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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
81

Manipulating Nonproportionality of Strontium Iodide Crystals with High-Flux Irradiation by 137Cs Gamma Rays

Caudel, David Dewayne 06 April 2017 (has links)
Strontium iodide doped with europium (SrI2:Eu2+) is a new scintillator being developed for use in high-energy astrophysical detectors with excellent energy resolution. Nonproportionality is the primary limiting factor to improving its energy resolution, although the physics of nonproportionality is not yet fully understood. Only in the past few years have the underlying physical models of nonproportionality been reported. Materials science solutions for improving nonproportionality are also being investigated, with the main approach being the incorporation of deliberate addition of crystal defects using codoping. In this study, we look at an alternative method that is similar to codoping, using radiation damage to alter proportionality. By irradiating a SrI2:Eu2+ sample with a 2,255 Ci 137Cs source, we explore both the crystalâs potential for space-based applications in a radiation environment and this new method of altering nonproportionality. We propose a novel method of altering the nonproportionality of scintillators, using radiation-induced F-centers in place of co-dopants. We demonstrate that radiation damage is an effective approach for defect engineering that may be used to study nonproportionality in scintillators. The advantage of this proposed approach is that one can use the same crystal to vary the concentration of defects, while codoping introduces additional variables, since it requires growing a different crystal for each codoping concentration.
82

Density Functional Theory Study of Dilute Transition Metal Phthalocyanines

Burrill, Daniel 01 January 2015 (has links)
Organometallic (OM) crystals are studied in fields ranging from spintronics to photovoltaics. This thesis focuses on studying a particular class of OM crystals known as transition metal-phthalocyanines (TM-Pc) - a molecular crystal composed of chains of planar OM molecules with a transition metal center and four coordinated pyrrole-aromatic rings joined by nitrogen atoms, similar to porphyrin. The structure resembles a dish rack pattern where the planar TM-Pcs of adjacent chains are oriented nearly perpendicular to each other. While TM-Pcs have been studied for decades due to their interesting optical properties and applications as dyes, there has been recent interest in understanding the magnetic properties with various transition metals. Due to crystal arrangement, inter-chain interactions among TM-Pcs are relatively weak when compared to intra-chain interactions. This property allows the chains of TM-Pcs to be isolated and approximated as a pseudo 1D system. The electronic structure and spin exchange are computationally examined along chains of CuPc when they have been diluted with the metal-free variant, H2Pc. Density functional theory is employed with the Hubbard U correction to account for electron interactions on the copper d-orbitals. Since the diluted systems are effectively 1D with narrow bands along their stacking axis, a 1D Heisenberg model is applied where the exchange coefficient is determined through the Broken Symmetry method. Additionally, the effect of non-local corrections, used to determine structural features, on the Hubbard U and Heisenberg exchange coefficient, J, are discussed.
83

The temperature and field dependence of thermionic and photoelectric currents

Fitch, R. K. January 1968 (has links)
No description available.
84

AFM, CAFM, and EFM Studies of the GaN System

Dickinson, John C. 01 January 2003 (has links)
This thesis discusses a variety of techniques based on the atomic force microscope (AFM), and their application to the GaN materials system. In particular, the local conductivity and contact potential of surfaces have been measured using the techniques of Conductive AFM (CAFM) and Surface Potential Electric Force Microscopy (SPEFM), respectively. CAFM studies of GaN surfaces have revealed that prismatic planes around islands and pits on surfaces can lead to enhanced conductivity, which may be related to leakage problems in device applications. With regard to SP-EFM work, the change in surface potential associated with inversion domains on Ga-polar GaN has been imaged, yielding voltage differences up to 90 mV. Given that such inversion domains increase carrier scattering and can degrade device performance, their identification using this technique is important. SP-EFM has also been used to map the local surface potential in the active region of Modulation Doped Field Effect Transistors (MODFET's). This is the first step in a proposed study to investigate the effects of current lag in such devices.
85

Precision Calculations for Electroweak Physics at Hadron Colliders

Zhou, Jia 26 October 2016 (has links)
<p> Electroweak (EW) corrections can be enhanced at high energies due to the soft or collinear radiation of virtual and real <i>W</i> and <i> Z</i> bosons that result in Sudakov-like corrections of the form &alpha;<sub> W</sub><sup>l</sup>log<sup>n</sup>(Q<sup>2</sup>/M<sub>V</sub><sup> 2</sup>), where &alpha;<sub>W</sub> = &alpha;/(4&pi;sin<sup>2</sup>&thetas;<sub> W</sub>) and <i>n</i> &le; 2l &ndash; 1. Here <i>M<sub>V</sub></i> denotes the <i>W</i> or <i>Z</i> boson mass, &thetas;<sub> w</sub> the weak mixing angle and Q<sup>2</sup> a typical energy scale of the hard process under consideration. The inclusion of EW corrections in predictions for the CERN Large Hadron Collider (LHC) is therefore especially important when searching for signals of possible new physics in distributions probing the kinematic regime Q<sup>2</sup> &raquo; M<sub>V</sub><sup> 2</sup>. Next-to-leading order (NLO) EW corrections should also be taken into account when their size (O(&alpha;)) is comparable to that of QCD corrections at next-to-next-to-leading order (NNLO) (O(&alpha;<sub>s</sub><sup>2</sup>)). To this end we have calculated and implemented in the parton-level Monte-Carlo program MCFM the NLO weak corrections to three key processes at the LHC: the Neutral-Current Drell-Yan process, top-quark pair production and di-jet production. This enables a study of their effects on LHC observables combined with the already available QCD corrections at NLO and NNLO. We provide both the full NLO weak corrections and their Sudakov approximation, since the latter is often used for a fast evaluation of weak effects at high energies and can be extended to higher orders. With both the exact and approximate results at hand, the validity of the Sudakov approximation can be readily quantified. In the case of top-quark pair production at the LHC we also calculated the NLO QED corrections.</p>
86

Optimization of electrostatic binding free energy : application to barnase and barstar

Lee, Lee-Peng, 1969- January 1999 (has links)
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 1999. / Includes bibliographical references. / by Lee-Peng Lee. / Ph.D.
87

RF edge physics on the Alcator C-Mod tokamak

Reardon, James C. (James Christian), 1969- January 1999 (has links)
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 1999. / Includes bibliographical references (p. 146-152). / by James Christian Readron. / Ph.D.
88

Tensor polarization of the deuteron at high momentum transfer in elastic (e,d) scattering

Zhao, Wenxia, 1968- January 1999 (has links)
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 1999. / Includes bibliographical references (p. 202-209). / by Wenxia Zhao. / Ph.D.
89

Multiband Models for Colossal Magnetoresistance Materials and Diluted Magnetic Semiconductors

Unknown Date (has links)
Several multiband models for Colossal Manetoresistance Materials and Diluted Magnetic Semiconductors are investigated within Dynamical Mean-Field Theory. The theoretical analysis of such models uses extensively the parametrization method for the bare Green's function to study the critical transition temperatures in a wide range of model's parameters; hoping amplitudes, couplings, and carrier concentrations. For both classes of materials, it is found within a two-band model that the transition temperature can be twice larger than the one predicted by the one band models and that its maximum is reached at a twice larger carrier concentration. It is also revealed that the off-diagonal hoppings, when considered, lead to a substantial boost of the ferromagnetic transition temperature. By accounting for the attractive Coulomb potential by acceptors in diluted magnetic semiconductors, the impurity to valence band crossover is studied as a function of doping and the critical values of doping, at which the crossover occurs, are estimated for various diluted magnetic semiconductors. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Degree Awarded: Summer Semester, 2007. / Date of Defense: May 30, 2007. / Coulomb Attraction by Magnetic Impurities, Ferromagnetic Transition Temperature, Colossal Magnetoresistance, Diluted Magnetic Semiconductors, Dynamical Mean-Field Theory / Includes bibliographical references. / Professor N. Bonesteel, Professor Directing Dissertation; Professor N. Dalal, Outside Committee Member; Professor P. Xiong, Committee Member; Professor J. Piekarewicz, Committee Member; Professor I. Chiorescu, Committee Member.
90

SO(10) Supersymmetric Grand Unified Theories: from Cosmology to Colliders

Unknown Date (has links)
Simple SUSY GUT models based on the gauge group SO(10) require t-b-t Yukawa coupling unification, in addition to gauge coupling and matter unification. The Yukawa coupling unification places a severe constraint on the expected spectrum of superpartners, with scalar masses ~ 10 TeV while gaugino masses are quite light. For Yukawa-unified models with μ > 0, the spectrum is characterized by three mass scales: i). first and second generation scalars in the multi-TeV range, ii). third generation scalars, μ and mA in the few-TeV range and iii). gluinos in the ~ 350−500 GeV range with chargino masses around 100−160 GeV. In such a scenario, gluino pair production should occur at large rates at the CERN LHC, followed by gluino three-body decays into neutralinos or charginos. Discovery of Yukawa-unified SUSY at the LHC should hence be possible with only 1 fb−1 of integrated luminosity, by tagging multi-jet events with 2–3 isolated leptons, without relying on missing ET . A characteristic dilepton mass edge should easily be apparent above Standard Model background. Combining dileptons with b-jets, along with the gluino pair production cross section information, should allow for gluino and neutralino mass reconstruction. A secondary corroborative signal should be visible at higher integrated luminosity in the X1±1X20 → 3l channel, and should exhibit the same dilepton mass edge as in the gluino cascade decay signal. A problem generic to all supergravity models comes from overproduction of gravitinos in the early universe: if gravitinos are unstable, then their late decays may destroy the predictions of Big Bang nucleosynthesis. We also present a Yukawa-unified SO(10) SUSY GUT scenario which avoids the gravitino problem, gives rise to the correct matter-antimatter asymmetry via non-thermal leptogenesis, and is consistent with the WMAP-measured abundance of cold dark matter due to the presence of an axino LSP. To maintain a consistent cosmology for Yukawa-unified SUSY models, we require a re-heat temperature TR ~ 106−107 GeV, an axino mass around 0.1−10 MeV, and a Peccei-Quinn breaking scale fa ~ 1012 GeV. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Degree Awarded: Fall Semester, 2008. / Date of Defense: September 12, 2008. / Supersymmetry (SUSY), Grand Unification (GUT), Dark Matter, Phenomenology, Collider Physics, Large Hadron Collider (LHC), Cern, Mulitlepton, Axino / Includes bibliographical references. / Howard Baer, Professor Directing Dissertation; Mark Sussman, Outside Committee Member; Laura Reina, Committee Member; Horst Wahl, Committee Member; Efstratios Manousakis, Committee Member.

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