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Lee, Soon-Gul. « Experimental study of the effects of disorder in weakly disordered superconducting aluminum films / ». The Ohio State University, 1988. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487588939089315.
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Rodriguez-Vega, Martin Alexander. « Disorder Effects in Dirac Heterostructures ». W&M ScholarWorks, 2016. https://scholarworks.wm.edu/etd/1477068246.
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In this dissertation, we study theoretically heterostructures based on Dirac mate- rials, i.e. materials, such as graphene in which the electrons behave as massless Dirac fermions at low energies. We first examine how the presence of long-range disorder affects the electronic ground state of a double layer graphene heterostruc- ture formed by two graphene layers separated by a thin dielectric film. We then identify the necessary conditions for the formation of an interlayer exciton conden- sate in such a system. We also comment on the effect of long-range disorder on the broken symmetry ground state induced by electron-electron interactions in bilayer graphene. Then, we study the transport properties of heterostructures obtained by stacking a graphene layer on the surface of a strong three-dimensional topological insulator (TI). In particular, we determine the non-equilibrium current-induced spin density accumulation for these systems using linear response theory and taking into account the effects of long- and short-range disorder both in the limit of strong and weak tunneling between the graphene layer and the TI. Finally, using some of the theoretical approaches developed to characterize the effect of long-range disorder in Dirac materials, we study the effect of long-range inhomogeneities in first-order phase transitions. In particular, we present a theoretical model to describe the ef- fect of inhomogeneities on the relaxation dynamics of vanadium dioxide films after a photo-induced metal-insulator transition.
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Klales, Anna. « A Classical Perspective on Non-Diffractive Disorder ». Thesis, Harvard University, 2016. http://nrs.harvard.edu/urn-3:HUL.InstRepos:26718765.
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The unifying themes connecting the chapters in this dissertation are the profound and often surprising effects of disorder in classical and quantum systems and the tremendous insight gained from a classical perspective, even in quantum systems. In particular, we investigate disorder in the form of weak, spatially correlated random potentials, i.e. far from the Anderson Localization regime.
We present a new scar-like phenomenon in quantum wells. With the introduction of local impurities to the oscillator, the eigenstates localize onto classical periodic orbits of the unperturbed system. Compared to traditional scars in chaotic billiards, these scars are both more common and stronger. Though the unperturbed system has circular symmetry, the random perturbation selects a small number of orientations which are shared by many scarred states -- dozens or even hundreds -- over a range of energies. We show, via degenerate perturbation theory, that the cause of the new scars is the combination of an underlying classical resonance of the unperturbed system and a perturbation induced coupling that is strongly local in action space.
Next we examine the same type of local perturbation applied to an open system: branched flow. Caustics in the manifold of trajectories have been implicated in the formation of strong branches. We show that caustic formation is intimately tied to compression of manifolds of trajectories in phase space, which has important implications for the position space density.
We introduce the "Kick and Drift" model, a generalization of the standard map. The model is a good approximation to the full two dimensional dynamics of a wave propagating over a weak random potential, but it provides a simpler framework for studying branched flow.
Next we develop a classical model for electrons executing cyclotron motion in a graphene flake and implement it numerically. We derive classical equations of motion for electrons moving through the graphene flake with a position dependent effective mass due to fluctuations in the background carrier density. I apply these methods to an experiment performed by the Westervelt group. They imaged the flow of electrons in a graphene flake by measuring the transresistance as they rastered a charged scanning probe microscope tip over the surface. My simulations show that the regions with the greatest change in transresistance do always coincide with the regions with the highest current density. Furthermore I show that the experimental results can qualitatively reproduced by treating the system classically.
Finally, we extend Heller's thawed Gaussian approximation from second order in the classical action to third order, in order to capture curvature in phase space. Such phase space dynamics are ubiquitous in systems with weak random potentials, such as those discussed above. We derive a closed form solution, but find that more work needs to be done to make it numerically tractable and competitive with other methods. A semiclassical method capturing phase space curvature could provide insight into the behavior of scars away from the hbar goes to zero limit.Physics
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Pyun, Deuk Soo. « Experimental study of the disorder effect in strongly disordered superconducting amorphous composite indium/indium oxide films / ». The Ohio State University, 1990. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487681148544033.
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Brinckerhoff, William B. « Dimensionality and disorder in molecule-based magnets ». The Ohio State University, 1995. http://rave.ohiolink.edu/etdc/view?acc_num=osu1343143081.
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Jones, Maximilian. « Disorder in multi-channel Luttinger liquids ». Thesis, University of Birmingham, 2018. http://etheses.bham.ac.uk//id/eprint/8147/.
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This thesis examines the effects of disorder upon a bundle of coupled one dimensional (ID) systems. Each 1D system is described as a Luttinger liquid, and the coupling between channels is weak enough such that this description remains valid. The coupling can be of either a density-density or current-current type. We consider continuous disorder in each channel, and derive renormalisation group (RG) equations governing the strength of the disorder. We analyse the effects of disorder in two specific examples: a lattice of identical channels, and two distinct channels. In both cases, close to the simultaneous metal-insulator transition, we arrive at coupled Berezinskii-Kosterlitz-Thouless (BKT) equations. Away from the simultaneous transition we analyse the RG equations numerically. Inter-channel interactions are found to shift the metal-insulator boundary, and destroy the mixed insulator-conductor phase close to the simultaneous transition.
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Willans, Adam J. « Disorder in an exactly solvable quantum spin liquid ». Thesis, University of Oxford, 2010. http://ora.ox.ac.uk/objects/uuid:8ea5b2cc-4843-44ef-aa0a-8535f00c6dc8.
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We investigate the properties of the Kitaev honeycomb model with both site dilution and exchange randomness. Embarking on this work, we review disorder in some strongly correlated electron systems, including spin-½ and spin-1 Heisenberg antiferromagnetic chains, two dimensional Heisenberg antiferromagnets, the cuprates and graphene. We outline some aspects of resonating valence bond phases, valence bond solids, spin liquids and quantum computation that are pertinent to an understanding of the Kitaev model. The properties of the Kitaev model without disorder are discussed and it is found to be a critical spin liquid, with algebraic correlations in two spin operators sigma^{alpha}_{i}sigma^{alpha}_{j}, where i and j,/em> are either end of a link of type alpha = x, y or z on the honeycomb lattice. The Kitaev model is exactly solvable and we show that this remains so in the presence of site dilution and exchange randomness. We find that vacancies bind a flux. In the gapped phase, a vacancy forms an effective paramagnetic moment. With two or more vacancies we describe the interaction of their effective moments and show that a finite density of vacancies leads to a divergent macroscopic susceptibility at small fields. In the gapless phase the effective moment has a susceptibility that is, to leading order at small fields, chi(h)~log(1/h). Interaction between the moments from two vacancies on opposite sublattices cuts off this divergence in susceptibility at a large but finite constant. Two vacancies on the same sublattice behave quite differently and we find the combined susceptibility is parametrically larger than that of an isolated vacancy, chi(h)sim [h(log(1/h))^{3/2}]^{-1}. We also investigate the effects of slowly varying, quenched disorder in exchange coupling. We demonstrate that this does not qualitatively affect the susceptibility but show that the heat capacity C ~ T^{2/z}, where z is a measure of the disorder and increases from one with increasing disorder strength.
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Vestergren, Anders. « From Order to Disorder in High Temperature Superconductors ». Doctoral thesis, KTH, Physics, 2004. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3833.
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Phase transitions in a number of models related to hightemperature superconductors are investigated, using scalingmethods and Monte Carlo simulations. This thesis considers twomain topics.
The first topic is phase transitions, phase diagrams, andvortex motion in high temperature superconductors at finitetemperature, subject to magnetic fields and disorder. We studya vortex glass model at finite temperature, with stronguncorrelated vortex pinning and a magnetic field. We find thatthe vortex glass exists at finite temperature and calculate thecritical exponents of the transition. We also investigate hightemperature superconductors with columnar disorder in zero andapplied magnetic fields. Some of these studies are alsorelevant for the superfluid to Mott insulator transition ofbosons in two dimensions. We find that the unscreened Boseglass transition belongs to a new universality class. Wecalculate the critical exponents of the superconductingtransition with columnar defects in zero applied magneticfield. The transverse Meissner transition is studied, and wefind an exotic universality class with a correlation volumethat is infinitely anisotropic in all directions.
The second topic is confinement-deconfinement transitions incompact Abelian Higgs models. We develop a new order parameter,related to a large Wilson loop for fractionalized charges, anduse it to study the concept of topological order. Thesetransitions may be relevant for strongly correlated electronsin two dimensions.
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Ward, S. N. E. « Spin ladder physics and the effect of random bond disorder ». Thesis, University College London (University of London), 2015. http://discovery.ucl.ac.uk/1462036/.
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This PhD thesis concerns the physics of low-dimensional quantum systems and especially quantum spin ladders. Novel metal-organic compounds (C5H12N)2CuBr4 and (C5H12N)2CuCl4 are investigated as model materials for low-dimensional quantum behavior by neutron scattering experiments and by measurements of magnetic and thermodynamic properties. The experimental results are compared quantitatively to calculations using a variety of theoretical and numerical methods (DMRG, ED) of the ground state and excitations of such systems. Key results are the determination of the Hamiltonian and its exchange parameters of (C5H12N)2CuCl4 studied here for the first time from quantitative modelling of magnetic susceptibility and neutron spectroscopy data. When a magnetic field is applied two quantum critical points occur at which fractionalization of the elementary quasi-particle excitations is ob- served. The characteristic excitation continua are explained by effective spin-chain and t-J models and are observed systematically as a function of magnetic field and temperature. Coherent and incoherent spin Luttinger-liquid physics is observed and for the first time modelled fully quantitatively. The chemical flexibility of the these metal-organic compounds allows continuous solid-state mixtures of Br and Cl resulting in systems with quenched disorder. The rung and leg exchange parameters assume discrete values given by the specific chemical composition of the exchange pathways. The influence of such quenched disorder on the excitations of quantum spin ladders has been studied experimentally. The observed spectra with damped excitations of the unperturbed ladder and more localized modes provide detailed insights into the physics that may be observed in such systems if a magnetic field is applied and so called Bose glass phases are induced.
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Cunha, Frederico. « A surface charge induced order-disorder phase transition in organic monolayers ». FIU Digital Commons, 1995. http://digitalcommons.fiu.edu/etd/2690.
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Adenine and 2-2' bipyridine monolayer films adsorbed on Au (111) have been studied by Scanning Tunneling Microscopy (STM) in solution as a function of surface charge and bulk concentration. Both molecules form polymer chains. The length of the polymer chains increases as the concentration increases. At low charges they are randomly oriented, but they begin to align in parallel as the charge density reaches a critical value. The transition, which resembles the isotropic-nematic phase transition encountered in liquid-crystals, is fully reversible by lowering the charge below the critical value.
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Povinelli, Michelle Lynn 1975. « Characteristics of defect modes, slow light, and disorder in photonic crystals ». Thesis, Massachusetts Institute of Technology, 2004. http://hdl.handle.net/1721.1/29449.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2004.Includes bibliographical references (p. 97-122).
Chapter 1 introduces the subject of photonic crystals and reviews the basic physical principles underlying the formation of a band gap and the creation of localized defect modes. Proposed applications, fabrication techniques, and numerical simulation methods are surveyed. Chapter 2 demonstrates the construction of 2D-like defect modes in a 3D photonic crystal with a complete gap. The modes are similar to those in 2D photonic crystals in terms of polarization, field profile, and projected band structures. The results should facilitate the implementation of 2D photonic-crystal devices in realistic 3D systems. Chapter 3 explores the possibility of using photonic-crystal defect modes to design magnetic metamaterials: structures that exhibit magnetic properties despite the non- magnetic character of their constituents. A synthetic magnetic moment is provided by a point-defect mode studied in Chapter 2. Quantitative analysis of the far-field radiation pattern proves that the mode has a primarily magnetic character: over 98% of the emitted power goes into magnetic multipole radiation. Chapter 4 calculates the radiation pressure on the surface of a waveguide formed by ornnidirectionally reflecting mirrors. In the absence of losses, the pressure goes to infinity as the distance between the mirrors is reduced to the cutoff separation of the waveguide mode. The divergence results from the reduction of the modal group velocity to zero, which causes slow-light magnification of the field intensity at constant power input.
(cont.) Chapter 5 analyzes slow-light, band-edge waveguides for compact, integrated, tun- able optical time delays. Slow group velocities at the photonic band edge give rise to large changes in time delay for small changes in refractive index, shrinking device size. Figures of merit are defined for tuning sensitivity and signal dispersion. Exact calculations for a realistic, three-dimensional grating structure are shown to be well predicted by a simple quadratic-band model, simplifying device design. Chapter 6 derives a general, coupled-mode theory for disorder-induced scattering in strongly periodic systems. The analytical results allow the comparison of photonic- crystal waveguides to similar index-guided waveguides. In the realistic limit of weak disorder, reflections are identical while transmission is higher for the photonic-crystal waveguide. The general results, verified by direct numerical simulations in an example system, suggest a new mechanism for the design of low-loss waveguides.
by Michelle Lynn Povinelli.
Ph.D.
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Van, Lierop Johan. « Separation of static and dynamic disorder in magnetic materials ». Thesis, McGill University, 2000. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=37853.
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Conventional transmission Mossbauer, selective excitation double Mossbauer (SEDM), and zero-field muon spin relaxation (ZF-muSR) spectroscopy were used to identify static and dynamic magnetic disorder. With the construction of an efficient SEDM spectrometer, a consistent description of static magnetic disorder in an amorphous alloy (a-Fe80B 20) was developed using transmission Mossbauer and SEDM spectroscopy. Both methods measure the effects of the random static distribution of local magnetic environments around the Mossbauer nuclei. Magnetic fine particle systems (Fe3O4 ferrofluids, polysaccharide iron complex) were examined using transmission Mossbauer spectroscopy, and a model was developed that describes the entire range of dynamic magnetic behavior, from blocked moments on towards collective excitations and superparamagnetic moments. SEDM has measured 180° moment flips in the ferrofluids, determining a model independent relaxation rate of superparamagnetic moments. With the spectral signatures of static and dynamic magnetic phenomena identified, SEDM spectroscopy has been used to unambiguously verify the existence (a-Fe 92Zr8) and absence (Fe65Ni35) of magnetic relaxation in chemically disordered alloys. Additionally, the static and dynamic disorder in a magnetic fine particle system (a polysaccharide iron complex) and a frustrated magnet system (a-FexZr100- x), have been measured with ZF-muSR spectroscopy. The effects of collective excitations have been independently verified with ZF-muSR and moment fluctuation rates are in agreement with transmission Mossbauer spectra fit results. Two magnetic transitions have been identified with ZF-muSR in the a-FexZr100- x system, one at TC and another at Txy corresponding to transverse spin freezing, in both static and fluctuating magnetic components of muSR spectra. SEDM has been used to verify the existence of a fluctuation peak at T xy, and the time-dependent hyperfine interactions due to transverse spin freezin
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Fischer, Andrea M. « Disorder and interactions in graphene and other quantum systems ». Thesis, University of Warwick, 2011. http://wrap.warwick.ac.uk/35238/.
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This thesis examines the topics of disorder and electron-electron interactions in three distinct quantum systems. Firstly, the Anderson transition is studied for the body centred cubic and face centred cubic lattices. We obtain high precision results for the critical disorder at the band centre and the critical exponent using the transfer-matrix method and finite size scaling. Comparing the critical disorder between the simple cubic, body centred cubic and face centred cubic lattices, an increase in the critical disorder is observed as a function of the coordination number of the lattice. The critical exponent is found to be v ≃ 1:5 in agreement with the value for the simple cubic lattice. Energy-disorder phase diagrams are plotted for both lattice types. Next, we consider the Aharonov-Bohm effect for an exciton in a 1D ring geometry. The aim is to determine how the addition of a constant electric field in the plane of the ring effects the Aharonov-Bohm oscillations, which occur as a function of the magnetic ux threading the ring. We develop a self consistent equation for the ground state energy, which is then solved numerically. Oscillations in the ground state energy have an increasing amplitude as a function of electric field strength until a critical electric field value. At this point, oscillations in the oscillator strength become inverted, with the oscillation minimum reaching zero at half a magnetic ux quantum. This suggests a possible process for controlling the formation and recombination of excitons through tuning the applied fields. The final and largest section of the thesis is concerned with collective excitations of graphene in a strong perpendicular magnetic field. The excitations, which are most strongly mixed are identified and used as a basis to diagonalise the Hamiltonian, which includes the Coulomb interaction between electrons and holes. In this way the oscillator strengths and energies of collective excitations are obtained. The good quantum numbers for collective excitations are identified. In particular, we study those arising from the SU(4) symmetry, which is due to two spin and two valley pseudospin projections. This enables us to determine the multiplet structure of the states. In addition to neutral collective excitations or excitons, we investigate the possible formation of charged collective excitations or trions from nearly full or nearly empty Landau levels. The localisation of neutral collective excitations upon a single Coulomb or δ-function impurity is also examined.
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Court, Steven James. « Physics of biological evolution ». Thesis, University of Edinburgh, 2014. http://hdl.handle.net/1842/9975.
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Part I: A remarkable feature of life on Earth is that despite the apparent observed diversity, the underlying chemistry that powers it is highly conserved. From the level of the nucleobases, through the amino acids and proteins they encode, to the metabolic pathways of chemical reactions catalyzed by these proteins, biology often utilizes identical solutions in vastly disparate organisms. This universality is intriguing as it raises the question of whether these recurring features exist because they represent some truly optimal solution to a given problem in biology, or whether they simply exist by chance, having arisen very early in life's history. In this project we consider the universality of metabolism { the set of chemical reactions providing the energy and building blocks for cells to grow and divide. We develop an algorithm to construct the complete network of all possible biochemically feasible compounds and reactions, including many that could have been utilized by life but never were. Using this network we investigate the most highly conserved piece of metabolism in all of biology, the trunk pathway of glycolysis. We design a method which allows a comparison between the large number of alternatives to this pathway and which takes into account both thermodynamic and biophysical constraints, finding evidence that the existing version of this pathway produces optimal metabolic fluxes under physiologically relevant intracellular conditions. We then extend our method to include an evolutionary simulation so as to more fully explore the biochemical space. Part II: Studies of population dynamics have a long history and have been used to understand the properties of complex networks of ecological interactions, extinction events, biological diversity and the transmission of infectious disease. One aspect of these models that is known to be of great importance, but one which nonetheless is often neglected, is spatial structure. Various classes of models have been proposed with each allowing different insights into the role space plays. Here we use a lattice-based approach. Motivated by gene transfer and parasite dynamics, we extend the well-studied contact process of statistical physics to include multiple levels. Doing so generates a simple model which captures in a general way the most important features of such biological systems: spatial structure and the inclusion of both vertical as well as horizontal transmission. We show that spatial structure can produce a qualitatively new effect: a coupling between the dynamics of the infection and of the underlying host population, even when the infection does not affect the fitness of the host. Extending the model to an arbitrary number of levels, we find a transition between regimes where both a finite and infinite number of parasite levels are sustainable, and conjecture that this transition is related to the roughening transition of related surface growth models.
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Nikolaou, Marios. « A Matter of Disorder : Monte Carlo Simulations of Phase Transitions in Strongly Disordered Systems ». Doctoral thesis, Stockholm : Royal Institute of Technology, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4313.
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Alyami, Arwa Saleh. « STUDY OF LOCAL ATOMIC ORDER AND DISORDER IN Ni-Cr AND Ni-V ALLOYS ». Kent State University / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=kent1555590952761642.
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Casas, Brian Wesley. « Effects of disorder and low dimensionality on frozen dynamics in Ca3Co2-xMnxO6 ». Scholar Commons, 2015. http://scholarcommons.usf.edu/etd/5657.
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Complex oxides represent an intersection of play grounds for the existence of exciting new fundamental physics and materials with potential technological implications. The realization of many exciting properties of these systems rely on the coupling of electronic, structural and magnetic degrees of freedom. Additionally, competing interactions within each type of coupling discussed previously lead to theoretically diverse ground states, which under the application of an external perturbation, can be tuned and probed.
Ca3Co¬2-xMnxO6 represent a quasi-one dimensional Ising spin chain system oriented in an antiferromagnetic triangular lattice. The exotic behavior of the undoped compound Ca3Co2O6 has inspired work on continuing the fundamental understanding of frustrated magnetic systems. Through chemical doping of Manganese ions, the magnetic properties, namely the exotic spin glass like behavior can be enhanced for a modest doping range of x
The effects of particle dimensionality were probed through the application of varied calcining conditions as to attempt to observe the altering of magnetic properties, mainly the out of equilibrium magnetization plateaus observed in Ca3Co1.75 Mn0.25O6. It appears that within the particle sizes studied the magnetic behavior is highly robust, even considering the inclusion of ionic disorder.
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Petersen, Greg M. « Anderson Localization in Low-Dimensional Systems with Long-Range Correlated Disorder ». Ohio University / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1365762218.
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Agarwal, Kartiek. « Slow Dynamics in Quantum Matter : The Role of Dimensionality, Disorder and Dissipation ». Thesis, Harvard University, 2016. http://nrs.harvard.edu/urn-3:HUL.InstRepos:33493505.
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A central goal in the study of modern condensed matter physics is the characterization of the dynamical properties of quantum systems. Many decades of effort towards this goal, studying a diverse range of (near-equilibrium) quantum matter, from Fermi liquids, to quantum two-level systems, to interacting spin models, and more, has revealed a remarkable pervasiveness of the simple dynamical description of these complex systems in terms of quasi-particles that carry spin, charge, and heat, and that are generally able to equilibrate systems.
This thesis is an examination of some exceptions to this rule. Specifically, we study a number of instances of quantum matter where equilibration phenomena happens at rather long time scales, or does not occur at all. Particular emphasis is laid on the role of dimensionality, disorder, and dissipation in engendering such novel dynamical behavior.
First, we consider non-equilibrium dynamics in one-dimensional quasi-condensates. Low dimensionality inhibits scattering in these systems, and low-energy excitations are long-lived phase fluctuations that exhibit an enriched conformal symmetry. Utilizing this symmetry, we generalize sudden quenches typically used to study non-equilibrium dynamics to quenches along general relativistic and conformal trajectories. Gases never truly equilibrate after such a quench; instead, they evolve into a `prethermal' state with thermal-looking correlations and a chiral asymmetry.
We then study the problem of the dynamical transition driven by disorder, from an ergodic to a non-ergodic phase, in one-dimensional quantum spin chains. In particular, in XXZ chains with on-site disorder, we find a unique intermediate phase straddling the boundary of the dynamical phase transition, wherein rare-region effects lead to long-time tails in equilibration and vanishing DC conduction before the onset of non-ergodicity. We propose generalizations of such `Griffiths' behavior to arbitrary dimensions. We also study the dynamics of random-bond Heisenberg chains by developing a strong-disorder renormalization group protocol for these systems. We discuss how magnetic noise from such disordered systems contains signatures of their anomalous dynamical properties.
Next, we re-examine the phenomenological theory of two-level systems in amorphous materials in the light of new experimental evidence that these states have large electric/magnetic dipole moments. We propose and justify an interpretation of the model as one of tunneling electrons slowed down by a large phonon drag and discuss the dynamical consequences of such polaronic effects.
Finally, we discuss how magnetic noise measurements can be used to non-invasively access the anomalous properties of systems such as those discussed above. In particular, we examine how scattering properties of isolated magnetic impurities and non-local transport in a variety of two-dimensional materials can be probed experimentally using NV centers as noise magnetometers.Physics
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Ellis, John. « Studies of surface order, disorder and dynamics using helium atom scattering ». Thesis, University of Cambridge, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.316784.
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Meetei, Oinam Nganba. « Metal-Insulator Transition and Novel Magnetism Driven by Coulomb Interactions, Spin-Orbit Coupling and Disorder ». The Ohio State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=osu1405698402.
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Armour, Andrew D. « Dynamics and disorder at the Kosterlitz-Thouless transition ». Thesis, University of Nottingham, 1999. http://eprints.nottingham.ac.uk/13849/.
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This thesis describes theoretical investigations into the dynamics of superfluid films and the effects of disorder on the roughening transition of crystal surfaces. The dynamic theory of superfluid helium films, due to Ambegaokar et al., is refined to improve the precision of the predictions made. A detailed comparison is made between the predictions of the modified theory and the results from experiments on helium films and on superconducting systems. It is found that, despite the modifications in the theory, agreement with experiments on helium films remains only qualitative. Consideration is then given to the effects on the roughening transition of disorder arising from screw dislocations. A crystal surface which is threaded by screw dislocation pairs may be in one of three different states depending on the temperature of the system and the way in which screw pairs are distributed. At high temperatures the interface is rough: it is not pinned to the lattice. At low temperatures the state of the interface depends on how the screw dislocations are distributed: when distributed as closely spaced pairs they lead to a faceted state with a single ground state energy; when distributed randomly they lead to a state of the interface which, though pinned to the underlying crystal lattice, has a degenerate ground state. It is then shown that the dynamic sine-Gordon formulation of the roughening transition can be used, via a Hubbard-Stratonovich transformation, to model the dynamic behaviour of superfluid systems. This method provides a re-normalization group framework within which the a.c. linear response can be studied. The ways in which the approach could be extended to study the effects of disorder and atomic layering are also discussed.
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Cockcroft, J. K. « Neutron scattering studies of order-disorder transitions in hexafluoride salts ». Thesis, University of Oxford, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.233503.
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Marmodoro, A. « Beyond single-site disorder effects in first-principles studies of solid state systems ». Thesis, University of Warwick, 2011. http://wrap.warwick.ac.uk/46881/.
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A generalization of the non-local coherent potential approximation (NL-CPA) for the treatment of short range ordering (SRO) effects in complex unit cell solids with different forms of disorder is proposed. The method is discussed with respect to fully first-principles Korringa, Kohn and Rostoker multiple scattering theory, and a tight-binding model hamiltonian. Examples from the context of applications to metallic alloys, diluted magnetic semiconductors, and half-metals in the presence of antisite disorder are discussed.
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Abdel-Salam, Mohamed Ahmed Ahmed. « Study of the vortex interaction with correlated disorder in high temperature superconductor ». Thesis, Imperial College London, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.250177.
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Watkin, Timothy L. H. « The theory of quenched disorder : spin glasses, neural networks and statistical inference ». Thesis, University of Oxford, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.315731.
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Lyon, Mary Elizabeth. « Electron screening and disorder-induced heating in ultracold neutral plasmas ». BYU ScholarsArchive, 2011. https://scholarsarchive.byu.edu/etd/2857.
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Disorder-induced heating (DIH) is a nonequilibrium, ultrafast relaxation process that occurs when laser-cooled atoms are photoionized to make an ultracold plasma. Its effects dominate the ion motion during the first 100 ns of the plasma evolution. Using tools of atomic physics we study DIH with ns time resolution for different plasma densities and temperatures. By changing the frequency of the laser beam we use to probe the ions, we map out the time evolution of the velocity distribution. We can compare this to a fluorescence simulation in order to more clearly determine the relationship between the fluorescence signal and the velocity distribution. In this study we observe and characterize effects due to electron screening on the ions during the equilibration process.
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Thomson, Steven. « The effects of disorder in strongly interacting quantum systems ». Thesis, University of St Andrews, 2016. http://hdl.handle.net/10023/9441.
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This thesis contains four studies of the effects of disorder and randomness on strongly correlated quantum phases of matter. Starting with an itinerant ferromagnet, I first use an order-by-disorder approach to show that adding quenched charged disorder to the model generates new quantum fluctuations in the vicinity of the quantum critical point which lead to the formation of a novel magnetic phase known as a helical glass. Switching to bosons, I then employ a momentum-shell renormalisation group analysis of disordered lattice gases of bosons where I show that disorder breaks ergodicity in a non-trivial way, leading to unexpected glassy freezing effects. This work was carried out in the context of ultracold atomic gases, however the same physics can be realised in dimerised quantum antiferromagnets. By mapping the antiferromagnetic model onto a hard-core lattice gas of bosons, I go on to show the importance of the non-ergodic effects to the thermodynamics of the model and find evidence for an unusual glassy phase known as a Mott glass not previously thought to exist in this model. Finally, I use a mean-field numerical approach to simulate current generation quantum gas microscopes and demonstrate the feasibility of a novel measurement scheme designed to measure the Edwards-Anderson order parameter, a quantity which describes the degree of ergodicity breaking and which has never before been experimentally measured in any strongly correlated quantum system. Together, these works show that the addition of disorder into strongly interacting quantum systems can lead to qualitatively new behaviour, triggering the formation of new phases and new physics, rather than simply leading to small quantitative changes to the physics of the clean system. They provide new insights into the underlying physics of the models and make direct connection with experimental systems which can be used to test the results presented here.
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Fulkerson, Matthew David. « Gas sensor array modeling and cuprate superconductivity from correlated spin disorder / ». The Ohio State University, 2002. http://rave.ohiolink.edu/etdc/view?acc_num=osu1486546889381639.
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Benini, Leonardo. « Disorder and localization in 1D long-range fermionic chains ». Master's thesis, Alma Mater Studiorum - Università di Bologna, 2017. http://amslaurea.unibo.it/13479/.
Texte intégralRésumé :
Il processo di termalizzazione è l’insieme di fenomeni microscopici che porta un sistema fisico verso quello che chiamiamo stato di equilibrio termodinamico. Nel corso degli anni ’90 il lavoro di J.M. Deutsch e M. Srednicki ha posto solide fondamenta teoriche per la comprensione della termalizzazione in sistemi quantistici, con la formulazione della cosiddetta Eigenstate Thermalization Hypothesis(ETH), fornendo un’elegante descrizione di questo meccanismo. Recentemente, progressi sperimentali nei campi della fisica atomica e dello stato solido hanno dato vita a nuove possibilità nello studio di sistemi quantistici a molti corpi isolati, con una grande precisione nel controllo dei parametri macroscopici. Questi avanzamenti in campo sperimentale hanno dato vita, nell’ultimo decennio, a un rinnovato interesse teorico nei confronti di sistemi che, non rispettando l’ETH, evitano la termalizzazione. Questo lavoro si concentra su un fenomeno caratterizzante di sistemi quantistici disordinati chiamato Localizzazione di Anderson. Nel primo capitolo viene fornita una introduzione al concetto di termalizzazione quantistica e alla derivazione dell’ETH, combinando elementi di teoria quantistica del chaos e Random Matrix Theory. Il secondo capitolo tratta la teoria della localizzazione e la sua recente estensione a sistemi interagenti, chiamata Many-Body localization: vengono spiegate le principali caratteristiche e implicazioni di questi fenomeni, introducendo i principali indicatori utilizzati per distinguere fasi localizzate da fasi ergodiche. Infine,nel terzo e nel quarto capitolo viene presentato e analizzato il modello di Kitaev unidimensionale disordinato con decadimento a lungo raggio del termine di pairing, oggetto del nostro studio. I risultati numerici presentati consistono principalmente in un’analisi spettrale delle autofunzioni dell’Hamiltoniana del nostro modello e in un’indagine sulle proprietà di entanglement, dimostrando l’esistenza di una fase localizzata.
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Tusch, Michael Andreas. « Interplay between disorder and electron interactions : theoretical studies of an Anderson-Hubbard model ». Thesis, University of Oxford, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.359510.
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O'Neal, Jared. « A Numerical Study of a Disorder-driven 2D Mott Insulator-to-Metal Quantum Phase Transition ». The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1492701913534985.
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Wan, Xuewen. « Effects of longitudinal disorder on the magnetic field distribution in bismuth strontium calcium copper oxide ». W&M ScholarWorks, 2002. https://scholarworks.wm.edu/etd/1539623413.
Texte intégralRésumé :
Transverse Field muon spin relaxation (TF-muSR) experiments were performed in external magnetic fields 1.0, 2.7, 3.0, 4.5, 5.5, 6.0 and 7.0 T along the Bi2212 crystalline c-axis. For the first time, the heterodyned fitting analysis technique shows that the field profiles on the ab basal planes of single crystal Bi2212 are symmetric in all experimental fields 1.0--7.0 T and at all experimental temperatures 2.0-90.0 K. The muon spin relaxation rates due to the mixed state of Bi2212 were found to increase linearly from 0 mus-1 at the transition temperature, 90.0 K, to about 1.0 mus -1 at the lowest temperature, 2.0 K. The relaxation rates have much less field dependence than the temperature dependence and the field dependence of the relaxation rate is of opposite sign to that seen for YBCO, which is undoubtedly due to vortex lattice disorder caused by the weak coupling between the CuO planes. The scaled magnetic field penetration depths ll0 were found to be independent of magnetic field B in the temperature range 0--50.0 K. Fitting ll0 by currently available models was attempted. A proposed pancake vortex disorder model strongly suggests pancake disordering at all temperatures including 2.0 K, the lowest temperature reached in our experiment. Our experiments and the computer simulation from the pancake vortex disorder model showed that muSR data in this temperature and field range are attributed to the 2-D anisotropic vortex characteristics of Bi2212.
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Gambino, Davide. « Structural and magnetic disorder in crystalline materials : a first principles study ». Licentiate thesis, Linköpings universitet, Teoretisk Fysik, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-156835.
Texte intégralRésumé :
Disorder in crystalline materials can take different forms and originate from different sources. In particular, temperature introduces disorder in any kind of material. This can be observed as the appearance of vacant lattice sites in an otherwise perfect crystal, or as a random distribution of different elements on the same lattice in an alloy; at the same time, if the material is magnetic, temperature induces disorder also on the magnetic degrees of freedom. In this thesis, different levels of disorder associated to structure and magnetism are investigated by means of density functional theory and thermodynamic models. I start with diffusion of Ti vacancies in TiN, which is studied by means of nonequilibrium ab initio molecular dynamics using the color diffusion algorithm at different temperatures. The result is an Arrhenius behavior of Ti vacancy jump rates. A method to perform structural relaxations in magnetic materials in their hightemperature paramagnetic phase is then developed based on the disordered local moments approach in order to study vacancies, interstitial atoms, and combinations of defects in paramagnetic bcc Fe and B1 CrN, as well as the mixing enthalpy of bcc Fe1−xCrx random alloys. A correction to the energetics of every system due to the relaxation in the disordered magnetic state is observed in all cases. Not related to temperature and disorder, but very important for an accurate description of magnetic materials, is the choice of the exchange and correlation functional to be employed in the first principles calculations. We have investigated the performance of a recently developed meta-GGA functional, the strongly constrained and appropriately normed (SCAN) functional, in comparison with the more commonly used LDA and PBE on the ferromagnetic elemental solids bcc Fe, fcc Ni, and hcp Co, and SCAN it is found to give negligible improvements, if not a worsening, in the description of these materials. Finally, the coupling between vibrational and magnetic degrees of freedom is discussed by reviewing the literature and proposing an investigation of the influence of vibrations on longitudinal spin fluctuations. These excitations are here studied by means of thermodynamic models based on Landau expansion of the energy in even powers of the magnitude of the local magnetic moments. We find that vibrational and magnetic disorder alter the energy landscapes as a function of moment size also in bcc Fe, which is often considered a Heisenberg system, inducing a more itinerant electron behavior.
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Parisen, Toldin Francesco. « Critical behaviour of magnetic systems in the presence of quenched random disorder ». Doctoral thesis, Scuola Normale Superiore, 2007. http://hdl.handle.net/11384/85889.
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Sutherland, Gregory J. « Characterization of Order-Disorder Phase Transition Temperature for Select Nanoparticles ». BYU ScholarsArchive, 2015. https://scholarsarchive.byu.edu/etd/5592.
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A method was found for creating ordered nanoparticles whose size and theoretical order-disorder temperature are ideal for study in the TEM. Specifically FePt, NiPt, FeNiPt and AuCu nanoparticles were studied. We were able to show how a nanoparticle's size affects its order-disorder temperature (Tod). When the particles were around 6 nm in diameter there was a shift downward of the Tod of 10-15 percent compared to the bulk. While particles around 10 nm in diameter experienced a downward shift of 0-6 percent compared to the bulk. One can approximate that particles less than 10-15 nm in diameter would show significant decreases in order-disorder temperature. We confirmed that alumina prevents copper losses, compositions were well within percent error. In addition we showed that when the alumina used is thin enough the images are not adversely affected and charging is not an issue.
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Loureiro, Bruno. « Disorder in holographic field theories : inhomogeneous geometries, momentum relaxation and SYK models ». Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/277911.
Texte intégralRésumé :
Holographic dualities are now an established tool in the study of universal properties of strongly coupled field theories. Yet, theories without translational symmetry are still poorly understood in this context. In this dissertation, we investigate three new approaches to this challenging problem. The first part of the dissertation concerns a class of phenomenological holographic models in which momentum relaxation can be achieved without breaking translational symmetry in the dual geometry. In particular, we focus on an example in which the dual geometry is similar to anti-de Sitter (AdS) Brans-Dicke theory. We study the thermodynamic and transport properties of the model and show that for strong momentum relaxation and low temperatures the model has insulator-like behaviour. In the second part, we go beyond the effective description and consider holographic theories which explicitly break translational symmetry. From the perspective of gravity, these theories translate to geometries that vary explicitly in the boundary space-like coordinates. We refer to these geometries as 'inhomogeneous' and investigate two approaches to study them. The first is motivated by the question: "what happens to a homogeneous geometry when coupled with a field varying randomly in space?". Starting from an AdS geometry at zero or finite temperature, we show that a spatially varying random Maxwell potential drives the dual field theory to a non-trivial infra-red fixed point characterised by an emerging scale invariance. Thermodynamic and transport properties of this disordered ground state are also discussed. The second is motivated by the complementary question: "how does a random geometry affect a probe field?". In the weak disorder limit, we show that disorder induces an additional power-law decay in the dual correlation functions. For certain choices of geometry profile, this contribution becomes dominant in the infra-red, indicating the breaking of perturbation theory and the possible existence of a phase transition induced by disorder. The third and last part of this dissertation switches from the gravity to the field theoretical side of the duality. We discuss the Sachdev-Ye-Kitaev (SYK) model, a disordered many-body model with distinctive black hole-like properties. We provide analytical and numerical evidence that these holographic properties are robust against a natural one-body deformation for a finite range of parameters. Outside this interval, this system undergoes a chaotic-integrable transition.
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Chapman, Brandon D. « The role of disorder in structural phase transitions in perovskite ferroelectrics / ». Thesis, Connect to this title online ; UW restricted, 2003. http://hdl.handle.net/1773/9692.
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Kissavos, Andreas. « Development and application of Muffin-Tin Orbital based Green’s function techniques to systems with magnetic and chemical disorder ». Doctoral thesis, Linköpings universitet, Teoretisk Fysik, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-8231.
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Accurate electronic structure calculations are becoming more and more important because of the increasing need for information about systems which are hard to perform experiments on. Databases compiled from theoretical results are also being used more than ever for applications, and the reliability of the theoretical methods are of utmost importance. In this thesis, the present limits on theoretical alloy calculations are investigated and improvements on the methods are presented. A short introduction to electronic structure theory is included as well as a chapter on Density Functional Theory, which is the underlying method behind all calculations presented in the accompanying papers. Multiple Scattering Theory is also discussed, both in more general terms as well as how it is used in the methods employed to solve the electronic structure problem. One of the methods, the Exact Muffin-Tin Orbital method, is described extensively, with special emphasis on the slope matrix, which energy dependence is investigated together with possible ways to parameterize this dependence. Furthermore, a chapter which discusses different ways to perform calculations for disordered systems is presented, including a description of the Coherent Potential Approximation and the Screened Generalized Perturbation Method. A comparison between the Exact Muffin-Tin Orbital method and the Projector Augmented-Wave method in the case of systems exhibiting both compositional and magnetic disordered is included as well as a case study of the MoRu alloy, where the theoretical and experimental discrepancies are discussed. The thesis is concluded with a short discussion on magnetism, with emphasis on its computational aspects. I further discuss a generalized Heisenberg model and its applications, especially to fcc Fe, and also present an investigation of the competing magnetic structures of FeNi alloys at different concentrations, where both collinear and non-collinear magnetic structures are included. For Invar-concentrations, a spin-flip transition is found and discussed. Lastly, I discuss so-called quantum corrals and possible ways of calculating properties, especially non-collinear magnetism, of such systems within perturbation theory using the force theorem and the Lloyd’s formula.
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Narayanan, Rajesh. « Influence of rare regions on the critical properties of systems with quenched disorder / ». view abstract or download file of text, 1999. http://wwwlib.umi.com/cr/uoregon/fullcit?p9948028.
Texte intégralRésumé :
Thesis (Ph. D.)--University of Oregon, 1999.Typescript. Includes vita and abstract. Includes bibliographical references (leaves 165-166). Also available for download via the World Wide Web; free to University of Oregon users. Address: http://wwwlib.umi.com/cr/uoregon/fullcit?p9948028.
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Pinheiro, Fernanda. « Multi-species systems in optical lattices : From orbital physics in excited bands to effects of disorder ». Doctoral thesis, Stockholms universitet, Fysikum, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-116436.
Texte intégralRésumé :
In this thesis we explore different aspects of the physics of multi-species atomic systems in optical lattices. In the first part we will study cold gases in the first and second excited bands of optical lattices - the p and d bands. The multi-species character of the physics in excited bands lies in the existence of an additional orbital degree of freedom, which gives rise to qualitative properties that are different from what is known for the systems in the ground band. We will introduce the orbital degree of freedom in the context of optical lattices and we will study the many-body systems both in the weakly interacting and in the strongly correlated regimes. We start with the properties of single particles in excited bands, from where we investigate the weakly interacting regime of the many-body p- and d-orbital systems in Chapters 2 and 3. This presents part of the theoretical framework to be used throughout this thesis, and covers part of the content of Paper I and of Preprint II. In Chapter 4, we study Bose-Einstein condensates in the p band, confined by a harmonic trap. This includes the finite temperature study of the ideal gas and the characterization of the superfluid phase of the interacting system at zero temperature for both symmetric and asymmetric lattices. This material is the content of Paper I. We continue with the strongly correlated regime in Chapter 5, where we investigate the Mott insulator phase of various systems in the p and d bands in terms of effective spin models. This covers the results of Paper II, of Preprint I and parts of Preprint II. More specifically, we show that the Mott phase with a unit filling of bosons in the p and in the d bands can be mapped, in two dimensions, to different types of XYZ Heisenberg models. In addition, we show that the effective Hamiltonian of the Mott phase with a unit filling in the p band of three-dimensional lattices has degrees of freedom that are the generators of the SU(3) group. Here we discuss both the bosonic and fermionic cases. In the second part, consisting of Chapter 6, we will change gears and study effects of disorder in generic systems of two atomic species. This is the content of Preprint III, where we consider different systems of non-interacting but randomly coupled Bose-Einstein condensates in 2D, regardless of an orbital degree of freedom. We characterize spectral properties and discuss the occurrence of Anderson localization in different cases, belonging to the different chiral orthogonal, chiral unitary, Wigner-Dyson orthogonal and Wigner-Dyson unitary symmetry classes. We show that the different properties of localization in the low-lying excited states of the models in the chiral and the Wigner-Dyson classes can be understood in terms of an effective model, and we characterize the excitations in these systems. Furthermore, we discuss the experimental relevance of the Hamiltonians presented here in connection to the Anderson and the random-flux models.At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Accepted. Paper 5: Submitted.
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Marten, Tobias. « Ab-initio study of disorder broadening of core photoemission spectra in random metallic alloys ». Thesis, Linköping University, The Department of Physics, Chemistry and Biology, 2004. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-2580.
Texte intégralRésumé :
Ab-initio results of the core-level shift and the distribution about the average for the 3d5/2 electrons of Ag, Pd and 2p3/2 of Cu are presented for the face-centered-cubic AgPd and CuPd random alloys. The complete screening model, which includes both initial and final states effects in the same scheme, has been used in the investigations.
The alloys have been modeled with a supercell containing 256 atoms. Density-functional theory calculations are carried out using the locally self consistent Green's function approach.
Results from the calculations clearly shows that the core-level shift distributions characteristic is Gaussian, but the components reveals a substantial difference in the FWHM (Full-Width at Half-Maximum). Comparison between the experimental and the calculated broadening shows a remarkable agreement.
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Asker, Christian. « Effects of disorder in metallic systems from First-Principles calculations ». Doctoral thesis, Linköpings universitet, Teoretisk Fysik, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-53584.
Texte intégralRésumé :
In this thesis, quantum-mechanical calculations within density-functional theory on metallic systems are presented. The overarching goal has been to investigate effects of disorder. In particular, one of the properties investigated is the bindingenergy shifts for core electrons in binary alloys using different theoretical methods. These methods are compared with each other and with experimental results. One such method, the so-called Slater-Janak transition state method relies on the assumption that the single-particle eigenvalues within density-functional theory are linear functions of their respective occupation number. This assumption is investigated and it is found that while the eigenvalues to a first approximation show linear behavior, there are also nonlinearities which can influence the core-level binding energy shifts. Another area of investigation has been iron based alloys at pressures corresponding to those in the Earth’s inner core. This has been done for the hexagonal close packed and face entered cubic structures. The effects of alloying iron with magnesium and nickel on the equation of state as well on the elastic properties have been investigated. The calculations have shown that the hexagonal close packed structure in FeNi is more isotropic than the face-centered cubic structure, and that adding Mg to Fe has a large impact on the elastic properties. Finally, the effects of disorder due to thermal motion of the atoms have been investigated through ab-initio molecular dynamics simulations. Within the limits of this method and the setup, it is found that the face-centered cubic structure of molybdenum can be dynamically stabilized at high temperature, leading to a metastable structure, on the average. The dynamical stabilization of face-centered cubic molybdenum also rendered it possible to accurately calculate the lattice stability relative to the body-centered cubic phase. Inclusion of temperature effects for the lattice stability using ab-initio molecular dynamics simulations resolves the disagreement between ab-initio calculations and thermochemical methods.
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Fulkerson, Matthew D. « Gas Sensor Array Modeling and Cuprate Superconductivity From Correlated Spin Disorder ». The Ohio State University, 2002. http://rave.ohiolink.edu/etdc/view?acc_num=osu1023379849.
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Dupont, Maxime. « Dynamics and disorder in quantum antiferromagnets ». Thesis, Toulouse 3, 2018. http://www.theses.fr/2018TOU30092/document.
Texte intégralRésumé :
La physique de la matière condensée, et notamment les systèmes fortement corrélés, amènent à des problèmes parmi les plus stimulants et difficiles de la physique moderne. Dans ces systèmes, les interactions à plusieurs corps et les corrélations entre les particules quantiques ne peuvent être négligées, sinon, les modèles échoueraient simplement à capturer les mécanismes physiques en jeu et les phénomènes qui en découlent. En particulier, le travail présenté dans ce manuscrit traite du magnétisme quantique et aborde plusieurs questions distinctes à l'aide d'approches computationnelles et méthodes numériques à l'état de l'art. Les effets conjoints du désordre (i.e. impuretés) et des interactions sont étudiés concernant un matériau magnétique spécifique : plutôt qu'une phase de la matière dite localisée, attendue à fort champ magnétique, une phase ordonnée induite par le désordre lui-même est mise en lumière, avec une réapparition inattendue de la cohérence quantique dans ledit composé. Par ailleurs, la réponse dynamique d'aimants quantiques à une perturbation externe, comme celle mesurée dans des expériences de résonance magnétique nucléaire ou de diffusion inélastique de neutrons est étudiéeCondensed matter physics, and especially strongly correlated systems provide some of the most challenging problems of modern physics. In these systems, the many-body interactions and correlations between quantum particles cannot be neglected; otherwise, the models would simply fail to capture the relevant physics at play and phenomena ensuing. In particular, the work presented in this manuscript deals with quantum magnetism and addresses several distinct questions through computational approaches and state-of-the-art numerical methods. The interplay between disorder (i.e. impurities) and interactions is studied regarding a specific magnetic compound, where instead of the expected many-body localized phase at high magnetic fields, a novel disorder-induced ordered state of matter is found, with a resurgence of quantum coherence. Furthermore, the dynamical response of quantum magnets to an external perturbation, such as it is accessed and measured in nuclear magnetic resonance and inelastic neutron scattering experiments is investigated
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Roshi, Aleksander. « Quenched Random Disorder Studies In Liquid Crystal + Aerosil Dispersions ». Link to electronic thesis, 2005. http://www.wpi.edu/Pubs/ETD/Available/etd-042705-123130/.
Texte intégralRésumé :
Dissertation (Ph.D.) -- Worcester Polytechnic Institute.Keywords: smectic-A to smectic-C; nematic to smectic-A; isotropic-nematic; phase transition; quenched random disorder; liquid crystal; gel structure; turbidity; gel dynamics; x-ray intensity fluctuation spectroscopy ( XIFS ); ac-calorimetry; x-ray diffraction Includes bibliographical references (p. 210-218).
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Knight, Christopher J. « Hydrogen bond topology order/disorder transitions in ice and the behavior of defects in a disordered ice lattice / ». Columbus, Ohio : Ohio State University, 2009. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1236788109.
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Sighinolfi, Giovanni. « Exploring brain network features in borderline personality disorder : a graph-based analysis of MR images ». Master's thesis, Alma Mater Studiorum - Università di Bologna, 2020. http://amslaurea.unibo.it/21714/.
Texte intégralRésumé :
A network approach to the modelling and the analysis of functional and structural Magnetic Resonance (MR) images is an increasingly popular technique, because of the solidity of the mathematical theory at its basis, the graph theory, which allows to explore a wide range of network properties. For this work, both Functional Connectivity (FC) and Structural Covariance (SC) networks were constructed.
The cohort of participants to this study was composed of 27 subjects affected by the Borderline Personality Disorder (BPD), a mental disorder causing behavioral and emotional dysregulation, and a matching group of 28 healthy controls.
Brain networks were analyzed using the methods provided by graph theory, both at global and nodal level, by exploring their topological and organizational properties mainly in terms of centrality, efficiency in information transfer and modularity. The outcomes obtained from such measures, in patients and controls separately, were compared in order to find statistically significant differences between the two groups, that may be characteristic of the disease. Additionally, the outcomes of the topological quantities were correlated with a series of clinical scores, evaluating the neuro-psychological condition of the subjects.
The results show significant differences between patients and controls mostly in the FC networks and especially located in the limbic system of the brain, which indeed has a fundamental role in emotion regulation. Node-specific variations tend to involve the amygdala, the caudal anterior cingulate cortex, the entorhinal cortex and the temporal pole. Such evident results were not retrieved from the SC networks, though they still supported a greater variability within the limbic system.
Therefore, the analysis of brain graphs allowed to achieve the detection of topological alterations in a young psychiatric population, which would be interesting to monitor in time.
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Song, Yi. « AC conductivity and dielectric constant of systems near the percolation threshold / ». The Ohio State University, 1986. http://rave.ohiolink.edu/etdc/view?acc_num=osu148726754698361.
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Dang, Mei-Zhen. « Interplay of spin structures, hyperfine magnetic field distributions and chemical order-disorder phenomena in face centered cubic Fe-Ni alloys studied by Mossbauer spectroscopy measurements and Monte Carlo simulations ». Thesis, University of Ottawa (Canada), 1996. http://hdl.handle.net/10393/10020.
Texte intégralRésumé :
The magnetic properties of fcc Fe-Ni alloys are studied by Mossbauer spectroscopy and Monte Carlo (MC) simulations. Both macroscopic (magnetization, paraprocess susceptibility, Curie points, etc.) and microscopic properties (hyperfine fields) are used to test simple local moment models under various assumptions. A non-linear composition dependence of the average hyperfine field is observed by Fe-57 Mossbauer spectroscopy. A microscopic vector hyperfine field model is proposed and used to model the measured average hyperfine fields and hyperfine field distributions (HFDs) in the collinear ferromagnetic Fe-Ni alloys (y $\le$ 0.45 in Fe$\sb{y}$Ni$\sb{1-y}).$ Modeling the liquid helium temperature average hyperfine fields and HFDs resolves the coupling parameters in the proposed hyperfine field model:$$\langle\vec H\sb{k}\rangle\sb{T}=A\langle\vec\mu\sb{k}\rangle\sb{T}+ B\sum\sb{j}\langle\vec\mu \sb{j}\rangle\sb{T}.$$To the extent that chemical short range order can be neglected in our rapidly quenched samples, the coupling parameters are $\rm A=A\sb0+A\sb1y\ (A\sb0=89$ kOe/$\mu\sb{B},$ A$\sb1={-}20$ kOe/$\mu\sb{B})$ and B = B$\rm\sb0=B\sb1y\ (B\sb0=4.4$ kOe/$\mu\sb{B},$ B$\sb1=3.2$ kOe/$\mu\sb{B}).$ MC simulations show the success and the limits of a simple local moment model, in characterizing the bulk magnetic properties of Fe-Ni. A new approach for simulating HFDs is developed. It combines MC simulation for the spin structure and the above phenomenological hyperfine field model for the site-specific hyperfine field values. Using this method, we calculated spin structures and HFDs in Fe-Ni alloys at different compositions and temperatures. Finally, interplay between the magnetic and the atomic ordering phenomena is studied in FeNi$\sb3,$ FeNi and Fe$\sb3$Ni, by considering the magnetic and chemical interactions simultaneously using MC simulations. Serveral new features that are not predicted by mean-field theory or MC simulations with chemical interactions only arise: (1) chemical order can be induced where using chemical interactions only leads to the prediction of no chemical order (2) chemical segregation can be induced where using chemical interactions only leads to the prediction of no chemical segregation, (3) FeNi$\sb3$ and Fe$\sb3$Ni are found to have significantly different chemical ordering temperatures where chemical interactions only lead to equal ordering temperatures, (4) chemical ordering temperatures are significantly shifted from their chemical interactions only values, even when the chemical ordering temperature is larger than the magnetic ordering temperature, (5) abrupt steps can occur in the spontaneous magnetization at the chemical ordering temperature, when the latter is smaller than the magnetic ordering temperature, and (6) nonlinear relations arise between the chemical ordering temperature and the chemical exchange parameter U $\equiv$ 2U$\sb{FeNi}-{\rm U}\sb{FeFe}-{\rm U}\sb{NiNi},$ where the U$\sb{ij}$ are the near-neighbour pair-wise chemical bonds.