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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.
21

Far infrared galaxies : star formation and AGN

Hyde, Ashley January 2014 (has links)
This thesis examines the AGN - star formation relation in galaxies out to z=4, probing the interplay between these two phenomena at various redshifts, for different galaxy morphologies, and at different luminosities. We have created a catalogue of 12,000 local infrared galaxies with optical morphological classifications, BPT analysis, and physical parameters derived from SED fits. We explore the variation in specific SFR and AGN fraction with morphology, compare star formation rate estimators, and provide a morphological breakdown of the IR luminosity distribution. Focusing on the Spirals and Barred Spirals from the M-IIFSCz, we find their distributions of specific star formation rates are statistically consistent. Conversely, Barred Spirals have a significantly higher AGN fraction than non-barred Spirals at both high and low stellar masses, but if we control for g-r colour, the AGN fraction is independent of bar presence. We discuss the implications of this result. We perform spectroscopic follow-up of 46 galaxies identified in the IIFSCz as ULIRGs and HLIRGs through photometric redshifts. By deriving spectroscopic redshifts, we show that their photometric redshifts are overestimated by an average factor of zphot/zspec=1.7. We quantify the implications for the Luminosity Function and show that applying a general correction for photometric redshift over- estimation reduces the number density of HLIRGs by 75%. At 1<z<4, we study 35 QSOs and find that rapid accretion is occuring onto relatively low-mass black holes. Their SFR:accretion rate ratios average 100:1; considerably lower than the typical value of 1000:1 seen in low-to-medium luminosity AGN in the local Universe, potentially due to star formation suppression.
22

Modelling of the upper atmosphere of gas-giant exoplanets irradiated by low-mass stars

Chadney, Joshua January 2015 (has links)
Upper atmospheres of Hot Jupiters are subject to extreme radiation conditions that can result in rapid atmospheric escape. The composition and structure of the upper atmosphere of these planets are affected by the high-energy spectrum of the host star. This emission depends on stellar type and age, which are thus important factors in understanding the behaviour of exoplanetary atmospheres. The work descried in this thesis details the development of a new 1D ionospheric model to describe the upper atmospheres of Extrasolar Giant Plants (EGPs). The model is time-dependent and includes photo-chemistry and diffusive transport. Electron-impact ionisation processes are taken into account through coupling with a suprathermal electron transport code. Neutral composition and temperature profiles are obtained by using a thermospheric model that incorporates atmospheric escape. Atmospheres composed of H, H2, He, and their associated ions are considered. Efforts have been made to obtain accurate X-ray and Extreme Ultraviolet (EUV) spectral irradiance of the stars studied. To this effect, synthetic spectra are used originating from a detailed coronal model for three different low-mass stars of different activity levels: epsilon Eridani, AD Leonis and AU Microscopii. This work is the first study of the ionosphere of EGPs that takes into account the different spectral energy distribution of low-mass stars. In planets subjected to radiation from active stars, the transition from slow, Jeans escape to a regime of rapid hydrodynamic escape at the top of the atmosphere is found to occur at larger orbital distances than for planets around low activity stars (such as the Sun). To correctly estimate the critical orbital distance of this transition, the spectral shape of stellar XUV radiation is important. A novel method to scale the EUV region of the solar spectrum based upon stellar X-ray emission is developed in this work. This new method produces an outcome in terms of the planet's upper atmosphere and escape regime that is very similar to that obtained using a detailed coronal model of the host star. EGP ionospheres at all orbital distances and around all stars studied are dominated by the long-lived H+ ion. In addition, planets in the Jeans escape regime also have a layer in which H3+ is the major ion at the base of the ionosphere. For fast-rotating planets, H3+ densities undergo significant diurnal variations, their peak value being determined by the stellar X-ray flux. In contrast, H+ densities show very little day/night variability and their value is determined by the level of stellar EUV flux. The H3+ peak in EGPs in the rapid hydrodynamic escape regime under strong stellar illumination is pushed to altitudes below the homopause, where this ion is likely to be destroyed through reactions with heavy species (C, O, etc.).
23

Theory and simulation of electronic and optical properties of thin film barium strontium titanate

Alsaei, Jawad January 2015 (has links)
In this thesis, we investigate, from first-principles, various means by which the energy gaps and the refractive index of BaTiO$_3$, SrTiO$_3$ and Ba$_{x}$Sr$_{1-x}$TiO$_3$ may be tuned, including epitaxial strain, alloying, and anionic doping. We start by discussing some limitations in the experimental spectroscopic ellipsometry technique that could affect the reliability of the measured optical spectra. Then, we investigate the effects of strain and alloying on the energy gaps of these materials. Interesting features in the gaps are observed as a function of strain, where it decreases and increases for small and high strains, respectively. These features are correlated with the spontaneous polarization of the system, whose behaviour is sensitive to the value of strain. The polarization also affects the behaviour of the bandgap of Ba$_{x}$Sr$_{1-x}$TiO$_3$ as a function of Sr content, which shows a non-monotonic trend. On the other hand, the refractive index of Ba$_{x}$Sr$_{1-x}$TiO$_3$ shows similar features with strain and alloying to those of the bandgaps. Strain is found to induce a decrease of up to $\sim 8\%$ in the refractive index and a birefringence of up to $\sim0.14$, whereas alloying has a smaller effect. Ferroelectric distortions are found to have a major role in decreasing the index, where much less effect is observed when they are inhibited. However, we show that the refractive index is insensitive to the ordering of Sr atoms in Ba$_{x}$Sr$_{1-x}$TiO$_3$. Finally, we investigate the effect of F and N codoping in BaTiO$_3$. The refractive index is sensitive to the atomic ordering of the dopant atoms, which prefer to be aligned in linear chains of F$-$Ti$-$N. The component of the index along the chain shows a dramatic increase with respect to pure BaTiO$_3$, while the other components are almost unaffected. Also, we show that the index can be further tuned by strain and doping concentration.
24

Performance of topological codes for quantum error correction

Watson, Fern January 2015 (has links)
In this thesis we present three main contributions to the field of topological quantum error correcting codes. We focus on some of the properties of such codes required for fault-tolerant quantum computation. Prior work has concentrated on determining error rate thresholds of particular models, but increasingly other parameters are gaining prominence. One of these is the overhead -- the quantity of a named resource required to achieve a desired level of performance from the code. We characterise the qubit overhead of the toric code in a fault-tolerant setting. These results provide a general framework for determining the overhead for other code constructions with more complicated noise models. Next we introduce a decoding algorithm, applicable to topological codes in a qudit architecture, specifically those where fault-tolerance is achieved through repeated syndrome measurements. It is computationally light and capable of decoding qudits of arbitrarily high dimension with negligible increase in its run time. The threshold of the decoder is limited by the percolation of the syndromes. Using local matching techniques we are able to overcome this limitation, increasing the threshold by almost a factor of two for high qudit dimensions. Finally, we turn our attention to a second family of topological quantum codes: the colour codes. In three and higher spatial dimensions such codes can support transversal non-Clifford gates. We show, using a technique that we call a star-bipartition of the vertices of the lattice, that any existing qubit colour code lattice can be used to define a qudit colour code. By generalising the notion of triorthogonal matrices we derive analogous transversality properties in the qudit codes.
25

Aspects of integrability in string sigma-models

Stepanchuk, Andrej January 2015 (has links)
The recent success in applying integrability-based methods to study examples of gauge/gravity dualities in highly (super)symmetric settings motivates the question of whether such methods can be carried over to more physical and less symmetric cases. In this thesis we consider two such examples of string sigma-models, which interpolate between integrable or solvable limits. First we consider classical string motion on curved p-brane backgrounds for which the sigma-model interpolates between the integrable flat space and AdS(k)xS(k) coset or WZW sigma-models. We find that while the equations for particle (i.e. geodesic) motion are integrable in these backgrounds, the equations for extended string motion are not. The second example we consider is string theory on AdS3xS3xT4 with mixed Ramond-Ramond (R-R) and Neveu-Schwarz-Neveu-Schwarz (NS-NS) 3-form fluxes, which interpolates between the integrable pure R-R and the pure NS-NS theory that can be solved using CFT methods. The dispersion relation and S-matrix for world-sheet excitations, which are the essential ingredients in solving for the string spectrum, are only partially fixed by integrability and symmetry arguments. By constructing the mixed flux generalisation of the dyonic giant magnon soliton, which we show can be interpreted as a bound-state of excitations, we determine the dispersion relation for massive excitations. We also construct the mixed flux generalisation of the folded string on AdS3xS1 and show that, at leading order in large angular momentum on AdS3, its energy is given by the pure R-R expression with the string tension rescaled by the R-R flux coefficient. Further, we derive the bound-state S-matrix and its 1-loop correction by considering the scattering of dyonic giant magnons and plane waves. From this we deduce the semiclassical and 1-loop dressing phases in the massive sector S-matrix, which we find to agree with recent proposals.
26

Search for supersymmetry in pp collisions with all-hadronic final states using the αT variable with the CMS detector at the LHC

Mathias, Bryn Lugh Shorney January 2014 (has links)
A search for supersymmetry in the exclusive hadronic and missing energy channel is presented on 5 fb-1 of data collected using the CMS detector at the LHC. The data were produced at a center-of-mass energy of 7 TeV. The kinematic discriminator αT is used to select signal events which are then binned in terms of the visible energy per event. The efficiency of the hadronic level one triggers is measured though-out the data taking period and a scheme to reduce the effects of multiple collisions per bunch crossing on the cross section of the trigger paths is studied, implemented and tested in situ. These efficiency measurements are considered in the development of an analysis specific trigger, the performance of which is measured in situ, with the final efficiencies taken into account in the presented analysis. A data driven background estimation method is used to predict the expected yield in the signal regions from Standard Model processes. In the absence of an observed excess, limits are set to the 95% confidence level on the production cross section and masses of new particles. In the context of the Constrained Minimal Supersymmetric Model (CMSSM), squarks and gluinos with a mass of up to 1 TeV are excluded. In terms of simplified models with various light and heavy flavour final states, squarks and gluinos are excluded at a mass of ≈1 TeV for a Lightest Supersymmetric Particle (LSP) mass of up to ≈500 GeV. Natural units (h = c = 1) are used though-out.
27

Searches for neutral Higgs bosons decaying to tau pairs and measurement of the Z+b-jet cross section with the CMS detector

Gilbert, Andrew January 2014 (has links)
The Compact Muon Solenoid (CMS) is a general-purpose particle detector at the CERN Large Hadron Collider. It is designed to search for the Higgs boson and evidence of new physics and to test the predictions of the standard model (SM) at the TeV scale. This thesis describes analyses of proton-proton collision data recorded by CMS during 2011 and 2012. A study of Z boson production in association with b jets, using 2.1 fb^{-1} of data recorded at a centre-of-mass energy of 7 TeV, is presented. The cross sections for production with exactly one, or at least two, b jets are measured, and the event kinematics are compared to the predictions of the madgraph event generator interfaced with pythia for hadronisation and parton showering. Searches for neutral Higgs bosons decaying to tau pairs are also presented. One search is in the context of the SM Higgs boson, for mass hypotheses in the range 90-150 GeV, and the other in the minimal supersymmetric standard model (MSSM), in which three neutral Higgs bosons are predicted and the search range is from 90 GeV to 1 TeV. Both searches use 4.9 fb-1 of data collected at 7 TeV and 19.7 fb^{-1} collected at 8 TeV. In the SM search an excess of events above the background expectation is observed and found to be compatible with the SM expectation for the 125 GeV Higgs boson. The observed (expected) local significance of this excess is 3.0 (3.1) standard deviations at 125 GeV. No significant excess is observed in the MSSM search. Upper limits at the 95% confidence level are determined, both in the m_A-tan(beta) parameter space of the m_h^max scenario and on the production cross sections in a model-independent interpretation.
28

Cylindrically convergent implosions of metal liners for quasi-isentropic compression of deuterium

Weinwurm, Marcus January 2014 (has links)
To date our understanding of strongly coupled, degenerate plasmas is incomplete. In particular considerable disagreement exists between theories of hydrogenic matter (HM) at pressures greater than 100 gigapascals (GPa) and temperatures below 3 electronvolts (eV). The predicted transition of fluid molecular hydrogen to a metallic atomic liquid has large implications for models of the interior structure and evolution of gas giants. Experimental confirmation of this transition is still pending. Furthermore, the properties of deuterium-tritium in this regime are of great interest to inertial confinement fusion. In this thesis we propose a scheme that can create strongly coupled, degenerate hydrogenic plasmas at terapascal (TPa) pressures on pulsed power machines. Our results show that this can be achieved by initiating cylindrically converging isentropic flow of sample material inside a metal liner. The liner acts as a pusher that drives a predefined compression of the fill, which is obtained by constructing an asymptotically self-similar implosion of cryogenically condensed HM. An empirical model that gives the required pulse shape for recreating this implosion inside the liner is introduced. Results of magnetohydrodynamic simulations demonstrate that a peak current of 10.8 megaamperes (MA) is sufficient for assembling nearly uniform HM at a stagnation pressure of 13 TPa and at temperatures of approximately 1.5 eV. A study of the stability of the implosion to imperfections of the liner's surface finds that liner-driven isentropic compression of hydrogen is robust to magneto-Rayleigh-Taylor growth for sufficiently thick liners. Since the methods in this work are readily adapted to a range of materials, an experimental realization could significantly extend our knowledge of degenerate, strongly coupled plasmas in general. Finally, we broaden our focus to the compression of the metal liner itself. Potential advantages of reducing shock heating and thereby increasing the degeneracy of liner material during a magnetized liner inertial fusion implosion are discussed.
29

Polymer/fullerene solution processing : impact on thin film morphology

Dattani, Rajeev January 2014 (has links)
Polymer:fullerene blends are commonly used in organic solar cells (OSCs) and solution based processing is currently the most widely used fabrication method (Vandewal et al., Macromolecules, 2013, Thompson et al. Angewandte Chemie, 2008) [1, 2]. However, the solution behaviour, polymer chain conformation and thermodynamic interactions within these systems, which critically impact thin film morphology, is not well understood. In this work, a model ternary system is systematically investigated polystyrene (PS), C60 and toluene. PS was chosen as it is a flexible polymer and has been extensively studied in solution, thin films and melt, and C60 is an exceptionally well defined molecule. A range of PS molecular weights is investigated, from 18 to 1000 kg/mol, dilute to semidilute polymer solution regimes and the fullerene solutions (below and above its miscibility limit in toluene). No change in the polymer chain dimensions (Rg, Rh or ξ) is found. C60 is shown to aggregate in solution below the miscibility limit in toluene and is shown to be dependent on PS Mw, PS concentration and time. The results are consistent with polymer/colloid theory in the protein-limit. (Dattani et al. Macromolecules, 2014) [3] Thin films with thicknesses ranging from 60 to 130 nm are fabricated from solutions in a homogenous and precipitated regime and the consequences for film formation and morphology under thermal annealing is investigated and quantified. C60 clusters are grown in solution via light exposure and are the result of light-induced oxidation. C60 epoxides are present in the solution and an aggregation mechanism is proposed. C60 epoxidation and aggregation is found to be dependent on C60 concentration, laser power, solvent, temperature and time. The knowledge gained from the model PS/C60/toluene system is applied to systems containing the much-studied poly-3-hexylthiophene (P3HT)/phenyl-C61-butyric acid methyl ester (PCBM) pair, with qualitatively similar results observed. The change in film morphology, accessible by a simple change in processing conditions, offers a unique method for tuning device efficiencies with respect to the size of the fullerene domains.
30

Quantum plasmonics : from quantum statistics to quantum interferences

Di Martino, Giuliana January 2014 (has links)
In this thesis, we focus our attention on the excitation of Surface Plasmon Polaritons (SPPs) and their propagation along metal stripes. Plasmons are characterized by losses into the metal, therefore an important step is to investigate the effect of these losses on their quantum properties. This is a field not yet fully investigated and the work presented here will give us the possibility to understand the effect of losses on the plasmons quantum properties. This will allow us to prove that plasmons can be used in the quantum information technology field, since they keep the quantum information regardless of their lossy character. Another key property yet to be fully investigated is the bosonic character of single surface plasmon polaritons (SPPs). The quasi-particle nature of SPPs, consisting of a photon (boson) coupled to a charge density wave of electrons (fermions), makes them an unusual type of quantum excitation. It is, as of yet, unclear whether SPPs are bosons, fermions, or a hybrid mixture. Here, we will prove the bosonic character of plasmons, making use of interference experiments. This study will open opportunities for controlling quantum states of light in ultra-compact nanophotonic plasmonic circuitry. First of all, the mean excitation rates, intensity correlations and Fock state populations are studied by using heralded single photons generated via spontaneous parametric down conversion as sources of light. One downconverted beam is used as a trigger, the other one is the signal we send to the metal stripes to excite the plasmons. After an introduction on the meaning of coherence functions, we explain how we couple photons into a gold waveguide with gratings on both sides, where the coupled plasmon is confined at the interface between gold and air. By measuring the second-order quantum coherence function g2(t), we demonstrated the ability to excite single SPPs. Moreover, the effect of losses incurred during propagation of the single SPPs is consistent with the classical exponential behaviour and does not change the value of g2(t), providing evidence that a linear uncorrelated Markovian loss model is valid for SPP damping at the single quanta level. Then, we move onto more complicated devices, such as X-shaped stripes that act as a plasmonic beamsplitter, in order to observe nonclassical effects in the interference of two single plasmons. This is an important step along the way to understanding better the behaviour of single surface plasmons at the quantum level and how one can build more complicated quantum interference networks, such as plasmonic-based quantum logic gates. In order to fully verify the bosonic nature of single excitations in the quantum regime it is vital to observe quantum interference. A natural thing to probe in the most basic type of scatterer (a 50/50 beamsplitter) operating in the quantum regime, is how it acts on two separate single surface plasmons. Here the launching method is the same as for the previous experiment, except that the waveguide structure is in the form of a 50/50 beamsplitter (X-shape) and both photons from the parametric down-conversion type-I generation are sent onto the two inputs of the plasmonic beamsplitter. By this way, both the beams, generated by the nonlinear crystal, act as signal beams. If the SPPs are truly bosonic and indistinguishable then they tend to bunch together when they interact at the beamsplitter -this is the well known Hong-Ou and Mandel quantum interference effect. In this work we report the first direct observation of quantum interference in the HOM effect for single SPPs, demonstrating by this way the bosonic nature of plasmons.

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