Physical Sciences and Mathematics Commons^™

Numerical Study Of Hubbard Model For Iron-Based Superconductors, Qinlong Luo

Doctoral Dissertations

This thesis addresses the numerical and analytical study of the iron-based superconductors, mainly using the multi-orbital Hubbard model and Hartree-Fock approximation. The text starts with an introduction to the new iron-based superconductors, followed by a theoretical description of the multi-orbital Hubbard model and Hartree-Fock approximations. The numerical study of the Hubbard model in momentum space is described next, where the phase diagrams for three- and five-orbital Hubbard models are provided. The physically ``realistic" regime of couplings is highlighted, to guide future theoretical work into the proper region of parameters of Hubbard models for iron-based superconductors. What goes next is the …

A Holographic Model Of Striped Superconductors, Suman Ganguli

Doctoral Dissertations

One of the most prominent distinguishing features in strongly correlated electron systems, such as the high Tc (critical temperature) cuprates and the most recent iron pnictides, is the presence of "competing orders" that are related to the breaking of the lattice symmetries. Does the ubiquitous presence of such inhomogeneous orders in strongly correlated superconductors have a deep connection to superconductivity? The answer to this question is crucial for identifying the mechanism of superconductivity, at least in the cuprates. Amidst serious difficulties within conventional theoretical framework to deal with strongly interacting degrees of freedom at finite density, "AdS/CFT correspondence" or "gauge/gravity …

Magneto-Optical Properties Of Complex Oxides, Peng Chen

Doctoral Dissertations

Complex oxides give rise to rich physics and exotic cross-coupled electronic and magnetic properties. This is because of the competing interaction between charge, structure, and magnetism in the materials. In this dissertation I present a spectroscopic investigation of several model complex oxides under external stimuli of magnetic field and temperature. The compounds of interest include BiFeO₃ [bismuth ferrite] nanoparticles and tetragonal film, Bi_1-xNd_xFeO₃ [neodymium doped bismuth ferrite], α-Fe₂O₃ [hematite], Ni₃V₂O₈ [nickel vanadate], and RIn_1-xMn_xO₃ [manganese doped rare earth indium oxide]. …

Neutron Scattering Study Of The Iron Based Superconductors, Miaoyin Wang

Doctoral Dissertations

In most iron-based and copper-oxide superconductors, the T_c [superconducting critical temperature] gradually increases upon charge carrier doping or isovalent doping. In the under-doped regime of electron-doped BaFe₂As₂ [Barium Iron 2 Arsenic 2], the superconductivity appears before the complete suppression of AF [antiferromagnetic] long-range order, creating a SC-AF [superconducting-antiferromagnetic] coexisting area. The interplay between long-range magnetic order and superconductivity was studied using a triple-axis thermal neutron spectrometer under an in-plane magnetic field. The suppression of superconductivity and the enhancement of long-range AF order were discovered under 11 Tesla, suggesting the competing nature of these two phases and …

Complex Magnetism In Noncentrosymmetric Magnets, Nirmal Jeevi Ghimire

Doctoral Dissertations

Broken inversion symmetry in a crystal lattice allows an extra term called the Dzyaloshinskii-Moriya (DM) interaction in the magnetic Hamiltonian. The DM interaction tends to align spins in perpendicular orientation and therefore competes with the exchange interaction that favors collinear spins. This competition results in different modulated chiral magnetic structures depending on the relative strength of the two interactions. Skyrmion, soliton and magnetic blue phases are some of the anticipated structures. This dissertation extends the search for these exotic magnetic structures using various techniques to study the magnetism in the noncentrosymmetric magnets Cr₁₁Ge₁₉ [chromium eleven germanium nineteen], …

The Measurement Of The Dielectric Constant Of Concrete Pipes And Clay Pipes, David Mcgraw

Doctoral Dissertations

To optimize the effectiveness of the rehabilitation of underground utilities, taking in consideration limitation of available resources, there is a need for a cost effective and efficient sensing systems capable of providing effective, in real time and in situ, measurement of infrastructural characteristics. To carry out accurate non-destructive condition assessment of buried and above ground infrastructure such as sewers, bridges, pavements and dams, an advanced ultra-wideband (UWB) based radar was developed at Trenchless Technology Centre (TTC) and Centre for Applied Physics Studies (CAPS) at Louisiana Tech University (LTU). One of the major issues in designing the FCC compliant UWB radar …

Novel Bimetallic Plasmonic Nanomaterials, Ritesh Sachan

Doctoral Dissertations

Plasmonic nanomaterials have attracted a lot of attention recently due to their application in various fields such as chemical and biological sensing, catalysis, energy harvesting and optical devices. However, there is a need to address several outstanding issues with these materials, including cost-effective synthesis, tunability in plasmonic characteristics, and long term stability. In this thesis, we have focused on bimetallic nanoparticles (NPs) of Ag and Co due to their immiscibility as well as their individual properties. First, a pulsed laser induced dewetting route was used to synthesize Ag-Co bimetallic plasmonic NPs. An synthesis parameter space was derived to show the …

Haldane Pseudopotentials And Landau Level Mixing In The Quantum Hall Effect, Rachel Elizabeth Wooten

Doctoral Dissertations

The discovery of the quantum Hall effect in 1980 opened to physics one of the simplest systems for studying many-body correlations. Numerical techniques and trial wavefunctions have proven useful for describing the novel collective behavior of the electrons, but have not fully explained all features of the fractional quantum Hall effect. For example, it is predicted that Landau level mixing should have a moderate effect on the system for all but the very strongest magnetic fields, but this effect has not been extensively studied. Among the tools most useful to modeling and describing the quantum Hall system is the Haldane …

Alkane Adsorption On Mgo(100): Volumetric Isotherm, Inelastic Neutron Scattering, And Computational Studies, Andrew Spencer Hicks

Doctoral Dissertations

Volumetric adsorption isotherms and computational molecular dynamics (MD) simulations were performed for nonane and decane adfilms on MgO(100) nanocubes. From the isotherms, variety of thermodynamic quantities are calculated. These values, along with visual inspection of the isotherms, indicate a layer by layer trend from 2D to 3D behavior. This is attributed to the increasing importance of vertical adsorbate-adsorbate interactions as distance from the surface increases. Additionally, a 2D phase transition is observed for the first adsorbed layer as indicated by the evolution of the widths of the isotherm first derivative peaks. These experimental results are complemented by the MD calculations, …

Growth And Electric Field Control Of Phase Separated Manganites, Hangwen Guo

Doctoral Dissertations

Perovskite Manganites have received numerous attentions due to exotic behaviors such as colossal magnetoreistance (CMR) and electronic phase separation (EPS). The purpose of my research is to answer fundamental questions about the growth properties of manganites and electric field control of the EPS properties.

Experimental study was conducted on controlling the growth mode of La_0.7Sr_0.3MnO₃[Lanthanum Strontium Manganese Oxide] thin films using pulsed laser deposition. Different thin film morphology, crystallinity and stoichiometry have been observed depending on growth parameters. To understand the microscopic origin, the thermodynamic processes were theoretically analyzed and a growth diagram was …