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Understanding and Control of Frustration and Topological Defects in Ferroic Matter
Get PDFFerroic matter are of great importance as they provide a platform to extend the understanding of many-body physics and also for their amazing functional properties relevant for technological advancements. The work in this dissertation is dedicated to two of such ferroic materials.
First is CrGeTe3 (CGT), a two dimensional (2D) magnet discovered in 2017. The discovery of two-dimensional magnetism has invigorated the research community as their low dimensionality offers a unique avenue to understand and investigate the origin of magnetism in these atomically thin systems and harvest diverse application such as information processing, storage and transfer using their magnetic spins. Here, we investigated the magnetic properties of 2D-CGT using density functional theory calculations to extract the parameters of interactions between the spins in this system as well as to manipulates these parameter by using strain as the tuning knob. The parameters clearly reveal that 2D-CGT is a ferromagnet (FM) in its unstrained state and can be transitioned into an antiferromagnet (AFM) via the application of compressive uniaxial strain. Further, we developed an effective Hamiltonian (Heff) using the parameters and preformed finite temperature simulation on this system employing Monte Carlo technique. Our simulations revealed that the FM-AFM transition is bordered by a magnetically frustrated region. The frustrated magnetic systems, due to many competing interaction, are in themselves an exciting research field where the inability of the system to form an ordered state can lead to the realization of exotic states. We present a magnetic phase diagram and demonstrate that strain engineering is a viable method to tune 2D magnets and harness interesting properties in them, specifically to deterministically design frustrated magnetic states in this case. Second is ultrathin films of ferroelectric PbZr0.4Ti0.6O3 (PZT). Ultrathin ferroelectric films in the quasi-two-dimensional limit have shown tremendous promise owing to the novel emergent properties that originate from their confined geometry. The interfacial effects play a vital role in these automictically thin polar films- the depolarizing field that arises from the surface electric dipoles forces them to reorganize themselves leading to the emergence of exotic phases in these materials such as the polar vortex, electric skyrmions, merons, bubbles and many more. The latter phases are a means to deepen the fundamental understanding of these complex systems and are promising for technological advancement. For example, these systems have potential for energy storage, ultra-high density information storage and ultrafast information processing. Here, we employ state-of-the-art first principles derived Heff for PZT and implement them in Monte Carlo (MC) and Molecular Dynamics (MD) simulations to explore its properties. It was found that epitaxially strained ultrathin PZT films host many exotic topological states that emerge out of the competition between long range and short range interactions between the local electric dipoles. Here, we studied the stability regime and control over various topological defects that occur in this polar system. We found thickness, stress, screening of depolarizing field and electric field can all be used as control parameters to fine-tune the stability and transition of the topological states. Our simulations were tested experimentally and were found to be in good agreement with the experimental outcomes. We also present that the topological phase transitions in PZT can be understood in terms of a unique residual depolarizing field conserving mechanism of the system. Polar vortex is a commonly observed state in the ultrathin ferroelectrics and is the precursor to the aforementioned exotic topological defects. Here, we also explored the dynamics of the vortex state in PZT, by employing MD simulations to obtain the phonon spectra, which unveiled the origin of the vortex state. We found that the vortex state results from the condensation of a phonon mode and is associated with a symmetry breaking mechanism. In such light, the vortex state can be understood as a condensed wave pattern, and we extended this understanding to explore a practical application. Specifically a terahertz ac-electric field was applied to the vortex state where we observed a resonant switching behavior of the polar vortex state between its two equivalent orientations. These predictions imply an exciting future in understanding of the dynamics of the topological defects and employing them for novel practical applications
Influence of Invasive Species, Drought, and Stream Permanence on Stream Crayfish Species with a Focus on the Imperiled Mammoth Spring Crayfish (Faxonius marchandi)
Get PDFFreshwater crayfish constitute one of the most-threatened taxonomic groups in North America. Invasion by other crayfish species as well as habitat degradation through alteration of natural flow regime are two of the most prominent threats to crayfish persistence. Additionally, extreme weather events like droughts can amplify threats posed by other stressors on crayfish populations and are expected to increase in frequency, magnitude, and/or duration in the future, which may negatively influence narrow-ranged endemic crayfish populations typical of the Southeast. The Mammoth Spring Crayfish, Faxonius marchandi, is a narrow-ranged endemic that occurs in the Spring River drainage of Arkansas and Missouri where it is both geographically and genetically isolated. Additionally, F. marchandi populations may be faced with a spreading invader, F. neglectus chaenodactylus, the Gap Ringed Crayfish. This dissertation sought to examine the influence of invasion and drying on F. marchandi growth and survival using mescosm experiments, the desiccation tolerance of the invader versus F. marchandi using experimental chambers, the influence of stream permanence and drying on crayfish density, occupancy, detection, colonization, and extinction using a multi-season dynamic occupancy model, and the predicted extinction risk and population viability of F. marchandi under future invasion and drought scenarios using a stage-based demographic metapopulation model. In the mesocosm experiment, there was a significant negative interspecific competition effect on change in length, and the experimental chamber experiment found differential tolerance to desiccation for all populations, with larger individuals tending to live longer. Additionally, the population models found negative effects under severe invasion and only under severe drought. These results indicate F. marchandi may show resilience when faced with F. neglectus invasion and drying will likely not play a part in their dynamics. The dynamic occupancy results showed context-dependent differences in density, detection, colonization, and extinction. This dissertation shows how crayfish may respond to environmental changes and invasive species in the coming years and gives potential implications about their persistence
Predicting IDS and IPS attacks using Machine Learning
Get PDFPredicting IDS and IPS attacks using Machine Learning
Presented at UA Power Group Undergraduate and REU Summer Research Poster Session on July 18, 2024.https://scholarworks.uark.edu/elegreu/1000/thumbnail.jp
Machine Learning-Based Prediction of Optimal Switch Configurations for Boost Converters by PowerSynth
Get PDFMachine Learning-Based Prediction of Optimal Switch Configurations for Boost Converters by PowerSynth
Presented at UA Power Group Undergraduate and REU Summer Research Poster Session on July 18, 2024.https://scholarworks.uark.edu/elegreu/1015/thumbnail.jp
Shell Translucency: A Novel Eggshell Quality Parameter in Broiler Breeders
Get PDFIn the United States, nearly 240 million commercial broiler breeder eggs are incubated weekly with an average hatchability of 79.1% (Third Party Reporting). Between 160 eggs/hen and 125 chicks/hen the loss is considerable. In 2007, an estimated 1% improvement in hatchability would escalate profits by over $25 million to hatcheries and markedly increase profitability in broiler operations (Schaal and Gherian, 2007). In order to tap into top hatchability performances, egg management in commercial hatcheries such as egg storage time and conditions, incubation temperature and humidity, as well as turning frequency are of utmost importance (Vick et al., 1993; Reis et al., 1997; Lourens et al., 2005; Elibol and Brake, 2006). However, regardless of how well the egg is handled in commercial hatcheries, eggshell quality is critical to improve hatchability (Orellana et al., 2023). The eggshell is not only a physical, but may also serve as a biological barrier, that regulates gas, water vapor diffusion and serves as an obstacle for bacterial colonization which impacts the survival of the developing embryo. Moreover, the eggshell is used as an indicator for hatchability through parameters such as specific gravity, vapor water conductance, color, weight, thickness, porosity, breaking strength, elastic modulus, static and dynamic stiffness, among others (McDaniel et al., 1979; Pebbles and Brake, 1987; King’ori, 2011; Sekeroglu and Duman, 2011; Liao et al., 2013). Understanding the relationship between shell quality parameters and hatchability could generate new opportunities for research on broiler breeder flock management and nutrition (Emery et al., 1984; Wilson, 1997; Ketta and Tumova, 2018). Currently, in North America, few to no commercial hatchery operations perform egg and eggshell quality measurements that are strategically used in daily decision making in business management. The main reason is the unpractical nature of the quality parameters implemented. In the search for a practical and non-destructive eggshell quality assessment that the poultry industry could easily adopt and rely on, eggshell translucency should be considered
Crazy Train: Managing an IR during transition
Get PDFCrazy, but that’s how it goes! The University of Arkansas’ organizational structure for the institutional repository and other publishing ventures underwent significant changes with the creation of a new department in July 2023, the resignation of the IR coordinator who had been the sole developer of the IR since its inception, the move of IR supervision to the department head in January 2024, and the hiring of a new IR coordinator in July 2024. We realized with the resignation of our coordinator that policy decisions, changes in template assignments, assigning metadata, and even the use of external forms and software were often not documented. If they were documented, the documentation was housed in over 1000 files in five different servers. How do you start making decisions on future priorities and codify practices with little knowledge of the work that had already been accomplished?
This presentation will describe the paths we have taken to get back on the rails through data analysis, surveys, and living through the trials and errors of starting in the middle of a large project. Attendees will learn about the pain points of restarting an IR “from scratch” and will also gain insights in management practices that might allow their IR trains to keep running on the tracks after they have moved on
Children at Work, Parental Rights—and Rhetoric
Get PDFStates are increasingly considering and enacting laws that reduce protections for child laborers, and the number of minors who have been employed in violation of existing child labor laws has been steadily growing. We argue that politicians deploy the rhetoric of parental rights in today’s legislative battles over child labor protections to create political cover for reforms that benefit businesses, not children or their families. Part I demonstrates that appeals to parental rights have a long history in child labor law. Through much of the battle to regulate children’s labor, child-labor opponents insisted that regulations infringed parents’ legitimate rights over the care, custody, and control of their children. Nevertheless, over time, it became clear that children’s wellbeing was best served by a combination of state and federal labor protections that restricted children’s work hours and conditions. Part II turns to the recent attacks on child labor protections in the name of parental rights. This Part considers the political benefits to legislators in framing efforts to roll back labor protections in the name of parental rights, as well as how this rhetoric hides a range of less politically popular forces driving this rollback. It then demonstrates that elimination of existing labor protections for children risks both their current wellbeing and their future prospects. Doing so, we argue, withdraws the state from the critical role it has long played—and should continue to play—in protecting children from market forces. Part III concludes by considering ways to counter the new rhetoric of parental rights and its impact on child labor laws
