Doctor of Philosophy, The Ohio State University, 2024, Materials Science and Engineering

This dissertation will center around the discussion of the investigation into the anomalous thermal and electrical transport phenomena in magnetic Weyl semimetal, YbMnBi2, as well as the characterization of its magnetization behavior. A theory-based experimental search for a new type of chiral anomaly in promising materials will also be covered. 1. Thermoelectrics (TEs) are solid-state devices that can realize heat-electricity conversion. Transverse TEs require materials with a large Nernst effect, which typically requires a strong applied magnetic field. However, topological materials with magnetic order offer an alternative pathway for achieving large Nernst via the anomalous Hall effect and the accompanying anomalous Nernst effect (ANE) that arise from band topology. Here, we show that YbMnBi2 with a low Hall density and a chemical potential near the Weyl points has the highest ANE-dominated Nernst thermopower of any magnetic materials, S_yx around 110 μV/K (T = 254 K, 5 T ≤ |(μ_0)*H| ≤ 9 T applied along the spin canting direction), due to the synergism between classical contributions from filled electron bands, large Hall conductivity of topological origin, and large resistivity anisotropy. In addition, an appreciable thermal Hall angle of 0.02 < ∇yT/∇xT (–9 T) < 0.06 was observed (40 K < T < 310 K). 2. How exactly the magnetization of YbMnBi2 changes with temperature and magnetic field remains indeterminate. Mysteries exist in the previous reports. Herein, through extensive magnetization characterization at various conditions, it was found that the magnetization behavior of YbMnBi2 showcases shared features in many aspects among multiple crystals in spite of a few sample-dependent details. The findings here hint at a more complex picture of the magnetic structure than what is currently known. This project hopefully can provide a foundation for future studies on thoroughly characterizing the magnetization behavior of YbMnBi2. 3. Chiral anomaly, a signat (open full item for complete abstract)

Committee: Joseph Heremans (Advisor); Maryam Ghazisaeidi (Committee Member); Michael Sumption (Committee Member); Roberto Myers (Committee Member) Subjects: Condensed Matter Physics; Materials Science; Physics

Doctor of Philosophy, The Ohio State University, 2023, Mathematics

Given a metric space $(X,d_X)$, the $n$-th curvature set is the set of $n$-by-$n$ distance matrices generated by a sample from $X$ with $n$ or less points. Similarly, the $(n,k)$ persistence set of $X$ is the set of $k$-dimensional persistence diagrams of all $n$-point samples from $X$. This dissertation has two major parts, each dedicated to persistence sets or curvature sets. A major obstacle that hampers the widespread use of topological techniques in data science is the sometimes prohibitive computational cost of persistent homology. Persistence sets aim to circumvent this limitation while retaining useful geometric and topological information from the input space. We study the experimental and theoretical properties of persistence sets, and compare them with the standard VR-persistent homology from the perspectives of computational efficiency and discriminative power, including in a practical shape classification task. We characterize several persistence sets of the circle, higher-dimensional spheres, surfaces with constant curvature, and a specific family of metric graphs, and show spaces that have different persistence sets but are indistinguishable by persistent homology. All in all, we believe that persistence sets can aid in data science tasks where the shape is important but the standard persistent homology algorithms are impractical. In the second part, we study the curvature sets $\Kn_n(\Sp^1)$ of the circle $\Sp^1$ as a topological space. We give several characterizations of $\Kn_n(\Sp^1)$ as quotients of tori under group actions, and use them to compute the homology of $\Kn_n(\Sp^1)$ with Mayer-Vietoris arguments. We construct an abstract simplicial complex $\St_n(\Sp^1)$ whose geometric realization is $\Kn_n(\Sp^1)$. Moreover, we give an embedding of $\St_n(\Sp^1)$ in $\R^{n \times n}$ and show that $\Kn_n(\Sp^1)$ is the union of the convex hulls of the faces this embedding. Lastly, inspired by the characterization of a persistence set of su (open full item for complete abstract)

Committee: Facundo Mémoli (Advisor); Matthew Kahle (Committee Member); David Anderson (Committee Member) Subjects: Mathematics

MS, University of Cincinnati, 2021, Engineering and Applied Science: Mechanical Engineering

Vibration-based damage detection methods have been extensively used in structural health monitoring as these are response-based techniques and can be applied to experimental/operational data. The conventional methods of obtaining full-field vibration measurements are limited due to the location and number of sensors. Advancements in imaging technology have enabled the use of the digital image correlation (DIC) technique to measure the full-field deformation of a vibrating structure. In this study, the DIC technique was used to obtain vibration measurements from an impact test of a cantilever beam for damage identification. The application of curvature mode shapes (CMSs) developed from the mode shapes (MSs) of the beam is studied to detect and locate the damage. The CMSs of the undamaged state of the beam are determined only from the damaged state of the beam, without prior information about the associated undamaged beam, provided the beam is geometrically smooth. It is shown that the polynomial fit of the appropriate order of measured MS is equivalent to the associated MS of the undamaged beam. The objective of this study was to investigate the use of DIC technique and CMSs to locate and detect damage in the form of a machined area with reduced thickness in a cantilever beam. The modal parameter estimation (MPE) was done using X-Modal software, based on the unified matrix polynomial approach (UMPA), to obtain mode shapes and natural frequencies from the vibration measurements. It is shown that the proposed method can successfully detect and locate damage in the beam, for the data obtained from a single-input impact test. The work focuses on understanding how the parameters used in the DIC technique and MPE influence the damage detection. The influences of parameters such as subset size and step size used in the DIC technique are studied. The influences of parameters such as type of MPE algorithm, frequency band selection and model order during MPE are studied. (open full item for complete abstract)

Committee: Yongfeng Xu Ph.D. (Committee Chair); Randall Allemang Ph.D. (Committee Member); Allyn Phillips Ph.D. (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2019, Mathematics

On the spectral triple of a noncommutative manifold $(\mathcal{A},H,D)$, despite the absence of underlying space of points, one can still consider its scalar curvature in terms of spectral information of the Dirac operator $D$, for example using short-time asymptotic expansion of the heat kernel $e^{-tD^2}$. In the recent decade, the conformal theory on a noncommutative two tori was firstly started by Connes and Tretkoff(nee Cohen), and later greatly developed by Connes, Moscovici and many others. Noncommutative conformal geometry on a noncommutative torus $\mathcal{A}_\theta$ is the study of quantized Gaussian curvature under noncommutative conformal change of ``metric'' by a positive operator-valued Weyl factor $k=e^h,h^*=h$. In this dissertation, by using Lesch and Moscovici's extension of Connes' pseudo-differential calculus to the Heisenberg modules, we will calculate the scalar curvature of a non-conformal change of metric by means of two commuting positive operator-valued factors $k_1,k_2$.\\ The first part of this paper, inspired by work by L.Dabrowski and S.Andrzej, contains extension of the rearrangement lemma that was systematized by M.Lesch, to non-conformal operators, by which we mean the elliptic operators with principal symbol $\sum_j k_j^2\xi_j^2$ with distinct $k_1,...,k_m$. By adapting the technique used by Y.Liu, we interpret the result of rearrangement as generalized hyper-geometric functions on Grassmannians, generalizing the conformal results of Y.Liu , namely when $k_1=k_2$. Second part of this paper consists of calculation of scalar curvature density associated to a non-conformal Laplacian operator $\Delta_{k_1,...,k_m}$ on a $m$-torus $\mathcal{A}_\Theta^m$. Third part is calculation of index density of a non-conformal Dirac operator $D_{k_1,k_2;\mathcal{E}(g,\theta)}$ on the Heisenberg module $\mathcal{E}(g,\theta)$. In appendix A, we will justify our terminology ``non-conformal''. W (open full item for complete abstract)

Committee: Henri Moscovici (Advisor); Ovidiu Costin (Committee Member); Michael Davis (Committee Member); David Penneys (Committee Member) Subjects: Mathematics

Doctor of Philosophy, Miami University, 2006, Botany

Gravitropism, the directed growth of a plant in response to gravity, can be divided into the following phases: gravity perception, signal transduction, and the growth response. In flowering plants, gravity sensing occurs in specialized cells termed statocytes, which include the columella cells of the root cap and the endodermis (= starch sheath) of stem-like organs. These statocytes contain organelles (amyloplasts) that are packed with dense starch grains and move in response to changes in the orientation of the plant organ relative to gravity. Signal transduction occurs when changes in the potential and/or kinetic energy of gravistimulated amyloplasts is converted into a biochemical signal. Numerous reports indicate that the process is mediated by actin microfilaments (F-actin), although the exact nature of F-actin involvement is unknown. This study was undertaken to assess the relationship between amyloplasts and F-actin in statocytes of Arabidopsishypocotyls before and during gravitropic stimulation by reorientation. A pharmacological approach was employed throughout this project. Disruption of F-actin with latrunculin B (Lat-B) caused a promotion of gravitropic curvature despite a reduction in growth. Cryofixation, histology and microscopy were used to determine the effects of F-actin disruption on amyloplast positions before and after gravistimulation. Amyloplast mobility in hypocotyls was virtually eliminated after Lat-B treatment. Reduced amyloplast mobility after cytoskeletal disruption is consistent with an active mechanism of intracellular statolith transport and suggests that amyloplast movement in endodermal cells is dependent upon F-actin. To determine whether amyloplasts require myosin motor proteins to move along actin filaments, Arabidopsisseedlings were exposed to myosin ATPase inhibitor 2,3-butanedione monoxime (BDM), and the effects of gravistimulation and myosin ATPase inhibition on growth rate, gravitropic curvature and amyloplast kinetics were (open full item for complete abstract)

Committee: John Kiss (Advisor) Subjects:

Master of Science, The Ohio State University, 2005, Graduate School

Committee: Not Provided (Other) Subjects:

Master of Science, The Ohio State University, 2006, Graduate School

Committee: Not Provided (Other) Subjects:

Doctor of Philosophy, The Ohio State University, 2024, Physics

This thesis is composed of two unrelated pieces of work: the first develops a semiclassical theory of transport in topological magnets, and the second presents a machine learning-based method of numerical analytic continuation. In Part I, we calculate the electric and thermal currents carried by electrons in the presence of general magnetic textures in three-dimensional crystals, including three-dimensional topological spin textures. We show, within a controlled, semiclassical approach that includes all phase space Berry curvatures, that the transverse electric, thermoelectric, and thermal Hall conductivities have two contributions in addition to the usual effect proportional to a magnetic field. These are an anomalous contribution governed by the momentum-space Berry curvature arising from the average magnetization, and a topological contribution determined by the real-space Berry curvature and proportional to the topological charge density, which is non-zero in skyrmion phases. This justifies the phenomenological analysis of transport signals employed in a wide range of materials as the sum of these three parts. We prove that the Wiedemann-Franz and Mott relations hold, even in the presence of topological spin textures, and justifying their use in analyzing the transport signals in these materials. This theory also predicts the existence of the in-plane anomalous and topological Hall effects in three-dimensional, low symmetry materials. We present a symmetry analysis which predicts when the in-plane Hall effect (IPHE) is forbidden, and predict which crystal structures could harbor an IPHE which is larger than the usual out-of-plane Hall effect. In Part II, we present a method of numerical analytic continuation of determinantal Quantum Monte Carlo (DQMC) Green's functions, utilizing an unsupervised neural network (NN). Many have used supervised machine learning methods to attack this problem, but the most interesting applications of DQMC are on systems in (open full item for complete abstract)

Committee: Mohit Randeria (Advisor); Chunning Lau (Committee Member); Ilya Gruzberg (Committee Member); Richard Furnstahl (Committee Member) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy, The Ohio State University, 2024, Mathematics

We are concerned with the study of \emph{Shephard groups}, a class of groups which encompasses the Coxeter groups, Artin groups, graph products of cyclic groups, and certain complex reflection groups. We extend a well-known result that Coxeter groups are $\mathrm{CAT}(0)$ to a class of Shephard groups that have ``enough'' finite parabolic subgroups. We also show that in this setting, if the underlying Coxeter diagram is type FC, then the Shephard group is cocompactly cubulated. Our method of proof combines the works of Charney-Davis on the Deligne complex for an Artin group and of Coxeter on the classification and properties of the regular complex polytopes. Along the way we introduce a new criteria (based on work of Charney) for a simplicial complex made of simplices of shape $A_3$ to be $\mathrm{CAT}(1)$. It is our hope that this begins the study of complex reflection groups through the lens of geometric group theory, as this has quickly shown to be a fruitful approach.

Committee: Jingyin Huang (Advisor); Jean-François Lafont (Committee Member); Michael Davis (Committee Member) Subjects: Mathematics

Doctor of Philosophy, The Ohio State University, 2024, Mathematics

In this thesis, we study essentially conformally symmetric (ECS) manifolds, that is, pseudo-Riemannian manifolds (M,g) with parallel Weyl curvature, which are not locally symmetric or conformally flat. Every ECS manifold carries a distinguished null parallel distribution D (called its Olszak distribution) whose rank, always equal to 1 or 2, is referred to as the rank of (M,g). More precisely, we focus on the rank-one situation: while the local structure of ECS manifolds of either rank was already well-known, little could be said about their global structure. In particular, examples of compact ECS manifolds were only known in dimensions of the form n = 3k + 2, with k ≥ 1, and they are all of rank one, geodesically complete, not locally homogeneous, and diffeomorphic to total spaces of torus bundles over S¹. In Chapter 4, combining analytic and combinatorial methods, we construct compact rank-one ECS manifolds of all dimensions n ≥ 5, and sorted in two distinct classes, called translational and dilational: such dichotomy refers to finiteness or infiniteness of the holonomy group of the natural flat connection induced on D. The translational examples appear in all dimensions n ≥ 5 and are all geodesically complete, without being locally homogeneous. The dilational examples, in turn, could only be produced in odd dimensions n ≥ 5, are all geodesically incomplete, and some of them are locally homogeneous, while others are not. In both cases discussed above, the resulting manifolds were diffeomorphic to total spaces of torus bundles over S¹ . This brings us to Chapter 5, where we show that this topological structure was not accidental: using some ideas from foliation theory, we prove that outside of the locally homogeneous case, and up to passing to a double isometric covering if needed, every compact rank-one ECS manifold must fiber over S¹ in such a way that the leaves of D^⊥ appear as the fibers. Chapter 6 brings the notion of genericity — originally intro (open full item for complete abstract)

Committee: Andrzej Derdzinski (Advisor); Jean François Lafont (Committee Member); Andrey Gogolev (Committee Member) Subjects: Mathematics

Master of Science, The Ohio State University, 2023, Mechanical Engineering

Hexagonal and hexagon-based structures are widely seen in nature and inspire various engineering designs by demonstrating the capabilities of tessellating complex two-dimensional (2D) and three-dimensional (3D) assemblies. While enabling functionality at the deployed state, folding hexagonal structures to a greatly reduced area or volume allows for space-saving for transportation. However, the study on an effective folding strategy is limited. In this work, a snap-folding strategy for the hexagonal ring is reported. The influence of geometric parameters, loading methods, and loading locations on the hexagonal rings' foldability and stability is first investigated systematically through finite element analysis and experimental validation. Then the snap-folding behaviors of modified hexagonal rings with residual strain and pre-twisted edges are investigated. Finally, the effect of edge curvature on folding behavior, folded configurations, and packing of curved hexagonal ring origami is studied. It is anticipated that the hexagonal ring origami could provide a new strategy for designing functional large foldable structures with self-guided deformation and excellent packing ability.

Committee: David Hoelzle (Advisor); Renee Zhao (Advisor) Subjects: Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2022, Mathematics

We explore lattice point counting and the method of stationary phase through the lens of questions about the number of lattice points on and near surfaces with vanishing curvature. Our focus is on spheres arising from the Heisenberg groups. In particular, we prove an upper bound on the number of points on and near large dilates of the unit spheres generated by the anisotropic Heisenberg norms for α ≥ 2. We accomplish this through a transformative process that takes a number theory question about counting lattice points and translates it into that of an analytical estimation of measure. This process relies on truncating and scaling the n-dimensional integer lattice to produce a fractal-like set. By introducing a measure on this resulting set and using elementary Fourier analysis, the counting problem is transformed into one of bounding an energy integral. This process uses principles of fractal geometry and oscillatory integrals. Primary challenges that arise are the presence of vanishing curvature and uneven dilations. Following a discussion and formal estimate of the curvature of the Heisenberg spheres, we utilize the method of stationary phase to compute a bound on the Fourier transform of their surface measures. Our work is inspired by that of Iosevich and Taylor (2011) and Garg, Nevo, and Taylor (2015). We present an extension of the main result in the former to surfaces with vanishing curvature. Furthermore, we utilize the techniques developed here to estimate the number of lattice points in the intersection of two such surfaces. Additionally, we present a mini-course on the basics of stationary phase—a quick-start guide to stationary phase in practice. This includes a discussion of the formulation of oscillatory integrals and their solutions with a focus on the impact of geometric properties (e.g. curvature) on the estimates for the decay of the Fourier transform. It further serves as a supplement to [Shakarchi and Stein, Functional Analysis: Chapter (open full item for complete abstract)

Committee: Krystal Taylor (Advisor); Rodica Costin (Committee Member); Barbara Keyfitz (Committee Member) Subjects: Mathematics

Doctor of Philosophy, The Ohio State University, 2021, Electrical and Computer Engineering

Recent reports from NHTSA state that approximately 37,000 fatalities occur each year as a result of traffic accidents. Around 6,000 of these fatalities are pedestrians and around 800 are bicyclists. Pedestrians and bicyclists are categorized as Vulnerable Road Users (VRU) in traffic. It should also be noted that the already high number of VRU fatalities is also increasing every year. In short, a serious safety risk for VRUs can be observed through these statistics where more pedestrian fatalities are present. This dissertation studies a pedestrian safety system for autonomous vehicles that non-autonomous vehicles can also partially utilize through warnings and speed profile recommendations provided to the driver. The safety system presented is designed to address both the safe stop condition and the emergency collision avoidance condition. Moreover, the approach was studied through several modules that covers aspects such as pedestrian path tracking and prediction, as well as real-world data processing for understanding human driver behavior and tuning the warning systems accordingly. On top of that, widely available mobile phones were utilized both in terms of their wireless communication and on-board sensor measurement capabilities which makes the safety system useful and available for a wider public at the current time while addressing dangerous no-line-of-sight or low visibility situations. For the cases where safe stopping is not possible or not preferable, a collision avoidance algorithm that executes modification of the pre-defined path based on the elastic band method for maneuvering around the pedestrian was proposed. Since autonomous vehicles are expected to have a pre-defined nominal path based on map information, modification of the pre-defined path is a more reasonable solution than creating a path from scratch because of the feasibility of the modified path and smooth departure from and connection to the pre-defined path. Pedestrians are treated as (open full item for complete abstract)

Committee: Levent Guvenc (Advisor); Benjamin Coifman (Committee Member); Keith Redmill (Committee Member); Bilin Aksun-Guvenc (Committee Member) Subjects: Automotive Engineering; Computer Science; Electrical Engineering

PHD, Kent State University, 2019, College of Arts and Sciences / Department of Computer Science

Recently, there has been extensive research on negative\ curvature (or hyperbolicity) of graphs, which measures the local deviation of a metric from a tree metric. In case of graphs (and general geodesic metric spaces), there exist several equivalent definitions of negative curvature parameters involving different but comparable values (they are within small constant factors from each other). In our research, we further study these parameters, and mostly we are investigating in the Rips' condition involving geodesic triangles, notion of slimness and thinness. We establish a relationship between slimness (and thinness) and other tree-likeness structure parameters including cluster-diameter Delta(G) of a layering partition, tree-length tl(G), and tree-breadth tb(G). We show a relationship between chordality and both slimness and thinness. We present sharp bounds on slimness and thinness of some structured classes of graphs such as AT-free graphs and HHD-free graphs. Additionally, we show that the slimness of every 4-chordal graph is at most 2 and characterize those 4-chordal graphs for which the slimness of every of its induced subgraph is at most 1. More interestingly, we provide exact and approximation algorithms for computing the slimness and thinness. Particularly, an algorithm computing the exact thinness of graphs runs in O(n^2m) time, an 8-approximation algorithm (with additive surplus 4) runs in O(n^2) time (provided the distance matrix is known in advance) and a 3-approximation algorithm runs in O(n^3) time. We present an algorithm to compute the exact slimness of graphs in O(n^4) time, a 24-approximation algorithm (with additive surplus 3) running in O(n^2) time (provided the distance matrix is known in advance) and an 8-approximation algorithm running in O(n^3) time. Additionally, we propose an efficient practical algorithm for computing the exact slimness in graphs using several tricks. The practical algorithm runs in O(n^3m) time in the worst case; howe (open full item for complete abstract)

Committee: Feodor Dragan (Advisor) Subjects: Computer Science

Doctor of Philosophy (PhD), Wright State University, 2018, Engineering PhD

The retail industry in the U.S. contributed 1.14 trillion in value added (or 5.9%) to the GDP in 2017, an increase of 3.7% from the previous year. While store closures have dominated the news in the recent past (e.g., Toys-R-Us, Sears, and Bon-Ton) due to ineffective supply chain practices, inadequate in-store experiences, and competition from e-tailers, other retailers such as Ross, T. J. Maxx, Burlington Coat Factory, and Kroger have been expanding their footprint. Brick-and-mortar stores are unique as they allow shoppers the ability to see, touch, and try products, in addition to exploring new products. Kohl's CEO has even indicated that 90% of their revenue is still generated in brick-and-mortar stores. Besides reducing supply chain costs, retailers have been paying considerable attention to redesigning their stores by varying layouts and displays to improve shopping experience and remain profitable. However, a lack of scientific methods that correlate layout changes to improved experience has often led to time-consuming and expensive trial-and-error approaches for the retailers. This research focuses on the design of such brick-and-mortar stores by developing a quantitative approach that models the visual interaction between a 3D shopper's field of view and the rack layout. This visual interaction has been shown to influence shopper purchasing habits and their overall experience. While some metrics for visual experience have been proposed in the literature, they have been limited in many ways. The objective of this research is to develop new models to quantify visual experience and employ them in layout design models. Our first contribution consists of quantifying exposure (which rack locations are seen) and the intensity of exposure (how long they are seen) by accounting for the dynamic interaction between the human 3D field of regard with a 3D rack layout. We consider several rack designs/layouts that we noticed at nearby retail stores, ranging from the (open full item for complete abstract)

Committee: Pratik Parikh Ph.D. (Advisor); Xinhui Zhang Ph.D. (Committee Member); Frank Ciarallo Ph.D. (Committee Member); Nan Kong Ph.D. (Committee Member); James Munch Ph.D. (Committee Member) Subjects: Industrial Engineering

MS, University of Cincinnati, 2018, Engineering and Applied Science: Aerospace Engineering

The predictable effect of surface curvature on the contiguous streamtube allows for the use of geometric curvature as a direct and aerodynamically meaningful parametric model to generate airfoil geometry. A novel and parsimonious parameterization technique driven by specifications of normalized meanline second derivatives, which is related to curvature, and a superimposed thickness distribution which explicitly eliminates or minimizes unintentional oscillations in curvature, is resented. The focus on smooth curvature control is inherently relevant to both internal and external flow geometries, enabling a unified method for generating both isolated airfoils and cascade sections. This technique is implemented and included in T-Blade3 which is an existing in-house open-source code. The underlying methodology for construction of camber-line is entirely analytical ensuring speed of execution. Two different thickness distributions are presented, one based on specifications of thickness B-spline control points, and another based on specifications of exact thickness. The B-spline thickness method requires a simple implementation and executes faster, but cannot impose an exact value of thickness at a specified location. The exact thickness method is capable of imposing specified thickness values both chord-wise and span-wise, while quantifying and optimizing the quality of the thickness curve based on curvature. Consequently, unintentional bumps on the airfoil and oscillations in curvature are eliminated or minimized. The parameterization ensures curvature and slope of curvature continuity on the airfoil surface which are critical for smooth surface pressure distributions. Consequently, losses due to unintentional pressure spikes are minimized and likelihood of separation reduced, resulting in a class of high-performance airfoils. The direct relationship between the parameterization and surface aerodynamics is demonstrated for both isolated and cascade airfoils. A framework (open full item for complete abstract)

Committee: Mark Turner Sc.D. (Committee Chair); Shaaban Abdallah Ph.D. (Committee Member); Donald French Ph.D. (Committee Member) Subjects: Aerospace Materials

Doctor of Philosophy, The Ohio State University, 2018, Physics

Topological Weyl semimetals have attracted substantial recent interest in condensed matter physics. In this thesis, we theoretically explore electronic and transport properties of these novel materials. We also present results of experimental collaborations that support our theoretical calculations. Topological Weyl semimetals (TWS) can be classified as type-I TWS, in which the density of states vanishes at the Weyl nodes, and type-II TWS, in which an electron pocket and a hole pocket meet at a singular point of momentum space, allowing for distinct topological properties. We consider various minimal lattice models for type-II TWS. We present the discovery of a type II topological Weyl semimetal (TWS) state in pure MoTe2, where two sets of WPs (W2$\pm$, W3$\pm$) exist at the touching points of electron and hole pockets and are located at different binding energies above $E_F$. Using ARPES, modeling, DFT and calculations of Berry curvature, we identify the Weyl points and demonstrate that they are connected by different sets of Fermi arcs for each of the two surface terminations. Weyl semimetals possess low energy excitations which act as monopoles of Berry curvature in momentum space. These emergent monopoles are at the heart of the extensive novel transport properties that Weyl semimetals exhibit. We show how the Nernst effect, combining entropy with charge transport, gives a unique signature for the presence of Dirac bands. The Nernst thermopower of NbP (maximum of 800 $\mu \textrm{V}\cdot \textrm{K}^{-1}$ at 9 T, 109 K) exceeds its conventional thermopower by a hundredfold and is significantly larger than the thermopower of traditional thermoelectric materials. The Nernst effect has a pronounced maximum near $T_M=90 \pm 20 \textrm{K}=\mu_{0}/ \kb$ ($\mu_0$ is chemical potential at $T=0$ K). A self-consistent theory without adjustable parameters shows that this results from electrochemical potential pinning to the Weyl point energy at $T\geq T_M$, driven (open full item for complete abstract)

Committee: Nandini Trivedi (Advisor); Rolando Valdes Aguilar (Committee Member); Mohit Randeria (Committee Member); Yuri Kovchegov (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2018, Mechanical Engineering

Morphing panels offer opportunities as adaptive control surfaces for optimal system performance over a broad range of operating conditions. This work presents a design framework for multifunctional composites based on three types of laminae, viz., constraining, adaptive, and prestressed. Based on this framework, laminate configurations are designed to achieve multiple morphing modes such as stretching, flexure, and folding in a given composite structure. Multiple functions such as structural integrity, bistability, and self-actuation are developed. The composites are developed through a concurrent focus on mathematical modeling and experiments. This research shows that curvature can be created in a composite structure by applying mechanical prestress to one or more of its laminae. Cylindrical curvature can be tailored using a prestressed lamina with zero in-plane Poisson's ratio. Analytical laminated-plate models, based on strain energy minimization, are presented in multiple laminate configurations to characterize composites with curvature, bistability, folding, and embedded smart material-driven actuation. Fabrication methods are also presented for these composite configurations. The mathematical models are validated experimentally using tensile tests and 3D motion capture. The mechanics of an n-layered composite is explained through modeling of all the stacking sequences of the three generic laminae. Actuation energy requirement is found to be minimal in the constraining-prestressed-adaptive layer configuration. Bistable curved composites are developed using asymmetric prestressed laminae on either face of a core layer; these composites address the drawbacks of thermally-cured bistable fiber-reinforced polymeric composites. When the prestressed directions are orthogonal, the stable curvatures are weakly-coupled. The composite's domain of bistability and actuation requirements are quantified using a non-dimensional high-order strain model. Active bi (open full item for complete abstract)

Committee: Marcelo Dapino Prof. (Advisor); Rebecca Dupaix Prof. (Committee Member); Ryan Harne Prof. (Committee Member); Haijun Su Prof. (Committee Member) Subjects: Aerospace Materials; Automotive Materials; Engineering; Mechanical Engineering; Mechanics

PHD, Kent State University, 2018, College of Arts and Sciences / Department of Physics

I investigate three systems that exhibit complex patterns in orientational order, which are controlled by geometry interacting with the dynamics of phase transitions, metastability, and activity. 1. Liquid Crystal Elastomers: Liquid-crystal elastomers are remarkable materials that combine the elastic properties of cross-linked polymer networks with the anisotropy of liquid crystals. Any distortion of the polymer network affects the nematic order of the liquid crystal, and, likewise, any change in the magnitude or direction of the nematic order influences the shape of the elastomer. When elastomers are prepared without any alignment, they develop disordered polydomain structures as they are cooled into the nematic phase. To model these polydomain structures, I develop a dynamic theory for the isotropic-nematic transition in elastomers. 2. Active Brownian Particles: Unlike equilibrium systems, active matter is not governed by the conventional laws of thermodynamics. I perform Langevin dynamics simulations and analytic calculations to explore how systems cross over from equilibrium to active behavior as the activity is increased. Based on these results, I calculate how the pressure depends on wall curvature, and hence make analytic predictions for the motion of curved tracers and other effects of confinement in active matter systems. 3. Skyrmions in Liquid Crystals: Skyrmions are localized topological defects in the orientation of an order parameter field, without a singularity in the magnitude of the field. For many years, such defects have been studied in the context of chiral liquid crystals—for example, as bubbles in a confined cholesteric phase or as double-twist tubes in a blue phase. More recently, skyrmions have been investigated extensively in the context of chiral magnets. In this project, I compare skyrmions in chiral liquid crystals with the analogous magnetic defects. Through simulations based on the nematic order tensor, I model both isolated skyrmions (open full item for complete abstract)

Committee: Jonathan Selinger (Advisor); John Portman (Committee Co-Chair) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 1984, Graduate School

Committee: Not Provided (Other) Subjects: Engineering