Condensed Matter Physics Commons^™

Nonlinear Charge And Spin Currents In Non-Centrosymmetric Electron Systems, Aniruddha Pan

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In this thesis, we discuss the existence of spin and charge currents in systems with broken spin inversion symmetry proportional to the magnitude square of the driving electric and thermal fields. This outcome is predicated on symmetry considerations in the momentum space, whereby the product between the current operator and the out-of-equilibrium distribution function has to be even.

First, we derive the second-order correction to the particle distribution function $\delta f^{(2)}$ in a semi-classical approximation, considering the local change in the equilibrium distribution function caused by external fields. Our approach departs significantly from the previous theory where $\delta f^{(2)}$ is …

Apparatus And Instrumentation Design For Investigation Of Surface Impact Effects On Superconductivity, Austin Back

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The effects of ion irradiation on the physical properties of materials make EBITs an invaluable tool for many scientific and engineering fields. Many experiments rely on the use of these lab setups to test for device reliability, explore surface physics phenomena, and replicate the environment for many physical systems that are not readily accessible. We seek to extend the capabilities of these experiments using the CUEBIT and a new sample holder installed in section 3.

This thesis begins by presenting an overview of the CUEBIT and the basic operations of the equipment. This is followed by a brief explanation of …

Quantum-Mechanical Evaluation Of Defects In Uranium-Bearing Materials, Megan Hoover

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Quantum-mechanical calculations using density functional theory with the generalized gradient approximation were employed to investigate the effects dopants have on the uranium dioxide (UO₂) structure. Uraninite is a common U⁴⁺ mineral in the Earth's crust and an important material used to produce energy and medical isotopes. Though the incorporation mechanism remains unclear, divalent cations are known to incorporate into the uranium dioxide system. Three charge-balancing mechanisms were evaluated to achieve a net neutral system, including the substitution of (1) a divalent cation for a tetravalent uranium atom and oxygen atom; (2) two divalent cations for a tetravalent …

Electrical And Optical Characterization Of Two-Dimensional Semiconductors Using Ultrafast Spectroscopy, Pan Adhikari

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The emergence of two-dimensional (2D) layered materials provides unprecedented opportunities for studying excitonic physics due to the strong Coulomb interaction between the electron-hole pair. Because of the reduced dimensionality and weak dielectric screening, the exciton is stable at room temperature, unlike bulk semiconductors. The evolution from low to high carrier density for optical gain in 2D semiconductors involves insulating exciton gas, exciton condensation, co-existence of various excitonic complexes, electron-hole plasmas (EHPs), or electron-hole liquids (EHLs), leading to the Mott transition. Strong interaction among the excitons, such as exciton-exciton annihilation (EEA), serves as a hot-carrier generation. A bound exciton dissociates into …

In Situ Study Of Ultrafast Carrier Transport Dynamics In Perovskite Thin-Films, Kanishka Kobbekaduwa

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Perovskites are a novel class of materials that have piqued the interest of researchers in photovoltaics, photodetectors, and optoelectronics. In this study, we measure and elucidate in situ ultrafast carrier dynamics in both organic and inorganic, lead, and non-lead-based halide perovskite thin films using ultrafast photocurrent spectroscopy (UPCS) with a sub-25 ps time resolution. The UPCS technique enables us to define carrier transport dynamics in spatial, temporal, and energy landscapes via measurements at different electric fields, laser intensities, and temperatures. Here we explore and analyze solution-processed bulk MAPbI₃ and nanocrystalline CsPbI₃-based devices and novel non-lead double-layered perovskite …

Highly Charged Ion Interactions With Ultrathin Dielectric Films, Russell Lake

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The excitations occurring at a solid surface due to slow highly charged ion (HCI) impacts are interesting from the perspective of fundamental processes in atomic collisions and materials science. This thesis focuses on two questions: 1) How much HCI potential energy deposition is required to form permanent surface modifications?, 2) How does the presence of a thin dielectric surface film change the classical over-the-barrier picture for neutralization above a clean metal?
I describe a measurement of craters in thin dielectric films formed by XeQ+ (26 ≤ Q ≤ 44) projectiles. Tunnel junction devices with ion-irradiated barriers were used to amplify …

Electrical Detection Of Mechanical Resonance In Nanotubes And Semiconducting Nanowires, Doyl Dickel

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In recent years, there has been substantial interest in the development of microelectro-mechanical systems (MEMS) and even nanoelectro-mechanical systems (NEMS) for use in a wide variety of applications both as experimental tools (refs) and in a continuing effort to decrease the size and cost and increase the efficiency of electrical components. In particular, cantilevered nanometer beams have been a recent focus due to a number of interesting properties, including enhanced field emission, high tensile strength, and piezoelectric properties. The ability to accurately determine the electrical and mechanical properties of these cantilevers is paramount in assessing their feasibility as MEMS and …

Effects Of Surface States, Defects And Dopants On The Optical And Magnetic Properties Of Low-Dimensional Materials, Ramakrishna Podila

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Nanomaterials have attracted the attention of researchers from various fields due to their unique features (that are otherwise absent in the bulk) such as quantum confinement, high surface to volume ratio, ability for surface modification etc. Since the discovery of fullerenes and carbon nanotubes, several synthesis techniques have been developed for nanomaterial growth. However, different control parameters in different synthesis techniques often result in nanostructures with varying defects that may alter their fundamental behavior. Such defects or disorder in the crystal lattice can lead to the disruption of lattice symmetry. The defect-induced symmetry lowering (or breaking) effects play a vital …

A Solution-Based Approach To The Fabrication Of Novel Chalcogenide Glass Materials And Structures, Nathan Carlie

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Chalcogenide glasses (ChGs) are well known for their large optical nonlinearities and high infrared transparency, and are candidate materials for next-generation thin film-based planar infrared (IR) optical applications. They are also known, however, to possess low thermal and mechanical stability as compared to oxide glasses. Traditional physical vapor deposition (PVD) methods used for the deposition of these materials as thin films often suffer from low deposition rates, deviation from stoichiometry, and cannot coat over complex surfaces. In order to retain the attractive optical properties of ChGs while enabling new fabrication routes and hybrid and composite material systems, we have developed …

The Dynamics Of Energy And Charge Transfer In Low And Hyperthermal Energy Ion-Solid Interactions, Matthew Ray

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The energy and charge transfer dynamics for low and hyperthermal energy (10 eV to 2 keV) alkali and noble gas ions impacting noble metals as a function of incident energy, species and scattering geometry has been studied. The experiments were performed in an ultra-high vacuum scattering chamber attached to a low and hyperthermal energy beamline.
The energy transfer was measured for K+ scattered from a Ag(001) surface along the [110] crystalline direction at a fixed laboratory angle of 90°. It was found that as the incident energy is reduced from 100 to 10 eV, the normalized scattered energy increased. Previous …

Effect Of Cosb3 Nanoparticles On The Thermoelectric Properties Of Filled And Unfilled Cosb3 Skutterudites, Paola Alboni

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This study explores the possibility of somewhat decoupling the electrical and thermal conduction, thereby being able to limit the thermal conduction while minimizing the effect on the electrical conduction. The approach is using a nanoparticle layer with a slight compositional mismatch as compared to the bulk skutterudite. A hydrothermal nanoparticle-plating technique has been employed to grow a layer of CoSb₃ nanoparticles on the surface of skutterudite bulk matrix grains. Skutterudites of various forms were fabricated and studied in order to assess the effect of this nano-plated layer as a viable method in the improvement of thermoelectric properties of CoSb …

The Thermoelectric Properties Investigation Of X0.05Mo3Sb7-YTeY (X=Mn, Fe, Ni, Co; Y=1.5, 1.6, 1.7), Tim Holgate

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With ever increasing energy costs, dwindling resources, and terms with the prefix 'green' filling headlines, the need for environmentally friendly energy sources has never been greater. Since the mid-20th century researchers have been pursuing greater efficiency in thermoelectric materials. Thermoelectric materials achieve energy conversion via the Seebeck effect (heat to electric power) and the Peltier effect (electricity to cooling power). Presented herein are the measured electrical and thermal transport properties and phenomenal analysis of the Ir₃Ge₇-type system: X_0.05Mo₃Sb_7-yTe_y (X=Mn, Fe, Ni, Co; y=1.5, 1.6, 1.7) where 'X' are interstitially …

Calculations Of Diffusion In Fcc Binary Alloys Using On-The-Fly Kinetic Monte Carlo, Erdi Bleda

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With the recent advancements in computer technologies and the improvements in simulation algorithms, along with the theories, has allowed scientists to carry out diffusion calculations more efficiently, especially for the cases where either the diffusion itself takes longer, or there is no available isotope to carry out the diffusion experiments such as tracer diffusion.
In this work, we present a systematic approach to diffusion in intermetallic alloys such as weakly clustered Cu — Ni and weakly ordered Au — Ag. We use Accelerated Molecular Dynamics (AMD) combined with the Embedded Atom Method (EAM) to find the necessary saddlepoint energies. With …

Lead Chalcogenide Nano-Composites: Synthesis, Thermal And Electrical Transport Properties, Bo Zhang

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This dissertation focused on the investigation of the thermoelectric properties of PbTe nanocomposites, presenting a series of experimental investigation and qualitative theoretical analysis regarding utilizing the nanostructures to enhance the thermoelectrical properties. Three innovative synthesis techniques, which are related to fabricating nanocomposites, were developed. These three techniques are the size-selective growth of PbTe nanoparticles by a convection CVD technique, the nanolayer coating technique by hydrothermal method, and the alkali-metal-nano-surface-treatment. In the future, these techniques can be applied to many other thermoelectric material systems.

We proposed to use the grain boundary engineering technique route in the thermoelectric nanocomposite field and successfully …

3-D Calculation Of Atom-Surface Collisions With Direct Scattering, Trapping And Desorption, Guoqing Fan

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In this work a scattering theory is developed to simulate the gas-surface scattering procedure in three dimensions, using an iterative algorithm and classical mechanics for the collision process, that describes both direct scattering and trapping-desorption of an incident beam of atomic particles. The initially trapped fraction of particles can be followed as they continue to make further collisions with the surface until they are all eventually promoted back into the positive energy continuum and leave the surface region.

With two different models, the discrete model and the smooth-surface model of the differential reflection coefficient, a series of calculation are made …

The Effects Of Nanoparticle Inclusions Upon The Microstructure And Thermoelectric Transport Properties Of Bismuth Telluride-Based Composites, Nick Gothard

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Research into materials that have high efficiencies of thermoelectric heat-energy conversion has been at a plateau since the middle of the last century. During this time, efficiencies have been engineered high enough for several interesting niche applications but not high enough for widespread adaptation into traditional power generation or refrigeration technologies. The past decade has seen considerable advancement as a number of theoretical works have suggested that lower dimensional structures could hold the key for enhanced efficiency, and several experiments have provided the proof of principle needed to inspire just such a research direction. The benefit of low dimensional structures …

Thermal Coulomb Drag In A Bi-Layer Semiconductor System, Florin Lung

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In this work, we investigate the existence of a thermoelectric effect that consists of the appearance of an electric field in one layer when a temperature gradient is applied in the other layer of a double quantum well system. This represents a generalization of the Seebeck effect to the case of two spatially separated electron systems allowed to interact only through the Coulomb repulsion. The induced electric field results from the momentum transfer between the electrons driven out of equilibrium by the temperature gradient and the electrons at rest in the passive layer, a mechanism known in the literature as …

Charge Effects In Bilayer Electric Transport, Christopher Czech

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The mathematical isomorphism between the layer index in a bilayer system and the spin index in a spin-polarized system is used to draw an analogy between phenomena that occur in the two systems as a result of the electron-electron (e-e) interaction. First, the classical case of the Coulomb drag, a phenomenon resulting from momentum transfer through e-e collisions, is reviewed for both bilayer systems and spin-polarized systems. Then, the spin-backflow phenomenon, a result of the interaction-induced changes in the local quasiparticle energy that affects the charge transport in spin-polarized systems is presented. Finally, the iso-spin correlation is exploited to establish …

Spin Density Wave Phases In Semiconductor Superlattice, Liqiu Zheng

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We discuss the existence of spin instabilities and of possible ground states with broken spin symmetry in the presence of a tilted magnetic field for several semiconductor heterostructures. In each instance, the fundamental premise of our study is the existence of a spin degeneracy, controlled by tuning various experimentally controllable parameters, that permits the apparition of spin-flip processes driven entirely by the many-body Coulomb interaction. If in the case of a single quantum well, the spin instabilities trigger an abrupt paramagnetic to ferromagnetic phase transition, we demonstrate that in superlattices, at low temperatures, a stable spin density wave ($SDW$) ground …

The Thermoelectric Properties Investigation Of Ir3Ge7 Typed Representatives, Huqin Zhang

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Energy conversion is employed to power generation utilizing the Seebeck effect or cooling utilizing the Peltier effect. Herein, we study several Ir₃Ge₇ typed semiconductor materials for potential thermoelectric energy conversion. Mo₃Sb₇, crystallizing in Ir₃Ge₇ type, can be rendered semiconducting by a partial Sb-Te substitution without noticeable structure changes, resulting in Mo₃Sb₅Te₂ with 55 valence electrons per formula unit. Meanwhile, large cubic voids in the Mo₃Sb₅Te₂ crystal structure provide the possibility of filling the voids with small cations to decrease the …

Comparison Of Thermoelectric Properties Of Arc-Melted And Hot-Pressed Half-Heuslers, Brad Edwards

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Relatively low thermal conductivity and great electrical properties in combination with wide tuning parameters through doping make half-Heuslers high potential thermoelectric materials. In this study we plan to investigate the effects of hot-pressing and arc-melting on the thermoelectric properties of ZrNiSn_0.975Sb_0.025, and Hf_0.75Zr_0.25NiSn_0.975Sb_0.025 samples. Also, the effects of electron and phonon scattering due to the insertion of small 50 nm particles within the sample were investigated in ZrNiSn_0.975Sb_0.025 +4% Al₂O₃ and Hf_0.75Zr_0.25NiSn_0.975Sb_0.025 +4% Al₂O …

Comparison Between Steady State And Laser Flash Techniques, Justine Andrews

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Thermoelectrics is a promising avenue of energy conservation. A material is deemed a good thermoelectric material when it portrays certain material characteristics. These characteristics include a high thermopower and electrical conductivity and a low thermal conductivity. This thesis focuses on the methods of measuring a material's thermal conductivity.
The steady state method is a measurement technique used in the low temperature thermal conductivity measurements. This technique assures that the system and sample are at equilibrium before a measurement point is taken. As a result the measurements are both precise and accurate.
The laser flash technique measures a sample's thermal conductivity …

Electronic Structure Of Mos2 Nanotubes, Lingyun Xu

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First-principles methods enable one to study the electronic structure of solids, surfaces, or clusters as accurately as possible with moderate computational effort.
So we used a first-principles electronic structure method to calculate the electronic structure of free-standing layer of MoS₂ with ABA and ABC stacking. Our results suggest MoS₂ with ABA stacking which appears as an insulator has an energy gap of 1.64 eV. The covalent bonds between molybdenum and sulfur atoms are strong enough to form this gap. The ABC stacking will break the symmetry and becomes metallic. The valance and impurities calculations show the rigid-band picture …

Raman Spectroscopic Evidence For Anharmonic Phonon Lifetimes And Blueshifts In 1d Structures, Rahul Rao

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Anharmonic effects in two different quasi-1D systems were probed via micro-Raman spectroscopy. In the first system, we observed upshifts of peaks in the Raman spectra for b-Ga2O3 nanowires grown along the [110] growth direction compared those present in bulk b-Ga2O3. Contrary to our Raman studies on -Ga2O3 nanowires, downshifts in the Raman spectrum for b-Ga2O3 nanowires grown along [401] direction has also been reported by other research groups. We attribute these Raman shifts to the growth direction-induced lattice strains (compressive and tensile) present in the nanowires, and present a model based on the quasi-harmonic density functional theory to support our …

Harmonic Detection Of Resonance In Micro- And Nano- Cantilevers, Jay Gaillard

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Over the past decade there has been an explosion in the study of cantilevered beams on the micron and submicron dimension. The applications and research that involve these structures include state-of-the-art electronic components, sensors, and more recently, studies aimed at elucidating the mechanical properties of cantilevered carbon nanotubes and semiconducting nanowires. In nanoelectro-mechanical systems (NEMS), it is desirable to develop a capacitive readout method involving only two electrodes that are fully compliant with standard CMOS technology. However, the main drawback with this method is the ability to detect resonance in the presence of parasitic capacitance, which is due to the …

Assembly And Modification Of A Hyperthermal And Low Energy Ion Beamline For Detecting Electron-Hole Pair Production In Schottky Diodes, Matthew Ray

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A hyperthermal and low energy ion beamline to measure electron-hole pair production due to ion bombardment of Schottky diodes has been modified and assembled.
A brief overview of the instrument is given and the reason for operating the ion beamline in the hyperthermal and low energy regime is discussed. The capabilities of the ion beamline are unique and provide the means to measure quantities that offer insight into various energy exchange mechanisms that occur at surfaces. Combined with the ability to accept Schottky diode samples, the beamline is capable of measuring both charge transfer and electron-hole pair production caused by …