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

Permanent URI for this collection

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

Now showing 1 - 5 of 5
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    Linear and Nonlinear Optics in a System of Massless Dirac Fermions
    (2014-08-10) Yao, Xianghan; Belyanin, Alexey; Lazarov, Raytcho; Ross, Joseph H.; Abanov, Artem G.
    Graphene electrons possess linear energy dispersion relation, and thus behave as two-dimensional (2D) Dirac fermions. Consequently, compared with the conventional 2D electron gas systems (2DEG) found in MOSFETs and quantum wells, graphene exhibits a variety of electronic and optoelectronic properties that are characteristic of 2D Dirac fermions. Similar 2D Dirac fermions are found at the surface layer of 3D topological insulator, and they are topological protected from backscattering due to spin-orbital coupling and time reversal symmetry. We here calculate the linear and nonlinear optical response of graphene in strong magnetic and optical fields, using a quantum-mechanical density-matrix formalism. We show that graphene in a magnetic field possesses a giant mid- or far-infrared optical nonlinearity, perhaps the highest among known materials. The high nonlinearity originates from the unique electronic properties and selection rules near the Dirac point. As a result, even one monolayer of graphene gives rise to an appreciable nonlinear frequency conversion efficiency for incident infrared radiation. Inspired by the highly efficient four-wave mixing process in the 2D Dirac fermion systems, we further propose a new mechanism of generating polarization-entangled photons based on the parametric generation process in the third section of this dissertation. Unique properties of quantized electron states in a magnetized graphene and optical selection rules near the Dirac point give rise to a giant optical nonlinearity and a high rate of photon production in the mid- or far-infrared range. A similar mechanism of photon entanglement may exist in topological insulators where the surface states have a Dirac-cone dispersion and demonstrate similar properties of magneto-optical absorption. In the absence of a magnetic field, the surface plasmon resonance provides an alternative method to enhance nonlinear frequency conversion efficiency. In the forth section of this dissertation, a graphene-based difference frequency generation (DFG) of terahertz plasmons is proposed as an example to study nonlinear photonplasmon interaction in 2D Dirac fermion systems. Our results demonstrate strong enhancement of the DFG efficiency near the plasmon resonance and the feasibility of phase-matched nonlinear generation of plasmons over a broad range of frequencies. Considering graphene plasmonics' superiorities in wave confinement, dissipation and tunability, a graphene-based nonlinear terahertz plasmon generation process promises applications in terahertz sources and sensors, as well as integrated photonic circuits.
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    Magnons in Ferromagnetic Films
    (2020-08-13) Li, Gang; Pokrovsky, Valery Leonidovich; Abanov, Artem G.; Naugle, Donald; Fulling, Stephen Albert
    The theory of magnons in ferromagnetic films has important applications to real magnets and a rather long history. In this dissertation, we first present a new version of the asymptotically exact theory of the spectrum and transverse distribution of magnetization in long-wave magnons. It is based on the exact analytical solution of the linearized Landau-Lifshitz equation in a film. We also studied and used symmetry of the Hamiltonian. Our new method simplifies all calculations and provides analytical results for the range of parameters most important for experiment. The quantization of the transverse wave vector and the role of evanescent waves at different values of parameters are studied. Another important motivation of this work was its application to the problem of Bose-Einstein condensation (BEC) and superfluidity of magnons. We use a classical modification of the Holstein-Primakoff transformation to solve the Landau-Lifshitz equation, the exact phase diagram for magnon condensate in Yttrium Iron Garnet Film is studied. We also collaborated with an experimental group that provides direct experimental evidence that magnons in a condensate exhibit a repulsive interaction resulting in condensate stabilization. We propose a mechanism, which is responsible for the interaction inversion. This mechanism supports their conclusions by the theoretical model based on the Gross-Pitaevskii equation.
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    NMR and Transport Measurements of Copper Chalcogenide and Clathrate Compounds
    (2016-10-18) Sirusi Arvij, Ali; Ross, Jr., Joseph H.; Naugle, Donald G.; Abanov, Artem G.; Arroyave, Raymundo
    Due to limited sources of fossil fuels worldwide and a large percentage wasted as heat energy, searching for efficient thermoelectric materials to convert heat to electricity has gained a great deal of attention. Most of the attempts are focused on materials with substantially lower lattice thermal conductivity and narrow band gaps. Among them, inorganic clathrates and copper-based chalcogenides possess intrinsic low thermal conductivity which makes them promising thermoelectrics. In this work, nuclear magnetic resonance (NMR), transport, and magnetic measurements were performed on clathrates and copper-based chalcogenides to investigate their vibrational and electronic charge carrier properties, as well as the unknown structures of Cu2Se and Cu2Te at low temperatures, and the effect of rattling of guest atoms in the clathrates. The NMR results in Ba8Ga16Ge30 indicate a pseudogap in the Ga electronic density of states, superposed upon a surprisingly large Ba contribution to the conduction band. Meanwhile, the phonon contributions to the Ga relaxation rates are large and increase more rapidly with temperature than typical semiconductors due to enhanced anharmonicity of the propagative phonon modes over a wide range. Moreover, the observed NMR shifts in the Ba8Cu5SixGe41-x clathrates change in a nonlinear way with increasing Si substitution: from x = 0 to about 20 the shifts are essentially constant, while approaching x = 41 they increase rapidly, demonstrating a significant change in hybridizations vs Si substitution. NMR studies of Cu2Se show an initial appearance of ionic hopping in a narrow temperature range above 100 K, coinciding with the recently observed low-temperature phase transition. At room temperature and above, this goes over to rapid Cu-ion hopping and a single motionally narrowed line both above and below the α-β structural transition. Furthermore, the NMR results on Cu2Te and Cu1.98Ag0.2Te demonstrate unusually large negative chemical shifts, as well as large Cu and Te s-state contributions in the valence band. The large diamagnetic chemical shifts coincide with behavior previously identified for materials with topologically nontrivial band inversion, and in addition, the large metallic shifts point to analogous features in the valence band density of states, suggesting that Cu2Te may have similar inverted features.
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    On the Controllable Spin-Wave Dynamics in Magnonic Crystals
    (2023-05-18) Liu, Ankang; Finkelstein, Alexander; Abanov, Artem G.; Naugle, Donald G.; Kuchment, Peter
    Spin waves are the collective wave excitations in the magnetically ordered system, which have the frequencies typically ranged from GHz up to even THz. In recent years, the study of spin waves, which is referred to as “magnonics”, has been significantly advanced; and the low-damping coherent spin waves are considered as a suitable candidate for performing rapid data processing and wave computing. The scalability of such spin-wave based computing devices is rather promising due to the possibilities of exciting spin waves with wavelengths down to the nanometer range. Magnonic crystals are various forms of spatial modulation of magnetic properties that can be seen as magnetic metamaterials. The magnonic crystals, as a widely used approach to tailor the spin-wave band structure and an effective way to control the spin-wave propagation, have been studied extensively. In this dissertation, we explore the possibilities of utilizing various kinds of magnonic crystals for the controllable spin-wave dynamics in different magnetic systems. We first develop a description of spin waves in a 3D quantum XY antiferromagnet (AFM) in terms of macroscopic variables, magnetization and Néel vector densities. We consider a layered AFM with spins located on the honeycomb lattice. We show that, in the discussed system, the spectrum of spin waves consists of four modes, all well captured by our macroscopic description. The gapless mode of the spin waves, i.e., magnons, is described by a system of equations, which has a structure general for the Goldstone mode in AFMs. We demonstrate that the parameters in the spin Hamiltonian can be evaluated by fitting the experimental data with the results obtained for the four modes using the macroscopic variable approach. The description of AFM in terms of macroscopic variables can be easily extended to the case when the lattice of the magnetic substance is deformed by an external strain or acoustic wave. Next, we study the spin-wave dynamics in such a layered AFM in the presence of a periodic lattice deformation. We suggest to use spatially modulated strain (a type of magnonic crystals) for the control of a spin wave propagating inside a bulk AFM. The modulation with the wave vector q, by virtue of magnetoelasticity, mixes spin waves with wave vectors near q/2 and −q/2. This leads to lifting the degeneracy of the symmetric and antisymmetric eigenstate combinations of these waves. Therefore, a moving spin wave being subjected to the lattice modulation after some time alters its propagation direction to the opposite one, and so on. The resulting picture reminds one of a tunneling particle in a symmetric double-well potential. The effect can be utilized for the control of the spin-wave propagation that can be useful for magnonic applications. The control may include a delay line element, filtering, and waveguide of the spin waves in AFM. For a ferromagnet (FM), we investigate its spin-wave dynamics under a switchable current-induced magnonic crystal. In this case, we consider a ferromagnetic (FM) sample with a metallic meander pattern (whose spatial modulation is described by a wave vector q) fabricated on its top surface. The magnonic crystal will be switched on and off by applying a current to the meander structure. For a conventional magnonic crystal with direct current (DC) supply, the spin waves around q/2 are resonantly coupled to the waves near −q/2, and similar to the periodically deformed AFM, a band gap is opened at k = ±q/2. We further demonstrate that if instead of the DC current the magnonic crystal is supplied with an alternating current (AC), then the band gap is shifted to k satisfying |ωs(k) −ωs(k − q)| = ωac; here ωs(k) is the dispersion of the spin wave, while ωac is the frequency of the AC modulation. The resulting gap in the case of the AC magnonic crystal is the half of the one caused by the DC with the same amplitude of modulation. The time evolution of the resonantly coupled spin waves controlled by properly suited AC pulses can be well interpreted as the motion on a Bloch sphere. The tunability of the AC magnonic crystal broadens the perspective of spin-wave computing.
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    On the Controllable Spin-Wave Dynamics in Magnonic Crystals
    (2023-05-18) Liu, Ankang; Finkelstein, Alexander; Abanov, Artem G.; Naugle, Donald G.; Kuchment, Peter
    Spin waves are the collective wave excitations in the magnetically ordered system, which have the frequencies typically ranged from GHz up to even THz. In recent years, the study of spin waves, which is referred to as “magnonics”, has been significantly advanced; and the low-damping coherent spin waves are considered as a suitable candidate for performing rapid data processing and wave computing. The scalability of such spin-wave based computing devices is rather promising due to the possibilities of exciting spin waves with wavelengths down to the nanometer range. Magnonic crystals are various forms of spatial modulation of magnetic properties that can be seen as magnetic metamaterials. The magnonic crystals, as a widely used approach to tailor the spin-wave band structure and an effective way to control the spin-wave propagation, have been studied extensively. In this dissertation, we explore the possibilities of utilizing various kinds of magnonic crystals for the controllable spin-wave dynamics in different magnetic systems. We first develop a description of spin waves in a 3D quantum XY antiferromagnet (AFM) in terms of macroscopic variables, magnetization and Néel vector densities. We consider a layered AFM with spins located on the honeycomb lattice. We show that, in the discussed system, the spectrum of spin waves consists of four modes, all well captured by our macroscopic description. The gapless mode of the spin waves, i.e., magnons, is described by a system of equations, which has a structure general for the Goldstone mode in AFMs. We demonstrate that the parameters in the spin Hamiltonian can be evaluated by fitting the experimental data with the results obtained for the four modes using the macroscopic variable approach. The description of AFM in terms of macroscopic variables can be easily extended to the case when the lattice of the magnetic substance is deformed by an external strain or acoustic wave. Next, we study the spin-wave dynamics in such a layered AFM in the presence of a periodic lattice deformation. We suggest to use spatially modulated strain (a type of magnonic crystals) for the control of a spin wave propagating inside a bulk AFM. The modulation with the wave vector q, by virtue of magnetoelasticity, mixes spin waves with wave vectors near q/2 and −q/2. This leads to lifting the degeneracy of the symmetric and antisymmetric eigenstate combinations of these waves. Therefore, a moving spin wave being subjected to the lattice modulation after some time alters its propagation direction to the opposite one, and so on. The resulting picture reminds one of a tunneling particle in a symmetric double-well potential. The effect can be utilized for the control of the spin-wave propagation that can be useful for magnonic applications. The control may include a delay line element, filtering, and waveguide of the spin waves in AFM. For a ferromagnet (FM), we investigate its spin-wave dynamics under a switchable current-induced magnonic crystal. In this case, we consider a ferromagnetic (FM) sample with a metallic meander pattern (whose spatial modulation is described by a wave vector q) fabricated on its top surface. The magnonic crystal will be switched on and off by applying a current to the meander structure. For a conventional magnonic crystal with direct current (DC) supply, the spin waves around q/2 are resonantly coupled to the waves near −q/2, and similar to the periodically deformed AFM, a band gap is opened at k = ±q/2. We further demonstrate that if instead of the DC current the magnonic crystal is supplied with an alternating current (AC), then the band gap is shifted to k satisfying |ωs(k) −ωs(k − q)| = ωac; here ωs(k) is the dispersion of the spin wave, while ωac is the frequency of the AC modulation. The resulting gap in the case of the AC magnonic crystal is the half of the one caused by the DC with the same amplitude of modulation. The time evolution of the resonantly coupled spin waves controlled by properly suited AC pulses can be well interpreted as the motion on a Bloch sphere. The tunability of the AC magnonic crystal broadens the perspective of spin-wave computing.