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Spin-Lattice Coupling In The Iron-Pnictide High-Temperature Superconductors, Daniel E Parshall

Doctoral Dissertations

The recent discovery of the iron-pnictide superconductors has generated tremendous excitement, in part because there are many tantalizing similarities to the cuprate superconductors. As with the cuprates, it is strongly suspected that the spins contribute to superconductivity.

There seems to be a strong relationship between the lattice and magnetism in this system. Several authors have discussed the possibility of spin-phonon coupling, but direct experimental evidence has remained elusive.

This work discusses the relationship between the spins and the lattice in the $BaFe_{2}As_{2}$ family. We demonstrate the presence of negative thermal expansion in these materials, which is a strong indicator of …

Interplay Between Structure And Chemistry Of Materials And Their Physical Properties, Alaska Subedi

Doctoral Dissertations

First principles calculations provide a powerful tool for sorting out the interplay of chemical composition and structure with the physical properties of materials. In this dissertation, I discuss the physical properties and their microscopic basis within this framework for following illustrative examples. (i) The Zintl phase hydrides, where I find H is anionic and the formation of covalent sp2 bonds in the Al/Ga/Al-Si planes, which is a highly unusual bonding configuration for these elements. (ii) PbTe, which shows strong coupling between the longitudinal acoustic and transverse optic modes that may explain its low thermal conductivity. (iii) The double perovskites BiPbZnNbO6 …

Towards Simulations Of Binary Neutron Star Mergers And Core-Collapse Supernovae With Genasis, Reuben Donald Budiardja

Doctoral Dissertations

This dissertation describes the current version of GenASiS and reports recent progress in its development. GenASiS is a new computational astrophysics code built for large-scale and multi-dimensional computer simulations of astrophysical phenomena, with primary emphasis on the simulations of neutron star mergers and core-collapse supernovae. Neutron star mergers are of high interest to the astrophysics community because they should be the prodigious source of gravitation waves and the most promising candidates for gravitational wave detection. Neutron star mergers are also thought to be associated with the production of short-duration, hard-spectral gamma-ray bursts, though the mechanism is not well understood. In …

Ads/Cft Correspondence And Hydrodynamics Of Relativistic Heavy Ion Collisions, James Ethan Alsup

Doctoral Dissertations

The experiments performed at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab have discovered a state of matter called the strongly coupled quark-gluon plasma (sQGP). The strong coupling has limited the ability of the standard theory to describe such matter, namely Quantum Chromodynamics (QCD). However, string theory's anti-de Sitter/conformal field theory (AdS/CFT) correspondence has provided a new way to study the situation and in an analytical manner. So far, hydrodynamic properties of RHIC's plasma, such as elliptic flow and longitudinal expansion, have been seen to follow from classical supergravity calculations. In this dissertation I discuss some of the …

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. …

Theoretical Models For Wall Injected Duct Flows, Tony Saad

Doctoral Dissertations

This dissertation is concerned with the mathematical modeling of the flow in a porous cylinder with a focus on applications to solid rocket motors. After discussing the historical development and major contributions to the understanding of wall injected flows, we present an inviscid rotational model for solid and hybrid rockets with arbitrary headwall injection. Then, we address the problem of pressure integration and find that for a given divergence free velocity field, unless the vorticity transport equation is identically satisfied, one cannot find an analytic expression for the pressure by direct integration of the Navier-Stokes equations. This is followed by …

Investigations Of The Dynamical Response In Solids By Time-Dependent Density-Functional Theory, Qinghong Kou

Doctoral Dissertations

The present dissertation studies a joint theoretical-experimental investigation of the dynamical structure factor of wide-gap insulators, using lithium-fluoride as a prototype. Ground-state (energy bands and electron densities) was calculated using Linear Augmented Plane Wave (LAPW) method and local density approximation (LDA) of density functional theory (DFT). Ab-initio principal is applied to obtain a realistic description of the band structure, which is central to the current research in the condensed matter physics. Dynamical response function has been evaluated within time-dependent density functional theory (DFT) with an adiabatic approximation (TDLDA), for the exchange-correlation kernel. Our TDLDA spectra contain one adjustable parameter: a …