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Petersen, Greg M.Anderson Localization in Low-Dimensional Systems with Long-Range Correlated Disorder
Doctor of Philosophy (PhD), Ohio University, 2013, Physics and Astronomy (Arts and Sciences)
It has been known for over a half century that when disorder is introduced into a crystalline system through impurities, vacancies, grain boundaries, or other mechanisms that break the translational invariance of the system, the electronic eigenstates will localize in space. This phenomenon is called Anderson localization after Phillip Anderson who first predicted it in 1958. Since then, Anderson localization has remained a vibrant topic of research due to new experimental methods of probing localized states, an increase in computational power, and its accurate phenomenological representation of many physical systems. To date, a large amount of effort has been dedicated to the study of uncorrelated, or short-range correlated, disorder distributions. However, more recent efforts on long-range correlated disorder distributions have yielded richer results, challenging the foundations of Anderson localization theory. In this document, we focus on characterizing the naturally occurring ~ 1/r correlation, and the phenomenologically rich ~ 1/k correlation in one-dimensional systems. We will discuss several important numerical and analytical methods for determining a localization-delocalization transition along with the localized phase itself. Finally, we will discuss on-going work related to novel two-dimensional disordered materials where random spin-orbit interactions are predicted to cause suppressed spin transport.

Committee:

Nancy Sandler (Advisor)

Subjects:

Condensed Matter Physics; Low Temperature Physics; Physics; Quantum Physics; Solid State Physics; Theoretical Physics

Keywords:

locailzation; long-range disorder; disorder; scale-free; scale free; long range; power-law; power law; graphene; 1D; one-dimension; one dimension; rashba; RRF; random; anderson localization; anderson; correlation; correlated disorder; correlated;

Nawarange, Amruta V.Optical Emission Spectroscopy during Sputter Deposition of CdTe Solar Cells and CuTe-Based Back Contacts
Doctor of Philosophy, University of Toledo, 2011, Physics

In this dissertation sputtering processes are studied in detail through optical emission spectroscopy. In order to extract plasma parameters, experimental data and simulations were matched together. We could extract excitation temperatures, vibrational temperatures and rotational temperatures of the plasmas. To explain the simulations and to understand the different mechanisms involved in the sputtering plasmas, relevant aspects of atomic spectroscopy and molecular spectroscopy are reviewed here. A mixture of argon and nitrogen gas was used to sputter a CuxTe target by RF magnetron sputtering. The emission data were then studied as a function of deposition pressure and RF power. These data show many non equilibrium aspects of the plasma; however, in most cases the data are consistent with energy distributions of the rotational, vibrational, and electronic systems that can be characterized individually by distinct temperatures.

We have also used sputter deposition of CuxTe thin-film layers instead of our standard Cu/Au metal layers for back contacts to look for an improved back contact. We prepared three different compositions of CuxTe target material and studied the properties of sputtered films using X-Ray Diffraction (XRD), Energy Dispersive X-Ray Spectroscopy (EDS), Scanning Electron Microscopy (SEM) and Hall measurements. At optimized deposition conditions for Cu2Te target sputtered films (2 nm thickness and 20 minutes annealing in vacuum) as determined from the thin-film properties, we sputtered this layer onto the back surface of the CdTe of the cell structure. We achieved efficiencies of 13.1% using Cu2Te target sputtered films followed by Au which is very close to our best efficiency achieved with Cu/Au contacts.

Committee:

Alvin Compaan, PhD (Advisor); Alvin Compaan, PhD (Committee Chair); Brian Bagley, PhD (Committee Member); Randall Ellingson, PhD (Committee Member); Sanjay Khare, PhD (Committee Member); Dean Giolando, PhD (Committee Member)

Subjects:

Alternative Energy; Condensed Matter Physics; Experiments; Materials Science; Molecular Physics; Physics; Plasma Physics; Solid State Physics; Theoretical Physics

Keywords:

OES

Bernard, BenjaminOn the Quantization Problem in Curved Space
Master of Science (MS), Wright State University, 2012, Physics

The nonrelativistic quantum mechanics of particles constrained to curved surfaces is studied. There is open debate as to which of several approaches is the correct one. After a review of existing literature and the required mathematics, three approaches are studied and applied to a sphere, spheroid, and triaxial ellipsoid.

The first approach uses differential geometry to reduce the problem from a three dimensional problem to a two-dimensional problem. The second approach uses three dimensions and holds one of the separated wavefunctions and its associated coordinate constant. A third approach constrains the particle in a three-dimensional space between two parallel surfaces and takes the limit as the distance between the surfaces goes to zero.

Analytic methods, finite element methods, and perturbation theory are applied to the approaches to determine which are in agreement. It is found that the differential geometric approach has the most agreement.

Constrained quantum mechanics has application in materials science, where topological surface states are studied. It also has application as a simplified model of Carbon-60, graphene, and silicene structures. It also has application as in semiclassical quantum gravity, where spacetime is a pseudo-Riemannian manifold, to which the particles are constrained.

Committee:

Lok Lew Yan Voon, PhD (Advisor); Morten Willatzen, PhD (Committee Member); Gary Farlow, PhD (Committee Member); Doug Petkie, PhD (Other)

Subjects:

Atoms and Subatomic Particles; Condensed Matter Physics; Mathematics; Nanoscience; Nanotechnology; Nuclear Physics; Physics; Quantum Physics; Solid State Physics; Theoretical Mathematics; Theoretical Physics

Keywords:

quantum mechanics; differential geometry; constrained quantum mechanics; schrodinger equation; finite element method; spheroid; ellipsoid; sphere; cylinder

Prabhakar, TejasStudy of Earth Abundant TCO and Absorber Materials for Photovoltaic Applications
Master of Science, University of Toledo, 2013, Physics
In order to make photovoltaic power generation a sustainable venture, it is necessary to use cost-effective materials in the manufacture of solar cells. In this regard, AZO (Aluminum doped Zinc Oxide) and CZTS (Copper Zinc Tin Sulfide) have been studied for their application in thin film solar cells. While AZO is a transparent conducting oxide, CZTS is a photovoltaic absorber. Both AZO and CZTS consist of earth abundant elements and are non-toxic in nature. Highly transparent and conductive AZO thin films were grown using RF sputtering. The influence of deposition parameters such as working pressure, RF power, substrate temperature and flow rate on the film characteristics was investigated. The as-grown films had a high degree of preferred orientation along the (002) direction which enhanced at lower working pressures, higher RF powers and lower substrate temperatures. Williamson-Hall analysis on the films revealed that as the working pressure was increased, the nature of stress and strain gradually changed from being compressive to tensile. The fall in optical transmission of the films was a consequence of free carrier absorption resulting from enhanced carrier density due to incorporation of Al atoms or oxygen vacancies. The optical and electrical properties of the films were described well by the Burstein-Moss effect. CZTS absorber layers were grown using ultrasonic spray pyrolysis at a deposition temperature of 350 C and subsequently annealed in a sulfurization furnace. Measurements from XRD and Raman spectra confirmed the presence of pure single phase Cu2ZnSnS4 . Texture analysis of as-deposited and annealed CZTS films indicated that the (112) plane which is characteristic of the kesterite phase was preferred. The grain size increased from 50 nm to 100 nm on conducting post-deposition annealing. CZTS films with stoichiometric composition yielded a band gap of 1.5 eV, which is optimal for solar energy conversion. The variation of tin in the film changed its resistivity by several orders of magnitude and subsequently the tin free ternary chalcogenide Cu2ZnS2 having very low resistivity was obtained. By carefully optimization of concentrations of tin, zinc and copper, a zinc-rich/tin-rich/copper-poor composition was found to be most suitable for solar cell applications. Etching of CZTS films using KCN solution reduced their resistivity, possibly due to the elimination of binary copper sulfide phases. CZTS solar cells were fabricated both in the substrate and superstrate configurations.

Committee:

Yanfa Yan (Committee Chair); Victor Karpov (Committee Co-Chair); Alvin Compaan (Committee Co-Chair)

Subjects:

Condensed Matter Physics; Electrical Engineering; Energy; Engineering; Experiments; Materials Science; Morphology; Nanotechnology; Physics; Plasma Physics; Solid State Physics; Sustainability; Technology; Theoretical Physics

Keywords:

Solar cells; Photovoltaic; CZTS; Thin films; Materials Science; TCO; Transparent conducting oxide; ZnO; AZO; Zinc oxide; Sputtering; Spray pyrolysis; Williamson-Hall; Stress; lattice strain; Argon flow rate; Working pressure; Earth abundant

Ruiz-Tijerina, David A.Kondo Physics and Many-Body Effects in Quantum Dots and Molecular Junctions
Doctor of Philosophy (PhD), Ohio University, 2013, Physics and Astronomy (Arts and Sciences)
In this document we present a study of the thermodynamic and transport properties of two kinds of quantum impurity systems in the Kondo regime. The first system consists of a spin-1 molecule in which mechanical stretching along the transport axis produces a magnetic anisotropy. We find that a generic coupling between a vibrational mode along this axis and the molecular spin induces a correction to the magnetic anisotropy, driving the ground state of the system into a non-Fermi-liquid phase. A transition into a Fermi-liquid ground state can then be induced by means of stretching, going through an underscreened spin-1 Kondo ground state at zero effective anisotropy. In the second system we study the effects of a charge detector, implemented by a quantum point-contact (QPC), on the Kondo state of a nearby spin-1/2 quantum dot (QD). While making the charge detection possible, the Coulomb interaction between the electrons traversing the QPC and those within the QD contribute to decoherence of the Kondo state. By modeling the QPC as two metallic terminals connected to an intermediate localized level, we can explore three transport regimes of the detector: a zero-conductance regime, a finite-conductance regime in mixed valence, and unitary conductance in a Kondo ground state that has been suggested as an explanation to the "0.7 anomaly" in QPCs. Transitions between these different ground states can be achieved by tuning the strength of a capacitive coupling that parameterizes the electrostatic interaction.

Committee:

Sergio Ulloa, Prof. (Advisor); Wojciech Jadwisienczak, Ph.D (Committee Member); Saw Hla, Prof. (Committee Member); Horacio Castillo, Ph.D. (Committee Member); Nancy Sandler, Ph.D. (Committee Member)

Subjects:

Condensed Matter Physics; Low Temperature Physics; Nanoscience; Nanotechnology; Physics; Quantum Physics; Solid State Physics

Keywords:

NRG; Numerical Renormalization Group; Kondo effect; many body physics; quantum phase transition; molecular junction; quantum dot; quantum point contact; electronic transport; highly correlated electrons; capacitive coupling; charge detector

Wijesundara, Kushal ChinthakaUltrafast Exciton Dynamics and Optical Control in Semiconductor Quantum Dots
Doctor of Philosophy (PhD), Ohio University, 2012, Physics and Astronomy (Arts and Sciences)

Device miniaturization with advanced fabrication techniques has revolutionized the semiconductor industry along with innovative concepts of carrier spin, potentially important at the fundamental physical limits of scalability. For spin-based information processing, semiconductor coupled quantum dots (CQDs) provide excellent control in spin dynamics due to 3-D confinement, discrete energy levels, and optical orientation and coupling. The research presented in this dissertation investigates spin interactions and exciton relaxation channels in semiconductor CQDs measured through optical control and time-resolved experimental techniques.

Our experiments involving photoluminescence (PL) and photoluminescence excitation (PLE) methods revealed effects arising from the structural properties of semiconductor nanostructures, including quantum rings and CQDs. High resolution PL measurements on positively charged exciton states demonstrated experimental evidence of isotropic exchange interaction. Controlling exchange interaction in different spin configurations is fundamental to quantum logic operations. Hence, polarization dependent PL experiments were executed and electric field tunable exchange interaction effects were reported on the neutral exciton states.

Next, time-resolved measurements were performed while pumping above the InAs wetting layer (WL) energy and probing below the WL to determine the dynamics of the optically generated electric field in CQDs. The observed, rapid onset of the optically generated electric field may provide the use of CQDs for optical switching applications.

Finally, carrier relaxations in the CQDs were identified through the dynamics of the spatially indirect exciton state using a mode-locked laser excitation source and standard time-resolved single photon counting technique. Wave function distribution, carrier tunneling, and phonon scattering led to the observed lifetime and intensity modulations. With time-resolved PLE, bi-exponential lifetime decay revealed non-monotonic phonon relaxations as a result of the structure factor of the CQDs. Furthermore, with resonant excitation, carrier tunneling into charged exciton states was also eliminated. These results demonstrated tunable exciton relaxation rates in CQDs, which are useful for quantum information, optoelectronics, and photonics applications.

Committee:

Dr. Eric A. Stinaff (Advisor); Dr. Alexander Govorov (Committee Member); Dr. Jeffrey Rack (Committee Member); Dr. Saw-Wai Hla (Committee Member)

Subjects:

Condensed Matter Physics; Materials Science; Molecular Chemistry; Molecular Physics; Nanoscience; Nanotechnology; Optics; Physics; Quantum Physics; Solid State Physics

Keywords:

Nanophotonics; Ultrafast Optics; Time-Resolved Photoluminescence; Photovoltaics; Semiconductor Quantum Dots; Coupled Quantum Dots; Quantum Dot Molecules; Spin Polarization; Excitons; Exciton Lifetimes; Tunable Exciton Dynamics; Phonon-Mediated Relaxation

Rolon Soto, Juan EnriqueCoherent Exciton Phenomena in Quantum Dot Molecules
Doctor of Philosophy (PhD), Ohio University, 2011, Physics and Astronomy (Arts and Sciences)
We investigate different aspects of the coherent dynamics of excitons in quantum dot molecules. A theoretical model is developed in order to extract the Forster energy transfer signatures in tunnel coupled quantum dot molecules in the presence of strong interdot tunneling. It is found that Forster coupling can induce spectral doublets in the excitonic dressed spectrum, which is suitable for detection in level anticrossing spectroscopy. The coherent exciton dynamics is investigated both in the closed and open quantum system approach by means of the Lindblad master equation. An adiabatic elimination procedure using the projection operator formalism allows us to extract effective Hamiltonians to describe analytically all relevant anticrossing gaps of the dressed spectrum. It is found that a pair of two indirect excitons can be used as the computational basis of a qubit. An adiabatic control pulse is constructed in order to manipulate the indirect exciton qubit and characterize its coherent dynamics, as well as its decoherence due to spontaneous recombination. On the other hand, recent experiments have shown that indirect excitons in hole tunnel coupled quantum dot molecules exhibit indirect exciton oscillatory relaxation rates, as function of an applied electric field. To this end we developed a model for the experimental results, in which we incorporate relaxation due to exciton-acoustic phonon coupling. We characterize the scattering structure factor and found that it contains an electrically tunable phase relationship between the phonon wave and the hole wave function, which leads to interference effects and oscillatory relaxation rates.

Committee:

Sergio Ulloa, Dr (Advisor); Alexander Govorov, Dr. (Committee Member); Eric Stinaff, Dr. (Committee Member); P. Gregory Van Patten, Dr. (Committee Member)

Subjects:

Materials Science; Molecular Chemistry; Molecular Physics; Nanoscience; Optics; Physics; Quantum Physics; Solid State Physics; Theoretical Physics

Keywords:

Feshbach; Projection Operator Formalism; Adiabatic Elimination; Indirect excitons; Excitons; Quantum Dot Molecules; Qubits; Forster; Resonant Energy Transfer; Qubits; Exciton Qubits; Coupled Quantum Dots; Tunable Exciton Relaxation; Exciton Lifetimes;

Vanfossen, Joseph ACHARM MESON PRODUCTION IN AU-AU COLLISIONS AT sqrt(s_NN) = 200 GEV AT RHIC
PHD, Kent State University, 2017, College of Arts and Sciences / Department of Physics
This research work is in the field of experimental nuclear physics, more specifically, the analysis of data taken with the Solenoidal Tracker at RHIC (STAR) apparatus at the Relativistic Heavy Ion Collider (RHIC) located at Brookhaven National Laboratory (BNL). There, we accelerate and collide beams of heavy ions (e.g. gold nuclei) at relativistic velocities. The collisions of heavy nuclei in the STAR Experiment compress nuclear matter to high densities, and heat it to extreme temperatures, over one trillion degrees Celsius. Under such conditions, Lattice QCD and other phenomeno- logical models predict a phase transition in nuclear matter, a transition, where quarks and gluons become deconfined, i.e. they freely move throughout the interaction volume and are no longer confined to individual nucleons, forming Quark Gluon Plasma (QGP), a new state of nuclear matter. The study of QGP, its properties and dynamics, will provide a better understanding of QCD, the strong force, and of the history of the early universe. Mesons containing heavy flavor (charm and bottom) quarks can be used in QGP searches. Heavy quarks are produced mainly in the early stages of a collisions via energetic parton-parton interactions; heavy flavor production in QGP or during hadronization is suppressed due to the high masses of the quarks. Heavy quarks can therefore be used to probe the whole evolution of the system and as a calibrated tool to better understand the nature of the early, hot matter formed in the collisions. A key finding by the experiments at RHIC is the anomalously low production of heavy flavor at high transverse momentum values. This was found by measuring the yields of the decay electrons from mesons containing either charm or bottom quarks. These measurements suffer from very large combinatorial backgrounds and conceal the parent’s kinematic properties. A suppression of particle production at high transverse momenta is likely caused by their interaction with the hot and dense surrounding medium, as the quarks traverse it. Such suppression is an indicator that the medium generated in relativistic heavy-ion collisions is strongly interacting. Theoretical models were successful in describing the suppression of light quarks but under-predicted the observed heavy-flavor suppression. The data triggered a new effort in modeling where theorists started taking into account the energy loss due to elastic collisions between the traversing parton and the surrounding medium. To fully understand the interplay between elastic and inelastic collision mechanisms of light and heavy partons and the hot medium, we needed precise data on heavy flavor production. Also, in order to be able to access the parent’s kinematic information, one needs to perform a full topological reconstruction of the parent’s decay. This will also allow for the separation of charm and bottom mesons. The study of D0 mesons, the lightest mesons with a charm quark, can be used to study the properties of the medium created in collisions, such as the density, flow, and thermalization of the medium. This dissertation presents an attempt to measure D0/D0bar ratios and D0 meson production in Au+Au collisions at sqrt(s_NN) = 200 GeV from fully reconstructed decays. For this purpose, we used a silicon tracker in STAR consisting of the Silicon Vertex Tracker (SVT) and the Silicon Strip Detector (SSD), along with the Time Projection Chamber (TPC) in a special run in the year 2007. We have developed new calibration and microvertexing techniques in the data analysis. We performed full secondary vertex reconstruction, to topologically reconstruct the secondary vertex of the D0 meson in the decay channel D0 -> K- + pi+ (B.R. = 3.89% and ct = 123 µm) and then performed a standard invariant mass analysis. At the same time we used a new tool (TMVA) in high energy physics for optimizing the signal to background ratio. However, precise measurements of open heavy flavor are difficult to obtain with the SVT due to a) the low yields and short lifespan of heavy hadrons, b) the huge combinatorial background, c) the poor statistics in the final data sample and d) the poor resolution of the SVT. STAR proposed and built a new generation vertex tracker, the Heavy Flavor Tracker (HFT). The HFT made its debut during the 2014 year’s run and has vastly improved the experiment’s heavy flavor capabilities making STAR an ideal detector to study the hot and dense matter created in heavy ion collisions. Taking advantage of the greatly improved pointing resolution from a dedicated microvertex detector, it is possible to directly track and reconstruct weak decay products from hadrons comprised of heavy `charm’ and `bottom’ quarks with low background. The HFT consists of three sub-detectors: PIXEL (PXL), the Intermediate Silicon Tracker (IST), and the Silicon Strip Detector (SSD) with 4 separate layers of silicon to guide tracks reconstructed in the Time Projection Chamber down to a pointing resolution of around 30 µm for 1 GeV/c pions, a requirement to distinguish between an event’s primary vertex and the position of a hadron’s decay. In this Dissertation we present the details of our SVT work, data analysis and results, and briefly show and discuss the recent results obtained with the HFT.

Committee:

Spyridon Margetis (Advisor); Declan Keane (Committee Member)

Subjects:

Experiments; High Temperature Physics; Nuclear Physics; Particle Physics; Physics; Quantum Physics

Keywords:

heavy ions; heavy flavor; charm; D0; flow; TMVA; Au Au; RHIC; STAR; BNL; high energy nuclear physics; hep; hep-ex

Kersell, Heath R.Alternative Excitation Methods in Scanning Tunneling Microscopy
Doctor of Philosophy (PhD), Ohio University, 2015, Physics and Astronomy (Arts and Sciences)
Since its inception, scanning tunneling microscopy (STM) has developed into an indispensible tool for surface science. Its sub-nanometer spatial resolution in both real space imaging and tip-sample interactions continue to demonstrate versatility in the study of novel phenomena at materials’ surfaces. This dissertation explores the expansion of experimental techniques in STM through application of two uncommonly exploited interactions at the tip-sample junction: X-ray absorption and the electric field between the tip and sample. This study begins by targeting the STM tip-sample junction with X-ray photons tuned to core level electron energies of atomic species on the sample. Interactions of atomic islands with the incident light are employed to introduce elemental sensitivity in STM with a resolution of just 2 nm. Elementally sensitive images are produced simultaneously with conventional STM images, and exploited to probe X-ray cross section behavior for structures measuring just a few tens of nanometers in the lateral directions. Additionally, point spectroscopic measurements of X-ray absorption behavior vs. incident photon energy facilitate the detection of local variations in emitted electron density due to the X-ray interactions. Locally measured electron emission densities measured by specialized SXSTM smart tips demonstrate a clear dependence on incident photon energy. Next, electric field interactions between the tip and sample are used to investigate the behavior of strongly dipolar molecular rotor networks. Symmetry and structure in the molecular networks are found to inhibit rotation of molecular rotors that exhibit thermally induced switching at temperatures as low as 5 K for isolated molecules. Additionally, inelastic electron tunneling is employed to induce controlled directional rotation in a different, single molecule motor system. The directionality is explained through the structure of calculated potentials in the motor system. The mechanisms used to investigate each system represent extensions beyond conventional techniques used in STM to probe novel phenomena. Additionally, the excitation efficiency, related to the quantum yield for the processes induced in each system, is determined.

Committee:

Saw-Wai Hla, Dr. (Advisor)

Subjects:

Condensed Matter Physics; Experiments; Low Temperature Physics; Molecular Physics; Molecules; Nanoscience; Nanotechnology; Physics

Keywords:

Scanning Tunneling Microscopy; STM; Synchrotron X-ray Assisted STM; SXSTM; X-ray Absorption Spectroscopy; XAS; Molecular Rotors; Molecular Motors; Elemental Sensitivity; Secondary Electron; Electron Emission; X-ray Cross Section; Directed Rotation

Nelson, Jocienne NOvertone Spectroscopy of Hydrogen in MOF-5
BA, Oberlin College, 2014, Physics and Astronomy
Metal-Organic Frameworks, or MOFs, are an exciting class of nanoporous crystalline materials with applications that include hydrogen storage and hydrogen isotope separation. The dynamics of adsorbed molecular hydrogen in the prototypical material known as MOF-5 have previously been studied using infrared spectroscopy. However, the rovibrational spectrum of the isotopologues, HD, and D2 were obscured due to overlap with the MOF peaks. Overtone infrared spectroscopy in conjunction with a diffuse reflectance geometry is used to observe the spectrum of H2, HD and D2. The overtone spectrum is shown to facilitate the identification of hydrogen peaks. Further, the spectrum of trapped H2 near the crystallographic metal site is greatly enhanced relative to other sites and displays a greater intensity relative to the fundamental spectrum than is seen in gas phase hydrogen. The ability of the MOF to catalyze ortho to para conversion of trapped species is also discussed.

Committee:

Stephen FitzGerald (Advisor)

Subjects:

Chemistry; Condensed Matter Physics; Molecular Physics; Physics; Quantum Physics

Keywords:

Metal-organic frameworks;overtone spectroscopy;hydrogen adsorption;molecular-hydrogen;induced absorption;deuterium;overtone;ortho to para conversion;trapped hydrogen;enhanced overtone;MOFs;Hydrogen deuteride;rovibrational spectrum;MOF-5

Miao, JiayuanTheory and Simulation of the Responses of Polymers to Electric Fields, Stress, Irradiation, and Diffusive Solvents
Doctor of Philosophy, Case Western Reserve University, 2017, Physics
This thesis focuses on analyzing the responses of polymers to the presence of electric fields, stress, irradiation, or diffusive solvents using numerical or molecular dynamics simulation techniques. The response of polymers to an electric field is studied to optimize energy storage in polymer dielectrics. We focus on the case of polymers containing permanent electric dipoles, which lie perpendicular to the chain axis, and study how the density of cross-linked sites, at which the chain is prevented from rotating, affects the stored energy. For the model considered here, which reflects some of the characteristics of polyvinylidene fluoride (PVDF) or its copolymers, the optimum energy storage is found within a very narrow range of densities of cross links. Electrospinning of PVDF nanofibers is accompanied with stress which promotes the formation of the ß phase and concurrent relaxation. We find the relaxation processes that follow formation of a nanofiber to be of two types. One type is a rapid rotation of chain segments that gives rise to twist defects at various points along the chain. In the second type, twist defects translate until they are near to other, at which point they stop and assemble into twist boundaries that include several chains. With the goal of elucidating the structure of PVDF nanofibers observed in high resolution transmission electron microscopy (TEM), all-atom molecular dynamics simulations and TEM image simulations of irradiated nanofibers were performed. The experimental and simulation results are compared. We find the features in the latter, such as the the orientations of elongated dots with respect to the chain axis; kinks along the experimental chain profile; varying sizes, shapes and brightness of dots; and distances between dots can all be very useful in unveiling the inherent structure of PVDF nanofibers. Different responses of a polymer matrix to small solvent molecules may lead to different types of diffusion. With this in mind, we build a model which could illustrate almost all observed diffusion types: Fickian diffusion, anomalous diffusion, Case II diffusion, and super Case II diffusion. This analysis provides an overview that incorporates the many specialized models interpreting specific types of diffusion into one unified model.

Committee:

Philip Taylor (Committee Chair)

Subjects:

Condensed Matter Physics; Physics; Polymers; Solid State Physics; Theoretical Physics

Mishra, RohanFirst Principles Study of Double Perovskites and Group III-V Compounds
Doctor of Philosophy, The Ohio State University, 2012, Materials Science and Engineering

The broad aim of this dissertation is to develop an atomic scale understanding of the magnetic and electronic properties of double perovskites, a class of materials that hold a lot of promise to realize new multifunctional devices. We have used first-principles density functional theory (DFT) calculations to develop this understanding.

In the first section, we focus on double perovskites with half metallic properties. We begin with the most widely studied half metallic double perovskite, Sr2FeMoO6. Even after more than a decade of extensive research, it has not yet been possible to realize the high degree of spin-polarization that has been theoretically predicted in Sr2FeMoO6. We find point defects to be playing a major role in degrading its half metallic properties. We determine transition-metal antisites and oxygen vacancies to be the thermodynamically stable defects, and predict Mo-rich Sr2FeMoO6 and stoichiometric Sr2FeMoO6 with antisite disorder to have poor spin-polarization.

We have then used our understanding of the magnetic interactions that result due to antisites disorder in Sr2FeMoO6 to predict Ca2MnRuO6 as a material which should allow high levels of spin-polarized conduction, even in a complete disordered form.

Next, we have studied the magnetic interactions in a recently discovered double perovskite Sr2CoOsO6, where the two magnetic ions Co and Os order independently of each other, a behavior that has never been observed in any other magnetic material. By calculating the strength of exchange interactions between different pairs of magnetic ions in the compound, we attribute the observed magnetic behavior to the weak nearest neighbor Co-O-Os interactions compared to the stronger Co-O-Os-O-Co and Os-O-Co-O-Os interactions. We explain this weak Co-O-Os interaction to be due to the small overlap of the Co 3d and Os 5d orbitals.

In the next section, we apply the method of determining the chemical potential of the constituent elements in multicomponent systems, which we had developed to calculate the stable defects in Sr2FeMoO6, to a simpler binary semiconductor InP and expand it into a more rigorous and general theory, which we found had been missing in the literature when we examined the point defects in Sr2FeMoO6. After identifying the stable neutral defects in both stoichiometric and non-stoichiometric InP, we determine their defect level within the experimental band gap, using a combination of computationally fast traditional exchange-correlation functionals used in DFT, which underestimate the experimental band gap, and computationally expensive but more accurate hybrid functionals. We combine the neutral-defect formation energies with the position of the defect states and suggest a method to calculate both, chemical potentials of the constituent elements, and the formation of energies of charged defects, as a function of the Fermi level.

In the final section, we suggest a method to study the electronic properties of interfaces, which is general enough to address the common questions at interfaces between complex materials such as e.g. found in semiconductor systems or complex oxide heterostructures. Using wurtzite AlN/GaN interface as a test system, we calculate the band line-up and interface charge density due to spontaneous and piezoelectric polarization, using a combination of first principles real-space band structure and electrostatics. Using this method, we show the interface charge density in the AlN/GaN system to be independent of the layer thicknesses of the two materials, with their value being identical to the interface charge density calculated using bulk constants.

Committee:

Wolfgang Windl, PhD (Advisor); Patrick Woodward, PhD (Committee Member); Siddharth Rajan, PhD (Committee Member); Roberto Myers, PhD (Committee Member); Jay Gupta, PhD (Other)

Subjects:

Chemistry; Condensed Matter Physics; Inorganic Chemistry; Materials Science; Nanotechnology; Quantum Physics; Solid State Physics; Theoretical Physics

Keywords:

DFT; spintronics; magnetism; electronic materials; polarization; chemical potential; interfaces

Langmack, Christian BishopUniversal Loss Processes in Bosonic Atoms with Positive Scattering Lengths
Doctor of Philosophy, The Ohio State University, 2013, Physics
In experiments with trapped ultracold gases, atoms can be lost through inelastic scattering processes. If the atoms have a scattering length that is much larger than the range of their interactions, then the system exhibits universal behavior that does not depend on details of their interactions. Dramatic enhancements in the loss rate are observed at special negative values of the scattering length for which there is a universal molecule at threshold. In some experiments, enhancements of the loss rate have also been observed at other positive values of the scattering length. A mechanism proposed to explain this enhancement is that the losses result from many collisions of an energetic diatomic molecule created by a 3-atom collision. In this thesis, I demonstrate the failure of this mechanism as a viable explanation of the enhancement seen at positive scattering length. I present an alternative explanation for these enhancements in experiments using a Bose-Einstein condensate of atoms. They result from inelastic scattering of universal diatomic molecules in a coexisting condensate of these molecules.

Committee:

Eric Braaten, Ph.D. (Advisor); Yuri Kovchegov, Ph.D. (Committee Member); Mohit Randeria, Ph.D. (Committee Member); Stanley Durkin, Ph.D. (Committee Member)

Subjects:

Atoms and Subatomic Particles; Condensed Matter Physics; Physics; Quantum Physics; Theoretical Physics

Keywords:

Universal Physics, Cold Atoms, Efimov Physics, Atomic Physics

Li, MingQuantum Theory of Ion-Atom Interactions
Doctor of Philosophy, University of Toledo, 2014, Physics
This thesis consists of a series of theoretical efforts aimed at reformulating the quantum theory of ion-atom interactions using quantum-defect theory that is based on the analytic solutions for the long-range, -1/R4, polarization potential. Ion-atom interactions, especially at cold temperatures of a few kelvin or lower, are complicated by the rapid energy variations induced by the long-range polarization potential, by the generally large number of contributing partial waves, and by the sensitive dependence of the interactions on the short-range potential. The standard numerical method is not only inefficient in addressing these issues, but can also miss important physics such as extremely narrow resonances. Ion-atom interaction at cold temperatures is further complicated by what is normally considered as "weak" interactions, such as the hyperfine interaction. While they may not be important at high temperatures, they become exceedingly important at 1 K or lower temperatures. The hyperfine effects, and the related effects of identical nuclei, have not been properly treated in existing theories. This thesis contains works that establish the quantum-defect theory for ion-atom interactions, including both its single-channel version, and its multichannel version. Through detailed comparison with numerical calculations, carried out for Na++Na and proton-hydrogen systems, we show how quantum-defect theories provide a systematic and an efficient understanding of ion-atom interactions. Such an efficient description is not only important for two-body systems, but also the key to a systematic understanding of quantum few-body systems, chemical reactions, and many-body systems involving ions. Proper treatments of hyperfine structure and identical nuclei are also developed as a part of these studies.

Committee:

Bo Gao (Advisor); Song Cheng (Committee Member); Steven Federman (Committee Member); Thomas Kvale (Committee Member); Biao Ou (Committee Member)

Subjects:

Atoms and Subatomic Particles; Molecular Physics; Physics; Quantum Physics; Theoretical Physics

Rytel, Alexander LHydrous Mineral Stability in Earth’s Mantle: Implications for Water Storage and Cycling
Master of Science, The Ohio State University, 2016, Earth Sciences
Understanding the storage and cycling of water and hydrogen in Earth is key to understanding Earth’s current and past global mantle dynamics. Constraining Earth’s deep mantle water cycle depends on constraining the amount of water coming out of the mantle, the amount of water carried into the mantle, the amount of water stored in the mantle, and the rate at which these fluxes have changed over Earth’s history. The water storage capacity in Earth’s interior and transport between reservoirs is dependent upon the stability of both hydrous and anhydrous minerals at high pressure. This study focuses on mantle water storage capacity. Constraint on the water storage capacity of the upper mantle depends upon the effects of cation disorder on the stability of hydrous and anhydrous silicates as a function of Si and Al coordination. In high temperature conditions, the lowest energy state of a crystal lattice is often disordered rather than ordered. For example, it has been demonstrated that the anhydrous mineral bridgmanite (MgSiO3, formerly referred to as perovskite), into which is dissolved about 15 mol% Al2O3 via substitution, is more energetically favorable than pure ordered bridgmanite at top of the lower mantle conditions [Panero et al. 2006]. This study models the stabilizing effect of disorder on the Gibbs free energy of hydrous aluminosilicates. Prediction of mineral stability is dependent upon accurately calculating the Gibbs free energy of a mineral as a function of pressure, temperature, and composition. Here I present the energetics of disorder at mantle pressure and temperature conditions from static-lattice energy minimizations using interatomic potentials modeled by the General Utility Lattice Program (GULP) coupled with modeling of configurational entropy of H-site and Si-Al disorder. This study focuses on three hydrous aluminosilicates that are stable at upper mantle pressure-temperature conditions: topaz-OH (Al2SiO4(OH)2), phase Egg (AlSiO3OH), and the delta-AlOOH – stishovite (SiO2) solid solution series. These calculations are benchmarked against anhydrous kyanite and anorthite in which Al-Si disorder and the aluminum avoidance principle have been thoroughly studied. This work evaluates the significance of the aluminum avoidance principle in determination of the stability fields of high-pressure hydrous aluminosilicates.

Committee:

Wendy Panero (Advisor); Michael Barton (Committee Member); Thomas Darrah (Committee Member)

Subjects:

Earth; Geological; Geology; Materials Science; Mineralogy; Molecular Physics; Molecules; Physics; Quantum Physics; Solid State Physics

Keywords:

topaz-oh; phase egg; delta-AlOOH; dAlOOH; d-AlOOH; AlOOH; Al-Si disorder; Tschermak defect; mantle water; hydrous mantle; hydrous aluminosilicate; general utility lattice program; deep earth water; configurational entropy

Leuty, Gary MAdsorption and Surface Structure Characteristics Toward Polymeric Bottle-Brush Surfaces via Multiscale Simulation
Doctor of Philosophy, University of Akron, 2014, Polymer Science
For decades, device design has focused on decreasing length scales. In computer and electronic engineering, small feature sizes allow increasing computational power in ever-smaller packages; in medicine, nanoscale in vivo devices and sensors and coatings have myriad applications. These applications all focus strongly on material/component interfaces. While recent advances in experimental techniques probing interfaces at nanometer and sub-nanometer scales have improved dramatically, computational simulation remains vital to obtaining detailed information about structure and energetics in nanometer-scale interactions at interfaces and the physical properties arising from interactions at larger scales. We start with all-atom molecular dynamics simulations of methane and chloromethane adsorption on the (100) surface of molybdenum to understand adsorbate polarity/geometry and substrate interaction potential effects on interfacial structure, packing and energetics. For featureless substrates, adsorbate geometry and orientation do not influence packing and affinity. Substrates with explicit surface structure show cooperation between substrate and adsorbate geometry via adsorption-site preference. Methane prefers sites over unit cell faces, roughly commensurate with the Mo surface, whereas chloromethane invites disorder, orienting its long axis along ”bridges” between surface Mo atoms. In the second phase, we used a coarse-grained bead-spring model to perform simulations of bottle-brush homopolymers tethered to a wall substrate at long time/length scales. We studied the intra- and intermolecular accumulation of tension in tethered bottle-brush backbones vs. bottle-brush dimensions and surface grafting density. Variations in bond force and bottle-brush/component shape and size descriptors uncovered three tension ”regimes”: (i) an isolated-brush regime (low surface grafting density), where intramolecular interactions dominate and tension is minimal; (ii) a ”soft-contact” regime, where neighboring bottlebrushes’ side chains overlap, compressing side chains and transmitting moderate tension to backbones; and (iii) a ”hard-contact” regime, where increased side-chain overlap forces reorientation, accumulating significant backbone tension. We then performed a small number of simulations of tethered bottle-brushes with two different side chain types to illustrate the morphologies available as a result of microphase separation, varying the strength of the interactions between side chain types. Continuing this work in the future should help discover other possible applications arising from varying the chemical nature of the side chains.

Committee:

Mesfin Tsige, Dr. (Advisor); Mark Foster , Dr. (Committee Member); Shi-Qing Wang, Dr. (Committee Member); Gustavo Carri, Dr. (Committee Member); Jutta Luettmer-Strathmann, Dr. (Committee Member)

Subjects:

Condensed Matter Physics; Materials Science; Molecular Physics; Physical Chemistry; Physics; Polymers; Theoretical Physics

Keywords:

Molecular dynamics simulation; bottle-brush polymers; adsorption; computer simulation; coarse-grained bead-spring molecular dynamics; all-atom molecular dynamics; surfaces and interfaces

Thota, Venkata Ramana KumarTunable Optical Phenomena and Carrier Recombination Dynamics in III-V Semiconductor Nanostructures
Doctor of Philosophy (PhD), Ohio University, 2016, Physics and Astronomy (Arts and Sciences)
Semiconductor nanostructures such as quantum dots, quantum wires and quantum wells have gained significant attention in the scientific community due to their peculiar properties, which arise from the quantum confinement of charge carriers. In such systems, confinement plays key role and governs the emission spectra. With the advancements in growth techniques, which enable the fabrication of these nanostructured devices with great precision down to the atomic scale, it is intriguing to study and observe quantum mechanical effects through light-matter interactions and new physics governed by the confinement, size, shape and alloy composition. The goal is to reduce the size of semiconductor bulk material to few nanometers, which in turn localizes the charge carriers inside these structures such that the spin associated with them is used to carry and process information within ultra-short time scales. The main focus of this dissertation is the optical studies of quantum dot molecule (QDM) systems. A system where the electrons can tunnel between the two dots leading to observable tunneling effects. The emission spectra of such system has been demonstrated to have both intradot transitions (electron-hole pair residing in the same dot) and interdot transitions (electron-hole pair participating in the recombination origin from different dots). In such a system, it is possible to apply electric field such that the wavefunction associated with the charge carriers can be tuned to an extent of delocalizing between the two dots. This forms the first project of this dissertation, which addresses the origin of the fine structure splitting in the exciton-biexciton cascade. Moreover, we also show how this fine structure can be tuned in the quantum dot molecule system with the application of electric field along the growth direction. This is demonstrated through high resolution polarization dependent photoluminescence spectroscopy on a single QDM, which was described in great detail by H. Ramirez (et al.) and also experimentally observed by N. Skold (et al.) for a fixed barrier thickness. However, we measured the strength of FSS as a function of barrier thickness in the strong tunneling regime. The results are discussed in chapter 4. The second project is carried out with an intention to generate entangled photon pairs from molecular states found in the emission spectra of a single QDM: A pair of photons, which reveals the information associated with the intrinsic property (polarization for example) of the other photon simultaneously and spontaneously when a measurement has been performed in either one of the two. The exciton-biexciton cascade not only has intradot transitions but the photoluminescence spectra also depicts interdot transitions, realizing the molecular nature of the system. Since the charge carriers are localized in different dots, the wavefunction overlap between the two is also reduced significantly. It is with this goal of enhancing the intensity of interdot or indirect transitions between the molecular biexciton-indirect exciton that we performed two color photoluminescence excitation studies and the results are discussed in chapter 5. Thirdly, the continuous creation of electron-hole pairs through photoexcitation leads to some local electric field effects, which arises due to the ionization of charge carriers inside the device structure. The advantage of the interdot transition in the emission spectra is the large Quantum Confined Stark Effect (QCSE) associated with it. This interdot QCSE is over an order of magnitude larger than for the intradot or direct transition and varies linearly with the applied electric field. By making use of the interdot exciton as a sensitive probe, the effects of optically generated electric field as a function of time are measured experimentally. Both rise time and fall time of the optically generated electric field as a function of excitation wavelength and applied field are studied in detail. The results are presented in chapter 6. Finally, carrier recombination dynamics in rare-earth doped nanostructures are measured by using ultrafast spectroscopy. Carrier dynamics in InGaN:Yb3+ nanowires and InGaN/GaN-Eu3+ superlattices are measured by frequency doubling the excitation laser, and the effects of implantation of rare-earth ions into the host material have been investigated. The results from the experimental measurements are presented in chapters 7 & 8. These experimental findings might help to understand the challenges associated with these nanostructured materials in the applications of quantum information processing, single photon emitters, and to integrate them into existing optoelectronic devices.

Committee:

Eric A. Stinaff, Prof. (Advisor); Sergio E. Ulloa, Prof. (Committee Member); Arthur R. Smith, Prof. (Committee Member); Wojciech M. Jadwisienczak, Prof. (Committee Member)

Subjects:

Condensed Matter Physics; Materials Science; Nanoscience; Nanotechnology; Optics; Physics; Quantum Physics; Solid State Physics

Keywords:

Quantum Dots; Quantum Dot Molecules; Light-Matter Interactions; Photoluminescence Excitation; III-V Semiconductor Nanostructures; Tunable Fine Structure Splitting; Time-Resolved Photoluminescence Measurements; Carrier Dynamics in III-V Nanostructures;

Ongkodjojo Ong, AndojoElectrohydrodynamic Microfabricated Ionic Wind Pumps for Electronics Cooling Applications
Doctor of Philosophy, Case Western Reserve University, 2013, EECS - Electrical Engineering
This work demonstrates an innovative microfabricated air cooling technology that employs an electrohydrodynamic (EHD) corona discharge or ionic wind pump that has the potential to meet industry requirements as a next generation solution for thermal management applications. A single ionic wind pump element consists of two parallel collecting electrodes between which a single emitting tip is positioned. A grid structure on the collector electrodes enhances the overall heat transfer coefficient and facilitates an IC compatible and batch process. The main purpose of the work presented here is thus to investigate whether an optimized ionic wind pump employed in an array configuration might exhibit performance comparable to a conventional CPU fan. The manufacturing procedure developed for the device uses a glass wafer, a single mask-based photolithography process, a low cost copper-based electroplating method, and explores the effect of employing a palladium coating on the device. Various design configurations and optimization processes were explored and modeled computationally to investigate their influence on the cooling phenomenon. The optimized single element device provides a convection heat transfer coefficient of up to 3200 W/m2-K and a COP of up to 46.7 (a maximum COP of 51.5 exhibited by the 6-element array) exhibiting an overall area of 5.35 mm x 3.61 mm, an emitter-to-collector gap of 500 ¿¿m, and an emitter radius curvature of 12.5 ¿¿m. When compared with other ionic wind pumps, the device developed for this work is superior in terms of heat transfer coefficient and COP. However, the overall performance of the array does not compare favorably to a conventional CPU fan except in terms of COP. Additionally, the lifetime experiments conducted demonstrate that additional work may be required to extend the operation of the device, and some form of non-porous coating may be required to protect the underlying copper material. Nonetheless, the device described herein exhibits a flexible and small form factor, low noise generation, high efficiency, large heat removal over a small dimension, relatively simple technology, high reliability (no moving parts), lower power consumptions, and low cost; these are characteristics required by the semiconductor industry for next generation thermal management solutions.

Committee:

Alexis Abramson, PhD (Advisor); Norman Tien, PhD (Committee Member); Christian Zorman, PhD (Committee Member); Jaikrishnan Kadambi, PhD (Committee Member)

Subjects:

Design; Electrical Engineering; Electromagnetics; Energy; Engineering; Experiments; Fluid Dynamics; Materials Science; Mechanical Engineering; Physics; Plasma Physics; Solid State Physics; Systems Design; Theoretical Physics

Keywords:

ionic wind pumps; thermal management; MEMS; electronics cooling; corona discharge; electrohydrodynamics; heat transfer coefficients; coefficient of performance (COP); FEM/FEA; optimization; lifetime test; microfabrication process

Cardellino, Jeremy DDynamics of Paramagnetic Spins: A Study of Spin Defects using Magnetic Resonance Force Microscopy
Doctor of Philosophy, The Ohio State University, 2015, Physics
Magnetic Resonance Force Microscopy (MRFM) is a challenging yet incredibly sensitive tool for characterizing and imaging magnetic materials down to the nanoscale. It combines the technology of scanned probe microscopy with the powerful spectral techniques of magnetic resonance. The MRFM can measure very small spin ensembles, down to a single electron spin, and the measurements are performed at thermal equilibrium. Instead of perturbing the polarization away from equilibrium, the `spin noise' or statistical spin fluctuations are used to generate a force signal. Here I show MRFM measurements on a nanoscale `spin wire', which is a narrow, high spin density region implanted in a diamond substrate. The spin wire measurements reveal an interesting interplay between the transport and lifetime of spins con fined within the nanoscale diamond wire which are relevant for the development of nanoscale spintronics. Additionally, I show measurements which resolve the hyper fine spectrum of the defects in the spin wire by measuring less than 100 net spins.

Committee:

Chris Hammel (Advisor); Ezekiel Johnston-Halperin (Committee Member); David Stroud (Committee Member); Richard Furnstahl (Committee Member)

Subjects:

Condensed Matter Physics; Low Temperature Physics; Physics

Keywords:

MRFM;spin; spintronics; defects; lifetime; force; detection; magnetic; resonance; microscopy; dynamics; paramagnetic;

Bharadwaja, SakethMolecular Dynamics Simulations of Si binding and diffusion on the native and thermal Silicon Oxide surfaces
Master of Science in Engineering, University of Toledo, 2012, Engineering (Computer Science)
Amorphous silicon (a-Si) thin-film solar cells grown via plasma-enhanced chemical vapor deposition (PECVD) are of significant technological interest. As a result, there is significant interest in understanding the physical processes which control the a-Si thin-film structure and morphology. In particular, since the early stages of a-Si growth on the silicon oxide substrate play a key role in determining the subsequent evolution, it is important to obtain a better understanding of this stage of a-Si growth. The key objectives of the work presented in this thesis are to obtain a better understanding of the structure and morphology of the silicon-oxide substrate used in a-Si growth via PECVD as well as of the key processes of Si diffusion on the substrate which control the nucleation of a-Si islands. In particular, motivated by experimental and simulation results, we have carried out molecular dynamics simulations of the formation of a thermal silicon oxide substrate (corresponding to oxide formation at high-temperature) as well as of the room-temperature oxidation of “native” silicon oxide thin-films. In addition, for the case of a native silicon oxide surface, we have studied the binding energies, binding sites, and diffusion barriers for Si diffusion in order to gain insight into the critical length-scales for a-Si island formation. In the case of thermal silicon oxide formed at high temperature, our molecular dynamics simulations were carried out using an effective Munetoh potential which takes into account the “average” charge transfer as well as bond angles and energies. In this case, due to the relatively high temperature the surface was found to be extremely rough and highly disordered, while the thin-film structure was found to be amorphous. In contrast, in our simulations of the formation of native silicon oxide thin-films at room temperature, a more sophisticated ReaxFF potential was used which properly takes into account the effects of O2 molecular dissociation and rebinding at the surface, as well as the long-range Coulomb interaction and local charge-transfer. We have also studied the binding and diffusion of Si atoms for this case in order to try to explain recent experiments and simulations in which it was shown that 3D a-Si islands with a typical island diameter of approximately 30 A are formed in the early stages of growth. For the case of native silicon-oxide our results for the oxygen penetration profile and surface roughness were found to be in good qualitative agreement with experiments. Our results also indicate that while the typical binding energies for Si adatoms on the SiO2 surface are significantly lower than for Si/Si(100), due to the disordered structure of the surface the barriers for diffusion are typically significantly higher. As a result, at the deposition temperature of 200oC used in low-temperature PECVD, these sites may act like “trapping sites” for deposited Si atoms. We note that these results are consistent with recent experiments on the relaxation of SiO2 microstructures at high temperatures. However, they also imply that the characteristic length-scale for 3D islands in the early stages of a-Si growth via PECVD cannot be explained by a combination of homogenous diffusion and a critical island-size, as is typically found in epitaxial growth.

Committee:

Jacques Amar (Committee Chair); Mohammed Niamat (Committee Co-Chair); Mansoor Alam (Committee Member)

Subjects:

Engineering; Particle Physics; Physical Chemistry; Physics; Plasma Physics

Keywords:

Amorphous; morphology; chemical vapor deposition; Coulomb interactions; critical island size; epitaxial growth.

Bhallamudi, Vidya PraveenSpins in heterogeneous landscapes: Consequences for transport and imaging
Doctor of Philosophy, The Ohio State University, 2011, Electrical and Computer Engineering

Spintronics is a burgeoning field that endeavors to use the spin of an electron for information processing. Remarkable progress has been made in understanding injection of spin-polarized electron populations/currents into semiconductors, which are typically non-magnetic, as well as manipulation and detection of said spins.

This dissertation presents a combination of numerical analysis and experiments aimed at understanding and developing imaging tools that utilize the direct coupling of spins with the spatially varying magnetic field of a micromagnetic probe. This coupling produces a spin detection technique which relies on the resulting magnetic force on a cantilever to which the magnetic probe is attached. While extensively used in materials research, such an imaging tool has lagged as a characterization tool for spintronics. This is because of the challenges associated with measuring extremely small forces from these samples, and a lack of understanding of the effects of the inhomogeneous vector field from the probe on the sample spin density.

I have developed a numerical framework, based on solving the spin-transport equation, for understanding behavior of spins in the presence of spatially varying parameters. Based on analysis of the analytically solvable case of spin in a uniform magnetic field, but with multiple vector components, I have identified a crucial quantity, θB. This captures the competition between parallel and perpendicular field components dictating the collective spin precession behavior in these spin ensembles.

The numerical simulations show that in the proximity of the micromagnet the Hanle response is broadening due to the field component parallel to the injected spin direction. This result also sheds light on the effect of magnetic injectors used in spintronics devices. Our simulations show that the Hanle response will also be artificially broadened in this case due to the magnetic field from the injector. Such broadening is the most likely cause for the larger than expected Hanle halfwidths measured in recent experiments, and should be taken into account while extracting spin lifetime from such data.

Insights from numerical analysis have led us to propose scanned magnetic perturbation imaging for spatially mapping spin properties. This technique enhances the imaging capabilities of existing detection methods by encoding local information in the field gradients from a micromagnet. The spin density is governed by a convolution of local spin properties and the local magnetic field that perturbs the spins. By scanning this local perturbation a contrast in the globally detected signal, resulting from sample inhomogeneity, can be seen.

On the experimental side I have measured the broadening of the Hanle response due to a micromagnetic tip, using a globally integrated spin photoluminescence signal from optically excited spins in n-GaAs epitaxial membranes. I have also conducted sensitive force detection experiments to measure the magnetic forces from the spins in these samples. I have measured small forces of the order femtonewtons, similar to that predicted from simulations. The Hanle response of this force signal behaves as expected. This may be the first force detection of non-equilibrium itinerant spin-polarized electrons in paramagnetic semiconductors.

Committee:

Dr. P. Chris Hammel (Advisor); Dr. Wu Lu (Committee Member); Dr. Steven Ringel (Committee Member)

Subjects:

Condensed Matter Physics; Electrical Engineering; Experiments; Materials Science; Nanoscience; Nanotechnology; Physics; Scientific Imaging; Solid State Physics

Keywords:

spintronics; magnetic force microscopy; scanned probe microscopy; spin PL; inhomogeneous magnetic fields; Hanle effect

Zhao, JunMulti-scale Molecular Dynamics Simulations of Membrane-associated Peptides
Doctor of Philosophy, University of Akron, 2013, Chemical Engineering
Biological membranes function as an essential barrier between living cells and their environments. The membrane associated peptides (MAPs) interact with membrane either to facilitate the energy and molecules exchange between the environments and cytoplasm (e.g. cell-penetrating peptide), or to disturb the membrane and cause deadly membrane leakage (e.g. amyloid peptides and antimicrobial peptides). The structures and activity of these peptides are essential to understand the membrane association mechanisms and to screen the drug candidates. However, the poor atomic details of MAPs membrane-bound structures and their complicated interactions with cell membranes leads to the difficulty to better understand their biological roles. As the structures of MAPs are the prerequisite, in this dissertation, the structure prediction and screening of MAPs were firstly performed in Chapter II, Chapter III, and Chapter IV. We selected amyloid peptides as they usually form complicated polymorphic oligomeric structures, which are the most toxic species. Misfolding and self-assembly of human islet amyloid polypeptide (hIAPP, one of amyloid peptides which belong to MAPs) monomer into polymorphic amyloid oligomers is pathologically linked to type II diabetes. We developed a structures-screening program base on GBMV implicit-solvent evaluation and a structure population evaluation program by Monte Carlo simulation to search the aggregated structures of hIAPP with dominant populations. After the structure search, the stacking-sandwich model and wrapping-cord model were proposed to describe polymorphic structures of hIAPP oligomers, and all-atom molecular dynamics simulations were used to examine the structure, dynamics, and association of the self-assembled hIAPP oligomers. Seven oligomers from the stacking-sandwich model and three oligomers from the wrapping-cord model were determined by their high structural stability with favorable peptide-peptide interactions, although all of them displayed completely different structures in symmetry and beta-sheet packing. These oligomeric structures can also serve as templates to present double- and triple-stranded helical fibrils via peptide elongation, explaining the polymorphism of amyloid oligomers and fibrils. Base on the predicted oligomeric structures, the mechanisms of amyloid toxicity can be studied. The leaking pore mechanism is more and more widely accepted, in which the amyloid peptides form an ion channel-like but unregulated pore structures. We further investigated the dynamic structures, ion conductivity, and membrane interactions of hIAPP pores in the DOPC bilayer using molecular dynamics simulations (Chapter V and Chapter VI). In the simulated lipid environments, a series of annular-like hIAPP structures with different sizes and topologies were compatible with the doughnut-like images obtained by atomic force microscopy (AFM) and with those of modeled channels for Abeta, K3 peptide, and antimicrobial peptide PG-1, suggesting that loosely-associated beta-structure motifs can be a general feature of toxic, unregulated channels. Base on the previous works of peptide aggregates, adsoption on membrane/artificial surfaces, and ion-leakage activity, the process how MAPs adsorb on membrane and further penetrate across the membrane is evaluated by the transmembrane potential mean force (PMF). Antimicrobial peptides (AMPs) are selected to be studied due to their simple aggregated structures and short length. We constructed an effect platform including adaptive biasing force (ABF) method which accelerates the membrane penetration process, umbrella sampling method which effectively generates trans-membrane PMFs, and MARTINI coarse-grained force field to measure the free energy required to transfer the AMPs from bulk water phase to water-membrane interface, and further to bilayer interior (Chapter VII). The results implied that biological activity (i.e. antimicrobial or cytolitic activity) appeared to be closely related to the trans-membrane ability indicated by the PMF profiles and the PMFs are instructive index to identify the activity of either existing or designed AMPs. This work provides a useful computational tool to effectively search polymorphic structures of MAPs, to better understand the mechanism and energetics of membrane insertion of MAPs and to rational design new effective MAPs or related inhibitors. Though the amyloid toxicity leaking pore mechanism was revealed by our simulations, how the pores formed from amyloid monomers or transient oligomers, and how these monomers or oligomers dynamically adsord on membrane and finally imbedded in membrane are still unknown. Due to the complicated components of cell membrane, it is better to simplify the interactions between amyloid-membrane to amyloid-artificial surfaces. Thus, in the last part of the dissertation, we further presented a series of exploratory molecular dynamics (MD) simulations to study the early adsorption and conformational change of Abeta oligomers from dimer to hexamer on three different self-assembled monolayers (SAMs) terminated with -CH3, -OH, and -COOH groups (Chapter VIII). Within the timescale of MD simulations, the conformation, orientation, and adsorption of Abeta oligomers on the SAMs was determined by complex interplay among the size of Abeta oligomers, the surface chemistry of the SAMs, and the structure and dynamics of interfacial waters. Energetic analysis of Abeta adsorption on the SAMs reveals that Abeta adsorption on the SAMs is a net outcome of different competitions between dominant hydrophobic Abeta-CH3-SAM interactions and weak CH3-SAM-water interactions, between dominant electrostatic Abeta-COOH-SAM interactions and strong COOH-SAM-water interactions, and between comparable hydrophobic and electrostatic Abeta-COOH-SAM interactions and strong OH-SAM-water interactions. Atomic force microscopy images also confirmed that all SAMs can induce the adsorption and polymerization of Abeta oligomers. Structural analysis of Abeta oligomers on the SAMs shows a dramatic increase in structural stability and beta-sheet content from dimer to trimer, suggesting that Abeta trimer could act as seeds for Abeta polymerization on the SAMs.

Committee:

Jie Zheng, Dr. (Advisor); Gang Cheng, Dr. (Committee Member); Richard Elliott Jr, Dr. (Committee Member); Ernian Pan, Dr. (Committee Member); Mesfin Tsige, Dr. (Committee Member)

Subjects:

Biogeochemistry; Bioinformatics; Biophysics; Biostatistics; Chemical Engineering; Computer Engineering; Engineering; Molecular Physics; Pharmaceuticals; Physical Chemistry; Physics; Quantum Physics

Keywords:

molecular dynamics; Monte Carlo simulation; potential mean force; amyloid oligomers; antimicrobial peptides; self assembled monolayer; type 2 diabetes; alzheimer disease; ion channel; hemolysis; amylin; membrane associated peptides;

Guenther, JustinMagnetoresistance in Permalloy/GaMnAs Circular Microstructures
Master of Science, Miami University, 2014, Physics
When two ferromagnetic materials are deposited directly on top of one another, their magnetic moments lock together and will no longer switch independently. This effect is known as exchange spring coupling. Reports in literature indicate that a bilayer composed of GaMnAs and permalloy may be a rare exception. Such a bilayer would allow for independent switching and, as a result, giant magnetoresistance. For this thesis, we verified the independent switching of continuous films of GaMnAs and expanded on existing literature. We also investigated GMR in bilayers. Samples were fabricated and measured using novel techniques and software developed specifically for this project. Transport measurements of GaMnAs/Py bilayers revealed a minimal to non-existent GMR effect; instead, the main discernible effect was due to AMR of the bulk substrate of the samples. This thesis also details the construction process of an inexpensive, temporary cleanroom environment.

Committee:

Khalid Eid, PhD (Advisor); Herbert Jaeger, PhD (Committee Member); Mahmud Khan, PhD (Committee Member)

Subjects:

Condensed Matter Physics; Electromagnetics; Materials Science; Nanoscience; Physics; Solid State Physics

Keywords:

Magnetoresistance; Giant Magnetoresistance; Permalloy; GaMnAs; GMR; AMR; Circular Microstructures; Bilayer; Exchange Coupling; Thin Films

Goble, Nicholas JamesELECTRONIC TRANSPORT AT SEMICONDUCTOR AND PEROVSKITE OXIDE INTERFACES
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 on small-scale devices. These three studies significantly contribute to the understanding of two-dimensional electronic transport. They elucidate scattering mechanisms at low temperatures, tuning of the metal-to-insulator transition in a perovskite oxide, and electrical transport through grain boundaries. Overall, this work continues to push our knowledge of two-dimensional systems further, toward achieving a complete understanding of low-dimensional transport in complex systems.

Committee:

Xuan Gao (Advisor); Harsh Mathur (Committee Member); Kathleen Kash (Committee Member); Alp Sehirlioglu (Committee Member)

Subjects:

Condensed Matter Physics; Physics; Solid State Physics

Keywords:

condensed matter; semiconductors; low dimensional; two dimensional; gallium arsenide; strontium titanate; electronic scattering; electronic transport; perovskite; oxides; domain wall; aluminum oxide; electron gas; strong interactions; low temperature

Quintero, AmilkarMEASUREMENT OF CHARM MESON PRODUCTION IN Au+Au COLLISIONS AT sqrt(SNN) =200 GeV
PHD, Kent State University, 2016, College of Arts and Sciences / Department of Physics
The study and characterization of nuclear matter under extreme conditions of temperature and pressure, and a full understanding of deconfined partonic matter, the Quark Gluon Plasma (QGP), are major goals of modern high-energy nuclear physics. Heavy quarks (charm and bottom) are formed mainly in the early stages of the collision. Open heavy flavor measurements, e.g. D0, D±, D*±, are excellent tools to probe and study the hot and dense medium formed in heavy ion collisions. Details of their interaction with the surrounding medium can be studied through energy loss and elliptic flow measurements thus providing valuable information about the nature of the medium and its degree of thermalization. Initial indirect reconstruction studies of heavy quark particles using the electrons from heavy flavor decays, showed a large magnitude of energy loss that was inconsistent with model predictions and assumptions, at the time. Precise measurements of fully reconstructed heavy mesons would provide better understanding of the energy loss mechanisms and the properties of the formed medium. In relativistic heavy ion collisions, the relatively low abundance of heavy quarks and their short lifetimes makes them difficult to distinguish from the event vertex and the combinatorial background; therefore the need for a high precision vertex detector to reconstruct their decay particles. In 2014 a new micro vertex detector was installed in the STAR experiment at Brookhaven National Lab. The Heavy Flavor Tracker (HFT) was designed to perform direct topological reconstruction of the weak decays of heavy flavor particles. The HFT improves STAR track pointing resolution from a few millimeters to ~30 microns for 1 GeV/c pions, allowing direct reconstruction of short lifetime particles. Although the results of the open charm meson reconstruction using the HFT improved dramatically there is still a lot of room for optimization, especially for reconstructed particles with low transverse momentum (<1 GeV/c). The standard reconstruction algorithm in the STAR experiment is based on a helix swimming of the reconstructed tracks. This method consists of finding the distance of closest approach between the two helices and defining the midpoint as the decay particle’s vertex position. In this work we are using an algorithm based on the Kalman filter to perform full vertex reconstruction. Although the Kalman filter is the most common fitting and filtering method used in tracking, it is not commonly used for particle reconstruction. By using the Kalman filter, the full error matrix for each track is taken into account in the calculations, performing a more complete approach to vertex reconstruction of the charm mesons by providing error estimates on all reconstructed quantities. Also in the traditional analyses, rectangular cuts are made to the reconstructed parameters of the candidate particle decay in order to improve the signal to background ratio and get the cleanest signal possible. In this analysis we use multivariate techniques (i.e. machine learning) to maximize the efficiency of the acquired signal. Machine learning techniques are widely used in many data analysis problems and are also in wide use in high-energy physics experiments. Different optimization methods are tested like Likelihood, Neural Networks. The one with the better performance for reconstruction of D0 mesons was found to be the Binary Decision Trees (BDT). We have applied these analysis techniques on our Run-14 data sample (~1.2 billion Au+Au events at 200 GeV) and we present results for D0 meson pT spectra and nuclear modification factor (RAA) for different event centralities. We discuss the obtained results and compare with current theory models.

Committee:

Spyridon Margetis (Advisor); Flemming Videbaek (Advisor); Declan Keane (Committee Member); Michael Strickland (Committee Member); Robert Twieg (Committee Member); Robin Selinger (Committee Member)

Subjects:

Nuclear Physics; Particle Physics; Physics

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