Transport Properties of the Infinite Dimensional Hubbard Model

Results for the optical conductivity and resistivity of the Hubbard model in infinite spatial dimensions are presented. At half filling we observe a gradual crossover from a normal Fermi-liquid with a Drude peak at $\omega=0$ in the optical conductivity to an insulator as a function of $U$ for temperatures above the antiferromagnetic phase transition. When doped, the ``insulator'' becomes a Fermi-liquid with a corresponding temperature dependence of the optical conductivity and resistivity. We find a $T^2$-coefficient in the low temperature resistivity which suggests that the carriers in the system acquire a considerable mass-enhancement due to the strong local correlations. At high temperatures, a crossover into a semi-metallic regime takes place.Comment: 14 page

High Performance Co-Polyimide Synthesis for Cosmic Radiation Shielding

Space radiation continues to be a problem for sustained space travel, harming humans and sensitive machinery. Overcoming this barrier is crucial to NASA’s goals for manned missions to asteroids and Mars. Therefore, more research is needed in order to safely protect our missions. Passive shielding through high performance polymers shows promise in providing lightweight shielding and strength for spacecraft. This thesis focused on the novel synthesis of aromatic, hydrogen rich co-polyimides. It was shown that through thermal co-polymerization, favorable physical and chemical properties increased due to the presence of branching and/or crosslinking chains in the polymer matrix

Phase Diagram of the Hubbard Model: Beyond the Dynamical Mean Field

The Dynamical Cluster Approximation (DCA) is used to study non-local corrections to the dynamical mean field phase diagram of the two-dimensional Hubbard model. Regions of antiferromagnetic, d-wave superconducting, pseudo-gapped non-Fermi liquid, and Fermi liquid behaviors are found, in rough agreement with the generic phase diagram of the cuprates. The non-local fluctuations beyond the mean field both suppress the antiferromagnetism and mediate the superconductivity.Comment: 4 pages, 5 eps figures, submitted to PR

Gap States in Dilute Magnetic Alloy Superconductors

We study states in the superconducting gap induced by magnetic impurities using self-consistent quantum Monte Carlo with maximum entropy and formally exact analytic continuation methods. The magnetic impurity susceptibility has different characteristics for T_{0} \alt T_{c0} and T_{0} \agt T_{c0} ($T_{0}$: Kondo temperature, $T_{c0}$: superconducting transition temperature) due to the crossover between a doublet and a singlet ground state. We systematically study the location and the weight of the gap states and the gap parameter as a function of $T_{0}/T_{c0}$ and the concentration of the impurities.Comment: 4 pages in ReVTeX including 4 encapsulated Postscript figure

Typical-medium, multiple-scattering theory for disordered systems with Anderson localization

The typical medium dynamical cluster approximation (TMDCA) is reformulated in the language of multiple scattering theory to make possible first principles calculations of the electronic structure of substitutionally disordered alloys including the effect of Anderson localization. The TMDCA allows for a systematic inclusion of non-local multi-site correlations and at same time provides an order parameter, the typical density of states, for the Anderson localization transition. The relation between the dynamical cluster approximation and the multiple scattering theory is analyzed, and is illustrated for a tight-binding model.Comment: 15 pages, 11 figure

Structural Steel Connection Design For Tensile Rupture by Advanced Inelastic Analysis

Connections are critical in structural steel buildings for transferring forces from member to member. Connections must be designed for safety and to ensure they serve their intended function. Many resources are available to engineers designing connections with common configurations and loads. But connection designers often encounter configurations and loading conditions for which there is little guidance. In these cases, design by advanced inelastic analysis can be advantageous. IDEA StatiCa is a steel connection design software for design by advanced inelastic analysis. In this software, some limit states are captured in the same manner as standard strength equations, while others are not. The net-section tensile rupture limit state is among the most basic limit states not captured using standard strength equations. It is not necessary to use standard strength equations in design by advanced inelastic analysis if the analysis provides a comparable or higher level of reliability. To date, no rigorous reliability analysis has been performed to show IDEA StatiCa, and the underlying component-based finite element method, provides a comparable or higher level of reliability than provided by the standard strength equations. Such a reliability analysis is performed in this work for the limit state of tensile rupture. Data from hundreds of previously published experimental results exhibiting tensile rupture in a variety of connection types were examined and analyzed. Strengths from both standard equations and IDEA StatiCa were compared to the experimentally obtained strengths and to each other. A reliability analysis based on Monte Carlo simulations was conducted using results from the strength comparisons. Additionally, the sensitivity of the IDEA StatiCa strength to mesh parameters and plastic strain limit was quantified. The results indicate that IDEA StatiCa does, in most cases, provide a comparable or higher level of reliability than the standard strength equations. Cases where it does not are identified and options for modifications are recommended. Documentation of the level of safety provided by IDEA StatiCa for the tensile rupture limit state presented in this work will bring confidence to the overall approach and enable the wider use of this helpful tool

Evaluative Assessment for NASA/GSFC Equal Opportunity Programs Office Sponsored Programs

The purpose of PREP (Pre-College Minority Engineering Program) is to upgrade skills of minority students who have shown an interest in pursuing academic degrees in electrical engineering. The goal is to upgrade skills needed for successful completion of the rigorous curriculum leading to a Bachelor of Science degree in engineering through a comprehensive upgrade of academic, study and interpersonal skills