100 research outputs found
Atomic-level Characterization of Fe(001)/MgO(001)/Fe(001) Tunneling Magnetoresistance Structures and Spin-polarized Scanning Tunneling Microscopy
This thesis seeks to understand the Fe-MgO-Fe system through a series of atomic level studies of the topographic, electronic, and magnetic properties of these epitaxial films. This multilayer system is uniquely important because of its huge tunneling magnetoresistance (TMR) arising from spin coherence and strong spin filtering through the structure. MgO-based magnetic tunnel junctions have been actively investigated and are now successfully applied to commercial products such as non-volatile magnetic random access memories and read-write heads for hard disk. However, despite its popularity most work has been done on macroscopic samples and has focused on the device-level performance. Yet very little effort has been devoted towards the understanding at the atomic length scales including the effects of atomic steps and local variation in stoichiometry. The primary goal of this work is to elucidate the interplay between morphology, stoichiometry, local magnetism, and local electronic properties. To this end a multifaceted approach was used involving atomic/magnetic force microscopy (AFM/MFM), scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), Auger electron spectroscopy, and low energy electron diffraction (LEED), which were operated in the cleanest possible conditions under an ultra-high vacuum. I linked the morphology directly to the formation of different magnetic domain configurations as a function of growth temperature and film thickness. I also correlated these atomic-level properties to the device-level performance. By investigating the topography and the surface electronic density of states with length scales in the nanometer regime, I found that the films had extremely inhomogeneous surface states. Because the structural defects such as surface steps, deep trenches and grain boundaries, as well as the existence of chemical impurities can perturb the spin-coherent tunneling, our observation of the electronic inhomogeneity can provide a direct clue for explaining the diminished TMR phenomenon on real systems compared to the theoretical expectation, which is one of longstanding problems to achieve high TMR in actual devices.
In addition to the Fe/MgO/Fe work, I also demonstrated spin polarized STM which revealed the anti-ferromagnetic spin-structure of single crystal chromium and the magnetic domains structure of permalloy film on silicon oxide
Structuring high-order harmonic generation with the angular momentum of light
Tesis por compendio de publicaciones[ES] Los pulsos láser ultracortos son una herramienta única para explorar
las dinámicas más rápidas de la materia. Sorprendentemente, los
pulsos de láser más cortos obtenidos hasta la fecha se producen a partir
del fenómeno no lineal de conversión de frecuencias de generación de
armónicos de orden alto (HHG), que resulta en la emisión de pulsos con
duraciones de attosegundo. Es importante destacar que estos pulsos
de attosegundo pueden exhibir una propiedad muy interesante, el
momento angular, que presenta dos formas diferentes, el momento
angular de espín (SAM) y el momento angular orbital (OAM), y que
abre nuevos escenarios para las interacciones luz-materia a escalas
espaciales nanométricas y temporales ultracortas.
En esta tesis desarrollamos un conjunto de esquemas para la crea-
ción de armónicos de orden alto y pulsos de attosegundo con nuevas
propiedades de momento angular mediante la estructuración del pro-
ceso de HHG a través de las características de los haces incidentes. Para
ese propósito, primero abordamos la descripción de los mecanismos
físicos fundamentales de la HHG. En particular, estudiamos la ioniza-
ción túnel en moléculas, descubriendo que depende de la ubicación
del electrón dentro de la molécula, debido a la naturaleza extendida
de estas. Esta característica deja huellas importantes en los espectros
de HHG y de fotoelectrones. Por lo tanto, hemos desarrollado una
receta para implementar este fenómeno en los modelos de campos
intensos existentes.
A continuación, predecimos y describimos teóricamente la gene-
ración de haces láser en el ultravioleta extremo (XUV) con nuevas
propiedades de momento angular que, en la mayoría de los casos,
son también creadas y caracterizadas experimentalmente por nuestros
colaboradores del grupo Kapteyn-Murnane en JILA, en la Universidad
de Colorado (EE. UU.), y del grupo del Prof. M.-Ch. Chen del Instituto
de Tecnologías Fotónicas de la Universidad Tsing Hua (Taiwán). Para
empezar, demostramos la generación, por primera vez, de haces de
luz con OAM variable en el tiempo, una propiedad que denominamos
como el auto-torque de la luz. Es importante destacar que los haces
con auto-torque surgen naturalmente en el régimen XUV cuando el
campo incidente para la HHG está formado por dos vórtices infrarro-
jos retardados en el tiempo. Bajo esta configuración, el OAM de los
armónicos de orden alto cambia a lo largo del tiempo en una escala de
tiempo de attosegundos, siendo la cantidad de auto-torque controlada
a través de las propiedades temporales de los pulsos incidentes. Por
lo tanto, creemos que los haces con auto-torque pueden servir como
nuevas herramientas para la manipulación láser-materia. Además,
mostramos cómo el OAM puede servir como instrumento para mani-
pular las propiedades espectrales y de divergencia de los armónicos
de orden alto. Empleando dos vórtices con el contenido adecuado
de OAM como pulsos incidentes, obtenemos peines de frecuencias de
armónicos de orden alto con un espaciado entre líneas espectrales
sintonizable y baja divergencia. Este control es particularmente intere-
sante para espectroscopía y formación de imagen en el XUV o incluso
en los rayos X blandos.
Además, presentamos varios esquemas para el control de la eliptici-
dad de los pulsos de attosegundo y de los armónicos de orden alto.
Utilizando la configuración no colineal contrarrotante, extraemos el
escalado de la elipticidad de los armónicos de orden alto con la de
los haces incidentes y desvelamos la información sobre la respuesta
dipolar oculta en esa conexión. Además, mostramos la generación
de vórtices polarizados circularmente a partir de la HHG usando un
campo incidente bi-circular vorticial. Destacablemente, al seleccionar
correctamente el OAM del campo incidente, podemos obtener, o bien
pulsos de attosegundo polarizados circularmente, o bien armónicos
de orden alto con baja carga topológica. Por último, demostramos
teóricamente la generación de trenes de pulsos de attosegundo con
estados de polarización ordenados temporalmente mediante la combi-
nación de dos campos incidentes bi-circulares vorticiales retardados en
el tiempo. Creemos que la generación de pulsos de attosegundo con
elipticidad controlada se puede emplear para el estudio de la dinámica
ultrarrápida de SAM en moléculas quirales o materiales magnéticos.
[EN] Ultrashort laser pulses are a unique tool to explore the fastest dy-
namics in matter. Remarkably, the shortest laser pulses to date are
produced from the non-linear frequency upconversion phenomenon
of high-order harmonic generation (HHG), which results in the emis-
sion of pulses of attosecond durations. Importantly, such attosecond
pulses can exhibit a very exciting property, the angular momentum,
which presents two different forms, the spin angular momentum (SAM)
and the orbital angular momentum (OAM), and that brings new sce-
narios for the light-matter interactions at the nanometric spatial and
ultrashort temporal scales.
In this thesis work, we develop a compilation of schemes for the
creation of high-order harmonics and attosecond pulses with novel
angular momentum properties by structuring the HHG process through
the characteristics of the driving beams. For that purpose, we first
address the description of the fundamental physical mechanisms
of HHG. In particular, we study the tunnel ionization in molecules,
finding that it is site-specific—its rate depends on the position of the
electronic wavefunction at the ion sites—, due to the extended nature
of the molecules. This characteristic leaves important signatures in the
HHG and photoelectron spectra. Therefore, we provide a recipe for
implementing the site-specificity in the existing strong-field models.
Afterwards, we theoretically predict and describe the creation of
extreme-ultraviolet (XUV) beams with novel angular momentum prop-
erties, which, in most of the cases, are experimentally generated and
characterized by our collaborators from the Kapteyn-Murnane group
in JILA, at the University of Colorado (USA) and from the group of
Prof. M.-Ch. Chen at the Institute of Photonics Technologies of the
Tsing Hua University (Taiwan). To begin with, we demonstrate the
generation, for the first time, of light beams with time-varying OAM, a
property which we denote as the self-torque of light. Importantly, self-
torqued beams arise naturally in the XUV regime from HHG driven by
two time-delayed infrared vortex beams. Under this configuration, the
OAM of the high-order harmonics changes along time in the attosec-
ond time-scale, being the amount of self-torque controlled through
the temporal properties of the driving pulses. Thus, we believe that
self-torqued beams can serve as unprecedented tools for laser-matter
manipulation. In addition, we show how the OAM can serve as an
instrument to manipulate the spectral and divergence properties of
the high-order harmonics. By driving HHG with two vortex beams
with properly selected OAM, we obtain high-order harmonic frequency
combs with tunable line-spacing and low divergence. Such control is
particularly interesting for XUV/soft-X-ray spectroscopy and imaging.
Moreover, we present several schemes for the ellipticity control of the
high-order harmonics and attosecond pulses. Using the non-collinear
counter-rotating scheme, we extract the scaling of the ellipticity of the
high-order harmonics with that of the driving beams’ and we unveil
the information about the non-perturbative dipole response hidden in
that connection. Also, we show the generation of circularly polarized
vortex beams from HHG driven by a bi-circular vortex field. Interest-
ingly, by properly selecting the OAM of the driving field we can obtain
either circularly polarized attosecond pulses, or high-order harmonics
with low topological charge. Finally, we theoretically demonstrate the
generation of attosecond pulse trains with time-ordered polarization
states by combining two time-delayed bi-circular vortex driving fields.
We believe that the generation of attosecond pulses with controlled
ellipticity can be employed for the study of ultrafast spin dynamics in
chiral molecules or magnetic materials
Fingerprints in the Optical and Transport Properties of Quantum Dots
The book "Fingerprints in the optical and transport properties of quantum dots" provides novel and efficient methods for the calculation and investigating of the optical and transport properties of quantum dot systems. This book is divided into two sections. In section 1 includes ten chapters where novel optical properties are discussed. In section 2 involve eight chapters that investigate and model the most important effects of transport and electronics properties of quantum dot systems This is a collaborative book sharing and providing fundamental research such as the one conducted in Physics, Chemistry, Material Science, with a base text that could serve as a reference in research by presenting up-to-date research work on the field of quantum dot systems
Stochastic modeling of the thermal and catalytic degradation of polyethylene using simultaneous DSC/TG analysis
Dissertação para obtenção do Grau de Mestre em
Engenharia Química e BioquímicaIn the present work a stochastic model to be used for analyzing and predicting experimental data from simultaneous thermogravimetric (TG) and differential scanning calorimetry (DSC) experiments on the thermal and catalytic degradation of high-density polyethylene (HDPE) was developed. Unlike the deterministic models, already developed, with this one it’s possible to compute the mass and energy curves measured by simultaneous TG/DSC assays, as well as to predict the product distribution resulting from primary cracking of the polymer, without using any experimental information.
For the stochastic model to predict the mass change as well as the energy involved in the whole process of HDPE pyrolysis, a reliable model for the cracking reaction and a set of vaporization laws suitable to compute the vaporization rates are needed.
In order to understand the vaporization process, this was investigated separately from cracking. For that, a set of results from TG/DSC experiments using species that vaporize well before they crack was used to obtain a global correlation between the kinetic parameters for vaporization and the number of C-C bonds in the hydrocarbon chain. The best fitting curves were chosen based on the model ability to superimpose the experimental rates and produce consistent results for heavier hydrocarbons. The model correlations were implemented in the program’s code and allowed the prediction of the vaporization rates.
For the determination of the global kinetic parameters of the degradation reaction to use in the stochastic model, a study on how these parameters influence the TG/DSC curves progress was performed varying those parameters in several simulations, comparing them with experimental data from thermal and catalytic (ZSM-5 zeolite) degradation of HDPE and choosing the best fitting. For additional improvements in the DSC stochastic model simulated curves, the thermodynamic parameters were also fitted.
Additional molecular simulation studies based on quantum models were performed for a deeper understanding on the reaction mechanism and progress.
The prediction of the products distribution was not the main object of the investigation in this work although preliminary results have been obtained which reveal some discrepancies in relation to the experimental data. Therefore, in future investigations, an improvement of this aspect is necessary to have a stochastic model which predicts the whole information needed to characterize HDPE degradation reaction
Quantum nuclear many-body dynamics and related aspects
Mémoire de HDR soutenu le 16 décembre 2010, Université de Caen-Basse NormandieThis review article is devoted to a compilation of recent advances in the nuclear many-body dynamical problem. The building block of any microscopic model is the nuclear mean-field theory, designed to provide proper description of one-body observables. Important aspects related to mean-field and its relation to observables evolutions are presented. Currently applied nuclear mean-field theories are formulated within a Density Functional Theory (DFT) framework, the so-called Energy Density Functional (EDF) theory. In addition, beyond mean-field approaches, that are introduced to account for direct nucleon-nucleon collisions, pairing and/or configuration mixing, are strongly guided by the theory of Open Quantum Systems (OQS). Both DFT and OQS are interdisciplinary concepts that play a key role in the nuclear many-body problem and are discussed in this review. Beyond mean-field theories are illustrated to fusion, transfer and break-up reactions. Finally, more phenomenological approaches dedicated to multifragmentation reactions and low energy spallation reactions are introduced
Spectral Line Shapes in Plasmas
International audienceFor the first two Spectral Line Shapes in Plasma workshops, participants submitted in total over 1,500 line-shape calculations. The studies collected in this Special Issue explore only a part of this immense work. This book is a reprint of the special issue that appeared in the online open access journal Atoms (ISSN 2218-2004) in 2014 (available at: http://www.mdpi.com/journal/atoms/special_issues/SpectralLineShapes)
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