A First-Principles Computational Study of Structural and Elastic Properties of ZnO

The purpose of this study is to determine structural and mechanical properties of zinc oxide (ZnO) using first-principles computational methods. ZnO is a semiconductor widely used in many electronic and optical applications. ZnO is also economically and environmentally desirable – first, both the constituent elements are abundant on Earth and therefore inexpensive for large-scale applications; second, it is non- toxic. The most significant contribution of this study is the simulations of the high-pressure phases. These high-pressure simulations are important because the rock salt phase of ZnO obtained at high pressure can be recovered at ambient pressure, and this new structural phase possesses different properties that may be more useful for certain applications

McNair Research Journal - Summer 2015

Journal articles based on research conducted by undergraduate students in the McNair Scholars Program Table of Contents Biography of Dr. Ronald E. McNair Statements: Dr. Neal J. Smatresk, UNLV President Dr. Juanita P. Fain, Vice President of Student Affairs Dr. William W. Sullivan, Associate Vice President for Retention and Outreach Mr. Keith Rogers, Deputy Executive Director of the Center for Academic Enrichment and Outreach McNair Scholars Institute Staf

Modeling the Quantum Behavior of Hydrogen using Density Functional Theory, Quantum Monte Carlo, and Machine Learning

Computation and simulation play a central role in physics. As applied to atoms, molecules, and condensed-matter systems, this is based on quantum mechanics. Many calculations are only possible though due to approximations made. Two of particular interest in this dissertation are the consideration of static and classical ions (nuclei) and the use of pseudopotentials to replace the divergent Coulomb interaction between ions and electrons. For many systems, these approximately work or are expected to work well. There are others though where things are less clear. Dense hydrogen is one such system, and one that is both rich in physics and has the potential for significant practical applications.In this dissertation, the aforementioned approximations as made in perhaps the most-used computational method, density-functional theory, and as applied to dense hydrogen are investigated. This includes the effect of the pseudopotential approximation on the phase diagram and superconductivity of solid hydrogen and also the nuclear quantum states of molecular hydrogen adsorbed to a surface. For the latter, quantum Monte Carlo is used in a novel way that supplements density-functional theory. As a supplemental study, a reduction of the computational cost of density-functional theory, from $O(N^3_e)$ to $O(N_e)$ where $N_e$ is the number of electrons, is considered using machine learning. Altogether, these results are expected to be significant for computational physics, the field of dense hydrogen, and other fields such a

Phase-Transition Induced Elastic Softening and Band Gap Transition in Semiconducting PbS at High Pressure

We have investigated the crystal structure and phase stability, elastic incompressibility, and electronic properties of PbS based on high-pressure neutron diffraction, in-situ electrical resistance measurements, and first-principles calculations. The refinements show that the orthorhombic phase is structurally isotypic with indium iodide (InI) adopting a <i>Cmcm</i> structure (<i>B</i>33). The cubic-to-orthorhombic transition occurs at ∼2.1(1) GPa with a 3.8% volume collapse and a positive Clausius–Clapeyron slope. Phase-transition induced elastic softening is also observed, which is presumably attributed to the enhanced metallic bonding in the <i>B</i>33 phase. On the basis of band structure simulations, the cubic and orthorhombic phases are typical of direct and indirect semiconductors with band gaps of 0.47(1) and 1.04(1) eV, respectively, which supports electrical resistivity measurements of an abrupt jump at the structural transition. On the basis of the resolved structure for <i>B</i>33, the phase transition paths for <i>B</i>1→<i>B</i>33→<i>B</i>2 involve translation of a trigonal prism in <i>B</i>1 and motion of the next-nearest neighbor Pb atom into {SPb₇} coordination and subsequent lattice distortion in the <i>B</i>33 phase