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

Phase transitions occur in many classical and quantum systems, and are the subject of many an open problem in physics. In the past decade, the conformal bootstrap has provided new perspectives for looking at the critical point of a transition. With this formalism, it is possible to exploit the conformal symmetry intrinsically present at the critical point, and derive general results about classes of transitions that obey the same symmetries. This thesis presents the application of this method to two problems of note in classical and quantum phase transitions. The first is a classical model of O(N) spins in the presence of a boundary. We use the numerical conformal bootstrap to prove rigorously the existence of a new boundary phase in three-dimensional Heisenberg (O(3)) and O(4) magnets, deemed the extraordinary-log universality class. The results agree well with a parallel numerical study but are more rigorous due to the bounded nature of the error. The second case is the inherently quantum problem of Anderson transitions between metals and insulators. It has been discovered that at criticality, the wavefunctions describe multifractal objects, that are described by infinitely many fractal dimensions. We use analytical constraints from conformal symmetry to predict the form of these fractal parameters in dimensions greater than two. Our exact prediction, which works in arbitrary dimensions, can be used as a probe for conformal symmetry at Anderson transitions. By studying these two problems, we demonstrate the power of conformal symmetry as a non-perturbative tool in the theory of phase transitions in arbitrary dimensions. Throughout the thesis, we have extended the domain of applicability of traditional bootstrap techniques for the purpose of non-unitary and non-positive systems.

Committee: Ilya Gruzberg (Advisor); Marc Bockrath (Committee Member); Samir Mathur (Committee Member); Yuanming Lu (Committee Member) Subjects: Condensed Matter Physics; Physics

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

In quantum many-body systems, fractionalization stands as a hallmark of quantum emergent phenomena, where an elementary degree of freedom, such as an electron, decomposes into distinct pieces with a fraction of quantum numbers due to frustration or strong quantum fluctuations. A canonical well-understood example of this is observed in one-dimensional quantum systems. In one-dimensional systems, the pronounced quantum fluctuations facilitate the deconfinement of these fractionalized quasiparticles, allowing them to exhibit independent dynamics, where electrons, carriers of both charge and spin, undergo spin-charge separation which results in the dynamical deconfinement of spinon and chargon. In two dimensions, however, the physics is more intricate. In the presence of frustrating interactions between spins, the interacting spins are unable to order. Instead, they create long-range patterns of entanglement leading to states of matter such as quantum spin liquids, heralding the topological quantum matter with novel fractionalized particles and emergent gauge fields. These states are characterized by topological order: ground state degeneracy on a manifold of non-zero genus, and fractionalized excitations with abelian and non-abelian quantum statistics. In these states, the original localized spin degrees undergo further fractionalization to give new degrees of freedom, such as Majorana fermions and spinons. In these states, both charges and spins are localized. However, the emergent fractionalized degrees of freedom can be remarkably delocalized and able to transport energy. Identifying and studying the phenomena of fractionalization presents a dual challenge: discerning fractionalized particles and finding material candidates that realize fractionalization. This dissertation presents a comprehensive theoretical study of fractionalization in both one and two dimensions, focusing on these challenges. In one-dimensional systems, we explore quantum and frustrated ma (open full item for complete abstract)

Committee: Nandini Trivedi (Advisor); Mohit Randeria (Committee Member); Marc Bockrath (Committee Member); Christopher Hirata (Committee Member) Subjects: Physics

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

Magnetic materials are ubiquitous and are essential to everyday life and are essential for the development of new technologies. The focus of this dissertation is to investigate the magnetic properties of various solid-state materials containing 5d metals. There are mainly two characteristics that distinguish 5d metals from their 3d and 4d counterparts: higher levels of spin-orbit coupling and relatively larger d orbitals. Those differences make the magnetic properties of 5d containing materials more unpredictable and intriguing. In chapter 2, the materials studied adopted the double perovskite structure, where one of the B-sites is a 5d1 metal and the other is a diamagnetic cation. A combination of high-resolution powder diffraction techniques and solid-state NMR was utilized to explore the links between crystal structure, orbital ordering, and reported magnetism in 5d1 double perovskite systems. In Ba2ZnReO6 a cubic-tetragonal transition was observed at 23 K that breaks the degeneracy of the t2g orbitals and presumably leads to a pattern of orbital ordering that stabilizes magnetic ordering at 16 K. A similar pattern was observed for Ba2MgReO6. Prior theoretical works indicate that only the pattern of orbital order seen in the P42/mnm space group could stabilize the canted ferromagnetism of these states. Unfortunately, powder diffraction data is not sensitive enough to differentiate between the I4/mmm and P42/mnm structural models as the distortions are subtle. The near complete lack of magnetic scattering detected in NPD combined with the symmetry lowering suggests that the magnetism in these two compounds is an ordering of local quadrupolar moments rather than the much more ordinary ordering of dipolar magnetic moments. In chapter 3, the materials studied also adopted the double perovskite structure. The effect of non-stoichiometry on the cation distribution, crystal structure, and magnetic properties of a series of Cr-rich Sr2Cr1+xRe1−xO6 samples have been (open full item for complete abstract)

Committee: Patrick Woodward (Advisor); Yiying Wu (Committee Member); Casey Wade (Committee Member) Subjects: Chemistry

Master of Science (MS), Bowling Green State University, 2023, Physics

One of the key aspects in developing advanced nuclear reactor technology is the stability of the materials, which directly impacts the reactor's lifespan. These materials are subject to coupled extreme environmental stresses such as high radiation, corrosive media, and large temperature gradients, which synergistically contribute to the buildup of defects and eventual material failure. To build a comprehensive understanding of defect evolution, it is important to study the early stages of defect evolution, beginning with the formation of vacancies, voids, dislocations, and non-equilibrium defects. On the experimental front, it is challenging to quantify these vacancy-type defects with standard characterization techniques, as it requires sub-nanometer resolution. Positron Annihilation Spectroscopy (PAS) bridges this gap, comprising a set of non-destructive techniques capable of directly detecting atomic-scale defects, at concentrations as low as 1 vacancy per ten million atoms. This thesis will detail the work done for the ongoing development of two positron beamlines, with an emphasis on the beamline used for in-situ investigations of ion-induced damage in nuclear materials. The first measurements using the newly developed beamline are presented for low-dose radiation-induced self-ion damage in Fe. Later, refinements to the apparatus and experimental design are also discussed with regards to revisiting the experiment. Additionally, two studies on a class of high-performance multi-principal element/high-entropy alloys are discussed. Using positron annihilation lifetime Spectroscopy (PALS) and Doppler broadening spectroscopy (DBS), the phase structure and chemical complexity effects on MoNbTi-based alloys are explored with evidence of radiation resistance in MoNbTiZr and MoNbTi as well as defect recovery observed in MoNbTiVZr. Finally, exploratory measurements using DBS on a ternary mutli-principal element alloys (MPEA), CoCrNi, have recently been performed (open full item for complete abstract)

Committee: Farida Selim (Advisor); Alexey Zayak (Committee Member); Marco Nardone (Committee Member) Subjects: Condensed Matter Physics; Experiments; Materials Science; Nuclear Chemistry; Nuclear Engineering; Nuclear Physics; Physics; Quantum Physics; Radiation; Solid State Physics

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

Micromachines open new and exciting avenues for experimentation by providing the tools to directly examine and manipulate microscopic environments. This work presents two projects that utilize magnetically actuated DNA origami constructs as micromachines. A third project focuses on development of a system for simultaneous control and translation of multiple individual magnetic microbeads to predetermined positions on a plane. The work presented here demonstrates the power and utility of magnetically controlled micromachinery. DNA origami is a powerful tool for the construction of micromachines thanks to its modifiability, stability, and ability to polymerize into larger structures. A lever arm, formed via polymerization of individual DNA origami bricks and attached to a surface on one end, is one such micromachine. This DNA origami lever is a basic tool that is here used for measuring the magnetic properties of a single superparamagnetic microbead. The lever is actuated by a superparamagnetic microbead attached to the opposite end which enables controlled movement of the lever about the surface attachment point through external magnetic fields. Two potential methods utilizing such DNA levers for determining the magnetic moment of a single discrete microbead, each with their own merits and drawbacks, were developed. The resulting magnetic moments were consistent with magnetic moment values expected from previous experiments. Additionally, these levers offer a potential method for examining the interactions between neighboring superparamagnetic microbeads. In the second DNA construct, a magnetically actuated DNA origami hinge was adapted as a device towards measuring individual molecular bond strengths. The hinge was customized to allow for attachment of specific interacting molecules to the top and bottom arms of the hinge. By closing the hinge with a molecule or molecules that attach to the upper and lower arm, the bond strength holding the hinge closed could (open full item for complete abstract)

Committee: Ratnasingham Sooryakumar (Advisor); Gregory Lafyatis (Committee Member); Fengyuan Yang (Committee Member); Yuan-Ming Lu (Committee Member) Subjects: Physics

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

Nominally metallic systems can be rendered insulating by electronic interactions, disorder, or both, leading to a myriad of interesting many-body phases. In this thesis, we present electronic transport data on a variety of such insulator materials, each with their own unique emergent phenomena. We start with few-layer graphene (FLG), the multilayer counterpart to monolayer graphene, and show that electronic interactions can lead to the development of an electronic energy gap in the band structure near charge neutrality. Previously, this has been associated with spontaneous inversion symmetry breaking, but has only been observed in suspended devices of the highest quality. Here, we show that similar physics can be observed in hexagonal boron nitride-encapsulated devices, alleviating the requirement for suspension. Moreover, in very thick FLG samples, typically thick enough to be considered as three-dimensional graphite, we show the existence of fractional quantum Hall states that are extended through the bulk of the material. Next, we turn to Pt-doped TiSe2, where the interplay between a charge density wave state and a newly discovered quasi one-dimensional insulating state gives rise to ultra slow time-scale physics, along with a strong resistance anisotropy. Finally, transport data as well as angle-resolved photoemission spectroscopy data on Se-doped Ge2Sb2Te5 devices are shown. Here, a disorder-induced metal-to-insulator transition exhibits unique properties, which we attribute to the onset of strong electronic interactions.

Committee: Stuart Raby (Committee Member); Nandini Trivedi (Committee Member); Roland Kawakami (Committee Member); Marc Bockrath (Advisor) Subjects: Physics

Doctor of Philosophy, Case Western Reserve University, 2022, Physics

In a world where modern micro- and nanoelectronics are reaching their limits, a spotlight has been focused on novel two-dimensional and layered materials. It is critically important to understand the electronic transport properties of potentially useful semiconductors in order to gauge their applicability for electronic devices. This dissertation explores layered semiconductor materials and characterizes their gate transport properties within the context of field effect transistor devices. One project examines MoO3 and its versatility as a layered gate dielectric material integrated onto a WSe2 field effect transistor. The following project concentrates on the anisotropic and ambipolar transport properties of ReSe2. Lastly, the subthreshold regime of the gate characteristic curve is explored for ambipolar devices, illustrating how to tune this behavior in WSe2 devices with respect to different contact metals. Overall, the electronic characterization of these materials will aid in developing new, functional nano-scale devices.

Committee: Xuan Gao (Advisor); Walter Lambrecht (Committee Member); Alp Sehirilioglu (Committee Member); Jesse Berezovsky (Committee Member); Shulei Zhang (Committee Member) Subjects: Condensed Matter Physics; Materials Science; Nanoscience; Nanotechnology; Physics; Solid State Physics

Master of Science, University of Akron, 2021, Physics

Classical thermodynamic phase transitions have been studied since the 19th century and are driven by thermal fluctuations. By contrast, the more recently discovered quantum phase transitions are driven by quantum fluctuations and may occur even at zero temperature. Both classical and quantum phase transitions are characterized by a change in symmetry of the state. In topological phase transitions the two phases on either side of the transition have the same symmetry. Rather than a change in symmetry the transition is characterized by a change in topology. In this thesis we demonstrate a remarkable correspondence between three models, each of which is considered an archetypal representative of a distinct transition type. These models are the classical Ising model in two dimensions, the transverse quantum Ising model in one dimension and the Kitaev model. Both Ising models describe magnets but differ in that the basic degrees of freedom are classical spins in one case and quantum spins in the other. The Kitaev model describes a superconducting wire; the basic degrees of freedom are fermionic quasiparticles. The correspondence between the classical Ising model in two dimensions and the quantum transverse Ising model in one dimension is based on the general correspondence between quantum statistical mechanics in D dimensions and classical statistical mechanics in D+1 dimensions. We review the well iiiknown derivation of this correspondence using a transfer matrix representation of the classical partition function and taking an anisotropic continuum limit. Surprisingly we find that there is also a correspondence between the quantum phase transition in the transverse Ising model and the topological phase transition undergone by the Kitaev model. We establish this by using the Jordan-Wigner transformation which transforms the spins of the quantum Ising model into the fermions of the Kitaev model. This mapping will allow us to investigate the counterp (open full item for complete abstract)

Committee: Yu-Kuang Hu (Advisor); Harsh Mathur (Advisor); Jutta Luettmer-Strathmann (Committee Chair) Subjects: Physics

Doctor of Philosophy, Case Western Reserve University, 2020, Physics

Within the past decade or so, semiconductor physics has turned a keen eye on two dimensional systems, with the pivotal investigation of atomically thin carbon films. The remarkable figures of merit produced by graphene in electronic and electrochemical applications, in contrast to bulk carbon properties, are indicative of the potential that layered materials might possess in their own right. Transition metal oxides offer a relatively unexplored facet of 2D semiconductor technology; these materials are often overlooked due to their wide band gaps when considering new subjects for nanostructure study. However, oxides offer a library of interesting properties, many of which are still not fully understood, and can be easily modified through doping to engineer new characteristics. Herein, three studies are discussed, where characterization of layered oxides, modified via various methods of doping, result in unique behaviors. The first study involves varying oxygen stoichiometry in α-MoO3, where transport is controlled by quantifiable reduction of grown α-MoO3 nanoflakes. The second details the study of LixCoO2, the staple cathode material used in lithium-ion batteries. This material exhibits unique charge-ordering phenomena as a function of lithium content, and is explored in its few-layer, single-crystal form for the first time. Finally, V2O5 is investigated, which displays p-type characteristics and a surface scattering effect when partially doped with sodium. The band structure is analyzed to explain these behaviors. The findings of these studies may play a key role in engineering thin oxide systems for future electronics applications.

Committee: Xuan Gao Professor (Advisor); Walter Lambrecht Professor (Committee Member); Jesse Berezovsky Associate Professor (Committee Member); Alp Sehirlioglu Associate Professor (Committee Member) Subjects: Condensed Matter Physics; Materials Science; Physics

Doctor of Philosophy, Case Western Reserve University, 2020, Physics

Before 1980, the principle of broken symmetry was the key concept for the classification of states of matter. The discovery of quantum Hall effect by Klaus von Klitzing, started a new paradigm in physics by providing the first example of a quantum phase transition where no spontaneous symmetry was broken. The unique feature of these quantum phases is the fact that even though two ground states share the same symmetry, they still belong to different phases (i.e. cannot be continuously tuned to each other through a smooth deformation of the Hamiltonian). This is a result of the fact that the wave functions that are defined on the n-dimensional Brillouin zone are "knotted" in non-trivial ways, despite having the same symmetry. In this talk, we start by describing a symmetry enforced nodal line semi-metal (NLSM) in the 2D flat form of honeycomb Group - V and its non trivial thermo-electric response. We will then proceed to show that, upon buckling, the system undergoes its first phase transition from NLSM to 2 sets of oppositely wound unpinned Dirac cones protected by C_2 symmetry. Further buckling leads to these unpinned Dirac cones annihilating in pairs at two distinct critical angle leading to a second topological phase transition to an insulating state. We then show that this seemingly innocuous insulating state is indeed a weak topological crystalline insulator. Furthermore, upon closer look, this insulating state turns out to be a Higher Order Topological Insulator (HOTI) that is protected by S_6 symmetry. HOTIs are d-spatial dimensional systems featuring topologically protected gap-less states at their (d-n)-dimensional boundaries with d>1. In a broader context, we will see that the the topological properties of buckled Group - V stem from the fact that they topologically belong to the class of Obstructed Atomic Limit (OAL) insulators. Combining all these, we will prove that annihilating pairs of Dirac fermions necessitate a topological phase transition from the (open full item for complete abstract)

Committee: Lambrecht Walter (Advisor); Mathur Harsh (Committee Member); Gao Xuan (Committee Member); Sehirlioglu Alp (Committee Member) Subjects: Condensed Matter Physics; Physics; Theoretical Mathematics; Theoretical Physics

Doctor of Philosophy, The Ohio State University, 2020, Physics

We probe the effects of applying an external magnetic field to Kitaev's spin S=1/2 model on a honeycomb lattice [1] and study this model's low-lying energy excitations across varying field strengths (and field orientations) using exact diagonalization. We go beyond Kitaev's original third order perturbation result (as well as some later studies [2]) in order to explore the effects of significantly stronger fields (up to the fully polarized limit), and we discover the presence of a novel gapless quantum spin liquid (QSL) phase sandwiched between Kitaev's lower-field gapped QSL phase and the forced ferromagnetic phase, for fields pointing along [111]. We discern this new intermediate-field strength gapless QSL by identifying signature features in the Kitaev honeycomb model's low-lying energy spectrum, topological entanglement entropy, on-site dynamical response, real-time on-site dynamics, and the average of plaquette flux operator expectations. We further explore the evolution of these measures away from a field pointing along [111] to gauge the robustness of the intermediate-field strength gapless QSL phase along varying field orientations. Finally, and in order to characterize the newfound intermediate-field gapless QSL phase, we perform a symmetry analysis of the Kitaev honeycomb model and derive a spinon hopping Hamiltonian consisting of five parameters. We vary these parameters to construct a many-body trial wavefunction whose energy, magnetization, and various other physical properties we calculate. We obtain the many-body ground state wavefunction in the completely polarized limit by minimizing the energy and then evolve away from this limit towards lower field strengths to find pockets at the M-points of the noninteracting Hamiltonian's band structure.

Committee: Nandini Trivedi (Advisor); Yuan-Ming Lu (Committee Member); Rolando Valdes Aguilar (Committee Member); Yuri Kovchegov (Committee Member) Subjects: Condensed Matter Physics; Physics

Master of Sciences, Case Western Reserve University, 2020, Physics

In this thesis we explore mechanical analogs of electronic topological insulators. We develop continuum models for the mechanical instability and spontaneous symmetry breaking for monolayer antimonene and bilayer graphene. We find that walls form between domains corresponding to different symmetry breaking minima. These domain walls are solitons in our model. Perturbations about the symmetry breaking equilibria propagate as waves with a gapped dispersion in the bulk but there is a gapless mode with linear dispersion that propagates along the domain wall in a manner reminiscent of the electronic edge modes of a topological insulator. We establish that monolayer antimonene is a mechanical topological insulator by demonstrating a mapping between our continuum model and an underlying Dirac equation of the symmetry class BDI which is known to be a topological insulator in one dimension and a weak topological insulator in two dimensions. Following a similar argument we expect that bilayer graphene as well is a weak topological insulator in two dimensions. We surmise that the effects studied here (namely low scale symmetry breaking, strain solitons and gapless edge modes) are not limited to antimonene and bilayer graphene but are common features of two dimensional materials.

Committee: Harsh Mathur (Advisor) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy, Case Western Reserve University, 2019, Physics

Soft condensed matter is the study of flexible materials that change their shape under the influence of a weak force. Typical examples are polymers and liquid crystals. Chromonic liquid crystals consist of molecules having a hydrophobic flat aromatic core to which have been attached ionic groups or non-ionic groups. These disk-like molecules self-assemble into ordered phases in the presence of solvents, aggregating in a face-to-face fashion, and creating stacked linear molecular aggregates. The degree of self-organization into columns depends on the concentration, temperature, and pressure. Depending on the types of terminal groups, the molecules can be dissolved in organic solvents or water. A theoretical study was performed to model the detailed effects of adding stacking, translation, and rotation energy on the length distribution of chromonic liquid crystals. A second derivative test was done to prove that all the solutions for isotropic phases are at a global minimum of the free energy. The free energies and length distributions for various nematic phase formulations are presented. Free energy and length distribution for both Maier-Saupe and Onsager interactions are presented. The free energy and length distribution for considering I-N phase co-existence and no phase co-existence are presented. It is shown that the nematic phase solutions all lead to a first-order phase transition. The nematic phase solution with the Maier-Saupe interaction causes the averaged aggregate length to increase when the system transitions from the isotropic phase to the nematic phase, while the nematic phase solution with the Onsager interaction causes the averaged aggregate length to decrease. Another major component of this thesis is the study of glass transitions. An analytical study that aims to describe the mean squared displacement at low temperatures is presented. The short-time and long-time limit of the analytical solution are discussed. Then, an app (open full item for complete abstract)

Committee: Philip Taylor (Advisor); Mesfin Tsige (Advisor); Wojbor Woyczynski (Committee Member); Harsh Mathur (Committee Member) Subjects: Mathematics; Physics; Polymers; Statistics

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

In crystalline materials, the symmetry of the crystal lattice imposes strict conditions on the observable properties of the material. These symmetry restricted conditions can be, in turn, probed by light via the electromagnetic interaction. Studying the electromagnetic excitations in solids can reveal many fundamental properties of these systems. A quick introduction and guide to symmetry in solids will be given, with an emphasis on how it can be used to interpret spectroscopic measurements. The measurement techniques used will also be described. Time domain Terahertz spectroscopy (TDTS) is the main technique used in this dissertation. Important experimental considerations pertaining to the construction of the THz spectrometer will be given. In the multiferroic Sr_2 FeSi_2O_7, we found multiple excitations in the few meV energy scale (THz), in the material's paramagnetic phase. Measurements with varying temperature and magnetic field revealed that these excitations are both electric and magnetic dipole active. By considering the ground state of the Fe 2+ magnetic ion in Sr 2 FeSi 2 O 7 , we concluded that our observation is coming from the spin-orbital coupled states of the ion. This realization demonstrated that spin-orbit coupling plays a crucial role in these exotic materials. Interestingly, these spin-orbital THz excitations persist into the magnetically ordered phase. The single-ion picture of the paramagnetic phase needs to be expanded theoretically to explain our observations. CaFe_2O_4 orders antiferromagnetically below ~ 200 K. Two co-existing magnetic structures (A and B phase) have been measured previously by neutron diffraction. The anti-phase boundaries between these two phases have been proposed to be the cause of the quantized magnetic excitations (magnons) measured by an inelastic neutron scattering study. We measured two antiferromagnetic resonances (magnons) with TDTS. Our observation can be explained by the orthorhombic crystal anisotropy of CaF (open full item for complete abstract)

Committee: Rolando Valdes Aguilar (Advisor); P. Chris Hammel (Committee Member); Nandini Trivedi (Committee Member); Douglass Schumacher (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2017, Physics

Spintronics research over the past two decades has focused on developing an understanding of spin transport in materials currently used in the semiconductor industry for potential spin-based applications. Recently, a surge of interest in spin transport in antiferromagnetic materials and spin interactions in novel materials have led to many promising discoveries. The work in this dissertation explores discrepancies in past discoveries, provides evidence to support recent theories on antiferromagnetic (AFM) spin transport, and develops the capabilities to further explore spin physics in novel states of matter. This dissertation focuses on four primary topics. First, a thickness dependence of the spin Hall angle in Au is discussed as a potential explanation for a large variance in the previously reported values. Second, evidence supporting a highly efficient mode of spin transport mediated by AFM fluctuations is found in the temperature dependence of the spin pumping signal in Pt/NiO/Y3Fe5O12 trilayers. Third, a new low damping metallic ferromagnet is developed and characterized as a potential platform for future spintronic research. Finally, a molecular beam epitaxy system is established with the capabilities to prepare topologically insulating materials that are predicted to host many novel phenomena.

Committee: Fengyuan Yang (Advisor) Subjects: Physics

Doctor of Philosophy, Case Western Reserve University, 2017, Physics

With the advancement of lithography technology and the higher requirement demands of switching devices, smaller scales of miniaturization and materials with higher mobility are required. 2D layered materials, as one of the platforms to satisfy such requirements have drawn attention from both theorists and experimentalists and unique, exotic phenomena on the material class becomes more relevant. This work explores such subjects on InSe, SnS and V2O5. In each material, different aspects of the field of study are visited. 1.) InSe: Demonstration of high mobility of the material, effect from dielectric. 2.) SnS: Debye screening and its effect on switching performances 3.) V2O5: Electrical and Optical anisotropic properties of 2D layered materials.

Committee: Xuan Gao PhD (Advisor); Walter Lambrecht PhD (Committee Member); Kathleen Kash PhD (Committee Member); Phillip Feng PhD (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2002, Graduate School

Committee: Not Provided (Other) Subjects: Physics

Doctor of Philosophy, Case Western Reserve University, 2016, Physics

The work discussed in this thesis represents the accumulation of research I performed throughout my doctoral studies. My studies were focused towards two-dimensional electronic transport in semiconductor and perovskite oxide interfaces. Electronic materials with low dimensionality provides experimentalists and theorists with incredible systems to probe physics at non-intuitive levels. Once considered “toy problems,” low-dimensional systems, particularly in two dimensions, are now treated as highly relevant, modern electronic materials on the verge of being used in next-generation technology. This thesis entails three main parts, each contributing new knowledge to the field of two-dimensional electronics and condensed matter physics in general. The first part, found in Chapter 3, analyzes short-range scattering effects in two-dimensional GaAs/AlGaAs quantum wells. The effect of aluminum concentration in the material is correlated to the non-monotonic resistance behavior at low temperatures through the short-range disorder potential. By accounting for different electronic scattering mechanisms, temperature-dependent resistance is shown to have a universal behavior, independent of short-range scattering. Chapters 4 transitions from two-dimensional electron gasses in GaAs to quasi-two-dimensional electron gasses in perovskite oxides, specifically gamma-Al2O3/SrTiO3 heterointerfaces. For the first time in that system, a metal-to-insulator transition is measured by backgating the strontium titanate. By measuring the carrier density, it is shown that immobile charge carriers are induced through backgating. Chapter 5 discusses my research on the cubic-to-tetragonal structural phase transition in LaAlO3/SrTiO3 heterointerfaces. By engineering micron-scale devices, I was able to measure the electronic transport properties of tetragonal domain walls below the structural transition temperature. Domain walls are shown to cause anisotropic resistance, which is measurable o (open full item for complete abstract)

Committee: Xuan Gao (Advisor); Harsh Mathur (Committee Member); Kathleen Kash (Committee Member); Alp Sehirlioglu (Committee Member) Subjects: Condensed Matter Physics; Physics; Solid State Physics

Doctor of Philosophy, The Ohio State University, 2015, Physics

The coupling of spin and charge as achieved by the giant and tunneling magnetoresistance effects revolutionized the data storage industry and ushered in the field of spintronics. In this dissertation, I present novel spin sensing and manipulation techniques.

Committee: P. Chris Hammel Professor (Advisor); Ezekiel Johnston-Halperin Professor (Committee Member); Klaus Honscheid Professor (Committee Member); Nandini Trivedi Professor (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2014, Physics

Co2FeSi is one of the most studied half metallic Heusler compounds with the potential for room temperature magnetoelectronic applications on account of it having the highest know Curie temperature (above 1000 K) among potential ferromagnets. Despite its promise, researchers have had mixed results in growing films of sufficiently high quality to realize its full potential. By identifying and controlling critical factors that complicate attempts to grow high quality films of Co2FeAlxSi1-x , we have overcome many obstacles including off-stoichiometry, impurity phase formation and disorder. All of which must be addressed in order to achieve half-metallicity. This dissertation reports an in-depth investigation that addresses several critical issues about the deposition of Co2FeAlxSi1-x epitaxial films using off-axis ultrahigh vacuum sputtering. We have discovered that, the choice of a lattice matched substrate plays a dominant role in the stoichiometry and phase formation of these films. Film disorder was categorized by nuclear magnetic resonance (NMR) as well as X-ray diffraction (XRD) to identify both short and long range order in the crystal. X-ray diffractometry shows the films to be epitaxial, pure-phase and well ordered. Magnetic characterization was done via vibrating sample magnetometry (VSM) and superconducting quantum interference device (SQUID) magnetometry. We observed a saturation magnetization of 5.5-6.0 mB per formula unit at T = 300 K, roughly in line with expectation for the film stoichiometry and ordering level and consistent with the Slater Pauling rules. X-ray diffraction and nuclear magnetic resonance gave high values (~74-87%) for the level of L21 and B2 ordering in these films. Point Contact Andreev reflections were also performed on these films to evaluate the level of spin polarization in these materials. The Co2FeAlxSi1-x films grown exhibit a combination of several desired properties and should be a suitable material candidate for spintronics (open full item for complete abstract)

Committee: Fengyuan Yang (Advisor); Ezekiel Johnston-Halperin (Committee Member); Ralf Bundschuh (Committee Member); Andrew Heckler (Committee Member) Subjects: Materials Science; Physics