Tesis Doctorals - Departament - Estructura i Constituents de la Matèria

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Response of simple-metal systems with plane symmetry to local one-body operators
(Universitat de Barcelona, 1997-09-09) Sellarès González, Jordi; Barberán Falcón, Núria; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèri
[eng] The objective of this work is to perform an alternative study of plasmon modes in systems with plane symmetry. There are still some aspects of this problem to which no conclusive answers have been given. The sum rule approach has some advantages over more sophisticated methods. This allows to address us some questions that are not easily answered using other methods. Limit cases, such as low momentum or step density profile, can be worked out often because results are analytical in many cases. Also, the observability of the modes can be discussed since the different contributions to the total energy can be analyzed on their own. Moreover, we will see that the numerical results obtained show an agreement with experimental data comparable to that of more involved methods, like TDLDA. This is due to the fact that realistic models for the density profile, the kinetic energy functional or the exchange and correlation functional can be employed. Therefore, the main motivation of this work is to exploit all these advantages to accomplish our goal. The contents are organized in the following way. In Chapter 2 we expose the sum rule formalism in a general way and some previous findings are commented in Chapter 3. This formalism is applied to study plasmon modes of simple-metal surfaces in Chapters 4 and 5. The Local RPA formalism is discussed and its application to simple-metal slabs is presented in Chapter 6. Finally, the main conclusions are summarized in Chapter 7. The relationship between atomic units and other systems is discussed in Appendix A and the expressions of the sum rules of the operators employed are given in Appendix B and C.
Quantum entanglement and quantum state estimation = Entrelazado cuántico y estimación de estados cuánticos
(Universitat de Barcelona, 2001-06-01) Acín dal Maschio, Antonio; Latorre, José Ignacio; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[spa] En esta Tesis se ha realizado en el marco de la Información Cuántica, una nueva disciplina de la Física Teórica en la que se funden la Mecánica Cuántica con la Teoría de la Información. El estudio se ha centrado en dos de los problemas más importantes en este campo: el análisis de las propiedades de los estados entrelazados, y la estimación de estados cuánticos desconocidos. Por lo que se refiere el primer problema, se ha efectuado un estudio de los estados cuánticos entrelazados de tres bits cuánticos, buscando generalizar conceptos ya existentes para el caso de estados de dos partículas. Se ha analizado tanto estados mezcla como puros, y para las dos situaciones se ha propuesto un esquema para la clasificación de las propiedades de entrelazado, así como varias herramientas matemáticas que simplifican el estudio. Como aplicación de los resultados, se ha analizado el estado resultante de la desintegración del positronio (partícula formada por un electrón y un positrón) a tres fotones. Respecto al segundo problema, se ha buscado el método óptimo para estimar un estado cuántico desconocido y se ha encontrado el aparato de medida que resuelve este problema. También se ha estudiado la situación en que no se está interesado en la determinación de todas las propiedades del estado, sino sólo en algunas de ellas, por ejemplo, el entrelazado en estados puros de dos bits cuánticos. De nuevo la estrategia óptima de medida ha sido encontrada. En resumen, la tesis se ha centrado en resolver aspectos teóricos de dos de los problemas más interesantes en el nuevo campo de la Información Cuántica, obteniéndose nuevas técnicas y resultados que ayudarán a mejorar su comprensión.
On Friedmann-Lemaître-Robertson-Walker cosmologies in non-standard gravity
(Universitat de Barcelona, 2011-03-03) Sáez Gómez, Diego; Elizalde, E. (Emili), 1950-; Odintsov, S. D. (Sergei D.); Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[spa] El proyecto de tesis está enfocado en el estudio de la evolución del Universo y más concretamente en la explicación del mecanismo que produce su actual expansión acelerada, y que probablemente sea el mismo que ha condicionado las distintas épocas de la historia del Universo, y que a su vez produjo la inflación. Partiendo de esta base, se trata de investigar a partir de teorías escalar-tensor y modificaciones de la relatividad general dicho mecanismo, y obtener nuevos efectos derivados de estas modificaciones que puedan ser contrastados con las medidas experimentales. Para ello se han construido modelos con la inclusión de campos escalares acoplados tanto débilmente como con acoplo fuerte , y por otro lado se han considerado modificaciones de la propia relatividad general a través de cambios en la dependencia de la acción de Hilbert-Einstein con respecto al escalar de Ricci, construyendo teorías alternativas cuya acción dependa de una función del escalar de Riccio y/o el invariante de Gauss-Bonnet, lo que se conoce como teorías f(R) y/o f(G). Por tanto, todo ello busca como objetivo la explicación de los resultados obtenidos a través de las observaciones. Además también se estudian las características y aspectos de la teoría de Horava-Lifshitz, que dice ser renormalizable, pero de la que aún sigue habiendo muchas cuestiones abiertas. Otros aspectos que tiene que ver con las métricas FLRW, como las singularidades son analizados.
Instabilities in Newtonian and non-Newtonian fluids
(Universitat de Barcelona, 2007-04-26) Torralba Cuello, Mireia; Ortín, Jordi, 1959-; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[spa] Durante la tesis se realizó un estudio experimental de inestabilidades hidrodinámicas en dos sistemas muy diferentes. Estudiamos inestabilidades laterales en dedos de Saffaman-Taylor sujetos a dos tipos de perturbación (desorden estático y perturbación periódica). Observamos una inestabilidad con una longitud de onda seleccionada del orden del ancho del canal. También estudiamos el flujo oscilatorio de un fluido de Maxwell en un tubo, comparándolo con uno Newtoniano. Observados grandes diferencias entre ambos casos. El flujo Maxweliano se hace inestable a números de Reynolds mayor 1.
Interplay between anisotropy and disorder in ferroelastics: structures and thermodynamics
(Universitat de Barcelona, 2010-06-15) Lloveras Muntané, Pol; Castán i Vidal, Maria Teresa; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[spa] Esta tesis se enmarca dentro del estudio de los efectos de inhomogeneidades en materiales funcionales, dentro de los cuales podemos citar a los materiales que presentan magnetoresistencia colosal, superconductores de alta temperatura, ferroicos como por ejemplo ferromagnetos, ferroeléctricos y ferroelásticos, y multiferroicos en general. Nos vamos a limitar aquí a los materiales ferroelásticos. Estos se caracterizan por una transición de fase estructural no difusiva con una fase producto multivariante que da lugar a una microestructura multidominio debido a un proceso de autoacomodación. Esto permite fenómenos de alta importancia tecnológica como la superelasticidad y la memoria de forma (SME), que consisten en la recuperación de deformaciones considerables, el primero como una gran respuesta a un campo de esfuerzos y el segundo debido a un proceso de cargado y posterior calentamiento. Encontramos usos como actuadores y sensores, entre muchos otros. Uno de los problemas que impiden un mayor desarrollo tecnológico consiste en la ausencia de materiales con un buen rango operativo en temperatura unido a un buen comportamiento a nivel de resistencia, ductilidad, repetición (ciclado), etc. Entre los materiales ferroelásticos funcionales encontramos sobretodo martensitas termoplásticos, que son básicamente aleaciones intermetálicas. Entre ellos, cabe destacar el TiNi como la aleación con memoria de forma (SMA) más utilizada a nivel tecnológico. Con propósito de hallar una aleación que presente buenas condiciones de trabajo se han investigado distintas familias de aleaciones. También es importante destacar que el cambio respecto la composición estequiométrica o el dopaje de materiales con un elemento adicional se ha observado que tiene un efecto muy grande sobre los rangos operativos de los materiales, así que es una vía que presenta muchas expectativas. Cabe destacar que precisamente este método incrementa el grado de las fluctuaciones composicionales (por tanto de desorden) en el sistema. Recientemente se ha observado el SME en Ti-Ni en un rango de composición que no presenta transición estructural, sino que el sistema queda anclado en un estado que presenta comportamiento de tipo vidrioso, cosa que es de importancia tanto a nivel de comprensión básica de estos sistemas así como a nivel tecnológico. Cabe destacar que una de las características fundamentales del Ti-Ni es que presenta una anisotropía cristalina muy baja en comparación con otros SMAs, cosa que influye, por ejemplo, en la morfología casi isótropa de las estructuras precursoras, que adquieren mayor relevancia en el rango de composición mencionado que inhibe la transformación estructural. En la presente tesis se ha estudiado mediante simulación numérica los efectos que tienen tanto la anisotropía como el desorden en sistemas ferroelásticos que presentan una transición cuadrada a rectangular. Para ello se ha utilizado un modelo de Ginzburg- Landau al que se añade un término de desorden acoplado al parámetro de orden (deformación rectangular) y interacciones anisotrópicas de largo alcance que surgen de considerar deformaciones adicionales de volumen y cizalla hasta orden armónico. Cabe señalar que el término de desorden introduce una distribución local de temperaturas características de estabilidad y equilibrio, y viene motivado por la alta sensibilidad que presenta dichas aleaciones a la composición específica, junto con la consideración de fluctuaciones composicionales intrínsecas a toda aleación. Asimismo, el peso del potencial de largo alcance es, a temperatura constante, directamente proporcional al factor de anisotropía elástico. El cálculo de dicho término se ha llevado a cabo en ele espacio de Fourier, que reduce mucho el tiempo de computación necesario (de un factora N^2 a NlogN, siendo N el número de celdas del sistema discretizado). Con dicho modelo se estudian las estructuras y respuestas termodinámicas del sistema. El estudio de las estructuras en función de la anisotropía da lugar a un paisaje bastante consistente con los experimentos: para valores altos de la anisotropía, en el régimen pretansicional se obtiene un patrón de modulaciones cruzadas llamado tweed, observando, por ejemplo, en Fe-Pd y Ni-Al. Al disminuir la temperatura, dicho patrón evoluciona hacia la fase procuro multivariante (twins) con unas interfases bien definidas entre variantes a lo largo de la diagonal. Este patrón es observado en un número muy amplio de SMA, incluyendo los ya mencionados. Para valores más bajos a la observada en Ti-Ni. A baja temperatura, los twins se empiezan a romper, siendo incapaces de correlacionar todo el sistema. Para valores muy bajos de la anisotropía, en la fase pretransicional se obtiene una estructura moteada, similar a la observada en Ti-Ni. A baja temperatura, los twins se empiezan a romper, siendo moteada sobreviven a baja temperatura. Dicho esquema se ha obtenido también al aumentar el desorden para un determinado valor de anisotropía. En particular, para valores altos de desorden, la transición es inhibida y la fase pretransicional sobrevive a Y-Ba-Cu-O por encima de un umbral de dopaje de ciertos elementos y en el caso de las estructuras moteada en aleaciones fuera de la estequiometría de Ti-Ni, o Ti-Ni dopado con Fe por encima de cierto valor crítico. Independientemente, se estudian otras características de sistemas ferroelásticos, como efectos de tamaño finito, que dan lugar a estructuras con longitudes características. Para ello se exploran varios métodos fenomenológicos, y se participa en el cálculo analítico de la solución de dicho problema de contorno para estos sistemas. También se obtienen agujas, todo ello consistente con observaciones experimentales. Para confirmar el esquema presentado anteriormente, se estudian varias respuestas termodinámicas. En particular, se obtiene que variaciones en la anisotropía dan lugar a anomalías en la capacidad calorífica C, consistentes en un desplazamiento y atenuación del pico de C hacia bajas temperaturas a medida que la anisotropía disminuye. Para valores muy bajos, el pico desaparece, consistente con la supresión de la transición. El comportamiento de la derivada de la fracción transformada da lugar a las mismas anomalías, cosa que apoya la validez de los resultados en C. Anomalías comparables se han observado en Ti-Ni(Fe) por encima de un cierto nivel de dopaje con Fe. Asimismo, se espera que resultados similares se obtuvieran aumentando el nivel de desorden para cierto valor de anisotropía. A continuación se estudia la respuesta elástica C' y se obtiene que al disminuir la anisotropía, C' se estudia la respuesta elástica C' y se obtiene que al disminuir la anisotropía, C' se aplana mucho, consistente también con la supresión de la transformación. Esto está de acuerdo con experimentos en Ti-Ni para diversos valores de la composición. Se estudia también la metaestabilidad de los estados obtenidos al variar la anisotropía y desorden. Los valores de dichos parámetros que suprimen la transición dan lugar a estados metaestables. Esto está parcialmente de acuerdo con los experimentos. Por un lado, el desplazamiento de la transición hacia bajas temperaturas observando experimentalmente se debe a una desestabilización termodinámica de la fase producto. Sin embargo, recientes experimentos termodinámicos en Ti-Ni (de SME) por la composición que no exhibe transformación dan lugar a creer que a temperaturas bastante bajas la fase pretransicional que sobrevive es metaestable y no termodinámica de equilibrio. Análogamente a los experimentos Zero-field-cooling (ZFC/FC) realizados en esta aleación en relación al comportamiento vidrioso, se obtiene un comportamiento cualitativo equivalente en nuestro sistema, consistente en una desviación de la curva ZFC respecto de la FC que es indicativa de congelación cinética, típica de comportamiento vidrioso. Esto es consistente con la metaestabilidad y comportamiento de las otras respuestas termodinámicas descritas anteriormente. se estudian las correlaciones entre las variantes de los dominios emergentes, y se confirma que el origen del comportamiento vidrioso es la congelación cinética y no la frustración geométrica, típica de sitemas vidriosos como el paradigmático triángulo antiferromagnético. Finalmente, se estudia el comportamiento termomecánico dels sistema mediante el análisis de las curvas de esfuerzo- deformación. Se observa superalasticidad en la transformación inducida por esfuerzo así como memoria de forma a baja temperatura. Variaciones en los valores de anisotropía y desorden se traducen en variaciones en el esfuerzo crítico de transformación en determinados rangos, etc. El comportamiento específico depende no trivialmente de los valores de anisotropía y desorden. Toda esta rica fenomenología se ha observado en varios SMAs al variar la composición específica o mediante dopaje, aunque también hay cierto comportamiento, como la disminución del esfuerzo de transformación al aumentar el desorden, que se contradice cualitativamente con los datos experimentales. Es importante remarcar que se obtiene SME para el rango de composición que suprime la transición, de acuerdo con las medidas realizadas en Ti-Ni. También se ha calculado el efecto estocástico durante la transición inducida por esfuerzo.
From molecular force generation to large scale cellular movements
(Universitat de Barcelona, 2006) Campàs i Rigau, Otger; Casademunt i Viader, Jaume; Joanny, J.-F. (Jean-François); Prost, J.; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[eng] The propulsion mechanisms that drive the movements of living cells constitute perhaps the most impressive engineering works of nature. Still, it is simply the interaction between molecules which is responsible for these complex and robust motility mechanisms. A question that arises naturally is thus how the underlying molecules self-organize to perform such highly coordinated tasks. Although a global understanding of cell behavior is still out of reach, the study of particular aspects of biological systems may help building up a more clear picture. Biologists have made lots of efforts to characterize the proteins involved in cellular movements, to identify their interactions and to understand their regulation. This information is very important and has explained several aspects of the motility of living cells. The discovery of proteins able to generate forces at molecular scales, known as motor proteins, provided essential information to understand the observed cellular movements. However, the force developed at the molecular level by a single protein is too weak to drive cellular movement on its own. Probably the clearest example is the functioning of muscles. The forces developed are about 12 orders of magnitude larger than the forces generated at molecular scales. This is possible because the contraction of muscles involves the collective action of many motor proteins (Alberts et al., 2004; Bray, 1992). Although each one of these proteins generates a small force (in the picoNewton range), the sum of their individual contributions leads to large forces. At the cellular scales something similar occurs. The necessary forces for the motion of a cell and even for intracellular movements, are larger than molecular forces. The collective action of molecular force generators is thus essential to understand most cellular movements. Here we study theoretically some examples of cellular movements and compare quantitatively, when possible, our results to the experimental observations. The work is divided in three parts: we first study the motion of oil drops propelled by an actin comet tail, which closely mimics the motility mechanism of several bacterial pathogens, as the bacteria Listeria. The second part is devoted to particular aspects of intracellular transport. We study the physical mechanism of membrane tube extraction by motor proteins, the traffic of motor proteins at large scales and the collective force generation of molecular motors pulling on fluid membranes. In the last part we address both the motion of chromosomes in eukaryotic cell division and the stability of spindle-like structures, as the mitotic spindle. Our aim is to understand how these movements arise from the cooperative action of molecular force generators. The forces developed by ensembles of force generators are not static, but depend on the dynamic state of the system. This is so because the kinetics of the individual force generators is strongly affected by the forces created by themselves. As we discuss below, this force-dependent kinetics imposes a highly non-linear dynamics for the system and, as a consequence, several dynamic instabilities occur. Our work shows that the collective behavior of molecular force generators is essential to understand some features of cellular movements.
Physico-chemical aspects on molecular motors
(Universitat de Barcelona, 2008-05-15) Ciudad Álvarez, Aleix; Sancho, José M.; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[eng] The discovery of molecular motors was not a single and isolated event but a set of gradual approaches from microscopic to nanoscopic systems. For example, the knowledge of the activity of muscle cells is pretty old, and even though the first myosin was identified about a century ago, it was not until 1986 when actin filaments were visualized under a microscope. But until 1995 the first single molecule measurement was not performed. Among other technical problems, to deal with such small objects implied thermal noise detection, that couldn’t be distinguished from the desired signal. Microscopic techniques were developed in order to decrease the background noise allowing a progressive improving in image resolution. However, some years before, optical trapping techniques allowed measurement of individual kinesin motion. In this text we will not cover a rigorous survey on the history of single molecule experiments.
Experiments on perturbed Saffman-Taylor flows
(Universitat de Barcelona, 1997-06-26) Ignés i Mullol, Jordi; Maher, J. V.; Ortín, Jordi, 1959-; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[eng] We have performed pattern formation experiments where a relatively well understood system (flow in a Hele-Shaw cell) is perturbed either by means of a lattice of grooves or by the use of viscoelastic fluids. We have extended the qualitative analysis found in the literature for anisotropic fingering patterns, presenting a more quantitative approach that may prove useful as a tool to attack more complex problems. We have analyzed the different morphological regimes and looked for signatures of the transition between phases, with partial success when we try to characterize a given morphology quantitatively. In our studies of viscoelastic Hele-Shaw flow with associative polymer solutions, we have observed a transition from viscous fingering patterns into a regime where the growing patterns resemble the fracture in brittle solids. We have been able to rescale the threshold for these transitions, and we have observed interesting properties in a regime of fracturelike patterns where, under sorne circumstances, we have measured a characteristic oscillation frequency which shows interesting regularities. We have also studied the pressure in the viscoelastic flow, and found consistent results that may be used to implement a better theoretical model to fully understand the dynamics.
Roughness experiments of viscous fluid interfaces in disordered Hele-Shaw cells
(Universitat de Barcelona, 2003-02-28) Soriano i Fradera, Jordi; Ortín, Jordi, 1959-; Hernández Machado, Aurora; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[spa] Se presenta un trabajo experimental sobre el crecimiento de interficies rugosas en medios desordenados. El experimento consiste en una celda de Hele-Shaw en cuya placa inferior se han distribuido una serie de obstáculos de cobre que hacen la función de desorden congelado. Un aceite de silicona es inyectado por uno de los extremos de la celda desplazando el aire inicialmente presente. El frente es estable por contraste de viscosidad pero se hace rugoso debido a la presencia del desorden. Los experimentos realizados se pueden clasificar en dos grupos principales según el tipo de mecanismo utilizado para la inyección del aceite: A- Velocidad constante y B- Presión constante. Para el Caso A- se han realizado multitud de experimentos variando la velocidad del fluido, el espaciado entre placas en la celda de Hele-Shaw y la configuración del desorden. Se ha observado que cuando la velocidad promedio del aceite es alta y/o la longitud de correlación del desorden en la dirección de crecimiento es baja (desorden débil), entonces el escalamiento de las fluctuaciones de la interfície sigue el escalamiento dinámico de Family-Vicsek, con un exponente de crecimiento de beta=0,50 independiente de la velocidad promedio de la interface, espaciado entre placas y configuración del desorden. También se ha estudiado del exponente de rugosidad, encontrando dos regímenes, alfa-1 a escalas pequeñas y alfa-2 a escalas grandes, con unos valores que dependen de la velocidad promedio, el espaciado entre placas y la configuración del desorden. Para velocidades grandes se obtienen un valor límite de alfa 1=1,3 y alfa 2=0 independientemente de los parámetros experimentales. También se ha caracterizado el punto de crossover entre alfa-1 y alfa-2, el cual escala con la velocidad con un exponente 0.5, de acuerdo con predicciones teóricas.
New Physics in the Electroweak Sector Under Scrutiny at LHC
(Universitat de Barcelona, 2019-10-17) Rosa Agostinho, Nuno Filipe; González García, Ma. Concepción; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[eng] For more than half a century, colliders have been in the forefront of studying the Standard Model (SM) predictions. The pinnacle of both SM and colliders occurred when on July the 4th 2012, at CERN Large Hadron Collider, the Higgs discovery was announced, almost 50 years after being postulated. Until 2012 the Higgs was the only missing piece of the SM, and its discov- ery was a milestone in the LHC. The complete analysis of Run 1 data and the preliminary ones from Run 2 data, indicate that this new particle is a scalar bo- son, with CP–even properties, as in the SM Higgs scenario. Furthermore, it seems that the observed state is directly connected to the electroweak symmetry breaking (EWSB), after analyzing interactions with gauge bosons. Until the moment there is no evidence of any physics BSM at the LHC in the form of new states. Clearly this disfavours any extension of the SM that predicts new particles at the TeV scale. This was the scenario when this Thesis was initiated. The Run 1 of LHC had reached its final luminosity, and the Run 2 was starting its operation. The lack of NP states at LHC served us as motivation to look for an alternative approach, instead of constraining ourselves to an specific SM completion. Here enters the model-independent philosophy, where through the use of an effective Lagrangian we start to confront all the existing available data and search for any possible de- viation of the SM predictions, by making use of the framework of Effective Field Theories (EFT). In the context of an EFT, we follow an atheist path: NP is expected to manifest directly at a scale Λ, which is higher than the scale at which the experiments are performed. Any effect of new physics (NP) at the low scale can be parametrized by a set of higher dimensional operators, that are suppressed by powers of the high energy scale. With this aim, in Chapter 2 we introduce the SM as an EFT at low energy. Our ignorance about the ultraviolet (UV) theory, and with no guidance of where the scale of NP could be laying, lead us to a bottom-up approach, where the higher dimensional operators used are driven by the existing data on the EW sector. This Chapter is supposed to set the roots of the analysis done in Chapters 3, 4 and 5. The results presented in these Chapters represent a step in the determination of the precision at which the different interactions in the EW sectors of the SM are being tested by the available data. The ultimate purpose of these analyses is to look for deviations that would be translated in the future on information regarding the UV completion of the SM. Up to this point our focus was on the scrutiny of the EWSB at LHC, and so the origin of the particles’ mass from a model-independent point of view. But still there are other alternatives to study physics beyond the SM, like direct searches at the LHC for some particular signature in a model. Clearly this approach only applies to neutrino models which are testable at LHC. In Chapter 6 we will study one such model of Type-III see-saw where the heavy states can live in the TeV region and, not less important, the couplings of these states are determined by the light neutrino masses and mixings.
Nucleation and cavitation in liquid helium
(Universitat de Barcelona, 1995-07-20) Guilleumas, Montserrat; Barranco López, Manuel; Pi Pericay, Martí; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[spa] Las transiciones de fase no siempre ocurren bajo condiciones de equilibrio. Un fluido puede ser conducido a un estado metastable. A pesar de ser internamente estable, existe otra configuración con un potencial termodinámico menor, por tanto energéticamente más favorable y a la cual tenderá el sistema. Esta transición está bloqueada por una barrera termodinámica que puede ser superada con resultado de la formación y crecimiento de pequeños nucleos de la nueva fase estable en la fase metastable. Este proceso se llama nucleación (formación de gotas en un vapor sub-enfriado) o cavitacion (burbujas en un líquido sobre-calentado). En esta tesis hemos estudiado los fenómenos de cavitación y nucleación en helio dentro del marco de la teoria del funcional de la densidad. Primero hemos estudiado la activación térmica en 3He y 4He puros y en las mezclas 3He-4He, estimando la presión a partir de la cual tiene lugar la transición de fase. Para los sistemas puros hemos estudiado también la activación cuántica a muy bajas temperaturas, estimando la temperatura de transición entre ambos regímenes.
Time Dependent Processes in Magnetic Systems
(Universitat de Barcelona, 2002-05-24) Iglesias, Òscar; Labarta, Amílcar; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[spa] En este trabajo se presentan modelos para el estudio de los procesos de dependencia en el tiempo de las propiedades magnéticas de diferentes tipos de materiales magnéticos, básicamente sistemas de partículas pequeñas y capas delgadas con aplicación en el campo del almacenamiento magnético. A lo largo del mismo se han estudiado las distintas problemáticas asociadas al creciente aumento de la densidad de grabación en soportes magnéticos. En primer lugar se han considerado los efectos de tamaño finito y de superficie asociados a la reducción del tamaño de las partículas magnéticas a través de simulaciones Monte Carlo de un modelo reticular para una única partícula de maghemita. Se han estudiado también los efectos de desmagnetización térmica a través de un modelo fenomenológico para la relajación magnética basado en llamada ley de escala Tln(t/to). A través de cálculos numéricos basados en este modelo y su aplicación a medidas experimentales, se ha mostrado como el modelo permite extraer información de las barreras de energía microscópicas responsables de la relajación, incluso cuando ésta se da en presencia de un campo magnético. La existencia de interacciones dipolares entre partículas puede también incorporarse dentro del marco del anterior modelo fenomenológico. A través de un análisis de la relajación calculadas mediante simulación en la ley de relajación y en la estructura de las distribuciones de barreras energéticas. Se han tratado también sistemas con más complejidad de interacciones magnéticas como son las capas delgadas, modelizadas a través de redes bidimensionales de espines con anisotropía perpendicular al plano. Las simulaciones realizadas para distintos valores de la interacción dipolar y de intercambio reproducen la gran variedad de estructuras magnéticas observadas en sistemas reales.
Entanglement, quantum phase transitions and quantum algorithms
(Universitat de Barcelona, 2006-07-20) Orús Lacort, Román; Latorre, José Ignacio; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[eng] From the seminal ideas of Feynman and until now, quantum information and computation has been a rapidly evolving field. While at the beginning, physicists looked at quantum mechanics as a theoretical framework to describe the fundamental processes that take place in Nature, it was during the 80’s and 90’s that people began to think about the intrinsic quantum behavior of our world as a tool to eventually develop powerful information technologies. As Landauer pointed out, information is physical, so it should not look strange to try to bring together quantum mechanics and information theory. Indeed, it was soon realized that it is possible to use the laws of quantum physics to perform tasks which are unconceivable within the framework of classical physics. For instance, the discovery of quantum teleportation, superdense coding, quantum cryptography, Shor’s factorization algorithm or Grover’s searching algorithm, are some of the remarkable achievements that have attracted the attention of many people, both scientists and non-scientists. This settles down quantum information as a genuine interdisciplinary field, bringing together researchers from different branches of physics, mathematics and engineering. While until recently it was mostly quantum information science that benefited from other fields, today the tools developed within its framework can be used to study problems of different areas, like quantum many-body physics or quantum field theory. The basic reason behind that is the fact that quantum information develops a detailed study of quantum correlations, or quantum entanglement. Any physical system described by the laws of quantum mechanics can then be considered from the perspective of quantum information by means of entanglement theory. It is the purpose of this introduction to give some elementary background about basic concepts of quantum information and computation, together with its possible relation to other fields of physics, like quantum many-body physics. We begin by considering the definition of a qubit, and move then towards the definition of entanglement and the convertibility properties of pure states by introducing majorization and the von Neumann entropy. Then, we consider the notions of quantum circuit and quantum adiabatic algorithm, and move towards what is typically understood by a quantum phase transition, briefly sketching how this relates to renormalization and conformal field theory. We also comment briefly on some possible experimental implementations of quantum computers.
Front Microrheology of biological Fluids
(Universitat de Barcelona, 2016-07-15) Trejo Soto, Claudia Andrea; Hernández Machado, Aurora; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[eng] Since Poiseuille times, several techniques has been developed to measure the viscosity of blood. During the 60's and 70's, with the appearance of the first rheomethers, the rheological properties of blood were accurately measured and a behavior dependent of the velocity gradient of the fluid flow was determined. As well, it was observed that blood had shear thinning properties, meaning, that if the velocity of the blood flow increases, the viscosity of blood decreases. From a medical point of view, blood and its blood plasma are the most effective fluids to detect global pathologies in human and animals. These pathologies may be related to their viscosities, their plasma proteins, or the properties of its red blood cells, as their aggregation, deformability or the elastic properties of its cellular membrane. Lately, with the birth of microfluidics at the beginnings of the 90’s, new techniques for the diagnostic of diseases has been developed. The avantages in the use of microfluidic devices for diagnostics are: the low amount of sample required to perform a measure, their portability, that they are easy to use and the low cost of its fabrication. The aim of this thesis project was to extend the study of front microrheology through the development of a device and method that describes accurately the non-linear rheology of biofluids, mainly blood, by means of a simple optical detection method based on tracking the fluid-air interface moving inside a microchannel. We centered in the fluid front (interface fluid-air) since is a direct, easy and cheap method to study fluid flows. In order to achieve this, we first had to developed a microfluidic device and method which would allow us to obtain a clear image of the fluid front. This was made, using a microscope and a high speed camera. The images obtained with the camera were analyzed by means of a computational code developed in Wolfram Mathematica©. The thesis work was mainly experimental comprinsing: fabrication of microfluidic devices and experiments with Newtonian and non-Newtonian fluids. Our results have been compared with theoretical and bibliographical results. The original results from this thesis are separated in two parts. The first part of the research was dedicated to the study the interface fluid-air of flows of Newtonian fluids. In order, to achieve reliable viscosity results with our device and method, and prove our system as a viscometer, we tested several fluids, including blood plasma which is known to be Newtonian. Our results were compared with the results obtained with a different viscometer to prove their reliability. This part of the study states our microfluidic device and method as a viscometer. The second part of this thesis was dedicated to extend the results for Newtonian fluid to non-linear hemorheology and comprises all the results for blood. Blood is essentially a difficult fluid to manipulate and study. In general, it presents non-Newtonian properties as shear thinning, meaning, that its viscosity decreases as the stress or the shear rate increases. This non-Newtonian properties are due to plasma proteins and especial characteristics of its red blood cells. With our device and method we were able to observe the non-Newtonian behavior of blood and to obtain its viscosity at different shear rates and stresses. As well, we related its viscosity to some of its red blood cells properties, as their tendency to form aggregates and the flexibility of their cellular membrane. The studies of blood were developed at different hematocrits, different dates from the extraction of the sample and with anemic blood and blood with alphathalessemia. In general, our device and method is usefull as a viscometer and rheometer, as well as, it enables to establish a relation between blood viscosity and its red blood cells characteristics.
Electronics control and signal processing for the LHCb fast calorimeter detectors
(Universitat de Barcelona, 2016-12-22) Picatoste Olloqui, Eduardo; Gascón Fora, David; Herms Berenguer, Atilà; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[eng] LHCb is one of the four large experiments of the Large Hadron Collider (LHC) based at CERN. The LHCb experiment is taking data at its nominal design luminosity. However, in order to distinguish among models of new physics, higher luminosity is needed. So the LHCb collaboration intends to upgrade the detector during the planned long LHC shutdown in 2019 in order to be able to operate at a luminosity about 10 times the nominal one. The objective the present thesis is the development of an integrated solution for the analog signal processing in the electronic and hadronic calorimeters of the LHCb. The analogue signal processing will be performed by a shaper ASIC in the calorimeters Front End (FE) boards. The signal pulse from a photomultiplier tube is clipped at its base and, then, is transmitted through a 12m 50Ω coaxial cable to the FE board located in the crates at the calorimeter platform. In order to reduce the PMT ageing after the increase in luminosity, the gain has to be decreased by a factor 5 with respect to the present operation in order to keep the same average current. Therefore, the preamplifier input noise must be decreased accordingly so that the total input referred noise voltage is smaller than 1nV/√Hz. Consequently, a 50Ω termination resistor is not acceptable. The main requirements for the analogue FE of the calorimeter system include a cali- bration of 4fC/2.5MeV per ADC count; a dynamic range of 12 bits; noise lower than 1 ADC cnt (ENC < 4 fC); and a spill-over residue level ±1% a linearity: < 1%. The implementation of the ASIC includes: four analog channels with programmable values to control the key parameters and compensate for process variations; a dedicated Delay Locked Loop (DLL) to synchronize each channel signal phase and a digital interface using SPI protocol. The analog channel is designed with an input amplifier that includes an electronically cooled termination input stage. A passive line termination would induce too large a noise and is avoided. Afterwards an alternated switched differential signal paths scheme permits the integration of the signal with no dead time between consecutive events. Each path includes a pole-zero filter in order to compensate for cable effects, a switched integrator with capacitive feedback, a Track-and-Hold for a 12-bit ADC and a MUX to select the correct sub-channel output signal. A fully differential signal processing is adopted in order to minimize the impact of common mode noise, which is important in a switched system. Each analog channel includes a delay line based on a DLL so the user can set a delay to compensate the delay introduced by PMT voltage settings, cable lengths or particle time of flight from the interaction point to the calorimeter cells. The DLL is adjusted by means of two control voltages to ensure that systematic process or environmental variations will not affect the channel time tuning. The radiation hardness expected from the selected technology (0.35µm AMS SiGe BiCMOS) is enough, but design techniques are used to ensure being able to tolerate SEUs, SETs and SELs. The design has been checked at different tests of a total of 30 pieces of the final pro- totype: at the laboratory using a signal obtained with a scope, with electron beams and ECAL channels in a dedicated facility at CERN, and its radiation hardness at Centre de Resources du Cyclotron at Louvain la Neuve. Dedicated boards were developed and the results are positive.
Functional organization and networ resilience in self-organizing clustered neuronal cultures
(Universitat de Barcelona, 2016-02-05) Teller Amado, Sara; Soriano i Fradera, Jordi; Casademunt i Viader, Jaume; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[eng] Major dynamical traits of a neuronal network are shaped by its underlying circuitry. In several neurological disorders, the deterioration of brain's functionality and cognition has been ascribed to changes in the topological properties of the brain's circuits. To deepen in the understanding of the activity-connectivity relationship, neuronal cultures have emerged as remarkable systems given their accessibility and easy manipulation. A particularly appealing configuration of these in vitro systems consists in an assembly of interconnected aggregates of neurons termed 'clustered neuronal networks'. These networks exhibit a complex dynamics in which clusters fire in small groups, shaping communities with rich spatiotemporal properties. The detailed characterization of this dynamics, as well as its resilience to perturbations, has been the main objective of this thesis. In our experiments we monitored spontaneous activity using calcium fluorescence imaging, which allows the detection of neuronal firing events with both high temporal and spatial resolution. The detailed analysis of the recorded activity, in the context of network theory and community analysis, allowed for the quantification of important properties, including the effective connectivity map and its major topological descriptors. As major results, we observed that these clustered networks present hierarchical modularity, assortative mixing and the presence of a rich club core, a series of features that have also been observed at the scale of the brain. All these characteristic topological traits are associated with a robust architecture that reinforces and stabilizes network activity. To verify the existence of such robustness in our cultures, we studied their resilience upon chemical and physical damage. We concluded that, indeed, clustered networks present higher resilience compared to other configurations. Moreover, these clustered networks exhibited recovery mechanisms that can be linked to the balance between integration and segregation in the network, which ultimately tend to preserve network activity upon damage. Thus, these in vitro preparations offer a unique scenario to explore vulnerability in networks with topological properties similar to the brain. Moreover, the combination of all these approaches can help to develop models to quantify damage upon network degradation, with promising applications for the study of neurological disorders in vitro.
Brassinosteroids role in arabidopsis root development : theoretical and experimental approaches
(Universitat de Barcelona, 2016-01-29) Pavelescu, Irina; Ibañes Miguez, Marta; Caño Delgado, Ana I.; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[eng] This PhD thesis represents an advance in the present understanding of the spatiotemporal control of model plant Arabidopsis thaliana root growth and development. The size and structure of a living organism are tightly controlled by the coordination between several highly dynamic molecular and cellular processes, such as cell division, movement, growth and deformation. At tissue level, a mesoscopic description of the system and these processes can be used, in terms of mechanical forces and energy minimization (see (Hamant & Traas, 2010) for a review focused on plants). How cells decide to switch from a cellular process to another is a fundamental question to understand the growth and shape of an organ. Because of the thermal fluctuations and finite number of molecules involved in the molecular reactions, cells take presumably these decisions in a stochastic manner, which makes it challenging to understand how morphogenesis generates organs with characteristic shapes and sizes. Plant roots grow due to cell division in the meristem and subsequent cell elongation up to terminal differentiation. The pleiotropic phenotypes of the short-root mutants available make it difficult to univocally assess which mechanism sets the transition from elongation to final differentiation. To elucidate it, in this thesis we use a novel approach based on the quantitative information associated to the phenotypic variability of wild type roots together with computational modeling of different mechanisms. In Chapter 1 we introduced the already published work in the field of root and meristem growth, at experimental and computational level. In Chapter 2 we have employed theoretical and computational models to analyze individual isogenic Arabidopsis seedlings and to quantify their heterogeneity, which we have quantified, together with their mean values. The quantification of heterogeneity has been crucial since it allowed the identification of dynamical mechanisms involved in Arabidopsis root growth. By analyzing these mechanisms in WT plants and Brassinosteroids (BRs) mutants, we found that growth defects in the BRs loss of function mutant are generated by defects related to cell differentiation. To deepen into this result, in Chapter 3 we investigated the mechanism through which cells decide to differentiate and achieve their final length. In this sense, we adopted a computational approach, combined with plant variability analysis, to test three putative mechanisms: Ruler (Band et al, 2012; De Vos et al, 2014), Timer (De Vos et al, 2014; Mähönen et al, 2014) and Sizer (Grieneisen et al, 2012). We compared the simulated data, based on the values extracted in Chapter 2, with experiments, and we found that Arabidopsis thaliana primary root uses a Sizer mechanism based on measuring cell sizes for final cell differentiation. We show this mechanism translates into specific correlations among phenotypic traits and explains why root growth is proportional to the meristem activity and displays mature cells of stereotyped length. We challenged our model by evaluating such correlations in a well-known BR signaling short-root mutant. We further show that BR signaling at the meristem is sufficient to recover some of the correlation slopes and hence root growth, yet it alters the mechanism. Together, our results establish a theoretical quantitative framework for stationary root growth and underscore the value of using computational modeling together with quantitative data. In Chapter 4 we analyzed the coupling between meristematic activity and telomere length by applying a novel quantitative fluorescence in situ hybridization to measure telomere length with tissue resolution in the primary root. The implementation of a new image analysis protocol contributed to revealing a telomere distribution map, with telomere length gradients along the meristem, and the longest telomeres localized in the stem cell niche (Gonzalez-Garcia et al, 2015). We applied this method to WT plants, several generations of telomerase deficient mutants, mutants with larger telomeres and cell differentiation mutants. Furthermore, we generated transgenic plants to check the localization of telomerase and we evaluated the relationship between telomere length and resistance to DNA damage. We also evaluated computationally the telomere distributions observed in WT and telomerase deficient mutants and we simulated the telomere dynamics which can generate such distributions. The conclusions of this thesis were contextualized in Chapter 5.
Dynamics of cellular decision making processes
(Universitat de Barcelona, 2016-02-02) Palau Ortin, David; Ibañes Miguez, Marta; Sancho, José M.; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[eng] Cells, either as unicellular organisms or as part of a tissue of a multicellular organism, can acquire different functions thanks to their capability of changing their expression state. The enzyme synthesis, cell division or cell differentiation are some examples of these functions. The turning on and off of them lie in the mechanisms by which cells are able to integrate the information they perceive from the environment. Frequently, cells exhibit different responses under the same stimulus or environment. These probabilistic processes, whose behaviours are not univocal, are known as "cellular decision making". We can classify these processes according to the range at which the decision is made. We denominate cell-autonomous decision those in which of each cell chooses its response independently of the choice of the other cells of the population. By contrast, if the decision is made collectively by the whole population, it is classified as non autonomous. This second type of decisions involve mechanisms of cell-to-cell communication that mediate in the choices the cells and so, some spatial distributions of the different cell states can arise. The capability of cellular decision making processes of performing a variety of responses under a same signal is given by the multistability and the stochasticity of their dynamics. While multistability is underlain by the nonlinear interactions of the elements involved in genetic regulation, stochasticity arises from the discrete nature of biochemical reactions and the thermal fluctuations of the cellular environment. These two characteristics motivate the study of these processes from Systems Dynamics the point of view, by identifying cell states with system attractors. This Thesis focuses on the study of the general dynamical mechanisms that control cellular decision making processes. The main goal is to connect the properties of the decision with the relevant dynamical behaviour of the system while it is being made. We have analysed the properties of cellular decisions in two systems: a system with cell-autonomous dynamics, where cells choose their state regardless the choice of the others; and a system where the decision is made jointly by all the tissue. In this second system, cells interact through a cell-to-cell communication that takes place at first neighbours. From these interactions, different pattern solutions arise, where different different cell types are spatially distributed along the tissue. Finally, it has been analysed the role that a specific choice, whose probability value is well known, plays in the functionality of an organism. The chosen system to study these consequences has been a process of differentiation that the parasite that causes malaria in humans performs.
Self-organization and cooperativity of cytoskeletal molecular motors
(Universitat de Barcelona, 2016-01-15) Oriola Santandreu, David; Casademunt i Viader, Jaume; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[eng] The present work deals with different aspects concerning the collective action of cytoskeletal molecular motors. The thesis is organized in two parts: the first part corresponds to the study of the cooperative action of molecular motors in intracellular transport, whereas the second part corresponds to the study of oscillatory dynamical instabilities driven by molecular motors. In the first part of the thesis, we carry out complete theoretical and experimental studies on the single-headed kinesin KIF1A, which constitutes a remarkable example of Brownian motor and a model motor to study intracellular transport. We provide a thorough numerical study of the collective action of single-headed KIF1A motors based on Brownian dynamics. We predict a dramatic improvement of the collective performance of these motors for tasks associated to the transport of membrane-bound cargoes. From a biological point of view, our results reinforce the hypothesis that the specificity of KIF1A to axonal vesicular trafficking is due to its unique adaptation to cooperative force generation. From a fundamental physics point of view, we show that Brownian motors based on two-state ratchets with independent switching and under unequal loading are remarkably adapted to cooperative force generation. We further test our predictions using a lattice model to study the dynamics of two interacting KIF1A motors. We show analytically the presence of cooperativity in the system and we consider a first extension of the problem to an arbitrary number of motors. Finally, we test our theoretical predictions experimentally, by using biomimetic tube pulling assays with single-headed KIF1A motors. We show that, despite the extreme inefficieny of the individual motors, they are able to cooperate collectively to extract membrane tubes, thus validating our theoretical predicitions. Additionally, we find the surprising formation of helical tubes around microtubules. This entails an impressive capability of single-headed KIF1A motors to exert significant off-axis by virtue of a diffusive state. Accordingly, this state affords two complementary strategies to overcome obstructions: brute force and manoeuvreing capability. In a series configuration (in line) it enables the generation of large forces by accumulation of motors, whereas in a parallel configuration (side by side) it enables lateral displacement of the cargo. In the second part of the thesis, we study the generation of dynamical instabilities driven by molecular motors. In particular, the spontaneous oscillations in a minimal in vitro actomyosin system and the self-organized flagellar beating driven by axonemal dynein. In the first case, we study theoretically an actomyosin system coupled to an elastic element, generating spontaneous oscillations in the presence of ATP via a Hopf bifurcation. This problem mimics the mechanism responsible of the asynchronous wing thrust observed in some insect species. We show that the theoretical model, based on an integro-differential system of equations, can be reduced to a simple three-dimensional ODE system. We find that both the complete and reduced systems exhibit subharmonic oscillations in some regimes. Remarkably, subharmonic peaks were reported experimentally in the signal power spectrum of a minimal in vitro actomyosin system. Hence, we provide an explanation for this phenomenon. In the second case, we study the nonlinear dynamics of axonemal beating driven by molecular motors. The explicit nonlinear equations for the flagellar shape and dynein kinetics are derived and solved numerically. Our analysis reveals the spatiotemporal dynamics of dynein kinetics and flagellum shape for different regimes of motor activity, medium viscosity and flagellum elasticity. We find that far from the bifurcation, linearized solutions fail to describe the flagellar shape and nonlinear effects arise in the system solely due to motor activity. Finally, we further characterize flagellar dynamics using principal component analysis and studying bending initiation.
Fluctuations, gene circuit architecture and stem cell quiescence
(Universitat de Barcelona, 2016-01-18) Frigola Tubert, David; Ibañes Miguez, Marta; Sancho, José M.; Universitat de Barcelona. Departament d'Estructura i Constituents de la Matèria
[eng] Biological development is a complex process in which, from a single cell, a whole multicellular organism arises. The formation of this intrincate structures requires a very precise regulation in space and time. This regulation involves a network of proteins and genes that interact. These interactions give rise to nonlinear behaviours, of the same kind that physicists have studied in other systems. Furthermore, cellular processes are often subject to fluctuations, a problem that has been studied by out of equilibrium statistical mechanics. These facts mean that the tools from nonlinear physics and statistical mechanics are suitable to study problems in the biological context and, at the same time, these problems are of interest to physcists. In this thesis we study small gene circuits that generate bistability, the property of having two stable states upon the same input. In the first part of the thesis, we study at a theoretical level how the architecture of these circuits interacts with stochasticity. We first study a simple autoactivating Positive Feedback Loop, in which a protein activates its own production. We compare how fluctuations affect the system when they are added as a simple perturbation to the deterministic system (additive noise) or when they are derived from the dynamics of the system (multiplicative noise). We also show that multiplicative noise is able to reproduce the experimentally observed result of asymmetric stochastic switching, in which it is easier to escape from a high concentration state to a low concentration state than vice versa. We also compare how noise from diferent sources affects five bistable circuits with different architectures, and find that the sensitivity of the stability of the states to each noise source depends greatly on the circuit considered. This suggests that they could be found in different biological contexts and functions. In a second part of the thesis, we study the quiescent centre of plant {\em Arabidopsis Thaliana}. This quiescent centre is formed by a group of stem cells that organize the different stem cell types surrounding them, and have the property of rarely dividing called quiescence. Thanks to a collaboration with the plant developmental biology group led by Dr. Ana I. Ca\-no-Delgado at Center for Research in Agricultural Genomics (CRAG), we are able to study a gene circuit that regulates quiescence. Our collaborators discovered BRAVO, a protein that regulates quiescence, and unveiled its interactions with BES1, that regulates many proteins according to the signalling of the Brassinosteroid hormone family and promotes divisions (releasing quiescence). Our mathematical modelling shows that the dimerization of BRAVO and BES1 enables strongly opposed states of these proteins, ensuring unequivocal regulation of quiescence, and a sharp transition from a [HIGH BRAVO, LOW BES1] state to a [LOW BRAVO, HIGH BES1] state upon Brassinosteroid signalling. We also add to this module a third protein, WOX5, that is only expressed in the quiescent centre and has been shown to directly regulate quiescence, and find that this opposed states and sharp transitions are preserved.

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