Interactions, external fields and disorder in low-dimensional systems

• Autores: Clara González-Santander de la Cruz
• Directores de la Tesis: Francisco Domínguez-Adame Acosta (dir. tes.)
• Lectura: En la Universidad Complutense de Madrid ( España ) en 2013
• Idioma: inglés
• Número de páginas: 163
• Títulos paralelos:
  • Interacciones, campos externos y desorden en sistemas de baja dimensionalidad
• Enlaces
  • Tesis en acceso abierto en: Docta Complutense
• Resumen
  • español

    La física de la materia condensada es uno de los campos más prolíficos de la física contemporánea. En los últimos 50 años más de 20 Premios Nobel han sido concedidos a investigaciones que se pueden enmarcar dentro de la física de la materia condensada. En 1956 se galardonó el descubrimiento del transistor, ejemplo tradicional de un dispositivo de estado sólido con una influencia decisiva para el avance de nuestro mundo cotidiano y por supuesto, de nuestra carrera profesional. Desde entonces hasta ahora, cada ciertos años se reconoce bien el desarrollo de instrumental científico fundamental para el estudio de la materia o el descubrimiento de nuevas fases de la materia o la propuesta de nuevas teorías que permitan entender los fenómenos físicos observados. Se ha establecido un nuevo paradigma científico, el campo de la física de la materia condensada, que incluye un amplio número de las investigaciones actuales en física. Este campo está en constante expansión y sus conexiones con otras disciplinas científicas, tales como la biología y la química, son cada vez mayores. Un área relacionada con la materia condesada con creciente interés es el de la nanociencia. Esto se debe tanto a los avances tecnológicos a escala nanométrica como a la formulación de teorías que permiten controlar y predecir los fenómenos físicos en dimensiones reducidas. Habitualmente se estudian sistemas de baja dimensionalidad en los que los portadores de carga están confinados en cero (0D), una (1D) o dos (2D) dimensiones. La mecánica cuántica, gran revolución científica del siglo XX, es el ingrediente fundamental de los modelos teóricos de la física de la materia condensada. Por tanto, se hace necesario plantear nuevas teorías y modelos que capturen la diversidad de fenómenos observados en sistemas de baja dimensionalidad. Esta Tesis está centrada en el estudio del comportamiento de los constituyentes fundamentales de la materia en sistemas de baja dimensionalidad. Este depende de tres aspectos fundamentales: la interacción con otras partículas, los efectos de campos externos y las contribuciones del desorden en sus funciones de onda. En concreto, el estudio se ha enfocado a cuatro sistemas tales como puntos (QDs), hilos (QWs), anillos (QRs) cuánticos y grafeno. La interacción entre partículas se ha explorado en excitones en QWs y QRs, impurezas hidrogenoides en QDs y átomos confinados en trampas armónicas. Se han considerado tres posibles campos externos: un haz láser intenso incidiendo sobre las nanoestructuras, un campo magn´etico en un QR y campos oscilantes en el tiempo en QRs y sistemas 1D relativistas. Los efectos del desorden y su consecuencias sobre la localización de la función de onda electrónica se han analizado en grafeno. El objetivo fundamental ha sido desarrollar modelos teóricos y procedimientos numéricos para aplicarlos a los sistemas anteriormente mencionados, y así comprender y proponer nuevas propiedades físicas de los mismos. Para ello hemos hecho uso tanto de teorías bien establecidas entre la comunidad científica como de los recursos computacionales disponibles en los centro de trabajo donde esta Tesis se ha desarrollado. Con este trabajo hemos entendido la aparición de fenómenos tales como el efecto Aharonov-Bohm (AB) de excitones en QRs bidimensionales, estados ligados al continuo (BIC) en sistemas dependientes del tiempo, el t´unel de Klein (KT) en barreras variables con el tiempo y la localización de Anderson en grafeno.

  • English

    Condensed matter physics could be distinguished as one of the most prolific fields of contemporary physics research. It is devoted to studying the physical properties of condensed phases of matter, both at the microscopic and the macroscopic levels. Physicists pursue to understand the diverse and unexpected phenomena that arise when matter is formed from its fundamental constituents. Discoveries of novel phases of matter and many technological inventions present in our everyday life have been developed as a result of the research in this branch of physics. Due to the diversity of topics with which is related, this active field is deeply connected to other scientific disciplines such as biology, chemistry, material science, nanotechnology and mathematics. A detailed revision of the scope of condensed matter physics and the fertile and multitudinous phenomena found under this name is given in the general and very complete book Introduction to Condensed Matter Physics by F. Duan and J. Guojun [1]. For a short inspect, in the webpage of the Institute of Physics (United Kingdom) a booklet that addresses the main challenges of this area of research and examples of some recent scientific advances can be found [2]. Traditionally, this field is divided in soft and hard condensed matter physics, differentiating between them as a function of the energy and length scale where the interactions between constituents happen. Hard condensed matter, usually known as solid state physics, deals with materials with structural rigidity such as crystalline solids. Their physical properties are fundamentally governed by atomic forces and quantum mechanics. However, soft condensed matter is centred on the study of liquids, membranes, polymers and biological materials. The energy scale at which their physical behaviours occur is comparable to room temperature multiplied by the Boltzmann constant. Therefore, quantum effects generally do not play any role, and their properties are mainly described by classical mechanics [1]. However, the boundary separating the two branches is becoming diffuse, and they are framed in the new paradigm of condensed matter physics, which emphasises many-body effects and broken symmetry [1]. This Thesis is embodied itself in the study of structures that operate in the quantum regime. The main purpose of this work is to explore different problems that have been faced in the past and in the present in the framework of theoretical hard condensed matter. The building blocks of this research are settled on the foundations of quantum mechanics, which in the early 20th century revolutionised the way we understand matter. Quantum physics was the seed from which many other theories and models emerged and provided insights on the interaction between particles and the effects of external potentials on microscopic systems [3]. On the other hand, during the second half of the 20th century another important step in understanding the physical properties of matter was gained by the advances in nanotechnology [4]. In artificial nanostructures electrons are confined in one, two or three dimensions. When the size of the confinement is large enough to modify the optical, transport and magnetic properties of the system compared to the ones observed in the bulk material, the nanosize structure is subjected to quantum effects. This opens the door to explore scales where the interaction among matter constituents becomes significant. Therefore, the better the understanding of the physical laws present at that scale, the better the tailoring of the physical properties and the subsequent designing of electronic devices. Theoretical studies, although having a smaller impact on commercial applications, constitute a fundamental step for understanding observed phenomena and predicting new ones. Inspired by the recent developments and experiments in the nanoscale regime, in this Thesis previous theoretical models are revised and new models are proposed to calculate, explain and augur physical properties in diverse quantum systems. The low-dimensional systems under study include two-dimensional (2D) systems such as graphene and quantum rings (QRs), one-dimensional (1D) structures like quantum wires (QWs) and zero-dimensional (0D) geometries such as artificial atoms or quantum dots (QDs). Specifically, this work is divided in three distinct areas of research related to fundamental mechanisms that govern the behaviour of particles in the quantum regime. We study different approaches for modelling the interaction with other particles, the effects of external fields on detectable properties of the system and finally, the way that imperfections could modify the particles’ wave function behaviour. Each of these themes is introduced below. The reader will see that the connection of the chapters with the main topics of this Thesis does not follow a chronological scheme because they appear simultaneously along all the work. However, the chapters are self-contained and a full description of their theoretical framework is given at their beginning in order to make them more understandable.