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SEMINARIO DFI: Fluctuations and Criticality of a Granular Solid-Liquid-like Phase Transition |
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Expositor: Gustavo Castillo DFI
Viernes 8 de junio, 16 h Sala F12 We present an experimental study of density and order fluctuations in the vicinity of the solid- liquid-like transition that occurs in a vibrated quasi-two-dimensional granular system. The two- dimensional projected static and dynamic correlation functions are studied. We show that density fluctuations, characterized through the structure factor, increase in size and intensity as the transition is approached, but they do not change significantly at the transition itself. The dense, metastable clusters, which present square symmetry, also increase their local order in the vicinity of the transition. This is characterized through the bond-orientational order parameter Q4, which in Fourier space obeys an Ornstein-Zernike behavior. Depending on filling density and vertical height, the transition can be of first or second order type. In the latter case, the associated correlation length ?4, relaxation time ?4, zero k limit of Q4 fluctuations (static susceptibility), the pair correlation function of Q4, and the amplitude of the order parameter obey critical power laws, with saturations due to finite size effects. Their respective critical exponents are ?? = 1, ?|| = 2, ? = 1, ? = 0.67, and ? = 1/2, whereas the dynamical critical exponent z = ?||/?? = 2. These results are consistent with model C of dynamical critical phenomena, valid for a non-conserved critical order parameter (bond-orientation order) coupled to a conserved field (density).
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SEMINARIO DFI: Morfología de la invasión del aire en un sistema granular inmerso en agua. |
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Expositor: Germán Varas Instituto de Física P. Universidad Católica de Valparaíso
Viernes 1 de junio, 16 h Sala F12 RESUMEN:La inyección de aire en un medio poroso sumergido en un fluido puede ser considerado, en términos generales, como un proceso de desplazamiento. Básicamente, el aire desplaza el agua atrapada en la matriz granular. Este tipo de proceso se puede encontrar en muchos sistemas naturales y aplicaciones industriales como por ejemplo la extracción de petróleo y la emisión de gases (ej, dióxido de carbono) en el fondo del mar. Un experimento típico que apunta a entender estos fenómenos consiste en llenar una celda con un material granular sumergido en un fluido. Luego, el mismo u otro fluido se inyecta en la parte inferior del recipiente. En esta presentación, se mostrará un estudio experimental para una configuración de tres y dos dimensiones. El análisis se centrará en la dinámica del aire a diferentes escalas de tiempo y en función de los parámetros del problema, como la altura de la capa de granos y agua, la velocidad de inyección y la gravedad efectiva. Estos resultados son extendidos con simulaciones numéricas en el caso donde la inyección del aire es finita. Finalmente, un modelo teórico simple es propuesto para dar cuenta de la dinámica del aire en el volumen (bulk) del sistema y la formación de un cráter en la superficie libre.
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Seminario Nolineal DFI: Quantum and thermal density fluctuations in an ultracold 1D Bose gas |
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Expositor: Julien Armijo Departamento de Física, Facultad de Ciencias, Universidad de Chile Martes 5 de Junio, 2012 - 16:15 hrs* Sala Oriente (Sala de Semniarios, 3er piso, Departamento de Física) Resumen: In this seminar, I will present recent results otained at Institut d'optique, Palaiseau, France.Using micro-magetic traps on an "atom chip", we capture and cool 87Rb atoms to ultralow temperatures (~10nK). Quantum degeneracy is reached when the de Broglie wavelength of atoms becomes larger than the interparticle distance. Then, we take absorption images of the "quantum gas" and, using hundred of pictures, we carefully analyse the fluctuations of atom number in each imaging pixel. Firstly, density fluctuations are a precise probe for the thermodynamics of the gas, and allow us to explore the rich phase diagram of repulsive 1D bosons, where the quantum statitical tendency of bosons to "bunching" competes with interparticle repulsion, which pushes towards anticorrelations. In particular we map out the "quasicondensation" transition, the analogue of Bose-Einstein Condensation in 1D. Secondly, at record low temperatures, we demonstrate the microscopic observation of quantum fluctuations, directly in each single picture, for the first time in a continuous system. *Café/té y Galletas a las 16:00 |
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SEMINARIO DFI: Supermassive Black Holes in Active Galactic Nuclei |
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Expositora: Paulina Lira Departamento de Astronomía, FCFM
Viernes 25 de mayo, 16 h Sala F12 Abstract: I will present the main motivations for the study of Supermassive BH in the centers of galaxies. In particular, what can be determined from the study of the accretion process in Active Galactic Nuclei. The most widely used method to determine the mass of the BHs will be discussed, together with a progress report of a current effort to determine the mass in the most massive and most powerful AGN known today. I will also discuss the relation between BH accretion and star-formation in high-z galaxies and the hope to further understand this correlation at the epoch of galaxy assembly.
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Seminarios
