Dissertations / Theses on the topic 'Phase transitions'
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Sang, Yan. "Phases and Phase Transitions in Quantum Ferromagnets." Thesis, University of Oregon, 2015. http://hdl.handle.net/1794/18716.
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In this dissertation we study the phases and phase transition properties of
quantum ferromagnets and related magnetic materials. We first investigate the effects of an external magnetic field on the Goldstone mode of a helical magnet, such as MnSi. The field introduces a qualitatively new term into the dispersion relation of the Goldstone mode, which in turn changes the temperature dependences of the contributions of the Goldstone mode to thermodynamic and transport properties. We then study how the phase transition properties of quantum ferromagnets evolve with increasing quenched disorder. We find that there are three distinct regimes for different amounts of disorder. When the disorder is small enough, the quantum ferromagnetic phase transitions is generically of first order. If the disorder is in an intermediate region, the ferromagnetic phase transition is of second order and effectively characterized by mean-field critical exponents. If the disorder is strong enough the ferromagnetic phase transitions are continuous and are characterized by non-mean-field critical exponents.
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Ran, Ying. "Spin liquids, exotic phases and phase transitions." Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1721.1/45404.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2007.Includes bibliographical references (p. 135-139).
Spin liquid, or featureless Mott-Insulator, is a theoretical state of matter firstly motivated from study on High-Tc superconductor. The most striking property of spin liquids is that they do not break any physical symmetry, yet there are many types of them, meaning a phase transition is necessary from one spin liquid to another. It was a long debate about whether these exotic states can serve as the ground states in real materials or even models. In this thesis I firstly discuss a large-N model, where we show the spin liquid states can be the ground states. Because the spin liquid phases cannot be characterized by symmetry breaking, the phase transitions associated with them are naturally beyond the traditional Laudau's paradigm. I discuss a few scenarios of these exotic phase transitions to show a general picture about what can happen for such exotic transitions. Those exotic phase transitions can actually serve as a way to detect these exotic phases. Then I move to a much more realistic model: spin-1/2 Kagome lattice, where we propose a U(1)-Dirac spin liquid as the ground state. The implications on the recent material ZnCu3(OH)6C12 are discussed. Finally, I come back to the high-Tc problem. A doped spin liquid can naturally be superconducting whose many properties have already been confirmed by experiments. Here I particularly study one experimental puzzle: the nodal-antinodal dichotomy in underdoped High-Tc material. This used to be one difficulty of the doped spin liquid theory. We show that a doped spin liquid can naturally has nodal-antinodal dichotomy due to further neighbor hoppings (t' and t").
by Ying Ran.
Ph.D.
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Knott, Michael. "Phases and phase transitions in charged colloidal suspensions." Thesis, University College London (University of London), 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.270941.
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Werner, Philipp. "Dissipative quantum phase transitions /." Zürich, 2005. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=16134.
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Palmer, David Cristopher. "Phase transitions in leucite." Thesis, University of Cambridge, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.357876.
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Ghaemi, Mohammadi Pouyan. "Phases and phase transitions of strongly correlated electron systems." Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/45456.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2008.Includes bibliographical references (leaves 169-174).
Different experiments on strongly correlated materials have shown phenomena which are not consistent with our conventional understandings. We still do not have a general framework to explain these properties. Developing such a general framework is much beyond the scope of this thesis, but here we try to address some of challenges in simpler models that are more tractable. In correlated metals it appears as strong correlations have different effect on different parts of fermi surface. Perhaps most striking example of this is normal state of optimally doped cuprates; the quasiparticle peaks on the nominal fermi surface do not appear uniformly. We try to track such phenomena in heavy fermion systems, which are correlated fermi liquids. In these systems, a lattice of localized electrons in f or d orbitals is coupled to the conduction electrons through an antiferromagnetic coupling. Singlets are formed between localized and conduction electrons. This singlet naturally have non-zero internal angular momentum. This nontrivial structure leads to anisotropic effect of strong correlations. Internal structure of Kondo singlet can also lead to quantum Hall effect in Kondo insulator, and formation of isolated points on the fermi surface with fractionalized quasiparticles. In the second part we study a phase transition in Heisenberg model between two insulating phases, Neel ordered and certain spin liquid state, popular in theories of the cuprates. The existence of such a transition has a number of interesting implications for spin liquid based approaches to the underdoped cuprates and clarifies existing ideas for incorporating antiferromagnetic long range order into such a spin liquid based approach. This transition might also be enlightening, despite fundamental differences, for the heavy fermion critical points where a second order transition between the heavy fermion phase and a metallic phase with magnetic antiferromagnetic order is observed.
by Pouyan Ghaemi Mohammadi.
Ph.D.
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Fliegans, Olivier. "Phase transitions in "small" systems." [S.l. : s.n.], 2001. http://www.diss.fu-berlin.de/2001/93/index.html.
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Rowley, Stephen Edward. "Quantum phase transitions in ferroelectrics." Thesis, University of Cambridge, 2011. https://www.repository.cam.ac.uk/handle/1810/252224.
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Oukouiss, Abdelkarim. "Phase transitions in ferromagnetic fluids." Doctoral thesis, Universite Libre de Bruxelles, 1999. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/211920.
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Casson, Brian Derek. "Phase transitions in surfactant monolayers." Thesis, University of Oxford, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.300797.
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Fairhurst, David John. "Polydispersity in colloidal phase transitions." Thesis, University of Edinburgh, 1999. http://hdl.handle.net/1842/1496.
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I have studied the effects of polydispersity on the phase behaviour of suspensions of PMMA colloidal spheres on their own and in the presence of non-adsorbed polymer. I systematically explored the volume fraction-polydispersity phase behaviour of hard spheres (with radii R =167, 244, 300 and 303nm) through direct observations and crystallography measurements. I observed normal crystallisation for sigma < 7:5%, and no crystals at sigma > 18%. Samples at sigma ~~ 9.5% showed crystal-fluid coexistence between 0:52 < phi < 0:56 but no fully crystalline be- haviour above this region. This may be explained by slow particle diffusion in the dense metastable fluid and a glass transition, possibly involving only the larger particles. The addition of random coil polymer (radius of gyration rg) to a suspension of single-sized spherical colloidal particles induces an attractive depletion potential which, for size ratios Xi = rg=R < 0:2, has the effect of expanding the crystal-fluid coexistence region. Surprisingly, when such a polymer solution (with Xi = 0:1), with a range of concentrations cp, is added to a polydisperse colloidal suspension (sigma ~~ 10%), crystal formation is actually suppressed. This can be explained by the fact that the polymer compresses the nascent crystal phase to volume fractions greater than the maximum phi permitted for polydisperse spheres. By modifying existing free energy equations to include the effects of colloidal polydispersity we also succeed in reproducing the observed phase diagram. Larger added polymer (Xi > 0:3) introduces a region of stable gas-liquid coexistence. In systems where crystallisation is suppressed due to polydispersity, this will theoretically be the only transition. By preparing many samples over a range of phi and cp this prediction was observed experimentally for Xi = 0:5. Fractionation studies on coexisting phases enabled verification of a recent universal law of fractionation in slightly polydisperse systems.
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Delivorias, Nikitas Alex. "Cosmic structure from phase transitions." Thesis, Durham University, 1997. http://etheses.dur.ac.uk/4702/.
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Motivated by recent observations suggesting that structures in the Universe appear to be concentrated on the walls of bubbles that surround giant voids, we examine the possibility that the observed structure may have resulted from a first order phase transition that occured after inflation and which proceeded by quantum tunnelling and the formation of bubbles of true vacuum. Since we lack a fundamental theory of particle physics that would define the scalar field responsible for the second phase change and predict the scale of the resulting structures, we instead examine two similar parametrised forms for the potential motivated by the standard Higgs model, and attempt to determine values of the parameters that can reproduce the kind of structures that are observed through bubble wall collisions. The method deployed is quite general and can be applied to any phase transition that occured after inflation. It is found that although the shape of the required potential and its coupling can be determined, the epoch of the proposed second phase transition is in general not specified by the observed structures. The full verification or otherwise of our proposal will require not only a more detailed consideration of its predictions for the large scale structure of the Universe and its compatibility with the cosmic microwave background radiation but also the embedding of our ideas in a credible theory of particle physics beyond the Standard Model.
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Stow, Simon John. "Phase transitions in relativistic systems." Thesis, Royal Holloway, University of London, 1985. http://repository.royalholloway.ac.uk/items/e6332754-d6f0-4d29-8960-dc05a5ca3390/1/.
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The BCS free energy for 3P2 paired neutron matter is derived taking account of relativistic effects. It is found that the values taken by the Ginzburg-Landau parameters are always in the region of the phase diagram correponding to a unitary phase. Phase transitions in the early universe are also discussed with inclusion of the effects of Higgs scalar chemical potentials as well as fermionic chemical potentials. The conditions for equilibrium, and the critical density to prevent symmetry restoration at high temperatures are studied. It is observed that the decay of pre-existing Higgs scalar asymmetries could greatly reduce baryon number and lepton number to entropy ratios from their initial values. Phase transitions in supersymmetric theories and the phenomenom of symmetry anti-restoration in a supersymmetric model with a U(1) gauge symmetry are studied at finite density.
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Fedorenko, T. O. "First-order magnetic phase transitions." Thesis, Сумський державний університет, 2014. http://essuir.sumdu.edu.ua/handle/123456789/34842.
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After the general properties of the first-order magnetic phase transitions between ordered structures were investigated, the number of possible models has been limited by supposing the sensitivity of the exchange energy to the interatomic distance to be responsible for the phase transition. It has shown that the magnetic phase transition occurs as a result of a sign-change in the proper combination of the exchange parameters. The transition may be influenced by the variation of the pressure and the magnetic field. The actual phase transition is, however, hindered in both directions by the elastic energy wall, separating the free energy minima of the different phases. The T – P phase diagram has been calculated and the conditions for the existence of a triple point in the T – P plane have been investigated in the case of antiferromagnetic ↔ ferromagnetic transitions. The temperature hysteresis, i.e. the difference between the upper (Tsup) and lower (Tinf) transition temperatures has been determined at different pressures and for both temperatures a linear dependence has been obtained.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/34842
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Littlewort, G. C. "Phase transitions in neural networks." Master's thesis, University of Cape Town, 1986. http://hdl.handle.net/11427/7617.
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The behaviour of computer simulations of networks of neuron-like binary decision elements is studied. The models are discrete in time and deterministic , but the sequence of states of neurons in a net is not generally reversible in time because of the threshold nature of neurons. Self-organisation, or activity-dependent modification of interneuronal connection strengths, is used. Cyclic modes of activity which emerge spontaneously, underlie possible mechanisms of short term memory and associative thinking. The transition from seemingly random activity patterns to cyclic activity is examined in isolated networks with pseudorandomly chosen connection matrices; and the transition is related to the gross properties of the network. Nets with inherent structure (from pseudorandom nature) and imposed structure are studied, when cycles of length greater than, say, 12 time units are considered separately from the less complex, shorter cycles; the aforementioned transitions appear to be consistently rapid, compared to the cycle length, unless architecture is imposed such that nearly independent groups of neurons exist in the same net.
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Xu, Hui. "Phase structure and phase transitions in semicrystalline isotactic polystyrene /." Thesis, Connect to Dissertations & Theses @ Tufts University, 2005.
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Thesis (Ph.D.)--Tufts University, 2005.Adviser: Peggy Cebe. Submitted to the Dept. of Physics. Includes bibliographical references. Access restricted to members of the Tufts University community. Also available via the World Wide Web;
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Haupt, Kerstin Anna. "Phase transitions in transition metal dichalcogenides studied by femtosecond electron diffraction." Thesis, Stellenbosch : Stellenbosch University, 2013. http://hdl.handle.net/10019.1/85608.
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Thesis (PhD)--Stellenbosch University, 2013.ENGLISH ABSTRACT: Low-dimensional materials are known to undergo phase transitions to differently or- dered states, when cooled to lower temperatures. These phases often show a periodic modulation of the charge density (called a charge density wave – CDW) coupled with a periodic perturbation of the crystal lattice (called a periodic lattice distortion – PLD). Although many experiments have been performed and much has been learnt about CDW phases in low-dimensional materials, the reasons for their existence are still not fully understood yet. Many processes, involving either strong electron–electron or electron–lattice coupling, have been observed which all might play a role in explaining the formation of different phases under different conditions. With the availability of femtosecond lasers it has become possible to study materials under highly nonequilibrium conditions. By suddenly introducing a known amount of energy into the system, the equilibrium state is disturbed and the subsequent relax- ation processes are then observed on timescales of structural and electronic responses. These experiments can deliver valuable information about the complex interactions between the different constituents of condensed matter, which would be inaccessible under equilibrium conditions. We use time resolved electron diffraction to investigate the behaviour of a CDW system perturbed by a short laser pulse. From the observed changes in the diffraction patterns we can directly deduce changes in the lattice structure of our sample. A femtosecond electron diffraction setup was developed at the Laser Research In- stitute in Stellenbosch, South Africa. Short laser pulses produce photo electrons which are accelerated to an energy of 30 keV. Despite space charge broadening effects, elec- tron pulses shorter than 500 fs at sample position can be achieved. Technical details of this system and its characterisation as well as sample preparation techniques and analysis methods are described in detail in this work. Measurements on two members of the quasi-two-dimensional transition metal di- chalcogenides, namely 4Hb-TaSe2 and 1T-TaS2, are shown and discussed. Both show fast (subpicosecond) changes due to the suppression of the PLD and a rapid heating of the lattice. When the induced temperature rise heats the sample above a phase tran- sition temperature, a complete transformation into the new phase was observed. For 4Hb-TaSe2 we found that the recovery to the original state is significantly slower if the PLD was completely suppressed compared to only disturbing it. On 1T-TaS2 we could not only study the suppression of the original phase but also the formation of the higher energetic CDW phase. Long (100 ps) time constants were found for the tran- sition between the two phases. These suggest the presence of an energy barrier which has to be overcome in order to change the CDW phase. Pinning of the CDW by de- fects in the crystal structure result in such an energy barrier and consequently lead to a phase of domain growth which is considerably slower than pure electron or lattice dynamics.
AFRIKAANSE OPSOMMING: Dit is bekend dat lae-dimensionele materie fase oorgange ondergaan na anders ge- ori¨enteerde toestande wanneer afgekoel word tot laer temperature. Hierdie fases toon dikwels ’n periodiese modulasie van die elektron digtheid (genoem ’n “charge density wave” – CDW), tesame met ’n periodiese effek op die kristalrooster (genoem ’n “peri- odic lattice distortion” – PLD). Alhoewel baie eksperimente al uitgevoer is en al baie geleer is oor hierdie CDW fase, is die redes vir hul bestaan nog steeds nie ten volle verstaan nie. Baie prosesse, wat of sterk elektron–elektron of elektron–fonon interaksie toon, is al waargeneem en kan ’n rol speel in die verduideliking van die vorming van die verskillende fases onder verskillende omstandighede. Met die beskikbaarheid van femtosekonde lasers is dit nou moontlik om materie onder hoogs nie-ewewig voorwaardes te bestudeer. Deur skielik ’n bekende hoeveel- heid energie in die stelsel in te voer, word die ewewigstaat versteur en word die daar- opvolgende ontspanning prosesse waargeneem op die tydskaal van atomies struktu- rele en elektroniese bewiging. Hierdie eksperimente kan waardevolle inligting lewer oor die komplekse interaksies tussen die verskillende atomiese komponente van ge- kondenseerde materie, wat ontoeganklik sou wees onder ewewig voorwaardes. Ons gebruik elektrondiffraksie met tyd resolusie van onder ’n pikosekonde om die gedrag van ’n CDW stelsel te ondersoek nadat dit versteur is deur ’n kort laser puls. Van die waargenome veranderinge in die diffraksie patrone kan ons direk aflei watse veranderinge die kristalstruktuur van ons monster ondergaan. ’n Femtosekonde elektronendiffraksie opstelling is ontwikkel by die Lasernavors- ingsinstituut in Stellenbosch, Suid-Afrika. Kort laser pulse produseer foto-elektrone wat dan na ’n energie van 30 keV versnel word. Ten spyte van Coulomb afstoting ef- fekte, kan elektron pulse korter as 500 fs by die monster posisie bereik word. Tegniese besonderhede van hierdie opstelling, tegnieke van die voorbereiding van monsters asook analise metodes word volledig in hierdie tesis beskryf. Metings op twee voorbeelde van kwasi-tweedimensionele semi-metale, naamlik 4Hb-TaSe2 en 1T-TaS2, word gewys en bespreek. Beide wys ’n vinnige (subpikosekon- de) verandering as gevolg van die versteuring van die PLD en ’n vinnige verhitting van die kristalrooster. Wanneer die ge¨ınduseerde temperatuur bo die fase oorgang tempe- ratuur styg, is ’n volledige transformasie na die nuwe fase waargeneem. Vir 4Hb-TaSe2 het ons gevind dat die herstelling na die oorspronklike toestand aansienlik stadiger is as die PLD heeltemal viernietig is in vergelyking met as die PLD net versteur is. Met 1T-TaS2 kon ons nie net alleenlik die vernietiging van die oorspronklike fase sien nie, maar ook die vorming van ’n ho¨er energie CDW fase. Lang (100 ps) tydkonstante is gevind vir die oorgang tussen die twee fases. Hierdie dui op die teenwoordigheid van ’n energie-versperring wat eers oorkom moet word om die CDW fase voledig te ver- ander. Vaspenning van die CDW deur defekte in die kristalstruktuur veroorsaak so’n energie versperring en gevolglik lei dit tot ’n fase van groeiende CDW gebiede wat heelwat stadiger as pure elektron of kritalrooster dinamika is.
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Qasim, Ilyas. "Structural and Electronic Phase Transitions in Mixed Transition Metal Perovskite Oxides." Thesis, The University of Sydney, 2013. http://hdl.handle.net/2123/10029.
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The reported multiferroic perovskite series Sr1-xAxTi1/2Mn1/2O3 has been the subject of numerous structural studies, without reaching consensus. In the current work, the cubic Pm3 ̅m is confirmed for end member SrTi1/2Mn1/2O3 in the Sr1-xAxTi1/2Mn1/2O3 ( A= Ca, La; 0 ≤ x ≤ 1) series. The Pm3 ̅m I4/mcm Pbnm structural evolution was observed with increased doping level of Ca. A cubic Pm3 ̅m rhombohedral R3 ̅c transition occurred when La is substituted instead of Ca. Interesting magnetic behaviours were observed and the major contribution to this was concluded to be the mixed Mn4+/Mn3+ ratio. Ru and Ir have almost identical ionic radii and behave similarly in many ways. Remarkably the structure and properties of SrRuO3 and SrIrO3 are different. The current study revealed that the divalent transition metal doped materials of the type SrR1-xMxO3 (R = Ru, Ir, and M = 3d transition metals) are isostructural. This was achieved by the synthesis of a number of new materials of the type SrIr1-xMxO3. Therefore, these two series are comparatively described in the thesis. The structure and physical properties of the iron doped series SrIr1-xFexO3 are found to be different from those of the divalent doped ones, and this was even true for Ru analogues. Therefore, Fe-doped SrRuO3 and SrIrO3, based on the results of the same level doped materials are presented in a separate chapter. In the final chapter, the impact of Cu2+ doping on the structure and electronic properties of LaCrO3 is described. In order to understand structure property relationships, all the materials structurally characterised have had magnetic and resistivity measurements conducted. Special attention is given to realise the correlations between structure, magnetism, and conductivity.
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Monceau, Pascal. "TRANSITIONS DE PHASE EN DIMENSIONS FRACTALES." Habilitation à diriger des recherches, Université Paris-Diderot - Paris VII, 2004. http://tel.archives-ouvertes.fr/tel-00521313.
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Parmi les méthodes de la théorie du groupe de renormalisation, les développements en epsilon sont basés sur des calculs dans l'espace réciproque et permettent de calculer les exposants critiques associés aux transitions magnétiques du second ordre pour des valeurs non entières de la dimension d'espace. Une interprétation physique naturelle consiste à se demander comment se comporte un système de spins en interaction dans un espace de dimension fractale. Or les structures fractales sont construites par itération d'une cellule génératrice dont les détails sont donc présents à plusieurs échelles ; la question qui se pose alors est de savoir ce qui se passe lorsque l'invariance par translation est remplacée par une invariance d'échelle géométrique. La convergence vers la limite thermodynamique se produit en même temps que le processus d'itération construit la structure. De ce fait, des simulations Monte Carlo fiables de ces transitions de phase n'ont pu être menées à bien que récemment, puisqu'elles nécessitent la simulation de très grandes tailles, lesquelles varient comme des séries géométriques avec l'étape d'itération. C'est en utilisant des algorithmes non locaux dits “d'amas” (Wolff, Swendsen-Wang), capables de réduire le ralentissement critique de manière significative, et des méthodes d'histogrammes pour traiter les données des simulationsMonte-Carlo que j'ai tout d'abord réalisé ces études. Il s'avère que le calcul précis des exposants critiques est rendu encore plus difficile par le fait que l'analyse en tailles finies du modèle d'Ising souffre de corrections d'échelle qui peuvent affecter fortement le comportement de certaines grandeurs thermodynamiques, en particulier lorsque la dimension fractale tend vers 1. J'ai montré que ces corrections d'échelle sont en partie liées à la très forte inhomogénéité du réseau sous jacent (due à l'existence de trous sur plusieurs ordres de grandeurs) et à la concomitance de la construction du fractal avec la convergence vers la limite thermodynamique. Les résultats que j'ai obtenus pour les exposants critiques, ou leurs bornes, sont toujours compatibles avec la relation d'hyperscaling dans laquelle on substitue la dimension de Hausdorff à la dimension d'espace. Le comportement critique en dimension non entière se décrit dans le cadre de l'universalité faible. Cela se manifeste par un désaccord net entre les exposants que j'ai obtenus par les méthodes Monte Carlo et les développements en epsilon. Les exposants critiques ne dépendent pas seulement de la dimension d'espace, des propriétés de symétrie du paramètre d'ordre et de la portée des interactions, mais aussi des propriétés géométriques de la structure fractale : Très récemment des calculs précis d'exposants critiques m'ont permis de montrer que des classes d'universalité différentes sont en général nécessaires pour décrire le comportement du modèle d'Ising sur des fractals de même dimension et de lacunarités différentes. Un tel résultat généralise le concept d'universalité faible proposé par Masuo Suzuki. L'hypothèse d'homogénéité qui sous-tend les lois d'échelle permettant de décrire un comportement critique se dérive par renormalisation. La procédure de renormalisation dans l'espace direct est naturelle dans les fractals, puisqu'elle suit exactement le processus inverse de construction de la structure. Avec mon étudiant Pai-Yi Hsiao, nous avons mené à bien l'étude du modèle d'Ising par une méthode de renormalisation Monte-Carlo sur une structure fractale de dimension voisine de 1, 89 ; il s'avère que l'exposant associé à l'une des directions propres peut être calculé avec une très bonne précision et est en accord avec les résultats de l'analyse en tailles finies. En revanche, la convergence est très lente dans l'autre direction, ce qui est lié aux corrections d'échelle mises en évidence lors de cette analyse. La cinétique stochastique associée à la formation des amas construits par l'algorithme de Wolff sous tend la compréhension du phénomène de ralentissement critique. J'ai montré que les distributions des tailles des amas de Wolff ont une propriété d'homogénéité qui fait intervenir l'un des exposants associé à une des directions propres du processus de renormalisation. Par ailleurs, les distributions des tensions de surface des amas vérifient une propriété analogue dans laquelle intervient un nouvel exposant critique. L'étude des fonctions d'autocorrélation m'a permis de calculer précisément les exposants dynamiques de Wolff lorsque la température critique est connue, et d'éclaircir l'évolution du ralentissement critique avec la dimension et la connectivité. Dans le cas de systèmes invariants par translation, l'ordre de la transition ferromagnétique du modèle de Potts est lié au nombre d'états de spin ; le passage du premier au second ordre est attendu pour des dimensions non entières. Par ailleurs, la présence de désordre peut, dans certaines conditions, induire une transition du second ordre à partir d'un système qui en présente une du premier. L'étude du comportement critique du modèle de Potts sur des fractals est donc particulièrement intéressante, puisque deux des paramètres qui le déterminent (dimensionnalité et désordre structurel) sont liés. Avec mon étudiant Pai-Yi Hsiao, nous avons montré que la transition associée au modèle de Potts à trois états sur une structure fractale de dimension voisine de 1, 89 est du second ordre. Les difficultés attendues lorsqu'on augmente le nombre d'états de spins se font déjà nettement sentir : Les corrections d'échelle empêchent de calculer la température critique avec une très bonne précision. Nous n'avons donc pu donner que des bornes pour certains exposants ; nous avons cependant clairement mis en évidence la différence entre les classes d'universalité de Potts à 2 et 3 états. L'étude de la percolation en dimension non entière est liée à la fois à celle du modèle de Potts et aux algorithmes d'amas. Elle est basée sur l'étude des moments de la distribution de taille des amas, ce qui nécessite la localisation de pics en fonction de la probabilité d'occupation. J'ai pu montrer que les corrections d'échelle n'affectent pratiquement pas le comportement des pics avec la taille des structures, et proposé de les interpréter en termes de "seuil effectif".
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Madamba, Maria Cecilia S. "Phase transitions in superionic PbSnF¦4." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp01/MQ39072.pdf.
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Chen, Leiming. "Tilt phase transitions in disordered systems /." view abstract or download file of text, 2006. http://proquest.umi.com/pqdweb?did=1251884301&sid=1&Fmt=2&clientId=11238&RQT=309&VName=PQD.
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Thesis (Ph. D.)--University of Oregon, 2006.Typescript. Includes vita and abstract. Includes bibliographical references (leaves 126-128). Also available for download via the World Wide Web; free to University of Oregon users.
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Jones, Nicholaos John. "Ineliminable idealizations, phase transitions, and irreversibility." Columbus, Ohio : Ohio State University, 2006. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1163026373.
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Lidmar, Jack. "Phase transitions in high-temperature superconductors." Doctoral thesis, Stockholm : Tekniska högsk, 1998. http://www.lib.kth.se/abs99/lidm0111.pdf.
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Li, Su. "Phase transitions of high temperature superconductors." College Park, Md. : University of Maryland, 2007. http://hdl.handle.net/1903/7276.
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Thesis (Ph. D.) -- University of Maryland, College Park, 2007.Thesis research directed by: Physics. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.
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Nikas, Yvonne Jiang. "Topics in phase transitions at interfaces." Connect to resource, 1989. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1232732921.
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Giddy, Andrew Peter. "Computational studies of structural phase transitions." Thesis, University of Cambridge, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239582.
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De, Dombal Richard Francis. "Phase transitions in nepheline and tridymite." Thesis, University of Cambridge, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239792.
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Ferreira, Pedro Tonnies Gil. "Observational consequences of cosmological phase transitions." Thesis, Imperial College London, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.338692.
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Karra, Glykeria. "Production of defects at phase transitions." Thesis, Imperial College London, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.298337.
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Larsson, Sebastian E. "Topological defects from cosmological phase transitions." Thesis, University of Oxford, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.298309.
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Adams, Jennifer Anne. "Cosmological phase transitions : techniques and phenomenology." Thesis, University of Oxford, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.306935.
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Swift, Michael Robert. "Surface phase transitions in novel geometries." Thesis, University of Oxford, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.279938.
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Hook, Daniel William. "Phase transitions in quantum microcanonical equilibrium." Thesis, Imperial College London, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.497906.
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Meier, Hannes. "Superfluid Phase Transitions in Disordered Systems." Licentiate thesis, KTH, Statistisk fysik, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-50051.
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This thesis presents results from large scale Monte Carlo simulations of systems subject to a superfluid phase transition in the presence of disorder. The simulations are performed by state-of-the-art, collective Monte Carlo algorithms treating phase degrees of freedom in effective models with amplitude fluctuations integrated out. In Paper I a model system for the possible solid to supersolid transition in 4He is presented.The Wolff cluster algorithm is used to study how the presence of linearly correlated random defects is able to alter the universality class of the 3-dimensional XY-model. In the pure case the superfluid density and heat capacity have singular onsets, which are not seen in the supersolid experiments where instead a smooth onset is obtained. Using finite size scaling of Monte Carlo data, we find a similar smooth onset in our simulations, governed by exponents ν=1 for the superfluid density and α=-1 for the heat capacity. These results are in qualitative agreement with experiments for the observed transition in solid 4He. In Paper II a systematic investigation of the scaling result z=d for the dynamic critical exponentat the Bose glass to superfluid quantum phase transition is performed. The result z=d has been believed to be exact for about 20 years, but although it has been questioned lately no accurate estimate of z has been available. An effective link current model of quantum bosons at T=0 with disorder in 2D is simulated using highly effective worm Monte Carlo simulations.The data analysis is based on a finite size scaling approach todetermine the quantum correlation time from simulationdata for boson world lines without any a priori assumption on the critical parameters. The resulting critical exponents are z=1.8 \pm 0.05, ν=1.15 \pm 0.03, and η=-0.3 \pm 0.1. This suggests that z=d is not satisfied.QC 20111206
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Zarikas, Vasilios. "Aspects of early universe phase transitions." Thesis, University of Newcastle Upon Tyne, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318257.
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Braun, Frank Nicholas. "Surface phase transitions in liquid crystals." Thesis, University of Southampton, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.242564.
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Gelb, Lev. "Theoretical studies of surface phase transitions." Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.387600.
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Gambhir, Manoj. "Models of phase transitions in silicates." Thesis, University of Cambridge, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.624645.
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Spalek, Leszek Jedrzej. "Emergent phenomena near selected phase transitions." Thesis, University of Cambridge, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.608135.
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Diener, Jacobus Petrus Willem. "Ferromagnetic phase transitions in neutron stars." Thesis, Stellenbosch : Stellenbosch University, 2012. http://hdl.handle.net/10019.1/71982.
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Thesis (PhD)--Stellenbosch University, 2012.ENGLISH ABSTRACT: We consider the ferromagnetic phase in pure neutron matter as well as charge neutral, betaequilibrated nuclear matter. We employ Quantum Hadrodynamics, a relativistic field theory description of nuclear matter with meson degrees of freedom, and include couplings between the baryon (proton and neutron) magnetic dipole moment as well as between their charge and the magnetic field in the Lagrangian density describing such a system. We vary the strength of the baryon magnetic dipole moment till a non-zero value of the magnetic field, for which the total energy density of the magnetised system is at a minimum, is found. The system is then assumed to be in the ferromagnetic state. The ferromagnetic equation of state is employed to study matter in the neutron star interior. We find that as the density increases the ferromagnetic field does not increase continuously, but exhibit sudden rapid increases. These sudden increases in the magnetic field correspond to shifts between different configurations of the charged particle’s Landau levels and can have significant observational consequences for neutron stars. We also found that although the ferromagnetic phase softens the neutron star equation of state it does not significantly alter the star’s massradius relationship. The properties of magnetised symmetric nuclear matter were also studied. We confirm that magnetised matter tends to be more proton-rich but become more weakly bound for stronger magnetic fields. We show that the behaviour of the compressibility of nuclear matter is influenced by the Landau quantisation and tends to have an oscillatory character as it increases with the magnetic field. The symmetry energy also exhibits similar behaviour.
AFRIKAANSE OPSOMMING: In hierdie studie het ons die ferromagnetiese fase in suiwer neutronmaterie, sowel as in ladingsneutrale, beta-ge¨ekwilibreerde neutronstermaterie, ondersoek. Vir die doeleindes het ons die Kwantum Hadrodinamika-model van kernmaterie gebruik. Dit is ’n relatiwistiese, veldteoretiese model wat mesone inspan om die interaksies tussen die protone en neutrone te bemiddel. Om die impak van die magneetveld te bestudeer, sluit ons ’n koppeling tussen die barioonlading en die magneetveld, asook barioondipoolmoment en die magneetveld, in by die Lagrange digtheid wat ons sisteem beskryf. Om die ferromagnetiese fase te ondersoek, varieer ons die sterkte van die barioondipoolmoment om ’n nie-nul waarde van die magneetveld wat energie digtheid sal minimeer te vind. Die ferromagnetiese toestandsvergelyking word toegepas op materie aan die binnekant van die neutronster en die impak hiervan op die waarneembare eienskappe van die ster word ondersoek. Ons vind dat die ferromagnetiese magneetveld nie kontinu toeneem soos die digtheid verhoog nie. Die skielike toenames in die magneetveld is die gevolg van die sisteem wat die konfigurasie van die gelaaide deeltjies se Landau-vlakke skielik verander en dit kan beduidende waarneembare gevolge vir die ster inhou. Ons vind ook dat die ferromagnetiese fase die toestandsvergelyking versag, maar dat die versagting die massa-radius verhouding van die ster nie grootliks beïnvloed nie. Die eienskappe van gemagnetiseerde kernmaterie word ook ondersoek. Ons bevestig dat gemagnetiseerde materie meer proton-ryk, maar minder sterk gebind word. Ons wys dat die saampersbaarheid van kernmaterie deur die teenwoordigheid van Landau-vlakke beïnvloed word en ossilerend saam met die magneetveld toeneem. Die simmetrie-energie manifesteer ook soortgelyke gedrag.
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Saunders, Timothy E. "Phase transitions in geometrically frustrated antiferromagnets." Thesis, University of Oxford, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.670121.
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Magou, Maria. "Holographic descriptions of chiral phase transitions." Thesis, University of Southampton, 2012. https://eprints.soton.ac.uk/345723/.
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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.
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Lusk, Mark T. Knowles James K. "Martensitic phase transitions with surface effects /." Diss., Pasadena, Calif. : California Institute of Technology, 1992. http://resolver.caltech.edu/CaltechETD:etd-11052004-161432.
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Cuvelier, Pierre. "Transitions de phase des cristaux plastiques." Grenoble 2 : ANRT, 1986. http://catalogue.bnf.fr/ark:/12148/cb375969616.
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Bailey, Delbert D. "Phase transitions of boolean satisfiability variants /." Diss., Digital Dissertations Database. Restricted to UC campuses, 2004. http://uclibs.org/PID/11984.
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Cortés, Huerto Robinson. "Phase transitions in many-electron systems." Thesis, Queen's University Belfast, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.527673.
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Rizzi, Matteo. "Quantum Phase transitions in Hubbard lattices." Doctoral thesis, Scuola Normale Superiore, 2007. http://hdl.handle.net/11384/85848.
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Thompson, Ian. "Dynamic phase transitions in biased ensembles of particle systems with repulsive interactions." Thesis, University of Bath, 2015. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.665407.
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We study dynamic phase transitions in the constant-volume and constant- pressure ensembles of two different systems: a one-dimensional system of diffusive hard particles and a three-dimensional glass-former of nearly-hard repulsive particles. The dynamic transitions are observed using ensembles of trajectories biased with respect to their dynamic activity, biasing to greater or lower activities than equilibrium allows us to sample different dynamic phases. We perform finite-size scaling of the transitions with respect to sys- tem size and observation time, and compare them to first-order phase tran- sitions. The two ensembles are not equivalent in the one-dimensional model. We compare our results to analytic predictions for diffusive systems in both the active and inactive phases, there are structural signatures for both dy- namic regimes. The active phases show hyperuniform ordering and the inac- tive regimes show jamming behaviour, local jamming in the constant-volume ensemble is achieved through phase separation. In the three-dimensional sys- tem we observe a dynamic transition to a glassy inactive phase, there is no obvious structural change and the structural relaxation time increases sig- nificantly. We take configurations from the active and inactive phases and subject them to a jamming protocol in order to compare the final density of the jammed packings. Previous work shows that the inactive phase of glass-forming systems have a different distribution of vibrational modes and a higher compressibility, this suggests that the jamming behaviour should differ between the two phases. We show that jammed packings generated from inactive configurations are denser than those generated from active configurations.
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Grover, Tarun Ph D. Massachusetts Institute of Technology. "Applied fractionalization : quantum phases and phase transitions of strongly interacting systems." Thesis, Massachusetts Institute of Technology, 2010. http://hdl.handle.net/1721.1/68973.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2010.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 131-136).
Strongly correlated systems present interesting challenges in condensed matter physics. On the one hand, the theoretical work in the last two decades suggests that strong interactions may lead to new phases and phase transitions of matter that don't fit paradigms such as Fermi liquid theory or Landau's theory of phase transitions. On the other hand, there are actual materials which are undoubtedly governed by strong interactions and indeed do not fit the conventional paradigms but whose behavior often doesn't quite match our theoretical expectations. This gap between theory and experiments is slowly narrowing owing to the discovery of new materials and recent advances in numerical simulations. As an example, the material K - (ET)2Cu 2(CN) 3 exhibits metallic specific heat in its insulating phase. This is indicative of the theoretically proposed phenomena of 'fractionalization' where elementary excitations in a phase carry quantum numbers that are fractions of that corresponding to an electron. Similarly, there is growing numerical evidence of the theoretical phenomena of 'deconfined quantum criticality', where quantum Berry phases lead to emergence of fractionalized particles right at the phase transition. In this thesis we study phenomena where the concept of fractionalization is a useful tool to explore new phases and phase transitions. Most of our examples are in the context of frustrated quantum magnets. Along the way, we also explore topics such as quantum numbers of topological defects and non-abelian phases of matter. Whenever possible, we compare theoretical predictions with experimental and numerical data. We also discuss deconfined quantum criticality in the context of metallic systems where it opens the route to phase transitions very different from the conventional spin-density wave instability of Fermi surface.
by Tarun Grover.
Ph.D.
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Arrachid, Abdessamad. "The phase transition analyzer : a tool to measure thermal transitions of biopolymers?" Thesis, University of Nottingham, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.435986.
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