Doctoral Dissertations

Date of Award

5-2015

Degree Type

Thesis

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

Takeshi Egami

Committee Members

Stephen Nagler, David Mandrus, Steven Johnston

Abstract

The world presents many natural and man-made crises and challenges that require scientific solutions. Condensed matter physics is one of the most influential and solution oriented disciplines in science. The field saw a significant rise in popularity especially during the past century as mankind enters the Information Age, when energy and computing related technologies become ubiquitous. This technological progress has been driven by efforts from scientists and engineers, through the synthesis, understanding, and implementation of new materials. Condensed matter physicists strive to solve puzzles at the frontier of material research. In the 21st century we have many advance tools at our disposal to help evaluate and solve these puzzles. For this thesis we focus on the forefront techniques of neutron scattering, along with other experimental methods to explore three frontier problems in condensed matter physics. The first problem is the investigation of phase transitions in Ru/Fe [Ruthenium/Iron] substituted PrFeAsO [Praseodymium Iron Arsenic Oxide]. PrFeAsO belongs to the 1111 family of iron pnictides, in which superconductivity can usually be induced by suppressing the magnetic and structural transitions via carrier doping. We have used neutron powder diffraction to investigate the effect of isoelectronic substitution on these transitions in PrFeAsO. Second is the study of the helimagnetic ordering in Cr [Chromium] doped FeGe [Iron Germanide]. Both CrGe [Chromium Germanide] and FeGe are in the B20 cubic structure, where CrGe exhibits no long range magnetic order down to at least 2 K, FeGe orders helimagnetically at 280 K with a periodicity of 700 Angstroms. We use small angle neutron scattering to study the Cr doping dependence of helimagnetism in FeGe. Finally, we examine the lattice dynamics in rocksalt structure compounds UC [Uranium Carbide] and US [Uranium Sulfide]. A recent inelastic neutron scattering experiment revealed quantum harmonic oscillator behavior of N [Nitrogen] atoms in UN [Uranium Nitride]. We deduce that other uranium rocksalts should also exhibit such behaviors. We use inelastic neutron scattering to extend the study on quantum harmonic oscillations in uranium rock salts.

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