Physical Sciences and Mathematics Commons^™

Topological States In Matter, Hasitha W. Suriya Arachchige

Doctoral Dissertations

Topologically nontrivial spin textures, mesoscopic spin configurations that cannot be continuously transformed to an elementary magnetic configuration such as a ferromagnet or antiferromagnet, are of interest due to their ability to exhibit magnetic solitons, with topological protection. Such properties have the potential for applications in future data storage and communication devices. For example, spin textures found in materials such as MnSi, Cu2OSe3, Co-Zn-Mn alloys, and GaV4S8, commonly known as skyrmions, are driven by the interplay of atomic-scale exchange interactions, single-ion anisotropy, and an applied magnetic field. Of particular importance to this class of materials is the presence of a Dyaloshinski …

The Effect Of Illumination On The Conformation And Thermodynamics Of Conjugated Polymers, Brian Frank Morgan

Doctoral Dissertations

This dissertation explores the effects which illumination has on the structure and conformation of conjugated polymers, both in solution and the bulk. Conjugated polymers are an important class of optoelectronic polymeric materials which make up the active layer of key organic electronic devices such as organic photovoltaics, transistors, and light emitting diodes. Considering the strong link between polymer chain structure and device efficiency in these materials, a comprehensive understanding of certain experimental conditions which may influence this conformation and thusly alter the functionality of the devices predicated upon them is of vital importance.

The first part of this dissertation provides …

Characterizing Local Order And Physical Properties Of Rare Earth Complex Oxides, Thomas Jacob Shamblin

Doctoral Dissertations

With more than 500 compositions, materials possessing the pyrochlore structure have a myriad of technological applications and physical phenomena. Three of the most noteworthy properties are the structure’s ability to resist amorphization making it a possible host matrix for spent nuclear fuel, its exotic magnetic properties arising from geometric frustration, and fast ionic conductivity for solid-oxide fuel cell applications. This work focuses on these three aspects of the pyrochlore’s many potential uses. Structural characterization revealed that pyrochlore-type oxides have a tendency to disorder from a high symmetry cubic structure to a lower symmetry orthorhombic arrangement in response to a variety …

Neutron Scattering Studies Of Phosphate Proton Conductors, Amal Bajes Al-Wahish

Doctoral Dissertations

Proton ceramic fuel cells operating in the intermediate temperature range of 300-500 °C offer potentially revolutionary advantages over existing fuel cells because expensive noble metal catalysts would not be needed, and in situ reforming of liquid bio-fuels such as ethanol or methanol would be possible.The chief obstacle facing intermediate fuel cells is the lack of a suitable electrolyte in the operating temperature range. A good electrolyte is thermally and chemically stable, inexpensive, environmentally friendly, and has a proton conductivity on the order of 10^-2 S cm^-1 [Siemens per centimeter] at 400 °C. Acceptor-doped lanthanum orthophosphate is an …

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 …

Structure And Dynamics Of High Temperature Superconductors, Jennifer Lynn Niedziela

Doctoral Dissertations

High temperature superconductivity in iron based compounds has presented a series of complex problems to condensed matter physics since being discovered in 2008. The stalwart basis of condensed matter physics is the “strength in numbers" aspect of crystalline periodicity. Perfect crystalline periodicity has made possible the reduction of the questions of structural and electronic properties to single dimensions, increasing the tractability of these problems. Nevertheless, modern complex materials stretch these assumptions to their limits, and it is at this point where our work starts. Using neutron and x-ray scattering, we have conducted a series of studies on the structural disorder …

Neutron Scattering Study Of The High Tc Superconductors, Jun Zhao

Doctoral Dissertations

We carried out systematic neutron scattering experiments to investigate the magnetic properties and their relationship to the high-$T_c$ superconductivity, when the materials are tuned from their antiferromagnetic (AF) parent compounds to the superconducting regime.

We observed resonance mode in the electron doped cuprate Nd$_{1.85}$Ce$_{0.15}$CuO$_4$, demonstrating that the resonance is a general phenomenon in cuprate superconductors regardless of hole- or electron-doping. In Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$, the local susceptibility displays two distinct energy scales that are broadly consistent with the bosonic modes revealed by scanning tunneling microscopy experiments. These results indicate the presence of very strong electron spin excitations couplings in electron doped cuprates. …