Physics

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    Microturbulent transport of non-Maxwellian alpha particles
    (2015) Wilkie, George John; Dorland, William; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A burning Deuterium-Tritium plasma is one which depends upon fusion-produced alpha particles for self-heating. Whether a plasma can reach a burning state requires knowledge of the transport of alpha particles, and turbulence is one such source of transport. The alpha particle distribution in collisional equilibrium forms a non-Maxwellian tail which spans orders of magnitude in energy, and it is this tail that is responsible for heating the plasma. Newly-born high-energy alpha particles are not expected to respond to turbulence as strongly as alpha particles that have slowed down to the bulk plasma temperature. This dissertation presents a low-collisionality derivation of gyrokinetics relevant for alpha particles and describes the upgrades made to the GS2 flux-tube code to handle general non-Maxwellian energy distributions. With the ability to run self-consistent simulations with a population of alpha particles, we can examine certain assumptions commonly made about alpha particles in the context of microturbulence. It is found that microturbulence can compete with collisional slowing-down, altering the distribution further. One assumption that holds well in electrostatic turbulence is the trace approximation, which is built upon to develop a model for efficiently calculating the coupled radial-energy turbulent transport of a non-Maxwellian species. A new code was written for this purpose and corrections to the global alpha particle heating profile due to microturbulence in an ITER-like scenario are presented along with sensitivity studies.
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    In Situ Growth and Doping Studies of Topological Insulator Bismuth Selenide
    (2015) Hellerstedt, John Thery; Fuhrer, Michael; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The past decade has borne witness to the rapid development of a new field of theoretical and experimental condensed matter physics commonly referred to as "topological insulators". In a (experimentalist's) single sentence a topological insulator can be thought of as material that behaves like an empty metal box: the i-dimensional material (i = 2,3) is insulating, but conducting states exist at the (i-1)-dimensional boundaries. These edge or surface states possess non-trivial properties that have generated interest and excitement for their potential utility in solving practical problems in spin electronics as well as the creation of condensed matter systems for realizing and testing new physics. The most extensively studied materials systems with these properties suffer a major drawback in that the interior of the metal box is not "empty" (insulating) but instead filled with metal. The goal of this work has been to understand why the box is full instead of empty, and explore a particular pathway to making it empty. To address these outstanding questions in the literature pertaining to the persistent n-doping of topological insulator Bi2Se3, a custom apparatus was developed for combined epitaxial thin film growth with simultaneous, in situ measurement of transport characteristics (resistivity, Hall carrier density and mobility). Bi2Se3 films are found to be n-doped before exposure to ambient conditions and this doping appears to be interfacial in origin. This work and methodology was extended to study the efficacy of an amorphous MoO3 capping layer. MoO3 acts as an electron acceptor, p-doping the Bi2Se3 up to a |∆n| = 1x10^13 cm−2 change in carrier density. Complimentary X-ray photoemission spectroscopy (XPS) on bulk crystals showed that this was enough to put the Fermi level into the topological regime. Thin films were too highly n-doped initially to reach the topological regime, but the same magnitude change in doping was observed via the Hall effect. Finally, a Bi2Se3 film with a 100 nm capping layer of MoO3 was vented to ambient, and found to retain a stable doping for days of ambient exposure, demonstrating the effectiveness of MoO3 for passivation.
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    Theory of graphene transport properties
    (2013) Li, Qiuzi; Das Sarma, Sankar; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Graphene is of great fundamental interest and has potential applications in disruptive novel technologies. In order to study the novel phenomena in graphene, it is essential to understand its electron transport properties and in particular the main factors limiting its transport mobility. In this dissertation, we study the transport properties of graphene in the presence of electron-hole puddles induced by charged impurities which are invariably present in the graphene environment. We calculate the graphene conductivity by taking into account the non-mean-field two-component nature of transport in the highly inhomogeneous density and potential landscape, where activated transport across the potential fluctuations in the puddle regimes coexists with regular metallic diffusive transport. Our theoretical calculation explains the non-monotonic feature of the temperature dependent transport, which is experimentally generically observed in low mobility graphene samples. Our theory also predicts the existence of an intriguing "disorder by order" phenomenon in graphene transport where higher-quality (and thus more ordered) samples, while having higher mobility at high carrier density, will manifest more strongly insulating (and thus effectively more disordered) behavior as the carrier density is lowered compared with lower quality samples (with higher disorder), which exhibit an approximate resistivity saturation phenomenon at low carrier density near the Dirac point. This predicted behavior, simulating a metal-insulator transition, arises from the suppression of Coulomb disorder induced inhomogeneous puddles near the charge neutrality point in high quality graphene samples. We then study carrier transport through graphene on SrTiO3 substrates by considering the relative contributions of Coulomb and resonant impurity scattering to graphene resistivity. We establish that the nonuniversal high-density behavior of &sigma(n) in different graphene samples on various substrates arises from the competition among different scattering mechanisms, and it is entirely possible for graphene transport to be dominated by qualitatively different scattering mechanisms at high and low carrier densities. Finally, we calculate the graphene conductivity as a function of carrier density, taking into account possible correlations in the spatial distribution of the Coulomb impurity disorder in the environment. We find that the conductivity could increase with increasing impurity density if there is sufficient inter-impurity correlation present in the system.
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    Trinity: A Unified Treatment of Turbulence, Transport, and Heating in Magnetized Plasmas
    (2009) Barnes, Michael; Dorland, William; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    To faithfully simulate ITER and other modern fusion devices, one must resolve electron and ion fluctuation scales in a five-dimensional phase space and time. Simultaneously, one must account for the interaction of this turbulence with the slow evolution of the large-scale plasma profiles. Because of the enormous range of scales involved and the high dimensionality of the problem, resolved first-principles global simulations are very challenging using conventional (brute force) techniques. In this thesis, the problem of resolving turbulence is addressed by developing velocity space resolution diagnostics and an adaptive collisionality that allow for the confident simulation of velocity space dynamics using the approximate minimal necessary dissipation. With regard to the wide range of scales, a new approach has been developed in which turbulence calculations from multiple gyrokinetic flux tube simulations are coupled together using transport equations to obtain self-consistent, steady-state background profiles and corresponding turbulent fluxes and heating. This approach is embodied in a new code, Trinity, which is capable of evolving equilibrium profiles for multiple species, including electromagnetic effects and realistic magnetic geometry, at a fraction of the cost of conventional global simulations. Furthermore, an advanced model physical collision operator for gyrokinetics has been derived and implemented, allowing for the study of collisional turbulent heating, which has not been extensively studied. To demonstrate the utility of the coupled flux tube approach, preliminary results from Trinity simulations of the core of an ITER plasma are presented.
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    semiconducting carbon nanotube transistors: electron and spin transport properties
    (2006-04-25) Chen, Yung-Fu; Fuhrer, Michael S.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Single-walled carbon nanotubes (SWNTs) have attracted great interest both scientifically and technologically due to their long mean free paths and high carrier velocities at room temperature, and possibly very long spin-scattering lengths. This thesis will describe experiments to probe the charge-and spin-transport properties of long, clean individual SWNTs prepared by chemical vapor deposition and contacted by metal electrodes. A SWNT field-effect transistor (SWNT-FET) has been shown to be sensitive to single electrons in charge traps. A single charge trap near a SWNT-FET is explored here using both electronic and scanned-probe techniques, and a simple model is developed to determine the capacitances of the trap to the SWNT and gate electrode. SWNTs are contacted with ferromagnetic electrodes in order to explore the transport of spin-polarized current through the SWNT. In some cases spin-dependent transport was observed, verifying long spin scattering lengths in SWNT. However, in many cases no spin-dependent effects were observed; these results will be discussed in the context of the present state of results in the literature. Semiconducting SWNTs (s-SWNTs) with Schottky-barrier contacts are measured at high bias. Nearly symmetric ambipolar transport is observed, with electron and hole currents significantly exceeding 25 µA, the reported current limit in m-SWNTs. Four simple models for the field-dependent velocity (ballistic, current saturation, velocity saturation, and constant mobility) are studied in the unipolar regime; the high-bias behavior is best explained by a velocity saturation model with a saturation velocity of 2 x 10^7 cm/s. A simple Boltzmann equation model for charge transport in s-SWNTs is developed with two adjustable parameters, the elastic and inelastic scattering lengths. The model predicts velocity saturation rather than current saturation in s-SWNTs, in agreement with experiment. Contact effects in s-SWNT-FET are explored by electrically heating the devices. These experiments resolve the origin of nanotube p-type behavior in air by showing that the observed p-type behavior upon air exposure cannot be explained by change in contact work function, but is instead due to doping of the nanotube. Modest doping of the SWNT narrows the Schottky Barriers and provides a high-conductance Ohmic tunnel contact from electrode to SWNT.