Browsing by Subject "Phase transition"
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Item Ordering in dense packings(2011-05) Aristoff, David Gregory; Radin, Charles, 1945-; Koch, Hans; de la Llave, Rafael; Gamba, Irene; Gonzalez, Oscar; Raizen, MarkWe examine various models of soft matter, and one model of quasicrystals, focusing on abrupt changes as density is varied. We consider in detail two models, one of granular matter and another of confined wires, showing that the models become ordered as density is increased, with crystalline order observed in the former and nematic order observed in the latter. We associate the phenomenon of random close packing with the onset of crystalline order in our granular model, and we conjecture that crumpled wires should exhibit a nematic transition with increasing compaction. We also consider two other models of granular matter: one which describes dilatancy onset as a second order phase transition, and one which describes random loose packing as a precise, well- defined density. Finally, we examine an equilibrium model of quasicrystals with a first order phase transition to a solid phase without any crystalline order.Item Phase transitions in holographic QCD and instanton crystals(2014-08) Alam, Muhammad Sohaib; Kaplunovsky, VadimWe investigate phase transitions in holographic models of QCD. In chapter I, we explore the effect of constant external U(1) fields on the physics of chiral symmetry breaking, as realized in the D3/D7 model. We discover that this model exhibits the phenomenon of magnetic catalysis, which is what one would expect from a weakly coupled field theory intuition. In chapter II, we continue exploring the effect of external U(1) fields but now on the backreacted D3/D7 model, where the backreaction is obtained via a smearing procedure. We again find the magnetic catalysis effect, however the results differ from the previous case depending on the backreaction parameters. In chapter III, we investigate lattices of instantons in the D4/D8 model of chiral symmetry breaking. These instanton lattices can change dimensionality, and in particular we investigate the 1D [right arrow] 2D transition as a simpler case of the more complicated 3D [right arrow] 4D transition which is conjectured to be holographically dual to the baryonic to quarkyonic phase transition. Besides this interpretation, one could also view this as a hypothetical condensed matter system. We have a lattice of instantons dominated by two-body forces, whose interactions depend not only on their mutual distance in physical space but also on their relative orientations in the internal isospace. We obtain a rich variety of instanton crystals whose description could serve to be useful beyond holography.Item Structural phase transitions in hafnia and zirconia at ambient pressure(2010-08) Luo, Xuhui; Demkov, Alexander A.; Chelikowsky, James R.; Ekerdt, John G.; MacDonald, Allan H.; Kleinman, LeonardIn recent years, both hafnia and zirconia have been looked at closely in the quest for a high permittivity gate dielectric to replace silicon dioxide in advanced metal oxide semiconductor field effect transistors (MOSFET). Hafnium dioxide or HfO2 is chosen for its high dielectric constant (five times that of SiO2) and compatibility with stringent requirements of the Si process. As deposited, thin hafnia films are typically amorphous but turn polycrystalline after a post-deposition anneal. To control the phase composition in hafnia films understanding of structural phase transitions is a first step. In this dissertation using first principles methods we consider three phase transitions of hafnia and zirconia: monoclinic to tetragonal, tetragonal to cubic and amorphous to crystalline. Because the high surface to volume ratio in hafnia films and powders plays an important role in phase transitions, we also study the surface properties of hafnia. We discuss the mechanisms of various phase transitions and theoretically estimate the transition temperatures. We find two types of amorphous hafnia and show that they have different structural and electronic properties. The small energy barrier between the amorphous and crystalline structures is found to cause the low crystallization temperature. Moreover, we calculate work functions and surface energies for hafnia surfaces and show the surface suppression of the phase transitions.