Crystal phases and glassy dynamics in monodisperse hard ellipsoids

We have performed Monte Carlo and molecular dynamics simulations of suspensions of monodisperse, hard ellipsoids of revolution. Hard-particle models play a key role in statistical mechanics. They are conceptually and computationally simple, and they offer insight into systems in which particle shape is important, including atomic, molecular, colloidal, and granular systems. In the high density phase diagram of prolate hard ellipsoids we have found a new crystal, which is more stable than the stretched FCC structure proposed previously . The new phase, SM2, has a simple monoclinic unit cell containing a basis of two ellipsoids with unequal orientations. The angle of inclination is very soft for length-to-width (aspect) ratio l/w=3, while the other angles are not. A symmetric state of the unit cell exists, related to the densest-known packings of ellipsoids; it is not always the stable one. Our results remove the stretched FCC structure for aspect ratio l/w=3 from the phase diagram of hard, uni-axial ellipsoids. We provide evidence that this holds between aspect ratios 3 and 6, and possibly beyond. Finally, ellipsoids in SM2 at l/w=1.55 exhibit end-over-end flipping, warranting studies of the cross-over to where this dynamics is not possible. Secondly, we studied the dynamics of nearly spherical ellipsoids. In equilibrium, they show a first-order transition from an isotropic phase to a rotator phase, where positions are crystalline but orientations are free. When over-compressing the isotropic phase into the rotator regime, we observed super-Arrhenius slowing down of diffusion and relaxation, and signatures of the cage effect. These features of glassy dynamics are sufficiently strong that asymptotic scaling laws of the Mode-Coupling Theory of the glass transition (MCT) could be tested, and were found to apply. We found strong coupling of positional and orientational degrees of freedom, leading to a common value for the MCT glass-transition volume fraction. Flipping modes were not slowed down significantly. We demonstrated that the results are independent of simulation method, as predicted by MCT. Further, we determined that even intra-cage motion is cooperative. We confirmed the presence of dynamical heterogeneities associated with the cage effect. The transit between cages was seen to occur on short time scales, compared to the time spent in cages; but the transit was shown not to involve displacements distinguishable in character from intra-cage motion. The presence of glassy dynamics was predicted by molecular MCT (MMCT). However, as MMCT disregards crystallization, a test by simulation was required. Glassy dynamics is unusual in monodisperse systems. Crystallization typically intervenes unless polydispersity, network-forming bonds or other asymmetries are introduced. We argue that particle anisometry acts as a sufficient source of disorder to prevent crystallization. This sheds new light on the question of which ingredients are required for glass formation.

Black hole evaporation and stress tensor correlations

General Relativity is one of the greatest scientific achievementes of the 20th century along with quantum theory. These two theories are extremely beautiful and they are well verified by experiments, but they are apparently incompatible. Hints towards understanding these problems can be derived studying Black Holes, some the most puzzling solutions of General Relativity. The main topic of this Master Thesis is the study of Black Holes, in particular the Physics of Hawking Radiation. After a short review of General Relativity, I study in detail the Schwarzschild solution with particular emphasis on the coordinates systems used and the mathematical proof of the classical laws of Black Hole "Thermodynamics". Then I introduce the theory of Quantum Fields in Curved Spacetime, from Bogolubov transformations to the Schwinger-De Witt expansion, useful for the renormalization of the stress energy tensor. After that I introduce a 2D model of gravitational collapse to study the Hawking radiation phenomenon. Particular emphasis is given to the analysis of the quantum states, from correlations to the physical implication of this quantum effect (e.g. Information Paradox, Black Hole Thermodynamics). Then I introduce the renormalized stress energy tensor. Using the Schwinger-De Witt expansion I renormalize this object and I compute it analytically in the various quantum states of interest. Moreover, I study the correlations between these objects. They are interesting because they are linked to the Hawking radiation experimental search in acoustic Black Hole models. In particular I find that there is a characteristic peak in correlations between points inside and outside the Black Hole region, which correpsonds to entangled excitations inside and outside the Black Hole. These peaks hopefully will be measurable soon in supersonic BEC.

Test der Speziellen Relativitätstheorie mittels Laserspektroskopie an relativistischen 7 Li +-Ionen am ESR

Die Invarianz physikalischer Gesetze unter Lorentztransformationen ist eines der fundamentalen Postulate der modernen Physik und alle Theorien der grundlegenden Wechselwirkungen sind in kovarianter Form formuliert. Obwohl die Spezielle Relativitätstheorie (SRT) in einer Vielzahl von Experimenten mit hoher Genauigkeit überprüft und bestätigt wurde, sind aufgrund der weitreichenden Bedeutung dieses Postulats weitere verbesserte Tests von grundsätzlichem Interesse. Darüber hinaus weisen moderne Ansätze zur Vereinheitlichung der Gravitation mit den anderen Wechselwirkungen auf eine mögliche Verletzung der Lorentzinvarianz hin. In diesem Zusammenhang spielen Ives-Stilwell Experimente zum Test der Zeitdilatation in der SRT eine bedeutende Rolle. Dabei wird die hochauflösende Laserspektroskopie eingesetzt, um die Gültigkeit der relativistischen Dopplerformel – und damit des Zeitdilatationsfaktors γ – an relativistischen Teilchenstrahlen zu untersuchen. Im Rahmen dieser Arbeit wurde ein Ives-Stilwell Experiment an 7Li+-Ionen, die bei einer Geschwindigkeit von 34 % der Lichtgeschwindigkeit im Experimentierspeicherring (ESR) des GSI Helmholtzzentrums für Schwerionenforschung gespeichert waren, durchgeführt. Unter Verwendung des 1s2s3S1→ 1s2p3P2-Übergangs wurde sowohl Λ-Spektroskopie als auch Sättigungsspektroskopie betrieben. Durch die computergestützte Analyse des Fluoreszenznachweises und unter Verwendung optimierter Kantenfilter für den Nachweis konnte das Signal zu Rauschverhältnis entscheidend verbessert und unter Einsatz eines zusätzlichen Pumplasers erstmals ein Sättigungssignal beobachtet werden. Die Frequenzstabilität der beiden verwendeten Lasersysteme wurde mit Hilfe eines Frequenzkamms spezifiziert, um eine möglichst hohe Genauigkeit zu erreichen. Die aus den Strahlzeiten gewonnen Daten wurden im Rahmen der Robertson-Mansouri-Sexl-Testtheorie (RMS) und der Standard Model Extension (SME) interpretiert und entsprechende Obergrenzen für die relevanten Testparameter der jeweiligen Theorie bestimmt. Die Obergrenze für den Testparameter α der RMS-Theorie konnte gegenüber den früheren Messungen bei 6,4 % der Lichtgeschwindigkeit am Testspeicherring (TSR) des Max-Planck-Instituts für Kernphysik in Heidelberg um einen Faktor 4 verbessert werden.

La Città Inclusiva. Educazione, democrazia e giustizia sociale: la ricerca di una prospettiva e di strategie che promuovano umanizzazione e inclusione

L'interrogativo da cui nasce la ricerca riguarda la possibilità di individuare, in controtendenza con la logica neoliberista, strategie per l'affermarsi di una cultura dello sviluppo che sia sostenibile per l'ambiente e rispettosa della dignità delle persone, in grado di valorizzarne le differenze e di farsi carico delle difficoltà che ognuno può incontrare nel corso della propria esistenza. Centrale è il tema del lavoro, aspetto decisivo delle condizioni di appartenenza sociale e di valorizzazione delle risorse umane. Vengono richiamati studi sulla realtà in cui siamo immersi, caratterizzata dal pensiero liberista diventato negli ultimi decenni dominante su scala globale e che ha comportato una concezione delle relazioni sociali basata su di una competitività esasperata e sull’esclusione di chi non sta al passo con le leggi di mercato: le conseguenze drammatiche dell'imbroglio liberista; la riduzione delle persone a consumatori; la fuga dalla comunità ed il rifugio in identità separate; il tempo del rischio, della paura e della separazione fra etica e affari. E gli studi che, in controtendenza, introducono a prospettive di ricerca di uno sviluppo inclusivo e umanizzante: le prospettive della decrescita, del business sociale, di una via cristiana verso un'economia giusta, della valorizzazione delle capacità delle risorse umane. Vengono poi indagati i collegamenti con le esperienze attive nel territorio della città di Bologna che promuovono, attraverso la collaborazione fra istituzioni, organizzazioni intermedie e cittadini, occasioni di un welfare comunitario che sviluppa competenze e diritti insieme a responsabilità: l'introduzione delle clausole sociali negli appalti pubblici per la realizzazione professionale delle persone svantaggiate; la promozione della responsabilità sociale d'impresa per l'inclusione socio-lavorativa; la valorizzazione delle risorse delle persone che vivono un’esperienza carceraria. Si tratta di esperienze ancora limitate, ma possono costituire un riferimento culturale e operativo di un modello di sviluppo possibile, che convenga a tutti, compatibile con i limiti ambientali e umanizzante.

Convergence results for stochastic particle systems with social interaction

We consider stochastic individual-based models for social behaviour of groups of animals. In these models the trajectory of each animal is given by a stochastic differential equation with interaction. The social interaction is contained in the drift term of the SDE. We consider a global aggregation force and a short-range repulsion force. The repulsion range and strength gets rescaled with the number of animals N. We show that for N tending to infinity stochastic fluctuations disappear and a smoothed version of the empirical process converges uniformly towards the solution of a nonlinear, nonlocal partial differential equation of advection-reaction-diffusion type. The rescaling of the repulsion in the individual-based model implies that the corresponding term in the limit equation is local while the aggregation term is non-local. Moreover, we discuss the effect of a predator on the system and derive an analogous convergence result. The predator acts as an repulsive force. Different laws of motion for the predator are considered.

From Complex to Stochastic Potential: Heavy Quarkonia in the Quark-Gluon Plasma

The in-medium physics of heavy quarkonium is an ideal proving ground for our ability to connect knowledge about the fundamental laws of physics to phenomenological predictions. One possible route to take is to attempt a description of heavy quark bound states at finite temperature through a Schrödinger equation with an instantaneous potential. Here we review recent progress in devising a comprehensive approach to define such a potential from first principles QCD and extract its, in general complex, values from non-perturbative lattice QCD simulations. Based on the theory of open quantum systems we will show how to interpret the role of the imaginary part in terms of spatial decoherence by introducing the concept of a stochastic potential. Shortcomings as well as possible paths for improvement are discussed.

Réstif de la Bretonne. La transición del espacio utópico experimental al espacio reducido

La crítica tradicionalmente ha sido muy severa con Réstif de la Bretonne, a quien ha relegado a un segundo plano durante más de cien años por considerarlo un autor marginal. A principios del siglo XX es rescatado de la oscuridad gracias a las teorías freudianas pero recién en los años sesenta se estudia su obra desde otros puntos de vista. Contribuyendo a esta línea, el presente artículo intenta aportar elementos que permitan devolver a Réstif de la Bretonne el lugar que le corresponde dentro de la historia de la literatura y del pensamiento político. A lo largo del siglo XVIII el espacio adquiere un valor fundamental porque se supone que debe ser conocido y dominado para poseer las leyes del mundo entero. Dentro de este marco la novela utópica de nuestro autor titulada El Descubrimiento Austral. Novela Filosófica, textualiza posturas aparentemente disímiles acerca del espacio que convierten a Réstif en uno de los símbolos del período de transición que representa. Las ideas vertidas en esta novela nos permiten afirmar que Réstif de la Bretonne se adelanta, sin duda, a movimientos del siglo XIX como el Socialismo Utópico, el Romanticismo y el Nacionalismo.