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.
Results
Showing 1 to 20 of 35 for “"Heisenberg model"”.
-
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 …
-
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 …
-
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 …
-
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 …
-
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 …
-
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 …
-
Untersuchungen zu uniaxial anisotropen Heisenberg-Antiferromagneten in zwei Dimensionen
… two-dimensional Ising and anisotropic Heisenberg models have been studied. Ising models with mobile and pinned defects are shown to describe the formation and thermal destruction of defect stripes. Modelling a specific "telephone number compound", a realistic Heisenberg model with …
-
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 …
-
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 …
-
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 …
-
Spin-gap materials from first principles: properties and applications of half-metallic ferromagnets
… FLEUR code. These parameters are then used in a Heisenberg model which describes the system at finite temperatures. Besides applying the random phase and the mean-field approximation, we seek the finite temperature properties by Monte Carlosimulations. To describe the system NiMnSb properly, we …
-
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 …
-
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.
-
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 …
-
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 …
-
Ab initio investigations of magnetic properties of ultrathin transition metal films on 4d substrates
… The trends are explained on the basis of the Heisenberg model with exchange parameters whose sign and value change systematically as function of the band filling across the transition-metal series. The Heisenberg model was extended by a Stoner-like term to include the induced magnetization of …
-
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 …
-
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 …
-
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 …
Page 1 of 2