Tesis Doctorals - Departament - Física Fonamental

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Infrared modifications of gravity
(Universitat de Barcelona, 2008-07-03) Blas Temiño, Diego; Garriga Torres, Jaume; Universitat de Barcelona. Departament de Física Fonamental
[spa] El propósito de la Tesis es estudiar las posibles modificaciones de la relatividad general a distancias largas. La motivación de dicho estudio se debe tanto a motivos teóricos (la rigidez de la relatividad general en comparación con el resto de interacciones) como a experimentales (la explicación de la actual expansión del universo). En la tesis nos hemos centrado en dos puntos principales: la gravedad unimodular y la bigravedad. La primera parte de la tesis consiste en el estudio de las teorías linearizadas de gravitones donde encontramos las posibles teorías de gravedad masiva que no sufren de inestabilidades. Además, para el caso sin masa hemos encontrado un caso diferente al proveniente de la relatividad general, si bien con idénticas predicciones físicas. También hemos extendido este resultado a otras teorías de spin alto para fermiones. En la segunda parte nos centramos en aspectos no-lineales. En primer lugar, para el caso sin masa, encontramos una alternativa a la relatividad general con idénticas predicciones excepto para la constante cosmológica: la gravedad unimodular. Intentamos, sin éxito, demostrar su unicidad. Para el caso de gravedad masiva nos hemos centrado en la bigravedad como modelo no-lineal. Hemos estudiado tanto aspectos globales como locales. En lo que se refiere a las perturbaciones, hemos encontrado que la bigravedad puede dar lugar a modificaciones de relatividad general consistentes.
Mesoscopic descriptions of complex networks
(Universitat de Barcelona, 2008-11-13) Fernández Sabater, Alberto; Gómez Jiménez, Sergio; Arenas, Àlex; Universitat de Barcelona. Departament de Física Fonamental
[spa] El objetivo de la presente tesis es el estudio de las subestructuras que aparecen a un nivel de resolución mesoscópico en las redes complejas. Dichas subestructuras, que en el campo de las redes complejas son denominadas comunidades, intentan agrupar los nodos de una red de manera que los nodos que forman parte de una misma comunidad estén más conectados entre ellos que con el resto de nodos de la red. La importada del análisis de estas estructuras radica en que nos permiten comprender mejor las redes complejas dándonos información sobre la funcionalidad de las comunidades que las componen. Hemos llevado a cabo el estudio de estas estructuras mesoscópicas utilizando la información topológica de las redes, y en cuanto a los métodos empleados éstos se pueden agrupar en dos grandes familias conocidas habitualmente como clustering jerárquico y clustering modular. Dentro de la primera familia de métodos nos hemos fijado en la existencia de un problema de no unicidad en el clustering jerárquico aglomerativo, y hemos propuesto una solución a dicho problema basada en el uso de una nueva herramienta de clasificación que denominamos multidendrograma. A continuación, hemos aplicado el resultado de una clasificación jerárquica para resolver un problema dentro de las redes complejas financieras. Más concretamente, hemos aprovechado una partición en clusters para resolver de manera más eficiente el problema de optimizar una cartera de valores. Por lo que respecta a la segunda familia de métodos de clustering estudiados, ésta se basa en la optimización de una función objetivo llamada modularidad El inconveniente que presenta la optimización de la modularidad es su elevado coste computacional, la cual cosa nos ha llevado a idear una reducción analítica del tamaño de las redes complejas de manera que se conserva toda la información necesaria en la red original de cara a hallar la estructura de comunidades que optimice la modularidad. A continuación hemos podido utilizar dicha simplificación de los cálculos en el análisis de toda la mesoescala topológica de las redes complejas. Dicho mesoescala la hemos estudiado añadiendo un mismo valor a todos los nodos de una red que mide su resistencia a formar parte de comunidades, La optimización de la modularidad para estas nuevas instancias de la red original obtenidas a partir de unos valores de resistencia acotados analíticamente, nos permite analizar la mesoescala topológica de las redes. Por último, hemos propuesto una generalización de la función de modularidad donde los bloques constituyentes ya no son solamente arcos sino que pueden ser distintos tipos de motifs. Esto nos permite obtener descripciones más generales de grupos de nodos que incluyen como caso particular a las comunidades.
Statistical and thermodynamical properties of DNA unzipping experiments with optical tweezers
(Universitat de Barcelona, 2011-02-04) Huguet Casades, Josep Maria; Ritort Farran, Fèlix; Universitat de Barcelona. Departament de Física Fonamental
[eng] Molecular biophysics is a scientific discipline that studies biomolecules. This discipline has experienced a revolution thanks to the development of single- molecule techniques. These techniques allows us to obtain new and valuable information that complements the traditional bulk assays. Optical tweezers is an experimental technique that uses the radiation pressure of light to exert forces on a tiny dielectric microsphere. Biomolecules can be bonded to such microspheres in order to perform pulling experiments at the single- molecule level. Minitweezers is a dual counter-propagating laser tweezers instrument that measures the force exerted on the microsphere by conservation of light momentum. The instrument has high stability and resolution (0.1 pN in force and 0.5 nm in distance) in the measurements. The properties of the DNA molecule can be studied with the Minitweezers at the single-molecule level. The DNA is a biomolecule that forms a double helix that stores the genetic information of the cells. DNA unzipping experiments consist in pulling apart the two strands of DNA by exerting mechanical forces on the extremities of the molecule. In such process, basepairs (bp) are disrupted sequentially, showing a succession of cooperative unzipping regions (CUR) of different sizes (between 1–100 bp). In a DNA unzipping experiment, we measure the force vs. distance curve (FDC) of the molecule, which has a characteristic sawtooth-like shape that is sequence-dependent. The FDC is analyzed with a Bayesian approach to infer the size distribution of the CURs. The experimental accuracy does not allow to observe CURs of sizes below 10 bp. Furthermore, the unzipping of one bp at a time can only be achieved by having an optical trap stiffness value higher than 0.1 N/m, which corresponds to the stiffness of a single nucleotide of DNA. This has been deduced from a toy model specifically introduced to study the CUR size distributions. In addition, the FDCs are theoretically predicted by the nearest-neighbor (NN) model adapted to unzipping experiments. The NN model describes the hybridization reaction of two strands of DNA. By fitting the experimental FDCs to the model, the unique 10 NN bp free energies are obtained with 0.1 kcal mol−1 precision between 10 mM–1 M of monovalent salt concentration. The results show that the unzipping FDCs and the melting temperatures of oligos are correctly described with a specific salt correction for each of the 10 NN bp free energies. Differently from the previous experiments, the unzipping of DNA can also be performed at con- trolled force. These last type of experiments exhibit large hysteresis and irreversibility. The free energy landscape is a tool that helps to understand the unzipping at controlled force. Finally, the work presented in this thesis can be extended to find practical applications of DNA unzipping, such as sequencing of DNA by force, and measurement of thermodynamic properties of biomolecules in conditions not accessible by bulk methodologies.
Estudi de sistemes de baixa dimensionalitat i correlació electrònica forta pel mètode de la matriu de transferència
(Universitat de Barcelona, 1989) Valentí i Vall, M. Roser; García Bach, Ma. de los Ángeles; Universitat de Barcelona. Departament de Física Fonamental
[cat] Els sistemes de baixa dimensionalitat han sigut i són atractius als física per diverses raons: - A nivell estrictament teòric són els sistemes més senzills d'estudiar, i, de fet, molts models no trivials en tres dimensions que sols poden tractar-se fent aproximacions intel·ligents, s'han resolt exactament en una dimensió. Aquesta característica els fa adients per a analitzar la eficàcia dels mètodes aproximats que després s'aplicaran a sistemes més complicats. Però, simultàniament, la baixa dimensionalitat els confereix unes propietats específiques no presents en sistemes de més dimensions. - Experimentalment s'han trobat sistemes reals que presenten un comportament de tipus uni i bidimensional. En particular, existeixen dues famílies de gran interès tant des del punt de vista teòric com del tecnològic. Aquests són els polímers conjugats (poliacetilè, poliacè, poliacenacè, polifenantrè, etc.), sistemes que poden classificar-se com a quasi-1 dimensionals. I els materials superconductors d'alta Temperatura de transició (òxids de coure principalment) que són sistemes fonamentalment bidimensionals. En aquest treball ens centrarem en aquestes dues famílies de sistemes esmentats.
Simulació de sistemes iònics
(Universitat de Barcelona, 1990) Trullàs Simó, Joaquim; Giró i Roca, Antoni; Universitat de Barcelona. Departament de Física Fonamental
[cat] La història de la simulació de sistemes de N cossos pot resumir-se breument dient que aquesta ha anat evolucionant dels primers estudis de líquids simples a l'estudi de sistemes cada vegada més complexes (sals foses, líquids moleculars, dissolucions, macromolècules, interfases, reaccions químiques, polímers, ...), Aquesta evolució ha estat possible gràcies, d'una banda, al gran progrés tecnològic dels ordinadors, i de l'altra, al desenvolupament de nous mètodes i tècniques de simulació. Aquesta línia d'evolució cap a l'estudi de sistemes més complexes és precisament la que s'està seguint en el grup de simulació integrat per membres de la Secció d'Informàtica del Departament de Física i Enginyeria Nuclear de la Universitat Politècnica de Catalunya i del Departament de Física Fonamental de la Universitat de Barcelona. Abans de l'inici d'aquest treball, s'havia aplicat la dinàmica molecular a l'estudi de líquids simples, i es començava a preparar el camí per a poder estudiar sistemes més complexes; d'una banda, s'havia desenvolupat un programa de dinàmica de Langevin per a poder simular dissolucions considerant explícitament només el solut, i de l'altra, s'acabava d'implementar el mètode d'Ewald al programa de dinàmica molecular per a poder simular sistemes iònics com les sals foses. Aleshores, seguint aquesta línia de progressió, en aquest treball hem estudiat: a) Sals foses aplicant la dinàmica molecular (AgI, Cul, CuBr, CuCI i NaCl). b) Dissolucions iòniques aquoses aplicant la dinàmica de Langevin (NaCI-aq).
Carácter asintótico de la invariancia adiabática
(Universitat de Barcelona, 1965) Navarro Veguillas, Luis; Garrido, L. (Luis), 1930-; Universitat de Barcelona. Departament de Física Fonamental
[spa] En el estudio de la Física del Plasma y, más concretamente, en la llamada aproximación del centro guía se trata el problema del movimiento de partículas cargadas en un campo magnético variable. Los resultados obtenidos se basan en la hipótesis de que la variación de dicho campo es lenta, tanto en el espacio como en el tiempo. Se obtienen magnitudes que son "casi" constentes. El problema principal consiste, por tanto, en el estudio de bajo qué condiciones es posible el establecimiento de dichos invariantes y qué grado de aproximación suministran los distintos procedimientos empleados. Constituye en conjunto lo que se conoce con el nombre de aproximación adiabática. El teorema adiabático ordinario, el de orden “m” y el generalizado, no son sino procedimientos para calcular el operador evolución del sistema físico que se estudia. El primero es clásico y suministra, en principio, una buena aproximación. El de orden “m”, presenta un mayor grado de validez, pero a costa de exigir al hamiltoniano unas condiciones extras que generalmente no tiene. En cambio el teorema adiabático generalizado suministra la aproximación deseada, sin otro requisito que repetir tantas veces como grado de aproximación se pretenda, el cambio de representrción conocido con el nombre de "representación de ejes giratorios". Nuestro objeto era ampliar el campo de utilización y las posibilidades de la aproximación adiabática. Nos hemos visto en la necesidad de asentar firmemente los fundamentos de la misma antes de abrir nuevas posibilidades. Y ello nos ha sido posible gracias a la observación de que el carácter asintótico y el rigor de los tres teoremas adiabáticos descansa en el de una integral común, salvo pequeños detalles. De este modo hemos dedicado el primer capítulo al estudio exhaustivo de la misma, apoyándonos en los modernos conocimientos que se poseen sobre las series asintóticas. Posteriormente, teniendo en cuenta les conclusiones del capítulo primero, nos he resultado relativamente sencillo presentar una demostración rigurosa, con la ventaja de ser común a los tres teoremas, salvo ligeros matices. En la última parte introducimos un exponente que al tomar distintos valores hace la demostración válida para cada uno de los tres teoremas.
Noves aplicacions de l'àlgebra geomètrica a la física matemàtica
(Universitat de Barcelona, 2002-01-24) Miralles Esteban, David; Parra Serra, Josep Manel; Vaz, Jayme; Universitat de Barcelona. Departament de Física Fonamental
[spa] En la presente tesis se han tratado los siguientes aspectos: 1,- Se ha reformulado la presentación de la teoría de espinores consiguiendo una única definición de espinor que da énfasis al observador y matiza el papel del espacio de representación. El formalismo utilizado ha permitido distinguir de manera efectiva entre espinores conceptualmente diferentes. Surge el concepto de espacio interno sin que se requiera ampliar las dimensiones del espacio-tiempo. 2,- En el marco de una teoría clásica de campos, el análisis algebraico de una generalización de la ecuación de Dirac-Hestenes ha permitido encontrar para la partícula y la antipartícula (con A=0) soluciones de energía positiva diferentes. Se demuestra que esta diferencia viene codificada por el ángulo de Yvon-Takabayasi. 3,- Se ha acotado la generalización anterior mediante un estudio de las posibles versiones multivectoriales de la ecuación de Dirac. La preocupación básica ha sido poder plantear la ecuación sobre espinores operadores. 4,- Hemos extendido los trabajos de Lounesto referentes al cambio de signatura. Ha sido realizado desde dos puntos de vista: Z-graduaciones y Z2-graduaciones, consiguiendo en ambos casos expresiones que permiten, dada una signatura, construir todas las estructuras algebraicas y diferenciales asociadas a todas las otras signaturas de la misma dimensión.
Fenómenos cuánticos de espín en sistemas mesoscópicos
(Universitat de Barcelona, 2001) Hernández Ferràs, Joan Manel; Tejada Palacios, Javier; Universitat de Barcelona. Departament de Física Fonamental
[spa] La Física Cuántica descansa sobre el principio de indeterminación que nos dice que no podemos conocer con total precisión y a la vez la posición y la velocidad de un objeto. Una consecuencia de la indeterminación en la posición es la probabilidad no nula que tiene una partícula de atravesar una región prohibida clásicamente; este es el denominado efecto túnel. El decaimiento de una partícula alfa de un núcleo, la conducción por "electron-hoping" en aislantes, las parejas de Cooper tuneleando a través de una unión de Josephson, etc, son algunos ejemplos de manifestaciones del efecto túnel. En los años 80, Caldeira y Leggett [1, 2] desarrollaron la teoría formal del efecto túnel macroscópico, que predecía que bajo determinadas circunstancias los objetos macroscópicos, podrían presentar propiedades cuánticas reservadas a los objetos microscópicos. La primera evidencia de efecto túnel macroscópico fue presentada por Clarke [3], quien mostró que, en uniones Josephson, la diferencia de fase superconductora a través de la unión puede tunelear entre el estado superconductor y el estado normal. Los físicos teóricos, guiados por el formalismo planteado por Caldeira y Legett, produjeron interesantes avances teóricos. Quiero destacar los trabajos de E. M. Chudnovsky dedicados al efecto túnel del momento magnético [4, 5]. En el ámbito experimental, hay sólidas evidencias experimentales del efecto túnel del momento magnético deducidas de los trabajos de relajación magnética a bajas temperaturas [6, 7, 8], muchas de ellas obtenidas por parte del grupo de investigación al que pertenezco. En ellas se observa la constancia de la viscosidad magnética por debajo de una determinada temperatura de "crossover", contrariamente a lo esperado clásicamente. Esto se atribuye a la inversión cuántica del momento magnético. La relajación magnética observada en la mayoría de materiales representa el promedio estadístico sobre un gran número de sucesos, con una ancha distribución de escalas temporales debido a factores aleatorios como el tamaño de las partículas o las interacciones. Por lo tanto, es difícil realizar una comparación directa entre la teoría y los experimentos. El formato elegido en esta memoria es el de compendio de artículos. Sin embargo, tal presentación ha sido completada con explicaciones para introducir, ampliar o justificar los resultados expuestos en las publicaciones. En el primer capítulo, se describe el diseño y operación de un sistema de medida de magnetización a ultra-bajas temperaturas. Su puesta a punto en el rango de muy bajas temperaturas lo ha convertido en un equipo experimental único en el mundo, lo que nos ha permitido obtener resultados muy novedosos en el panorama científico internacional. En el segundo capítulo muestro un modelo teórico que describe el comportamiento de los sistemas de partículas magnéticas a bajas temperaturas. Hago especial hincapié en la obtención de resultados para los diferentes protocolos de medida usados habitualmente en los experimentos. Analizo su utilidad para extraer los parámetros intrínsecos del material así como para llegar al esclarecimiento de los mecanismos que gobiernan su relajación. Los resultados de este modelo en comparación con resultados de experimentos en sistemas de partículas permiten una fácil interpretación de los mismos en términos del efecto túnel de la magnetización. El tercer capítulo describe tanto los resultados de los experimentos como el modelo teórico que explica los fenómenos cuánticos en clústeres moleculares. Tales compuestos han abierto la puerta para un estudio detallado de los fenómenos de relajación cuántica presentando además nuevas características asociadas al carácter discreto de su bajo momento magnético (del orden de la decena de magnetones de Bohr). Presento también un nuevo fenómeno en la Física, el efecto túnel resonante de la magnetización, basado en el hecho de que, cuando los niveles a ambos lados de la barrera dada por el hamiltoniano de espín están degenerados, la probabilidad de efecto túnel es máxima. Presento los resultados de caracterización obtenidos mediante diferentes técnicas: magnetización d.c. y a.c. al helio líquido, capacidad calorífica, experimentos bajo campos magnéticos pulsados, y, en el caso del compuesto Fe8, magnetización d.c. a ultra-bajas temperaturas obtenida en el magnetómetro descrito en el primer capítulo. El estudio de la respuesta de estos materiales bajo la acción de campos magnéticos pulsados, nos ha revelado que la magnetización puede relajarse en forma de avalanchas asociadas a la gran cantidad de energía liberada en el proceso. Las medidas a ultra-bajas temperaturas demuestran que el efecto túnel en dicho rango de temperaturas se produce por el nivel fundamental, definiendo el marco necesario para el desarrollo del experimento descrito en el capítulo 4. En el cuarto capítulo, presento los resultados de las medidas de susceptibilidad a.c. a alta frecuencia en el compuesto Fe8. Dichas medidas muestran dos picos en la dependencia de la susceptibilidad con el campo magnético y pueden ser interpretadas como una absorción de energía por los niveles que aparecen como consecuencia de la superposición cuántica de los estados a ambos lados de la barrera. Este hecho constituye la primera evidencia experimental de la coherencia cuántica mesoscópica del momento magnético.
The motion sensing problem in spherical gravitational wave detectors
(Universitat de Barcelona, 1999-05-28) Serrano Moral, Ma. Ángeles (María Ángeles); Lobo Gutiérrez, José Alberto, 1953-; Universitat de Barcelona. Departament de Física Fonamental
[spa] La tesis presenta una cuidadosa descripción del comportamiento dinámico de las antenas esféricas resonantes de ondas gravitatorias cuando son excitadas por radiación gravitatoria, o también por señales de calibración, centrando especialmente la atención en el problema de los resonadores y desarrollando un procedimiento general aplicable a cualquier propuesta. La primera parte del trabajo incluye los preliminares y una breve supervisión de la teoría de Radiación Gravitatoria según la Relatividad General. La atención se centra luego en la Detección de Ondas Gravitatorias, y se describe el estado actual revisando las diversas técnicas, especialmente la detección resonante con detectores esféricos. El resto de la tesis se dedica exclusivamente a estos últimos, que se consideran como un sistema acoplado esfera-resonadores. Se establecen el conjunto de ecuaciones diferenciales que describen el sistema bajo suposiciones generales, y se restringen a una situación ideal. Su resolución en base a series perturbativas proporciona las resonancias del dispositivo acoplado, así como las amplitudes vibracionales en las posiciones de los resonadores. También se discute la posibilidad de construcción de canales de modo. A continuación, presentamos nuestra propuesta PHCA, que como la antena TIGA presenta una distribución mínima interesante de resonadores, y se analizan ambas propuestas como particularizaciones de los resultados generales. FInalmente, emprendemos el tratamiento de otras dos cuestiones importantes: como pequeñas desviaciones de la situación ideal afectan las respuestas del sistema, y el problema de la deconvolución de señales, también cuando se tiene en cuenta la presencia de ruido. El primer tema es especialmente interesante por incrementar el grado de aplicabilidad del modelo ideal a sistemas reales. La memoria se cierra con una breve discusión sobre los resultados.
Dynamic Properties of Magnetic Colloidal Particles and Holes
(Universitat de Barcelona, 1985-10-06) Miguel López, María del Carmen; Rubí Capaceti, José Miguel; Universitat de Barcelona. Departament de Física Fonamental
[eng] Our main goal is to study certain aspects of the dynamics of fluids with magnetic particles in suspension, based on their promising practical applications as new materials as welI as on its fundamental scientific interest. In the introduction we brief the reader on the most essential properties of the system. We have characterized the monodomain magnetic particles and the time scales inherent to magnetic fluids. Having introduced the rotational diffusion equation as the most convenient tool to take into account the different mechanism inftuencing the dynamics of the particles, we have also proposed a fruitful approach for solving it in any general situation. We have also highlighted the macroscopic properties of the magnetic fluid treated now as a continuous medium and showed up the different phenomena associated with the lack of stability in the system. In Chapter I we concentrate on two limit cases whose analysis is easier but very illustrative. The first part of the chapter is devoted to the study of a suspension of rigid dipoles, in which the magnetic moments are rigidly attached to the body of the particles themselves. In these conditions, if we apply an external magnetic field both the magnetic moment and the particle move together so that the magnetic torque acting upon it becomes zero. Thermal fluctuations tends to disrupt this order, and it turns out that, for instance, that the effective viscosity of the suspension depends on the dimensionless parameter comparing magnetic and thermal energies. In the second part we consider magnetic materials with finite anisotropy energy at high magnetic fields. For such monodomain particles the magnetic moments rapidly orient along the direction of the external field, and then as a second step the mechanical rotation of the particles takes place. In this case, the effective viscosity of the suspension is a function of the magnetic anisotropy constant of the material, of the volume of the particles as well as the thermal energy. Our results are compared to experimental measurements. The second chapter is concerned with the determination of the viscosity and of some magnetic and optical properties of magnetic fluids in the whole range of possible experimental situations. The magnetic moments and the particles inside the liquid reorient separately but their dynamics are coupled thus giving rise to a more intricate relaxation process. We have compared part of our results with available experimental data for different ferrofluids showing quite a good agreement. In Chapter III we joint to our discussion of magnetic fluids the presence of nonmagnetic particles of micrometer size and study their motion through the ferrofluid. The ferrofluid is considered now as a continuous medium with new transport coefficients already determined in the previous sections. Under the action of a rotating external magnetic field, we study the rotational motion of the nonmagnetic particles and compare our expressions to sorne measurements carried out in these composite systems. In this chapter we are also con cerned with the characterization of the hydrodynamic interactions among these particles in a carrier ferrofluid. Chapter IV is intended as a brief introduction to the multiple problems which arise when one handle the aggregation phenomena which may take place in these systerns. We study the kinetics of the forrnation of the aggregates by rneans of the Smoluchowski theory of coagulation in colloids. But we account for hydrodynarnic interactions which are not usually considered when studying such process and that gives rise to sorne corrections for high concentrations of particles. In addition, the rheology of the chains that are usually observed in systerns with dipolar interactions is given for a rather simplified situation in order to elucidate the effects of the dipolar magnetic interactions. Finally, we sum up our main conclusions and indicate some of the perspectives stimulated by the contents of this monograph and in which we plan to pursue work in the near future.
Mesoscopic Nonequilibrium Kinetics of Nucleation Processes
(Universitat de Barcelona, 2001-11-23) Reguera, D. (David); Rubí Capaceti, José Miguel; Universitat de Barcelona. Departament de Física Fonamental
[spa] Esta tesis está dedicada al estudio de la cinética de la nucleación centrada en sus aspectos mesoscópicos y fuera equilibrio. Hemos desarrollado y presentado un marco teórico, basado en la Termodinámica Mesoscópica y de No Equilibrio (MNET), que garantiza una descripción flexible, bien fundamentada y conectada con las simulaciones de la dinámica de procesos fuera de equilibro, y en particular de la cinética de la nucleación. Este marco se ha mostrado especialmente útil para desvelar las sutilezas mesoscópicas y las influencias potenciales del carácter de no equilibrio del proceso. En particular, hemos analizado y resuelto algunos aspectos importantes y controvertidos en el ámbito de la nucleación, como son el Teorema de la Nucleación, donde hemos presentado una prueba termodinámica que ratifica su validez general; o la controversia que rodea a la incorporación de las contribuciones asociadas a los grados de libertad traslacionales y rotacionales, donde hemos incorporado por primera vez los efectos del movimiento y la rotación de los núcleos. También hemos analizado la influencia que puede tener la dinámica y las inhomogeneidades del medio en el proceso de nucleación. En particular, hemos estudiado detalladamente dos situaciones de interés práctico y experimental, como son la nucleación en presencia de gradientes térmicos, y en presencia de flujos cortantes, comparando las predicciones teóricas con los experimentos. Finalmente, también hemos señalado cómo se pueden incorporar estas influencias en las etapas avanzadas de la transición de fase, más allá de la etapa inicial de nucleación, lo cual permite una descripción más realista del proceso de cristalización.
La hipótesis adiabática de Paul Ehrenfest: historia de una transformación
(Universitat de Barcelona, 2007-04-24) Pérez Canals, Enric; Navarro Veguillas, Luis; Universitat de Barcelona. Departament de Física Fonamental
[spa] La pregunta que ha inspirado que ha venido a dar en este trabajo ha sido: ¿Qué papel tuvo la hipótesis adiabática de Paul Ehrenfest en el desarrollo de la teoría cuántica? Para ello se ha tratado de precisar el significado mismo de la hipótesis y se ha reconstruido una especie de "ruta adiabática" hacia la teoría cuántica. En definitiva, se ha situado el origen, se han señalado los antecedentes, el impacto, etc., de esta contribución de Ehrenfest. En la primera parte se trata la gestación de la hipótesis adiabática. Para ello se da una somera visión del escenario en que ésta tuvo lugar; el nacimiento y primera evolución de la hipótesis cuántica, introducida en la física por Max Planch en el año 1990. Ehrenfest publicó sus primeros trabajos sobre teoría cuántica en 1905 y 1906. En ellos encontramos una crítica severa al proyecto planckiano de dar cuenta de la irreversibilidad en el sistema de la cavidad radiante. En 1991 Ehrenfest publica un extensísimo análisis de la teoría de la radiación térmica de Planck, en un artículo en el que sin duda hay que situar el origen de la hipótesis adiabática. Otros de los resultados que encontramos en este artículo son: Comparación exhaustiva entre las hipótesis cuánticas de Einstein (hipótesis de los quanta de energía) y de Planck (cuantización y justificación de la forma (de la dependencia con la frecuencia) del quantum. En la segunda parte se reconstruye el proceso que Ehrenfest llegó a formular con precisión su hipótesis en 1916. El escenario corresponde al de los años 1913-1918, y podría caracterizarse como los años de extensión y generalización de la hipótesis cuántica. En la tesis se comentan también artículos de Ehrenfest de 1913 y 1914 en los que prácticamente encontramos todos los resultados que publicará juntos un par de años después.
Non-binary maximum entropy network ensembles and their application to the study of urban mobility
(Universitat de Barcelona, 2016-07-22) Sagarra Pascual, Oleguer Josep; Díaz Guilera, Albert; Universitat de Barcelona. Departament de Física Fonamental
[eng] Complex networks grow subject to structural constraints which affect their measurable properties. Assessing the effect that such constraints impose on their observables is thus a crucial aspect to be taken into account in their analysis, if one wants to quantify the effect a given topological property has on other observed network quantities observed in empirical datasets. Null models are needed for this end. A well understood analytical approach to face the generation and development of flexible models for binary networks is based on considering ensembles of networks obtained using an entropy maximization principle. In this work, we explore the generalization of maximum entropy ensembles to networks where multiple or non-dihcotomic connections among nodes are allowed. We develop a statistical mechanics framework where it is possible to get information about the most relevant observables given a large spectrum of linear and non-linear constraints including those depending both on the weight per link and their binary projection. We furthermore identify three different relevant cases that lead to distinctively different edge statistics, depending on the distinguishable nature of the events allocated to each link. For each case, we perform an extensive study considering microcanonical or hard constrained ensembles as well as grand canonical or soft constrained ones. We provide tools for the generation an analysis of network instances belonging to each model which are implemented and available in the form of open-source software packages, and we provide also analytical tools to obtain null model expectations to later compare to real data. Developing the theory developed, we apply the obtained insights to the analysis of urban mobility considering four large datasets of taxi displacements in the cities of New York, Singapore, San Francisco and Vienna. We show that, once they are appropriately transformed, mobility patterns are highly stable over long time scales and display common features across the studied datasets which are very conveniently represented using one of the cases earlier studied maximum entropy ensembles. We furthermore perform a critical review on existing mobility demand forecasting models and discuss their strengths and weaknesses when adapted to the urban environment, while showing how entropy maximizing models display the best descriptive power of the datasets using a number of network-based, information and matrix similarity metrics to assess the accuracy of the predicted vehicle flows. Based on our observations, we develop two practical applications based on our theoretical work. On the hand, we envisage a supersampling methodology to reliably extrapolate mobility records from a reduced sample which opens the possibility to scale up data from limited records when information on the full system is required. On the other hand, we adapt previous work on graph filtering to our proposed models that allows to extract random contributions from the observed empirical data. This allows to obtain simplified network backbones which contain the most relevant features of mobility datasets not explained by the considered constraints imposed in the maximum entropic models considered. Such a filter is useful for easing the analysis, computational handling and visualization of dense datasets, as well as assessing the degree of proximity between a model and empirical data using suitable hypothesis testing arguments.
The role of clustering in the stucture and function of complex networks
(Universitat de Barcelona, 2016-06-09) Colomer de Simón, Pol; Díaz Guilera, Albert; Universitat de Barcelona. Departament de Física Fonamental
[eng] The study of a system from a network perspective focuses on the impact that connectivity between the elements has on the function of the system. The observation and measurement of parameters of real-world networks reveals that these systems have highly complex structures that differ from those of lattices and random graphs, and which have striking effects on their behaviour. Moreover, some common topological properties shared by networks with completely different natures have been found. This suggests the existence of common fundamental principles that determine the structure and evolution of networks. One of the most common features of real networks is the high presence of triangles or strong clustering. However, in contrast to other topological properties of real networks, little was known about the emergence of clustering and its effect on network structure and function. The reason for this was twofold. First of all, the mere presence of triangles in networks contradicts assumptions that are used almost across the board in mathematical tools that are applied in network theory, and therefore it hinders any analytical treatment. Second, there was a lack of appropriate clustered network models that allow empirical study. Therefore, clustering was the main factor that thwarted the possibility of applying network theories to real situations and became one of the most important challenges facing network science. In this thesis we studied the role played by clustering in the structure and function of complex networks. In this direction, we first analyse the clustering generated by one of the most popular random network model: the configuration model. Our results show that, contrary to common believes strong heterogeneity can be enough to generate moderate levels of clustering. Then, we studied the distribution of triangles within real networks. Interestingly enough, real networks tend to be closer to maximally random clustered graphs, although clear differences are evident. This fact have an impact on the study of clustering on network processes since it casts doubt on previous results derived from clustered network models in which triangles were organized in a very specific way. Finally, we focus on the effect of clustering on the classical bond percolation problem. Our choice was based on the direct relation that this simple process has with robustness and epidemics dynamics of networks. Our results show that clustering makes weakly heterogeneous networks more fragile to random failure of their connections and less prone to spread infected agents. However, clustering in strongly heterogeneous networks can induce a core-periphery organization in which the core and periphery percolates independently. This phenomenon, namely a multiple percolation transition, has not been observed before. In this situation clustering makes the core more robust and the periphery more fragile. Furthermore, I analytically prove that such multiple percolation transitions are possible in networks that are sufficiently weakly connected. This new scenario has very important implications for different aspects of the analysis of the percolation properties of complex networks. On the one hand, the existence of multiple critical points changes the way we need to address percolation as a critical phenomenon. We should not develop theories to find the true and unique percolation threshold, but to reveal the set of critical points and the nodes involved in each one of them. On the other hand, this new phenomenon implies that previous empirical methods for finding the percolation threshold are obsolete. The obvious incapacity to perform finite size scaling in a real finite system, together with the existence of multiple transitions, implies that no existent empirical method can be used to measure percolation thresholds.
Magnetic deflagration in Mn₁₂-ac and Nd₅Ge₃ : new techniques and phenomena
(Universitat de Barcelona, 2016-02-03) Villuendas Pellicero, Diego; Hernández Ferràs, Joan Manel; Universitat de Barcelona. Departament de Física Fonamental
[eng] The goal of this work is to investigate the magnetic deflagration phenomena in two very different magnetic systems, using two very different experimental techniques. In the first chapter of this dissertation I introduce the concept of magnetic deflagra-tion, together with a description of the state of the art of the field. In the second chapter of this thesis I present the study of magnetic deflagration in single crystals of the prototypical single molecule magnet Mn12—ac, using the magneto-optical imaging method; never done before. In the chapter the reader will find out that, effectively, the deflagration process can be explored using this technique. The second part of the thesis include the other five chapters. Such a difference in length between both parts is due two factors. The first factor is that the Mn12—ac is a very well known system, with few new experiments to study; while on the contrary, the Nd5Ge3 intermetallic compound has been barely studied in its single crystalline form, and therefore there are many experiments to conduct to understand its dynamics. The second factor is the rich phenomena that this intermetallic compound presents directly related with the abrupt magnetic changes that possesses. In this system sudden changes occur in every studied physical property when the magnetization changes in a stepped manner. Two fundamental properties of the magnetic dynamics of the Nd5Ge3 system make it so remarkable. First, it is one of few systems with a large irreversible AFM-+FM transition induced by the magnetic field, and second, the magnetic changes during its magnetization processes occur in a stepped manner. Therefore, the motivation to investigate the possibility of finding for the first time magnetic deflagration phenomena in a ferromagnet, and also for the first time in two different magnetic phases of a given material, was very high. In the first two chapters dedicated to the Nd5Ge3 I explore the magnetic properties, the heat capacity and the electric resistivity. Some of the measurements were not previously reported in a single crystal. In the fifth chapter I explore explicitly the spon-taneous field-induced avalanches in magnetism, heat capacity and resistivity. The reason of this three chapters is to have as much ingredients as possible to be able to answer the questions that the possible magnetic deflagrations would raise. I say possible because it is not until the fifth chapter of this thesis when we get the experimental confirmation that the field-induced spontaneous avalanches that the compound present correspond to magnetic deflagration phenomena. In this chapter the magnetic deflagrations were studied in detail in the AFM-+FM transition and in the FM reversal, as a function of the magnetic field and the temperature. The spontaneous and induced deflagrations are explored, obtaining good estimations of the propagation speed and the temperature of the deflagration flame. Using the theoretical framework of the magnetic deflagrations we fit the speed propagation of the front to the experimental data using only one pa-rameter, the heat diffusivity of the material; obtaining a value within the range of heat diffusivities found in other intermetallic compounds.
Microscopic analysis of rotating black holes
(Universitat de Barcelona, 2016-02-08) Maccarrone Heredia, Alessandro; Emparan García de Salazar, Roberto A.; Universitat de Barcelona. Departament de Física Fonamental
[eng] The main objective of this thesis is to understand from a microscopic point of view some of the characteristic phenomena of rotating black holes. The inclusion of rotation gives rise to physics that allows a more precise and detailed understanding of the microscopic string theory of black holes. In this thesis we focus on two models of particular interest: one is based on the D0-D6 system and the other on the D1-D5-P system. The former is interesting because, through its connection to M-theory, it yields a statistical-mechanics description of neutral black holes. The latter allows to have better control over the microscopic conformal field theory and yields a cleaner picture of the origin of superradiance. We extend the microscopic analysis of extremal dyonic Kaluza-Klein (D0-D6) black holes to cover the regime of fast rotation in addition to slow rotation. Fastly rotating black holes, incontrast to slow ones, have non-zero angular velocity and possess ergospheres, so they are more similar to the Kerr black hole. The D-brane model reproduces their entropy exactly, but the mass gets renormalized from weak to strong coupling, in agreement with recent macroscopic analyses of rotating attractors. We discuss how the microscopic model accounts for the fact that fastly rotating extremal KK black holes possess an ergosphere and exhibit superradiance while slow ones don't. In addition, we show in full generality how Myers-Perry black holes are obtained as a límit of Kaluza-Klein black holes, and discuss the slow and fast rotation regimes and superradiance in this context. A, perhaps surprising, consequence of our analysis is that both slowly and fastly-rotating KK black holes provide microscopic accounts of the entropy formula of MP black holes, even if they correspond to rather different microscopic states. As we discuss, this does not pose any problem, since the microscopic theory always retains a memory of how the 5D black hole is embedded within Taub-NUT. For a more detailed and quantitative study of black hole superradiance from the stringy microscopic side, we consider the D1-D5-P system. In order to disentangle superradiance from finite-temperature effects, we consider an extremal, rotating D1-D5-P black hole that has an ergosphere and is not supersymmetric. We explain how the microscopic dual accounts for the superradiant ergosphere of this black hole. The bound 0 < ω < mΩH on superradiant mode frequencies is argued to be a consequence of Fermi-Dirac statistics for the spin-carrying degrees of freedom in the dual CFT. We also compute the superradiant emission rates from both sides of the correspondence, and show their agreement. This is an extension of previous analyses of radiation from the D1-D5-P black holes. We generalize those results to include momentum for the bulk scalar. It would be interesting to extend our picture for superradiance to the smooth SUGRA solitons with D1-D5-P charges which correspond to CFT states such that both sectors are in pure states. Another issue to be investigated would be the absence of fermionic superradiance emission by the previously considered systems with ergoregion.
Magnetization dynamics at the nanoscale in nanoparticles and thin films: single-molecule magnets, magnetic vortices, and magnetic droplet solutions
(Universitat de Barcelona, 2016-01-20) Lendínez Escudero, Sergi; Tejada Palacios, Javier; Macià Bros, Ferran; Universitat de Barcelona. Departament de Física Fonamental
[eng] Research in magnetic materials leads to new devices and technologies. As the technology progresses, the devices become smaller and this miniaturization allows more storage capacity and lower costs in the production of new technologies. As new and smaller materials are fabricated, new phenomena appear and thus new physics is needed to describe them. Nanomaterials meet characteristics of both the microscopic quantum world and the macroscopic classic world. This intermediate length scale is known as mesoscale. Nanomaterials can be obtained in a variety of forms, being nanoparticles and magnetic ultra-thin films some of the most used. These magnetic systems are very different in their composition: nanoparticles are grown with chemical reactions, and thin films are grown on a substrate by nanofabrication techniques such as sputtering or electron-beam evaporation. The magnetization might not be uniform in a magnetic thin film or in a large magnetic nanoparticle leading to the formation of magnetic domains. Magnetic domains are static structures that appear due to competition of the different magnetic energies and can be used to store and transport information. In all these systems, the magnetization dynamics gives rise to new behavior not visible in static measurements: quantum steps of the magnetization in molecular magnets; characteristic resonant frequencies that can be used to control the magnetic state of vortices; and formation of magnetic droplet solitons in thin films with perpendicular magnetic anisotropy. Understanding the dynamics of nanomaterials and the evolution of the magnetization is a key process to develop faster devices and technologies. The early studies of molecular magnets showed quantum effects at the macroscopic scale, which have allowed a better understanding of spin. Magnetic vortices have been proposed for multiple applications, from magnetic storage of information to cancer cell destruction. The recently discovered magnetic droplet soliton is also a very good candidate for technological applications due to the low current and magnetic field needed for its generation, and it is now a system with a growing interest in spintronics. In this dissertation we show some new dynamic phenomena. In the first part of the thesis we study systems that allow a macroscopic-spin model where spatial variations of magnetization are neglected. We develop a theory that sets the requirements for the observation of the rotational Doppler effect in a ferromagnetic system and we measure quantum effects in randomly oriented nanoparticles of a single-molecule magnet, which might be a good candidate for the observation of the Doppler effect. In the second part of the thesis, we study the magnetization dynamics in macroscopic systems that require a spatial dependence of the magnetic moment. We generate and control the dynamic states of the magnetic domains with oscillating fields, in the case of magnetic vortices, and with electrical currents, in the case of droplet solitons.
Computational study of the emergent behavior of micro-swimmer suspensions
(Universitat de Barcelona, 2016-02-08) Alarcón Oseguera, Francisco; Pagonabarraga Mora, Ignacio; Universitat de Barcelona. Departament de Física Fonamental
[eng] It is known that active particles induce emerging patterns as a result of their dynamic interactions, giving rise to amazing collective motions, such as swarming or clustering. Here we present a systematic numerical study of self-propelling particles; our main goal is to characterize the collective behavior of suspensions of active particles as a result of the competition among their propulsion activity and the intensity of an attractive pair potential. Active particles are modeled using the squirmer model. Due to its hydrodynamic nature, we are able to classify the squirmer swimmer activity in terms of the stress it generates (referred to as pullers or pushers). We show that these active stresses play a central role in the emergence of collective motion. We have found that hydrodynamics drive the coherent swimming between swimmers while the swimmer direct interactions, modeled by a Lennard-Jones potential, contributes to the swimmers' cohesion. This competition gives rise to two different regimes where giant density fluctuations (GDF) emerge. These two regimes are differentiated by the suspension alignment; one regime has GDF in aligned suspensions whereas the other regime has GDF of suspensions with an isotropic orientated state. All the simulated squirmer suspensions shown in this study were characterized by a thorough analysis of global properties of the squirmer suspensions as well as a complementary cluster analysis. Active matter refers generically to systems composed of self-driven units, active particles, each capable of converting stored or ambient free energy into systematic movement. Examples of active systems are found at all length scales and could be classified in living and nonliving systems such as microorganisms, tissues and organisms, animal groups, self- propelled colloids and artificial nanoswimmers. Specifically, at the micro and nano scale we find an enormous range of interesting systems both biological and artificial; e.g. spermatozoa that fuse with the ovum during fertilization, the bacteria that inhabit our guts, the protozoa in our ponds, the algae in the ocean; these are but a few examples of a wide biological spectrum. In the artificial world we have self- healing colloidal crystals and membranes as well as self- assembled microswimmers and robots. Experiments in this field are now developing at a very rapid pace and new theoretical ideas are needed to bring unity to the field and identify "universal" behavior in these internally driven systems. One important feature of active matter is that their elements can develop emergent, coordinated behavior; collective motion constitutes one of the most common and spectacular example. Collective motion is ubiquitous and at every scale, from herds of large mammals to amoeba and bacteria colonies, down to the cooperative behavior of molecular motors in the cell. The behavior of large fish schools and the dance of starling flocks at dusk are among the most spectacular examples. From a physical perspective collective motion emerges from a spontaneous symmetry breaking that allows for long-range orientational orden The different mechanisms responsible for such symmetry breaking are still not completely understood. We have performed a systematic numerical study of interactive micro-swimmer suspensions building on the squirmer model, introduced by Lighthill. Since the squirmer identifies systematically the hydrodynamic origin of self-propulsion and stress generation it provides a natural scheme to scrutinize the impact that the different features associated to self-propulsion in a liquid medium have in the collective dynamics of squirmer suspensions. In this abstract we describe the simulation scheme and how squirmers are modeled, then some of the main results are discussed and finally we conclude emphasizing the main implications of the results obtained.
Correspondences in higher-dimensional gravity
(Universitat de Barcelona, 2015-11-12) Di Dato, Adriana; Emparan García de Salazar, Roberto A.; Universitat de Barcelona. Departament de Física Fonamental
[eng] In this thesis we have made progress on the study of higher dimensional gravity by focusing on the properties of black holes and branes and their dynamics. We have developed two main projects: • provide several maps between different spacetimes • determine the hydrodynamical behavior of fluids dual to some classes of black holes This work improves the current understanding of GR in spacetimes with general dimension and gives hints for holography in spacetimes different from AdS. Here we give a brief summary of the work developed underling the main results achieved. In Chapter 2, we introduce the techniques applied for studying black brane hydrodynamics. In the long-wavelength regime, black hole dynamics can be related to fluid dynamics and one can develop effective theories which capture the hydrodynamical description of such black holes. We review two of these: the fluid/gravity correspondence and the blackfold approach. We have hence learnt that black holes behave as fluids under certain circumstances. One can therefore compute the effective stress energy tensor associated to the fluid, extract the corresponding dissipative transport coefficients and possibly perform a stability analysis. In Chapter 3, we have introduced the AdS/Ricci flat correspondence, which is a relation between a class of AdS spacetimes and Einstein solutions with zero cosmological constant. Remarkably, we have developed an extension of such correspondence to spacetimes with positive cosmological constant, including scalar matter. This AdS/dS correspondence may possibly give hints to improve our understanding of holography in dS space. We have also found a new Kerr/AdS solution with hyperbolic horizon from a known Kerr/dS one through the map. The hydrodynamics of fluids using the KK dimensional reduction was studied in Chapter 4. Choosing a generic relativistic fluid, performing a boost in N internal dimensions, compactifing them and reducing on an N dimensional torus we have obtained a charged fluid with N charges. Therefore, we have investigated the variation of the transport coefficients, the shear and bulk viscosity, of the original theory and we were also able to compute the thermal conductivity. The same analysis has been applied to a particular fluid: the fluid dual to a black p-brane. We were able to compute the shear viscosity, bulk viscosity and thermal conductivity matrix for a black p-brane with N charges in the compact directions. This method is particularly interesting since it allows studying the hydrodynamics of charged objects without performing a perturbative analysis but only applying dimensional reduction techniques. Using the AdS/Ricci flat correspondence we have checked that our mapped transport coefficients coincide with the ones obtained for a known charged AdS black branes. In Chapter 5 we have investigated the hydrodynamics properties of fundamentally charged (dilatonic) black branes and branes with Maxwell charge smeared over their worldvolume. We have determined the dissipative behavior of the effective fluids associated to those branes in terms of the transport coefficients of the effective stress energy tensor. Studying the response to small long-wavelength perturbations we have analyzed the dynamical stability of both classes of charged black branes. We have moreover modified the AdS/Ricci flat correspondence to include charged cases using a non-diagonal KK reduction. In this thesis we have shown how higher dimensional gravity is surprisingly rich of new phenomena and bizarre features. Playing with spacetime dimension is the key to probe GR. Hopefully, we will able to improve our comprehension of this mysterious and powerful theory. Holography is an extremely useful tool available for this aim. Mapping apparently unrelated theories living in different number of dimensions has revealed various successful predictions and results but above all opens new perspective for our perception and understanding of GR.
Structure versus Magnetism in Magnetic Nanoparticles
(Universitat de Barcelona, 2015-12-01) Moya Álvarez, Carlos; Labarta, Amílcar; Universitat de Barcelona. Departament de Física Fonamental
[eng] From the fundamental point of view, NPs formed by MFe2O4 with (M= Co, Fe) are ideal system models to study the new magnetic phenomena associated with the so-called particle-like behaviour, which emerges from the size reduction towards the nanometre scale and contrasts with the well-established magnetic properties of their bulk-counterparts. It is well known that most of the particle-like behaviour and in general the large variability of the magnetic properties observed in this kind of nanomaterials are related to structural features of the NPs rather than being originated from intrinsic finite-size or surface effects, at least for NPs bigger than a few nanometers. These structural features, such as crystallographic defects, polycrystalline nature of the NPs, lack of crystallinity at the particle surface, etc., have strong influence on their magnetic properties and can be modified at will through the synthesis method. Therefore, whenever this particle-like behaviour is unwanted for applications with highly demanding requirements, the choice of a suitable synthesis method is of key importance to obtain NPs of high-crystalline quality. On the contrary, particle-like behaviour controlled by the crystalline nature of the NPs could be useful to tailor their magnetic properties for specific applications. Among the common synthesis methods, high-temperature decomposition of metal-organic precursors results the best alternative due to the remarkable final properties of the obtained NPs, such as narrow size distribution, high crystallinity and relatively simple tuning of their size and shape. So this will be the chemical route chosen in this work to study the capabilities of this synthesis method to control the final properties of the NPs through their nanostructure. In addition, to get a deeper insight in the magnetic and structural properties of those materials and to shed light on relevant issues that are still under discussion (dynamic response, magnetic frustration or inter¬particle interactions) it could be useful to combine experimental techniques enabling the characterization of the system from macroscopic scales towards single-particle structures. Within this framework, we present this work that is divided into three main parts. First, it is studied the effect of the concentration of two common reactants, involved in the thermal decomposition method, on the final properties of magnetic NPs based on iron oxides aiming at optimizing the synthesis procedure and getting a good control of the structure of the final product. In the second part, those NPs obtained in the former way are applied to demonstrate the crucial role of the nanostructure on the physical properties of nanoparticulate systems; specially, the strong interplay existent between structure and both magnetic frustration and interparticle interactions. Finally, in the third part, MFM experiments with an external applied magnetic field have been performed to directly observe the reversal of the magnetization of isolated particles and the dynamic behaviour of small aggregates.

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