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Orientador: Adriano A. Natale / Mestre

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En physique du solide, l’étude des effets du désordre a mené à la découverte d’une transition de phase. A faible désordre le solide est conducteur. A fort désordre ce dernier devient isolant. Cette dernière porte le nom de “transition d’Anderson” ou encore de “transition métal-isolant”. Elle peut être caractérisée par un exposant critique ν. Il est prédit théoriquement que sa valeur est universelle, autrement dit, qu’elle n’est pas dépendante des détails microscopiques caractérisant le désordre, mais seulement des symétries satisfaites par le hamiltonien. La réalisation expérimentale d’un tel système est délicate. Des effets de décohérence trop nombreux viennent fausser la mesure de l’exposant critique. Pour contourner ces difficultés, nous réalisons un rotateur frappé avec des atomes froids. La dynamique quantique de ce système est connue pour être la même que celle de l’électron dans un potentiel désordonné. Nous testons alors différents jeux de paramètres régissant le désordre microscopique, et montrons que l’exposant critique ν en est indépendant. Ainsi nous prouvons expérimentalement l’universalité de la transition, ainsi que son appartenance à une classe d’universalité : l’ensemble gaussien orthogonal. Nous détaillons par ailleurs un changement de taille dans le dispositif : la réalisation d’une onde stationnaire verticale et d’une détection vélocimétrique par temps de vol. / In solid state physics, the study of the effects of disorder led to the discovery of a phase transition. For weak disorder, the solid is a conductor, whereas for strong disorder it becomes an insulator. This is known as the “Anderson transition” or as the “metal-insulator transition”, and can be characterized by a critical exponent ν. It is theoretically predicted that this exponent’s value is universal, i.e., that it is not determined by the microscopic details, but only by the symmetries of the Hamiltonian.The experimental realization of such a system in condensed matter is rather difficult. Decoherence effects cannot neglected and affect critical exponent’s value. To circumvent this phenomenon, we use cold atoms to experimentally realize a kicked rotor. The quantum dynamics of such a system are known to mimic those of the solid state problem. We hence test different sets of parameters controlling the statistical properties of the disorder, and show that the critical exponent ν is independent. We hereby prove the universality of the transition, and determine experimentally its universality class : the Gaussian Orthogonal Ensemble. We will then detail an important change in the experimental setup : the installation of a vertical standing wave, and of a time-of-flight velocimetric detection.

Amateurism acted as the guiding principle for the many sporting clubs and governing bodies that were created and developed by the late Victorian middle-classes. While some forms of coaching and training were tolerated, many organisations, such as the Amateur Rowing Association, expressed a preference for amateur honorary coaches rather than professionals. Through the use of archival material, oral history interviews, and ongoing case studies in athletics and swimming, this thesis traces the trajectory of the less than harmonious relationship between amateurism and professional coaching in Britain throughout the twentieth century. In the pre and interwar period, a number of proposals for coaching schemes emerged, especially after poor Olympic performances, but continuing resistance within the amateur establishment meant that these initiatives were uncoordinated and experienced short life-spans. Even in the post-war period, characterised by an increasing number of centralised coaching schemes and the appointment of national coaches, amateur officials sought to maintain strict control over their appointments. A reluctance to accept advice from professional coaches, coupled with a struggling economy and a government determined to remain distant from sport, contributed to a further decline in international sporting performance. British athletes had long proved unable to compete with the Americans and the emergence of another sporting superpower at the 1952 Olympics, the Soviet Union, finally prompted a number of responses, including the 1960 Wolfenden Report. The government subsequently took a more active role in sport, resulting in an inevitable shift towards greater specialisation as centralised funding became inextricably linked with targets and results. Although this encouraged a more widespread utilisation of professional coaches and improved the integration of sports science, the ethos of amateurism proved farreaching, even at elite levels. The evidence suggests that, while it is no longer considered a guiding principle, its legacy continues to impact on the working lives of many British coaches.

University, Diss., 2004--Jena.

Universiẗat, Diss., 2005--Münster (Westfalen).

abstract: Quark matter at sufficiently high density and low temperature is expected to be a color superconductor, and may exist in the interior of neutron stars. The properties of two simplest possible color-superconducting phases, i.e., the color-flavor-locked (CFL) and two-flavor superconducting (2SC) phases, are reviewed. The effect of a magnetic field on the pairing dynamics in two-flavor color-superconducting dense quark matter is investigated. A universal form of the gap equation for an arbitrary magnetic field is derived in the weakly coupled regime of QCD at asymptotically high density, using the framework of Schwinger-Dyson equation in the improved rainbow approximation. The results for the gap in two limiting cases, weak and strong magnetic fields, are obtained and discussed. It is shown that the superconducting gap function in the weak magnetic field limit develops a directional dependence in momentum space. This property of the gap parameter is argued to be a consequence of a long-range interaction in QCD. / Dissertation/Thesis / Ph.D. Physics 2012

Made available in DSpace on 2014-06-11T19:32:10Z (GMT). No. of bitstreams: 0 Previous issue date: 2004-03Bitstream added on 2014-06-13T20:23:06Z : No. of bitstreams: 1 aguilar_ac_dr_ift.pdf: 1161016 bytes, checksum: 84481c94c9eb08c4b2373598d751b55b (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / O sistema complexo de equações de Schwinger-Dyson nos fornece uma abordagem não perturbativa na qual as propriedades infravermelhas da QCD, tais como comportamento de propagadores e vértices podem ser estudados. Utilizando essas equações, o comportamento do propagador do gluon é estudado, no gauge de Landau, do regime perturbativo ao não-perturbativo. Entretanto para fazer essa torre de equações tratável, algumas aproximações são necessárias. Aqui nós vamos discutir dois esquemas diferentes que foram aplicados na equação de Schwinger-Dyson: a aproximação de Mandelstam e o sistema acoplado gluon-ghost. A primeira aproximação consiste em desprezar todas as contribuições que vêem dos campos de férmions e ghosts, enquanto que na segunda aproximação, os campos de ghosts são levados em conta, nos levando a um sistema de equações integrais acopladas. Em ambos os casos, nós mostraremos que uma massa dinâmica para o gluon surge como solução, e algumas de suas propriedades, tais como sua dependência com a escala da QCD, 'lâmbda IND.QCD', e seu comportamento perturbativo serão discutidos. / The complex system of Schwinger-Dyson equations (SDE) provides a non-perturbative framework in which we can study the infrared properties of QCD such as propagators and vertex. Using SDE, the behavior of gluon propagator is studied in Landau Gauge from perturbative to non-perturbative regimes. However to make this tower of equations tractable some approximations are needed. Here we discuss two different schemes which were applied for the SDE: the Mandelstam and gluon-ghost approximaions. The former consist in neglecting all contributions that come from fermions and ghosts fields while in the latter, the ghosts field are taken into account which lead to a coupled system of integral equations. In both cases, we show that a dynamical mass for the gluon propagator arises as a solution, and some of its properties, such as its dependency on 'lâmbda IND.QCD' and how is its behavior at large momenta, will also be discussed.

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