Global ETD Search
Search theses and dissertations gathered from participating repositories worldwide. Every result links back to the library that holds it. No account is needed.
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Showing 1 to 20 of 28 for “"Heisenberg model"”.
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High-temperature critical indices for the classical anisotropic Heisenberg model
The classical anisotropic Heisenberg model is studied by means of high-temperature expansions. The purpose of the work is to determine (in the context of the model) how many phase transitions (as characterized by the critical exponents) there are, and which features of the dynamics and kinematics …
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Dynamics of vortices in the two-dimensional anisotropic Heisenberg model with magnetic fields.
… dynamics may be described in terms of a core model which takes into account a coupling between the vortex polarization dynamics and the motion of the vortex center. We showed that a reduced core model, which is valid near the threshold of the IP-OP vortex instability (lambda ~ lambda c), can …
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Beyond the isotropic two-dimensional Heisenberg model: Raman scattering studies of the transition between isotropic and anisotropic spin dynamics in Sr2IrO4
… mode shows behavior consistent with S=1/2 2D Heisenberg model. However, for H<1.5 T, it shows anisotropic behavior as a function of external field due to interlayer couling. Ths study showed that while for even for moderate field, the magentic behavior is consistent with suggested …
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Behavior of the time-dependent Heisenberg spin system Stein's method and physically interesting processes
This dissertation explores the mean field Heisenberg spin system and its evolution in time. We first study the system in equilibrium, where we explore the tool known as Stein's method, used for determining convergence rates to thermodynamic limits, both in an example proof for a mean field Ising …
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Exploring spin physics with ultracold atoms
… phenomena, but they also provide paradigmatic models with deep connections to high-T_c superconductivity, optimization problems, neural networks, and more. Experiments with ultracold alkali atoms in optical lattices have realized spin models with great success. In particular, the isotropic …
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Spin Dynamics in a Tunable Heisenberg Magnet Realized with Ultracold Atoms
… interactions. This system is described by the Heisenberg model, a paradigmatic model, which has been studied for almost a century now. But so far, experiments on spin dynamics have been mostly limited to isotropic spin-spin interactions. In this work we use ultracold atoms to implement the …
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Competition and unification of Kondo coherence and magnetic ordering
… across the transition. In the quest for a toy model to envision this simultaneous collapse, we study the Kondo-Heisenberg model on the honeycomb lattice at half-filling. We study a fixed point controlling the quantum phase transition of a Kondo coherent phase to a critical phase called …
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Realizing quantum spin models with ⁷Li atoms in an optical lattice
Quantum spin Hamiltonians are paradigmatic models, which display different kinds of quantum phase transitions, strongly-correlated and topological ground states, and various regimes of transport. Expanding their significance, many mappings exist between quantum spin models and other systems in …
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Magnetic super-exchange with ultra cold atoms in spin dependent optical lattices
… a system that behaves according to the Hubbard model. By loading the atoms into a state dependent lattice, it may be possible to explore the full phase space of the Heisenberg model and see magnetic super exchange-driven magnetic ordering in a variety of lattice geometries. The aim of this …
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On numerical methods in quantum spin systems
… solvable point in a phase diagram of an extended Heisenberg model on a kagome lattice. Chapter 6 studies a quantum Heisenberg model on the stuffed honeycomb lattice. Phase transitions in quantum many body systems manifest themselves as a reorganization of low energy spectra of the system. They …
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Magnetic surface relaxation and reconstruction phenomena in frustrated magnetic systems
… a phase diagram for a frustrated square lattice Heisenberg model. Regions of the phase diagram are found to exist in which the reconstruction is expected to occur, and furthermore the results can be mapped directly to type-1 FCC lattice systems.
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Local Magnetism in Strongly Correlated Electron Systems with Orbital Degrees of Freedom
… degrees of freedom through the use of effective models. My dissertation focuses on the sub-class of these materials where electrons appear to be localized by interactions, and magnetic ions have well measured magnetic moments. This suggests that we can capture the low-energy physics of the …
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Strongly coupled systems : from quantum antiferromagnets to unified models for superconductors
… the significance of the antiferromagnetic Heisenberg model (AFHM) in both high-energy and condensed-matter physics, and proceed to describe an efficient cluster algorithm used to simulate the AFHM. This is one of two algorithms with which my collaborators and I were able to obtain numerical …
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Chern-Simons theory of magnetization plateaus on the kagome lattice
… lattice using a spin-1/2 antiferromagnetic XXZ Heisenberg model in the presence of an external magnetic field as well as other perturbations. Such a system is expected to give rise to magnetization platueaus which can exhibit topological characteristics in certain regimes. We will first develop …
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Phases and phase transitions of strongly correlated electron systems
… 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 …
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Spin liquids : mean field and beyond
… diagram of different spin-1 generalizations of Heisenberg model [2]. For an SU(3)-invariant model with sufficiently strong ring-exchange terms, we find a d+id paired quantum spin liquid with a Fermi surface of deconfined spinons. Chapter 3 addressed the behavior of Kondo S = 1/2 impurity …
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Creating novel quantum states of ultracold bosons in optical lattices
… Next, I report on the realization of a spin-1 Heisenberg Hamiltonian which emerges as the low energy effective theory describing spin ordering in the doubly-occupied Mott insulator of two spin components. This integer spin Heisenberg model is qualitatively different from the half-integer spin …
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Multiscale approaches on two-dimensional magnetic materials: from atomistic to mesoscale level
… a multiscale approach involving atomistic scale models with Hubbard-U corrected Density Functional Theory (DFT+U) and mesoscale models with Monte-Carlo (MC) simulations. Firstly, we performed a high throughput screening over 150 compounds, identifying 59 potential two-dimensional magnets. To …
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Spectral and dynamical properties of disordered and noisy quantum spin models
… In the first part, the quantum random energy model (QREM), a mean-field spin glass model with a many-body localisation transition, is studied. In Chapter 2, we attempt a diagrammatic perturbative analysis of the QREM from the ergodic side, proceeding by analogy to the single-particle theory of …
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