities that have been measured in inelastic light scattering and electron-spin resonance experiments on quantum wells. In particular, we have evaluated the spin-orbit contribution to the spin splitting of the Landau levels and to the splitting of charge- and spin-density excitations. We also discuss how the spin-orbit effects change if the applied magnetic field is tilted with respect to the direction perpendicular to the quantum well.

Coverage effects on the magnetism of Fe/MgO(001) ultrathin films

Different aspects of the structure-magnetism and morphology-magnetism correlation in the ultrathin limit are studied in epitaxial Fe films grown on MgO(001). In the initial stages of growth the presence of substrate steps, intrinsically higher than an Fe atomic layer, prevent the connection between Fe islands and hence the formation of large volume magnetic regions. This is proposed as an explanation to the superparamagnetic nature of ultrathin Fe films grown on MgO in addition to the usually considered islanded, or Vollmer-Weber, growth. Using this model, we explain the observed transition from superparamagnetism to ferromagnetism for Fe coverages above 3 monolayers (ML). However, even though ferromagnetism and magnetocrystalline anisotropy are observed for 4 ML, complete coverage of the MgO substrate by the Fe ultrathin films only occurs around 6 ML as determined by polar Kerr spectra and simulations that consider different coverage situations. In annealed 3.5 ML Fe films, shape or configurational anisotropy dominates the intrinsic magnetocrystalline anisotropy, due to an annealing induced continuous to islanded morphological transition. A small interface anisotropy in thicker films is observed, probably due to dislocations observed at the Fe¿MgO(001) interface.

MgO/Ag(001) interface structure and STM images from first principles

The interface of MgO/Ag(001) has been studied with density functional theory applied to slabs. We have found that regular MgO films show a small adhesion to the silver substrate, the binding can be increased in off-stoichiometric regimes, either by the presence of O vacancies at the oxide film or by a small excess of O atoms at the interface between the ceramic to the metal. By means of theoretical methods, the scanning tunneling microscopy signatures of these films is also analyzed in some detail. For defect free deposits containing 1 or 2 ML and at low voltages, tunnelling takes place from the surface Ag substrate, and at large positive voltages Mg atoms are imaged. If defects, oxygen vacancies, are present on the surface of the oxide they introduce much easier channels for tunnelling resulting in big protrusions and controlling the shape of the image, the extra O stored at the interface can also be detected for very thin films.

Room-temperature continuous-wave operation of GaInNAs/GaAs quantum dot laser with GaAsN barrier grown by solid source molecular beam epitaxy

We present the results of GaInNAs/GaAs quantum dot structures with GaAsN barrier layers grown by solid source molecular beam epitaxy. Extension of the emission wavelength of GaInNAs quantum dots by ~170nm was observed in samples with GaAsN barriers in place of GaAs. However, optimization of the GaAsN barrier layer thickness is necessary to avoid degradation in luminescence intensity and structural property of the GaInNAs dots. Lasers with GaInNAs quantum dots as active layer were fabricated and room-temperature continuous-wave lasing was observed for the first time. Lasing occurs via the ground state at ~1.2μm, with threshold current density of 2.1kA/cm[superscript 2] and maximum output power of 16mW. These results are significantly better than previously reported values for this quantum-dot system.

El Acto Legislativo 001 de 2003 y el comportamiento electoral de partidos y movimientos políticos, en las elecciones a la Cámara de Representantes 2002 y 2006. Estudio de caso : Antioquia

El presente trabajo tiene como objetivo mostrar los efectos del acto legislativo 01 de 2003, en las elecciones a la Cámara de Representantes de 2006 en el Departamento de Antioquia. Esto con el fin de mostrar el comportamiento de los partidos y movimientos políticos antes y después de entrar en vigencia la reforma. El objetivo principal de la investigación es identificar y entender la cotidianidad y las prácticas políticas a las que los partidos tradicionales y las nuevas organizaciones estaban acostumbradas después de la constitución de 1991. Estas reglas de juego planteadas por las Constitución Política, cambiaron el panorama político tanto para los partidos tradicionalistas, como para las nuevas organizaciones que querían y reclamaban reconocimiento, participación y poder. Reglas que se transformaron con la reforma política, y específicamente con el Acto legislativo 01 de 2003, cambiando el escenario político y por ende el comportamiento electoral.

EdShare workshop 001

Materials and links related to staff workshop on using EdShare.

Characterisation of low temperature and semi-insulating GaAs lateral photo-Dember THz Emitters

Poster for IRMMW-THz conference in Mainz, Germany 2013

Implantación de un sistema de gestión ambiental de acuerdo con la Norma UNE-EN ISO 14 001: 2004 en institutos de secundaria : una experiencia en IES Ramón y Cajal, de Valladolid.

Se presentan las principales fases del proyecto de implantación de un sistema de gestión ambiental de acuerdo con al normativa española, en el IES Ramón y Cajal de Valladolid. En una primera fase se procede al diagnóstico ambiental inicial y a la formación por objetivos y áreas de trabajo de los grupos establecidos. A continuación, se procede a la elaboración de la documentación necesaria, entre ella, la política ambiental del IES, y la implantación del sistema. La fase de seguimiento del sistema se realiza a través de auditorías internas y revisiones por parte de la dirección. Finalmente, se obtiene la certificación de la norma por parte de SGS.