Electronic Theses, Dissertations, and Records of Study (2002– )

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http://hdl.handle.net/1969.1/2

Description

This collection contains Texas A&M University theses and dissertations written after 2002.

History

In 2002, the Texas A&M University’s Office of Graduate and Professional Studies (OGAPS) began accepting electronic submission of theses and dissertations. In 2004, electronic submission became a requirement, and OGAPS now also accepts electronically submitted records of study.

Access

Most theses and dissertations in this collection are open access. However, Texas A&M University students have a right to place their work under embargo in certain circumstances. The full-text of theses and dissertations under embargo is restricted until the embargo period has expired.

Website: Thesis and Dissertation Services

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Now showing 1 - 7 of 7

Dirac Operator with Concentrated Nonlinearity and Bifurcation of Embedded Eigenvalues from the Bulk of the Essential Spectrum
(2018-08-06) Pekmez, Hatice Kubra; Comech, Andrew; Howard, Peter; Abanov, Artem G
We study the nonlinear Dirac equation with Soler-type nonlinearity in one dimension (which is called the Gross-Neveu model), with the nonlinearity localized at one and at two points. We study the spectral stability of the solitary wave solutions in these models. As a consequence, we obtain the result that the eigenvalues of the equation with the Soler-type nonlinearity move along the imaginary axis. We also construct solitary waves under perturbations of the model and look for relations between components of solitary waves in light of the techniques which we use for analyzing the Gross-Neveu model. We apply the same analysis to the Dirac equation with the concentrated nonlinearity of the same type as in the massive Thirring model. We find the same spectrum of linearization at solitary waves as that in the nonlinear Dirac equation with Soler-type nonlinearity.
Electron Irradiation Induced Changes of the Electrical Transport Properties of Graphene
(2014-08-06) Woo, Sung Oh; Teizer, Winfried; Ross , Joseph H; Wang, Haiyan; Abanov, Artem G
This research investigates the effect of electron irradiation on transport properties in graphene Field Effect Transistor (FET) devices. Upon irradiation, graphene is doped with electrons and adsorbs molecules by transfer of accumulated electrons in graphene to environmental molecules, resulting in the deterioration of transport properties. Molecules adsorbed after electron irradiation in ambient condition were observed by Atomic Force Microscopy (AFM). In-situ transport properties were measured in a vacuum after electron irradiation. In addition, hysteresis in the transport properties appeared as a result of electron irradiation. The origin of the hysteresis was investigated by exposing the electron beam irradiated graphene to ambient condition. As environmental molecules are adsorbed on graphene, the hysteresis disappears. In addition, annealing in a vacuum also removes the hysteresis. We conclude that the hysteresis is the result of polar adsorbates on graphene induced by electron irradiation. In addition, the effect of electron irradiation on a PMMA (Poly Methyl Methacrylate)/Graphene bilayer was studied. We observed a deterioration of the electrical transport properties of a graphene FET. Prior to electron irradiation, we observed that the PMMA layer on graphene did not degrade the carrier transport of graphene but improved its electrical properties instead. As a result of the electron irradiation on the bilayer, defects could be observed by Raman spectroscopy. We suggest that the degradation of the transport behavior originates from the binding of atoms or molecules generated by the PMMA backbone secession process.
Magnetic and Spectroscopic Properties of Heterometallic Lanthanide Compounds and Towards Utilizing Synchrotron Mössbauer Spectroscopy for Magnetic Relaxation Measurements
(2018-08-03) Dickie, Courtney Marie; Nippe, Michael; Gabbai, François P; Hughbanks, Timothy; Abanov, Artem G
Single-molecule magnets (SMMs) have attracted considerable attention for their fascinating properties and exciting potential applications in next generation high-density data storage and quantum computing. SMMs are a class of paramagnetic compounds defined by a bistable magnetic ground state and an energy barrier (U) to reorientation of their molecular spin. Towards the utilization of SMMs in devices, the development of methods for the control of magnetic properties via external stimuli is required. A major objective of this research project has been the development of lanthanide-based SMMs featuring a redox-active transition metal center, with the ultimate goal of utilizing the redox properties of the transition metal to modulate the magnetization dynamics at the nearby lanthanide center. Early results in developing mixed d-block/f-block systems led to the isolation of the dysprosium-isocarbonyl compound, [(thf)v5Dy((u-OC)W(CO)v2Cp)v3]·thf. Notably, this was the the first structurally characterized dysprosium-isocarbonyl compound. The static and dynamic magnetic properties of [(thf)v5Dy((µ-OC)W(CO)v2Cp)v3]·thf were characterized, revealing field-induced slow relaxation only in a magnetically dilute sample (12:1 Y^3+:Dy^3+) with Uveff = 12.6 cm^-1. However, due to instability towards oxidation/reduction, these isocarbonyl compounds were determined to be unsuitable for redox-switchability applications. A possible route to improved magnetization dynamics is through the development of low-coordinate lanthanide-isocarbonyl compounds. Towards this goal, the new, bulky tungsten hydride compound, HWCp^BIG(CO)v3 (HCp^BIG = HCv5(4-n-Bu-Cv6Hv4)v5) was isolated. Gratifyingly, HWCp^BIG(CO)v3 is soluble in non-coordinating solvents, including hexanes and toluene. The highly reversible redox properties of ferrocence, FeCpv2, make it ideal for use in redox-switchability applications. Towards this goal, dysprosium(III) and erbium(III) bis(diamidoferrocene) complexes have been developed and their dynamic magnetic properties investigated. The Dy^3+ compound exhibits either "on/off" or "slow/fast" switching in the presence or absence of a dc field, respectively. The Er^3+ compound exhibits "on/off" functionality in the presence of a dc field. UV-vvis-NIR spectroscopy, ^57Fe Mössbauer spectroscopy, and electrochemical studies support the presence of electronic communication between the mixed-valent Fe ions in both solution and in the solid state. This molecular level study is intended to provide the foundation for future switchable solid materials based on the redox properties of ferrocene/ferrocenium. The cyclopentadienide (Cp^-) substitution chemistry of chromocene was investigated as a synthetic route for the generation of transition metal precursors for d-block/f-block complexes. The reaction between chromocene and two equivalents of K(N(SiMve3)v2) generated the polymeric species, [(N(SiMev3)v2)v2K]v∞ Carrying out the reaction in the presence of cryptand-222 afforded the mononuclear compound [K(crypt-222)][CpCr(N(SiMev3)v2)v2]. Despite the similar coordination environments and only slight differences in geometric parameters, the polynuclear species showed no evidence of slow relaxation whereas the mononuclear cryptand species displayed field-induced slow relaxation with Uveff = 9.8(6) cm^-1 (Hvdc = 1000 Oe). Notably, this is only the third reported mononuclear chromium(II)-based compound to display field-induced slow relaxation and first example that is not square planar at the Cr^2+ center. Another major objective of this research project was the development of new techniques for the characterization of SMMs. Mössbauer spectroscopy presents an exciting opportunity to study magnetization dynamics at higher temperatures than currently achievable through ac magnetometry, as the lifetime of the Mössbauer spectroscopy measurement is 5-6 orders of magnitude shorter. In this first of its kind study, the magnetization dynamics of Dy^3+-based hexafluoroacetylacetonate compounds were studied by ^119Sn and ^161Dy synchrotron Mössbauer spectroscopy. In the zero-field ^119Sn Mössbauer spectra, no measurable magnetic hyperfine field from the Dy^3+ center was detected at the nearby diamagnetic Sn center. While detailed analyses of the collected ^161Dy Mössbauer spectra are currently ongoing, early observations suggest a relationship between the loss of count rate (intermediate relaxation regime) and the relaxation times/Uveff determined through ac magnetometry studies. It remains to be determined whether or not ^161Dy synchrotron Mössbauer spectroscopy can be utilized to extract relaxation times and calculate Uveff in Dy^3+-based SMMs.
On Magnon Superfluidity in Ferromagnetic Films
(2018-05-16) Sun, Chen; Pokrovsky, Valery L; Abanov, Artem G; Wu, Wenhao; Berkolaiko, Gregory
More than ten years ago, Bose-Einstein condensation (BEC) of magnons (or quantized spin waves) was experimentally observed in yttrium iron garnet (YIG) films at room temperature. Since BEC and superfluidity are closely related phenomena, it is natural to ask whether such magnon condensates can transport as superfluid and, if so, how such superfluid looks like. In this work, we study theoretically superfluidity of magnons in ferromagnetic films. We first give an review of the basic theory of magnons in ferromagnetic films. We then discuss BEC of magnons from both experimental and theoretical points of view. Then we study superfluidity of magnons in ferromagnetic films by starting from a model of spins in ferromagnetic films. Model in terms of magnon operators is then introduced, and a Hamiltonian describing the condensed magnons is derived. Focusing on the one-dimensional (1D) stationary case, we study behaviors of superfluid formed by the condensed magnons. We found an unconventional soliton-like profile of the magnon superfluid, as compared to a uniform superflow, which we argued to be due to the dipolar interaction. We also show by estimates that in YIG films it is possible to have a superfluid current that strongly exceeds the current of normal magnons, so the magnon superfluidity could possibly be observed.
On the Magnon Bose Einstein Condensation in Ferromagnetic Film
(2014-11-12) Li, Fuxiang; Pokrovsky, Valery L; Abanov, Artem G; Wu, Wenhao; Kuchment, Peter
Bose-Einstein condensation (BEC) is one of the most intriguing macroscopic quantum phenomena. It has been observed in a variety of different systems, including ultracold atoms and ensembles of quasiparticles. In this work we concentrate on the magnon Bose-Einstein condensation observed in ferromagnetic yttrium iron garnet (YIG) film. In contrast to the cold atomic system, the magnon BEC proceeds at room temperature. We first review the basic theory of magnons in ferromagnetic film and discuss the recent experimental results on magnon BEC. The magnon spectrum in YIG film has two minima of energy at nonzero wavevectors Q and -Q. Therefore, in principle two condensates can appear. It is very important for observable condensation phenomena how the condensed magnons are distributed between the two minima and whether two condensates are coherent. Previous theoretical and experimental studies ignored both these problems. In this dissertation we address these important questions. Starting from the microscopic model describing the ferromagnetic film, we analytically calculate the interaction of condensates. It depends on thickness of the film d and external magnetic field H0. In comparatively thick films (1-5 µm) the magnons of the same condensate attract each other, whereas the magnons of different condensate repulse. It leads to spontaneous violation of the mirror symmetry predicted by our theory. As a consequence, the numbers of condensed magnons in the two minima are not equal. This result explains the rather low contrast in the interference pattern observed in experiments by the real space Brillouin light scattering methods. We also find that the dipolar interaction that does not conserve the magnon number generates a special type of interaction that leads to the coherence between two condensates and to the existence of two types of condensates with sum of their phase equal to either 0 or π. The existence of the interference pattern violates also the translational symmetry of the condensate. The corresponding excitations are Goldstone modes that we call "zero sound'. We calculated its spectrum. We also calculated how the condensate depends on the thickness of film and external magnetic field and discovered that, in the range of thickness 0:1 - 03 µm the phase transition to the phase with equal condensate densities proceeds. This transition as well as transition between 0- and π- phases can be driven by external magnetic field. Next we study the relaxation rate of condensed magnons. There are two important time scales in the formation of magnon BEC, that is, the thermalization time τth and the life time τl. In order to generate and observe BEC, the condition τth >> τl must be fulfilled. Experimentally the thermalization time is of the order of 100 ns. The relaxation is due to the magnon-magnon interaction conserving the magnon numbers. The lifetime is found to be of the order of 1 µs, and was thought to be due to the magnon-phonon interaction which doesn't conserve the magnon numbers. However the calculation of lifetime due to magnon-phonon interaction disagrees with the experimental values. Here we calculate the lifetime due to three magnon processes in a ferromagnetic film with finite thickness. Our calculation gives a lifetime of the order of 10 µs, which is almost of the same order of magnitude with the one provided by magnon-phonon interaction. This means that the three magnon processes provide an important channel for the relaxation of condensed magnons.
Spintronics in Half-Passivated Graphene
(2016-12-02) Hemmatiyan, Shayan; Sinova, Jairo; Abanov, Artem G; Naugle, Donald; Cagin, Tahir
In this thesis, I propose a practical way to stabilize half passivated graphene (graphone). I show that the dipole moments induced by a hexagonal-boron nitride (h-BN) substrate on graphene stabilize the hydrogen atoms on one sublattice of the graphene layer and suppress the migration of the adsorbed hydrogen atoms. I also present the substrate effect of h-BN that reduces distortion induced by fluorination of graphene and stabilizes half-passivated graphene in a single sublattice. Then using spin-polarized density functional calculations I investigate magnetic properties of graphone. I show the system has different magnetic order which can be described by either super-exchange or double exchange mechanisms depending on the type of add atom. The hybridization of graphene changes from sp^2- to sp^3- type hybridization due to the buckling induced by passivation. The change in hybridization together with add-atom orbitals induces a fairly large spin orbit coupling (SOC). Based upon first principle spin-polarized density of states calculations, I show that the graphone obtained in different graphene/h-BN heterostructures exhibits a half metallic state. I propose to use this exotic material for spin valve systems and other spintronics devices.
Statistical Physics Models Governed by Diffusion
(2020-05-14) Ye, Jiayan; Procaccia, Eviatar B; Abanov, Artem G; Berkolaiko, Gregory; Paouris, Grigoris
In this article we consider two probability models: stationary diffusion limited aggregation (SDLA) and finitary random interlacements (FRI). SDLA is a stochastic process on the upper half planar lattice, growing from an infinite line, with local growth rate proportional to stationary harmonic measure. We first prove that stationary harmonic measure of an infinite set in the upper planar lattice can be represented as the proper scaling limit of the classical harmonic measure of truncations of the infinite set. Then we construct an infinite SDLA that is ergodic with respect to left-right integer translation. For FRI, we prove a phase transition in the connectivity of FRI FI^{u,T} on Z^d with respect to the average stopping time T .

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Browsing Electronic Theses, Dissertations, and Records of Study (2002– ) by Author "Abanov, Artem G"

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