<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>

xiii, 110 p. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / This dissertation investigates the interplay between, and the possible coexistence of, magnetic and superconducting order in metals. We start with studying the electromagnetic properties of s-wave superconductors near a ferromagnetic instability. By using a generalized Ginzburg-Landau theory and scaling arguments, we show that competition between magnetic order and superconducting order can change the scaling of observables. For instance, the exponent for the temperature dependence of the critical current can deviate from the Ginzburg-Landau value of 3/2. These results may be relevant to understanding the observed behavior of MgCNi 3 . We then study the nature of the superconductor-to-normal-metal transition in p-wave superconductors. Although the phase transition is continuous at a mean- field level, a more careful renormalization-group analysis in conjunction with large-n expansion techniques strongly suggest that the transition is first order. This conclusion is the same as for s-wave superconductors, where these techniques also predict a first-order transition. In p-wave superconductors, topological excitations known as skyrmions are known to exist in addition to the more common vortices. In the third part of this dissertation, we study the properties of skyrmion lattices in an external magnetic field. We propose iv experiments to distinguish vortex lattices from skyrmion lattices by means of their melting curves and their μSR signatures. / Adviser: Dietrich Belitz

xvi, 127 p. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / Brownian ratchets generate transport at the micron scale with the help of thermal motion. The Brownian ratchet studied here is the flashing ratchet which transports particles by switching on and off a spatially asymmetric, periodic potential. Experimental work in the literature indicates that interdigitated electrode arrays can been used to create such potentials in solution, but no detailed study of particle trajectories has accompanied such experiments. Here, interdigitated electrode array devices were fabricated. Analysis of the trajectories of individual particles moving in response to a switching voltage revealed that the transport is likely due to electroosmotically driven fluid flow, not the Brownian ratchet effect. Simulation work in the literature predicts that polymers in a ratchet potential will exhibit qualitatively different transport from the particle case. Here, polymer transport was tested experimentally using interdigitated electrode array devices, collecting images of individual à à à à à à » DNA molecules and applying a flashing voltage. The DNA was observed to move in response to the applied potential and the resulting images contain DNA trajectories and also information about its conformations and dynamics. Conformations were analyzed using principal components analysis, extracting the normal modes of the variations amongst large sets of polymer images. These results iv show no conformational changes indicative of the polymer ratchet mechanism, despite the polymer motion. This result and detailed analysis of the DNA trajectories, suggest that the observed motion was driven by bulk flow generated through electroosmosis, in agreement with results from experiments using particles in similar devices. Deterministic Lateral Displacement (DLD) uses an array of obstacles in a microfluidic channel to sort micron-scale objects with à à à à à ¢ à à à à à ¼10nm precision. However, very little work has been done to quantitatively address the role of diffusion in DLD sorting. Here, modeling of transport in DLD arrays has shown that using arrays of obstacles that are small compared to their separation can create sorting that is robust against changes in flow velocity. Also, novel sorting modes were revealed when the model was applied to unconventional array geometries that have not been discussed in the literature. / Adviser: Heiner Linke

abstract: Proton and fluorine diffusivity and ionic conductivity of 2-fluoropyridinium triflate (2-FPTf) and proton and fluorine diffusivity, ionic conductivity, and viscosity of trifluoromethanesulfonic acid (TFMSA) monohydrate have been measured over a wide range of temperatures. Diffusivities were measured using the pulsed-gradient spin-echo (PGSE) technique on a 300 MHz NMR spectrometer. Conductivities were measured using electrochemical impedance spectroscopy (EIS) on standard equipment and viscosities were determined using a Cannon-Ubbelohde viscometer. For 2-FPTF, the diffusivity of mobile protons increased from 1.84+/-0.06 x 10(-11) m2/s at 55 degC to 1.64+/-0.05 x 10(-10) m2/s at 115 degC while the diffusivity of 2-fluoropyridine fluorines increased from 2.22+/-0.07 x 10(-11) m2/s at 70 degC to 9.40+/-0.28 x 10(-11) m2/s at 115 degC. For TFMSA monohydrate, the diffusivity of protons increased from 7.67+/-0.23 x 10(-11) m2/s at 40 degC to 3.92+/-0.12 x 10(-10) m2/s at 110 degC while the diffusivity of fluorines increased from 4.63+/-0.14 x 10(-11) m2/s at 40 degC to 2.79+/-0.08 x 10(-10) m2/s at 110 degC, protons diffusing approximately 1.5 times faster than fluorines over the entire temperature range. NMR spectra indicate that proton diffusion occurs via direct hopping from TFMSA molecule to molecule. The conductivity of 2-FPTf varied from 0.85+/-0.03 mS/cm to 35.9+/-1.08 mS/cm between 25 and 110 degC. The conductivity of TFMSA monohydrate varied from 6.60+/-0.2 mS/cm to 84.6+/-2.5 mS/cm between 23 and 139 degC and its viscosity varied from 27.5+/-1.4 mPa.s to 4.38+/-0.22 mPa.s between 49 and 121.5 degC, in good agreement with literature values. Temperature dependences of the measured properties showed Arrhenius behavior with activation energies for proton diffusion, fluorine diffusion and ionic conduction for 2-FPTf above the melting point of 16.9+/-0.8 kJ/mol, 48.0+/-2.4 kJ/mol and 27.8+/-1.4 kJ/mol respectively. Activation energies for proton diffusion, fluorine diffusion, ionic conduction and viscosity for TFMSA monohydrate were 23.4+/-1.2 kJ/mol, 26.0+/-1.3 kJ/mol, 22.1+/-1.1 kJ/mol, and 26.9+/-1.3 kJ/mol respectively. The degree of dissociation of the charged species, calculated using the Nernst-Einstein relation, varied from 13 to 24% for 2-FPTf and from 25 to 29% for TFMSA monohydrate over the temperature range. / Dissertation/Thesis / Doctoral Dissertation Physics 2015

abstract: The work contained in this dissertation is focused on the optical properties of direct band gap semiconductors which crystallize in a wurtzite structure: more specifically, the III-nitrides and ZnO. By using cathodoluminescence spectroscopy, many of their properties have been investigated, including band gaps, defect energy levels, carrier lifetimes, strain states, exciton binding energies, and effects of electron irradiation on luminescence. Part of this work is focused on p-type Mg-doped GaN and InGaN. These materials are extremely important for the fabrication of visible light emitting diodes and diode lasers and their complex nature is currently not entirely understood. The luminescence of Mg-doped GaN films has been correlated with electrical and structural measurements in order to understand the behavior of hydrogen in the material. Deeply-bound excitons emitting near 3.37 and 3.42 eV are observed in films with a significant hydrogen concentration during cathodoluminescence at liquid helium temperatures. These radiative transitions are unstable during electron irradiation. Our observations suggest a hydrogen-related nature, as opposed to a previous assignment of stacking fault luminescence. The intensity of the 3.37 eV transition can be correlated with the electrical activation of the Mg acceptors. Next, the acceptor energy level of Mg in InGaN is shown to decrease significantly with an increase in the indium composition. This also corresponds to a decrease in the resistivity of these films. In addition, the hole concentration in multiple quantum well light emitting diode structures is much more uniform in the active region when Mg-doped InGaN (instead of Mg-doped GaN) is used. These results will help improve the efficiency of light emitting diodes, especially in the green/yellow color range. Also, the improved hole transport may prove to be important for the development of photovoltaic devices. Cathodoluminescence studies have also been performed on nanoindented ZnO crystals. Bulk, single crystal ZnO was indented using a sub-micron spherical diamond tip on various surface orientations. The resistance to deformation (the "hardness") of each surface orientation was measured, with the c-plane being the most resistive. This is due to the orientation of the easy glide planes, the c-planes, being positioned perpendicularly to the applied load. The a-plane oriented crystal is the least resistive to deformation. Cathodoluminescence imaging allows for the correlation of the luminescence with the regions located near the indentation. Sub-nanometer shifts in the band edge emission have been assigned to residual strain the crystals. The a- and m-plane oriented crystals show two-fold symmetry with regions of compressive and tensile strain located parallel and perpendicular to the ±c-directions, respectively. The c-plane oriented crystal shows six-fold symmetry with regions of tensile strain extending along the six equivalent a-directions. / Dissertation/Thesis / Ph.D. Physics 2013

Estudamos o efeito do campo aleatório sobre um modelo de vidro de spin com interações de p spins de alcance infinito e distribuição de probabilidade gaussiana. O caso p = 2 corresponde ao modelo de Sherrington-Kirkpatrick na presença de um campo aleatório. O caso p \'SETA\' \'INFINITO\' corresponde ao REM (Random Energy Model) de Derrida na presença de um campo aleatório. Além da interação de p spins, consideramos a presença de interações uniformes ferro ou antiferromagnéticas de alcance infinito. Tanto no caso ferro quanto antiferromagnético, empregamos dois procedimentos para tratar o problema: o método de réplicas no ensemble canônico e o método da contagem de estados no ensemble microcanônico. No método de réplicas resolvemos o problema para qualquer valor de p tanto sem quebra da simetria de permutação entre réplicas, quanto com um passo de quebra de simetria de Parisi. Deste modo, recuperamos resultados conhecidos para alguns modelos já estudados na literatura. Em seguida, tomamos o limite p \'SETA\' \'INFINITO\' que fornece uma solução completa para o problema do REM na presença de um campo aleatório. No método da contagem de estados, aplicável apenas no limite p \'SETA\' \'INFINITO\', mostramos que podemos estender a solução de Derrida mesmo na presença de um campo aleatório. Isso nos permitiu fazer a contagem de estados evitando assim o problema da \"catástrofe da entropia negativa\" presenta na solução réplica simétrica. Além disso, mostramos que qualquer sistema que seja solúvel sem a interação aleatória de p spins continua solúvel na presença dessa interação no limite p \'SETA\' \'INFINITO\'. Portanto, concluímos que a interação aleatória de p spins é somente adicionar um carácter vidro de spin ao sistema. Obtivemos expressões gerais válidas para qualquer distribuição do campo aleatório, embora a análise numérica tenha sido restrita às distribuições duplo-delta e gaussiana. Estudamos a influência do campo aleatório sobre os diagramas de fases e, em particular, mostramos que podem surgir pontos tricríticos no caso de uma distribuição duplo-delta. / We studied the effect of a random field on spin-glass models with infinite-ranged p spin interactions with a Gaussian probability distribution. The case p = 2 corresponds to the Sherrington-Kirkpatrick model in the presence of a random field. The case p \'SETA\' \'INFINITO\' corresponds to the REM (Random Energy Model) introduced by Derrida in the presence of a random field. Besides the p-spin interactions we also included uniform infinite-ranged ferromagnetic and antiferromagnetic interactions. Both in the case of ferromagnetic and antiferromagnetic interactions we employed two different approaches: The replica method in the canonical ensemble and the method of counting of the states in the microcanonical ensemble. In the replica method we solved the problem for arbitrary p both in the case of replica symmetry and in the first step of Parisi\'s replica-symmetry breaking scheme. This allowed us to rederive results for some models already known in the Literature. Next we took the limit p \'SETA\' \'INFINITO\' which yielded a complete solution to the REM in a random field. In the method of counting of the states, which is effective only in the limit p \'SETA\' \'INFINITO\', we showed that we can extend the Derrida\'s solution even in the presence of a random field. This allowed us to do the counting of the states avoiding the so called negative-entropy catastrophe present in the replica-symmetric solution. We also showed that any solvable model without random p-spin interactions is also solvable in the presence of such interactions in the limit p \'SETA\' \'INFINITO\'. Therefore, we conclude that the p-spin random interactions only add a spin-glass character to the system. We have obtained general expressions valid for any random-field distributions, although we limited the numerical analysis to double-delta and Gaussian distributions. We studied the effects of the random field on the phase diagrams, and in particular, we showed the possibility of tricritical point in the case of double-delta distributions.

Transparent conducting oxides (TCOs) are an important class of materials with many applications such as low emissivity coatings, or transparent electrodes for photovoltaics and flat panel displays. Among the possible TCO materials, Al-doped ZnO (AZO) is studied due to its relatively low cost and abundance of the raw materials. Thin films of AZO are commonly produced using physical vapour deposition techniques such as magnetron sputtering. However, there is a problem with the homogeneity of the films using reactive direct current magnetron sputtering (DCMS). This homogeneity problem can be related to the bombardment of the growing film with negative oxygen ions, that can cause additional acceptor defects and the formation of insulating secondary phases. In this work AZO films are deposited by high power impulse magnetron sputtering (HiPIMS), a technique in which high instantaneous current densities are achieved by short pulses of low duty cycle. In the first part of this thesis, the possibility to improve the homogeneity of the deposited AZO films by using HiPIMS is demonstrated. This improvement can be related to the high instantaneous sputtering rate during the HiPIMS pulses, so the process can take place in the metal mode. This allows for a lower oxygen ion bombardment of the growing film, which can help to avoid the formation of secondary phases. Another problem of AZO is the stability of the properties in humid environments. To assess this problem, the degradation of the electrical properties after an aging procedure was investigated for films deposited by both DCMS and by HiPIMS. A method was proposed, to restore the properties of the films, using a low temperature annealing under N2 atmosphere. The improvement of the electrical properties of the films could be related to a diffusion process, where water is diffusing out of the films. Then, the influence of the substrate temperature on the properties of AZO films deposited by HiPIMS was studied. The electrical, optical and structural properties were found to improve with increasing substrate temperature up to 600 ◦C. This improvement can be mostly explained by the increase in crystalline quality and the annealing of defects. Finally, the deposition of AZO films on flexible PET substrates was investigated. The films are growing as a thick porous layer of preferentially c-axis oriented columns on top of a thin dense seed layer. The evolution of the sheet resistance of the films after bending the films with different radii was studied. There is an increase in the sheet resistance of the films with decreasing bending radius, that is less pronounced for thicker films.

<p> Quantum computers will potentially outperform classical computers for certain applications by employing quantum states to store and process information. However, algorithms using quantum states are prone to errors through continuous decay, posing unique challenges to engineering a quantum system with enough quantum bits and sufficient controls to solve interesting problems. A promising platform for implementing quantum computers is that of circuit quantum electrodynamics (cQED) using superconducting qubits. Here, two energy levels of a resonant circuit endowed with a Josephson junction serve as the qubit, which is coupled to a microwave-frequency electromagnetic resonator. Modern quantum circuits are reaching size and complexity that puts extreme demands on input/output connections as well as selective isolation among internal elements. Continued progress will require adapting sophisticated 3D integration and RF packaging techniques found in today's high-density classical devices to the cQED platform. This novel technology will take the form of multilayer microwave integrated quantum circuits (MMIQCs), combining the superb coherence of three-dimensional structures with the advantages of lithographic integrated circuit fabrication. Several design and fabrication techniques are essential to this new physical architecture, notably micromachining, superconducting wafer bonding, and out-of-plane qubit coupling. This thesis explores these techniques and culminates in the design, fabrication, and measurement of a two-cavity/one-qubit MMIQC featuring qubit coupling to a superconducting micromachined cavity resonator in silicon wafers. Current prototypes are extensible to larger scale MMIQCs for scalable quantum information processing.</p><p>

One-dimensional (1D) nanostructures (NSs) of Zinc Oxide (ZnO) such as nanorods (NRs) have recently attracted considerable research attention due to their potential for the development of optoelectronic devices such as ultraviolet (UV) photodetectors and light-emitting diodes (LEDs). The potential of ZnO NRs in all these applications, however, would require synthesis of high crystal quality ZnO NRs with precise control over the optical and electronic properties. It is known that the optical and electronic properties of ZnO NRs are mostly influenced by the presence of native (intrinsic) and impurities (extrinsic) defects. Therefore, understanding the nature of these intrinsic and extrinsic defects and their spatial distribution is critical for optimizing the optical and electronic properties of ZnO NRs. However, identifying the origin of such defects is a complicated matter, especially for NSs, where the information on anisotropy is usually lost due to the lack of coherent orientation. Thus, the aim of this thesis is towards the optimization of the lowtemperature solution-based synthesis of ZnO NRs for device applications. In this connection, we first started with investigating the effect of the precursor solution stirring durations on the deep level defects concentration and their spatial distribution along the ZnO NRs. Then, by choosing the optimal stirring time, we studied the influence of ZnO seeding layer precursor’s types, and its molar ratios on the density of interface defects. The findings of these investigations were used to demonstrate ZnO NRs-based heterojunction LEDs. The ability to tune the point defects along the NRs enabled us further to incorporate cobalt (Co) ions into the ZnO NRs crystal lattice, where these ions could occupy the vacancies or interstitial defects through substitutional or interstitial doping. Following this, high crystal quality vertically welloriented ZnO NRs have been demonstrated by incorporating a small amount of Co into the ZnO crystal lattice. Finally, the influence of Co ions incorporation on the reduction of core-defects (CDs) in ZnO NRs was systematically examined using electron paramagnetic resonance (EPR).