No description available.

No description available.

No description available.

No description available.

No description available.

Ceramics in the solid solution series, Ag(NbxTal-x)03 were fabricated to study their phase transitions and functional properties. As Ta concentration increased, the onset temperature of the M-phase transitions decreased in agreement with the Pawelcyck phase diagram. Electron diffraction and Raman spectroscopy revealed that the reduction in onset temperature may be attributed to a decrease in the correlation length and magnitude of cation displacements as the less polarisable Ta replaced the highly polarisable Nb ion in the B06 octahedra. For compositions with x = 0.75, a previously unreported phase transition was observed at ~ 3 l5K. Further studies were also performed on Li doped AgNb03, AgNbl/2 Ta 1/203 and AgTa03 compositions to determine whether their functional properties could be modified or enhanced. For > 0.05% Li doped AgNb03, the structure transformed from orthorhombic, Pbcm (√2ap,√2ap,4ap) to a new structure with a unit cell, √2ap√2ap,6ap, where ap is the pseudocubic perovskite lattice parameter. The new cell may be envisaged as arising from a complex modulated tilt system in which blocks of 3 in- phase tilted octahedra are rotated in antiphase with respect to each other around the c- axis. The onset of this transition resulted in asymmetry of the M3-M2 transition but there was no discernible improvement in ferroelectric properties at room temperature. For Li doped AgNbl/2 Ta1/2O3, the correlation length of cation displacements and to a lesser extent octahedral tilting decreased according to electron diffraction and Raman spectroscopy as Li concentration increased. The decrease in correlation length manifested itself as a highly perturbed domain structure at the solubility limit (~ 15 mol% Li) and resulted in convergence of the M-phase transitions to give a single broad dielectric response at the solubility limit. For Li doped AgTaO3, there were no apparent changes in the structure at room temperature. However, temperature dependent dielectric measurements clearly indicated that Li substitution had induced the onset of a ferroelectric phase transition in contrast to the incipient behaviour of AgTaO3. The temperature of the ferroelectric transitions increased with increase in Li concentration up to the solubility limit (~ 15 mol %.). The onset of the ferroelectric transition was also observed by Raman spectroscopy but further work is required to determine its precise nature.

No description available.

No description available.

Quantum Chromodynamics (QCD) poses a challenge in calculating physical phenomena in low energy scales due to its strongly coupled character. The tools available for understanding this region of QCD are limited. One such tool is gauge/gravity duality which promises to attack strongly coupled related phenomena, at least in a qualitative level, by using the conjectured equivalence between string theory and some classes of quantum field theories (gauge/gravity duality). In this thesis strongly coupled 3+1d and 2+1d field theories are explored by using D3/D7 and D3/D5 brane systems respectively. These theories exhibit some QCD-like characteristics like chiral symmetry breaking and confinement. The main focus of the following chapters is understanding chiral phase transitions in those theories and constructing their phase diagrams in finite temperature and chemical potential. Chiral symmetry breaking is induced in these holographic brane setups by turning on a background magnetic field or by choosing an appropriate running dilaton profile. The phase diagrams for each field theory considered are mapped, giving a rich structure of first, second and BKT holographic transitions. Some successful attempts where made to reproduce the standard QCD phase diagram, in the running dilaton scenario. Also, in the running dilaton case wrapped D5 branes where used to introduce holographic baryons. The baryonic phase, for some range of the parameter space, participates in the phase diagram and it is found in the regime expected from QCD. Finally, chiral phase transitions with energy scale where explored as well as their holographic effective potentials for various D3/D7 and D3/D5 systems.

En présence de désordre, la diffusion des particules peut être complètement annihilée, donnant lieu à la fameuse localisation d’Anderson. En dimension trois, une transition de phase sépare une telle phase isolante du régime diffusif. À partir de différentes approches théoriques et numériques, cette thèse a pour objectif de déterminer l’effet des interactions entre particules sur la localisation d’Anderson et sur la transition d’Anderson, dans le contexte expérimental des condensats de Bose-Einstein. Dans le cas unidimensionnel, la compétition entre désordre et interaction induit l’existence de trois régimes dynamiques dont les caractéristiques sont étudiées grâce à une approche spectrale. En nous appuyant sur le modèle du rotateur frappé quasi-périodique, nous caractérisons l’émergence du régime sub-diffusif qui tend à remplacer le régime localisé dans le cas tridimensionnel. Nous étudions également la dynamique des excitations du système et démontrons l’universalité de la transition d’Anderson vis-à-vis des quasi-particules de Bogoliubov. Dans l’objectif d’étudier la validité de l’équation de Gross-Pitaevskii, nous nous sommes enfin intéressés à une nouvelle approche, la méthode de la troncature d’Husimi. Celle-ci nous permet d’envisager une étude de la compétition entre désordre et interaction enrichie par la prise en compte du bruit quantique. / In a disordered potential, the diffusive transport of non-interacting particles can be inhibited, a phenomenon known as Anderson localization. In three dimensions, there exists a quantum phase-transition between localized (insulator) and diffusive (metal) dynamics. A long-standing question is the effect of interactions on such dynamics. The goal of this thesis is to investigate this problem theoretically and numerically in the experimental framework of Bose-Einstein condensates. In one dimension, the interplay between disorder and interactions leads to the existence of three regimes which are characterized with a spectral approach. In three dimensions, using a “quantum simulator” of the 3D Anderson model we show the emergence of sub-diffusion in lieu of Anderson localization. Considering the excitations of the system in the very weakly interacting regime, we also demonstrate that the concept of universality of the Anderson transition also applies to Bogoliubov quasi-particles. Finally, we show the relevance of a new method, the truncated Husimi method, in order to take into account the effect of quantum noise on interacting disordered systems.