<p> In the past three decades, spintronic devices have played an important technological role. Half-metallic alloys have drawn much attention due to their special properties and promised spintronic applications. This dissertation describes some theoretical techniques used in first-principal calculations of alloys that may be useful for spintronic device applications with an emphasis on half-metallic ferromagnets. I consider three types of simple spintronic materials using a wide range of theoretical techniques. They are (a) transition metal based half-Heusler alloys, like CrMnSb, where the ordering of the two transition metal elements within the unit cell can cause the material to be ferromagnetic semiconductors or semiconductors with zero net magnetic moment, (b) half-Heusler alloys involving Li, like LiMnSi, where the Li stabilizes the structure and increases the magnetic moment of zinc blende half-metals by one Bohr magneton per formula unit, and (c) zinc blende alloys, like CrAs, where many-body techniques improve the fundamental gap by considering the physical effects of the local field. Also, I provide a survey of the theoretical models and numerical methods used to treat the above systems.</p>

<p> Historically our understanding of the microscopic world has been impeded by limitations in systems that behave classically. Even today, understanding simple problems in quantum mechanics remains a difficult task both computationally and experimentally. As a means of overcoming these classical limitations, the idea of using a controllable quantum system to simulate a less controllable quantum system has been proposed. This concept is known as quantum simulation and is the origin of the ideas behind quantum computing. </p><p> In this thesis, experiments have been conducted that address the feasibility of using devices with a circuit quantum electrodynamics (cQED) architecture as a quantum simulator. In a cQED device, a superconducting qubit is capacitively coupled to a superconducting resonator resulting in coherent quantum behavior of the qubit when it interacts with photons inside the resonator. It has been shown theoretically that by forming a lattice of cQED elements, different quantum phases of photons will exist for dierent system parameters. In order to realize such a quantum simulator, the necessary experimental foundation must rst be developed. Here experimental eorts were focused on addressing two primary issues: 1) designing and fabricating low disorder lattices that are readily available to incorporate superconducting qubits, and 2) developing new measurement tools and techniques that can be used to characterize large lattices, and probe the predicted quantum phases within the lattice. </p><p> Three experiments addressing these issues were performed. In the rst experiment a Kagome lattice of transmission line resonators was designed and fabricated, and a comprehensive study on the effects of random disorder in the lattice demonstrated that disorder was dependent on the resonator geometry. Subsequently a cryogenic 3-axis scanning stage was developed and the operation of the scanning stage was demonstrated in the final two experiments. The rst scanning experiment was conducted on a 49 site Kagome lattice, where a sapphire defect was used to locally perturb each lattice site. This perturbative scanning probe microscopy provided a means to measure the distribution of photon modes throughout the entire lattice. The second scanning experiment was performed on a single transmission line resonator where a transmon qubit was fabricated on a separate substrate, mounted to the tip of the scanning stage and coupled to the resonator. Here the coupling strength of the qubit to the resonator was mapped out demonstrating strong coupling over a wide scanning range, thus indicating the potential for a scanning qubit to be used as a local quantum probe.</p>

<p> Silicon nitride passivation on AlGaN\GaN heterojunction devices can improve performance by reducing electron traps at the surface. This research studies the effect of displacement damage caused by 1 MeV electron irradiation as a function of the variation of passivation layer thickness and heterostructure layer variation on AlGaN/GaN HEMTs. The effects of passivation layer thickness are investigated at thicknesses of 0, 20, 50 and 120 nanometers on AlGaN\GaN test structures with either an AlN nucleation layer or a GaN cap structures which are then measured before and immediately after 1.0 MeV electron irradiation at fluences of 10¹⁶cm^-2. Hall system measurements are used to observe changes in mobility, carrier concentration and conductivity as a function of Si₃N₄ thickness. Models are developed that relate the device structure and passivation layer under 1 MeV radiation to the observed changes to the measured photoluminescence and deep level transient spectroscopy. A software model is developed to determine the production rate of defects from primary 1 MeV electrons that can be used for other energies and materials. The presence of either a 50 or 120 nm Si₃N₄ passivation layer preserves the channel current for both and appears to be optimal for radiation hardness.</p>

Thesis (Ph.D.)--Lehigh University, 2007. / Adviser: Himanshu Jain.

Thesis (P.D.)--Lehigh University, 2008. / Adviser: Nelson Tansu.

Thèse (M.Sc.A.)--Université de Sherbrooke (Canada), 2007. / Titre de l'écran-titre (visionné le 1 février 2007). In ProQuest dissertations and theses. Publié aussi en version papier.

Thesis (Ph. D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-11, Section: B, page: 6874. Adviser: Kenneth S. Schweizer. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007. / Source: Dissertation Abstracts International, Volume: 69-02, Section: B, page: 1071. Adviser: Sheldon Katz. Includes bibliographical references (leaves 117-123) Available on microfilm from Pro Quest Information and Learning.

<p> With first-principles calculations based on density functional theory, we can predict with good accuracy the electronic ground state properties of a fixed arrangement of nuclei in a molecule or crystal. However, the potential of this formalism and approach is not fully utilized; most calculations are performed on experimentally determined structures and stoichiometric substitutions of those systems. This in part stems from the difficulty of systematically generating 3D geometries that are chemically valid under the complex interactions existing in materials. Designing materials is a bottleneck for computational materials exploration; there is a need for systematic design tools that can keep up with our calculation capacity. Identifying a higher level language to articulate designs at the atomic scale rather than simply points in 3D space can aid in developing these tools. </p><p> Constituent atoms of materials tend to arrange in recognizable patterns with defined symmetry such as coordination polyhedra in transition metal oxides or subgroups of organic molecules; we call these structural motifs. In this thesis, we advance a variety of systematic strategies for understanding complex materials from structural motifs on the atomic scale with an eye towards future design. </p><p> In collaboration with experiment, we introduce the harmonic honeycomb iridates with frustrated, spin-anisotropic magnetism. At the atomic level, the harmonic honeycomb iridates have identical local geometry where each iridium atom octahedrally coordinated by oxygen hosts a <i>J_eff</i> = 1/2 spin state that experiences interactions in orthogonal spin directions from three neighboring iridium atoms. A homologous series of harmonic honeycomb can be constructed by changing the connectivity of their basic structural units. </p><p> Also in collaboration with experiment, we investigate the metal-organic chalcogenide assembly [AgSePh]&infin; that hosts 2D physics in a bulk 3D crystal. In this material, inorganic AgSe layers are scaffolded by organic phenyl ligands preventing the inorganic layers from strongly interacting. While bulk Ag₂Se is an indirect band gap semiconductor, [AgSePh]&infin; has a direct band gap and photoluminesces blue. We propose that these hybrid systems present a promising alternative approach to exploring and controlling low-dimensional physics due to their ease of synthesis and robustness to the ambient environment, contrasting sharply with the difficulty of isolating and maintaining traditional low-dimensional materials such as graphene and MoS₂. </p><p> Automated density functional theory via high throughput approaches are a promising means of identifying new materials with a given property. We automate a search for ferroelectric materials by integrating density functional theory calculations, crystal structure databases, symmetry tools, workflow software, and a custom analysis toolkit. Structural distortions that occur in the structural motifs of ferroelectrics give rise to a switchable spontaneous polarization. In ferroelectrics lattice, spin, and electronic degrees of freedom couple leading to exotic physical phenomena and making them technologically useful (e.g. non-volatile RAM). </p><p> We also propose a new neural network architecture that encodes the symmetries of 3D Euclidean space for learning the structural motifs of atomic systems. We describe how these networks can be used to speed up important components of the computational materials discovery pipeline and generate hypothetical stable atomic structures. </p><p> Finally, we conclude with a discussion of the materials design tools deep learning may enable and how these tools could be guided by the intuition of materials scientists.</p><p>

<p> The fractional quantum Hall effect (FQHE) in the half-filled second Landau level (filling factor &nu; = 5/2) offers new insights into the physics of exotic emergent quasi-particles. The FQHE is due to the collective interactions of electrons confined to two-dimensions, cooled to sub-Kelvin temperatures, and subjected to a strong perpendicular magnetic field. Under these conditions a quantum liquid forms displaying quantized plateaus in the Hall resistance and chiral edge flow. The leading candidate description for the FQHE at 5/2 is provided by the Moore-Read Pfaffian state which supports non-Abelian anyonic low-energy excitations with potential applications in fault-tolerant quantum computation schemes. The Moore-Read Pfaffian is the exact zero-energy ground state of a particular three-body Hamiltonian and explicitly breaks particle-hole symmetry. In this thesis we investigate the role of two and three body interaction terms in the Hamiltonian and the role of particle hole symmetry (PHS) breaking at &nu; = 5/2. We start with a PHS two body Hamiltonian (<i>H</i>₂) that produces an exact ground state that is nearly identical with the Moore-Read Pfaffian and construct a Hamiltonian H(&alpha;) = (1 &ndash; &alpha;)<i>H</i>₃ + &alpha; <i>H</i>₂ that tunes continuously between <i>H</i>₃ and <i> H</i>₂. We find that the ground states, and low-energy excitations, of <i>H</i>₂ and <i>H</i>₃ are in one-to-one correspondence and remain adiabatically connected indicating they are part of the same universality class and describe the same physics in the thermodynamic limit. In addition, evidently three body PHS breaking interactions are not a crucial ingredient to realize the FQHE at 5/2 and the non-Abelian quasiparticle excitations.</p><p>