• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 6669
  • 939
  • 700
  • 677
  • 6
  • 4
  • 3
  • 2
  • 1
  • 1
  • 1
  • 1
  • Tagged with
  • 12202
  • 6787
  • 3134
  • 3132
  • 3132
  • 2087
  • 2028
  • 1852
  • 1105
  • 632
  • 593
  • 587
  • 473
  • 363
  • 353
  • 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.
31

Dynamic pressure pulses in Earth's dayside magnetosheath

Archer, Martin January 2014 (has links)
Solar wind mass, energy and momentum can be transferred to Earth's magnetosphere at the magnetopause with the shocked magnetosheath acting as an interface between the two regions. In particular the magnetosheath pressure is important in terms of the position and motion of the magnetopause, which in turn can have effects throughout the dayside magnetosphere. A variety of transient phenomena often occur in the magnetosheath and in this thesis one example is studied, namely pulses in the magnetosheath dynamic pressure, using multipoint spacecraft observations to investigate their origins and magnetospheric impacts and illuminate dayside magnetospheric dynamics. Simultaneous observations in the solar wind, foreshock and magnetosheath reveal an interval of dynamic pressure pulses that did not exist upstream of the bow shock in the pristine solar wind or foreshock and appear consistent with previous simulations of solar wind discontinuities interacting with the bow shock, which predict large amplitude pulses when the local geometry of the shock changes. A statistical study of these structures, however, reveals their predominant origin near the quasi-parallel shock, typically under steady interplanetary magnetic fields, suggestive that the foreshock is important in their generation. The enhanced pressure on the magnetopause due to these pulses can perturb the boundary, exciting ultra-low frequency waves in the magnetosphere and travelling convection vortices in the ionosphere, similar to the response to pressure variations of solar wind origin. However, in this case the response is smoother and on longer timescales than the sharp, impulsive pressure variations and often a collective effect of numerous pulses. Conditions at the magnetopause are often inferred from suitably time lagged measurements of the pristine solar wind taken far upstream of Earth at the L1 Lagrangian point. However, such methods cannot predict the precise locations and times of dynamic pressure pulses in the magnetosheath, which directly drive magnetospheric dynamics.
32

Photon coupling effects and advanced characterisations of multiple-quantum-well multi-junction solar cells

Lee, Kan-Hua January 2014 (has links)
Achieving optimal band-gap combinations of multi-junction solar cells at production level is the most difficult challenge in concentrator photovoltaics. To improve the state-of-the-art InGaP/InGaAs/Ge triple-junction cells, it requires that the band gaps of the top and middle junction to be lower or an additional 1 eV junction. This involves lattice-mismatch growth or introducing dilute nitrides materials, which makes it difficult to scale up to production at low cost. Strain-balanced multiple quantum wells (MQWs) in the middle junction has been very well-studied as a means to adjust the absorption edges of the middle junction in multi-junction solar cells. To fully optimise the efficiency of solar cells with MQW GaAs subcell, an InGaP top cell with MQWs also has to be introduced to achieve current-matching. The aim of this thesis is to address the issues of production multi-junction cell with MQWs. We studied the material properties of MQW InGaP subcells and demonstrated its strong photon coupling effects in multi-junction devices. Several characterisation techniques were developed to acquire deeper understanding of the material qualities and sheet resistance of MQW solar cells.
33

Electrical detection of surface plasmon polaritons via the plasmon drag effect

Lupi, Antonio January 2014 (has links)
This thesis concerns the measurement and characterization of the Surface Plasmon Drag Effect (SPDE) in metallic structures and its application to the electrical detection of surface plasmon polaritons (SPPs). We demonstrate that SPPs absorbed in a metallic structure generate an electric current, which polarity depends on the propagation direction of the absorbed SPP, without the need of any applied voltage. We investigate the effect in gold and silver thin films of different thickness and on various metallic bilayers, which are deposited on right angle prisms and hemispheres to allow coupling of light to SPP through the Kretschmann-Raether configuration. We then simultaneously measure the angular spectrum of the reflected light and the electric current generated by the effect. The accuracy of the experiment allow us to determine the effect efficiency and thus to quantitatively compare different samples. In an attempt to clarify the mechanism giving rise to the current generation, we compare our experiments with existing models of the Photon Drag Effect (PDE). This is a similar phenomenon mediated by photon absorption where the current is the result of momentum transfer from the photon to conduction electrons. We find that the model qualitatively predicts our results and thus SPDE can be interpreted as the result of quasimomentum transfer from SPPs to the electrons, but care must be taken for considering the prediction of the model quantitatively. In addiction, we discovered that the effect shows local efficiency enhancement and even change of the current polarity in the presence of films with defects. Those results suggest a different interpretation to previous literature results and overall deepen the understanding of the phenomenon. A clear comprehension of the mechanisms leading to current generation is crucial for designing future applications in sensing and photonic circuitry. Despite the low efficiency in the visible range, in fact, this effect can be attractive since it promises to have an ultrafast response, to retain its sensitivity at longer wavelengths and has the peculiar ability of sensing the propagation direction of the SPP.
34

New frequencies and geometries for plasmonics and metamaterials

Francescato, Yan January 2014 (has links)
The manipulation of light at the nanoscale has become a fascinating research field called nanophotonics. It brings together a wide range of topics such as semiconductor quantum dots or molecular optoelectronics and the study of metal optics, or plasmonics, on one hand and the development of finely designed structures with specifically engineered optical properties called metamaterials on the other. As is often the case, it is at the boundary of these two domains that most novel effects can be observed. Plasmonics has for instance enabled the detection of single molecules due to the large field enhancement which exists in the vicinity of nanostructured metals. Thanks to the confinement of electromagnetic waves below the diffraction limit plasmonic systems are also foreseen as ideal conduits connecting electronic and photonic systems. On another hand, when a material is patterned on a scale smaller than the wavelength, its optical properties are reflections of the structure of the patterned material rather than the material itself, a concept known as metamaterial. This has allowed researchers to obtain exotic optical properties such as negative refractive indices and can be implemented in devices acting like invisibility cloaks or perfect lenses. While the prospects for nanophotonics are far-reaching, real-life applications are severely limited by the intrinsic absorption of metals and the current fabrication methods mostly based on electron-beam lithography which is slow and costly. In this thesis, we investigate these issues by considering the potentials of other polaritonic materials such as semiconductors, silicon carbide and graphene for field confinement applications. This is achieved through the combination of both numerical studies and sample fabrication and testing with the help of international collaborators. Our results show much improvement over the metallic structures typically used, with an operating range covering the near- and mid-infrared as well as the terahertz. The field compression can also be much greater compared to conventional plasmonic materials, with near-field enhancements reaching four orders of magnitude. Furthermore, we analyse theoretically the optical properties of metallic gyroids which are obtained by self-assembly - a promising chemical route for fabricating large-scale 3D structures with molecular sized resolution. These materials exhibit unexpected properties such as negative refraction and could in consequence be used as thin lenses or wave-plates. Last, we develop and apply a theoretical formulation of Fano theory for the case of plasmonics. It allows a clear and simple physical understanding of the interference spectra which are commonly encountered in nanooptics.
35

The role of galaxy mergers in the evolution of massive galaxies

Carpineti, Alfredo January 2014 (has links)
This Thesis presents a study of the nature of the different stages of galaxy mergers that lead to the formation of massive galaxies. In particular we look into the properties of infrared bright mergers, spheroidal post-mergers and star-forming early-types and how their properties compare and contrast with the properties of regular late and early-type galaxies. The aim of this thesis is to expand our knowledge of the merging process and to find a justification for the variability of the more active early-type galaxies. These studies were performed by extracting all the possible information from different surveys. For the optical analysis we used the Sloan Digital Sky Survey (SDSS), while we used surveys conducted by IRAS and GALEX for infrared and ultraviolet data respectively. To better understand the mergers/massive galaxies connection we performed the first detailed analysis of spheroidal post-mergers, as well as the first infrared- blind study of the properties of merging galaxies and produced a multi-wavelength catalogue of local star-forming early-type galaxies. We also looked at the more general galaxy population by constructing the largest morphological survey of far- infrared selected objects, which provided us with the first estimate of how different morphologies (but mergers in particular) contribute to the local SF budget. The results show the pivotal role played by mergers in the formation of stars and evolution of galaxies in the local Universe.
36

Kinetic study of the source-collector sheath system and its application on the charging of large dust grains

Rizopoulou, Nikoleta January 2014 (has links)
Plasma-wall interactions play an important role in plasma physics. In many theoretical studies an infinite unbounded system is assumed. However in reality, it is often the case that we need to include the physics of the plasma -wall interaction in order to be able to realistically model a plasma system. Such studies are quite complicated due to the non-linear nature of the associated physics. One such example is the source-collector sheath system which describes the plasma between an infinite wall and a Maxwellian source. Such a system comprises of two distinct areas; the first is an electron rich region near the Maxwellian source, the source sheath, and the other is an ion rich area near the wall, the collector sheath. In the first part of this work, we model this system theoretically using truncated Maxwellian distributions for both electrons and ions to describe the collisionless, ionization-free plasma, and we also include flows. Furthermore, we study the problem using simulation results from our Vlasov kinetic code Yggdrasil using a perfectly absorbing wall as a boundary. In the second part of this work, we study the effect of an electron emitting surface in the source-collector sheath system, without plasma flows. Electron emission plays a crucial role for many plasma applications, for example in fusion devices it is an important part of the physics of the interaction of the plasma with the walls of the reactor. A theoretical model is constructed for a range of ion and electron temperatures and for a variety of electron emission characteristics. Furthermore, we use our kinetic code Yggdrasil with an electron emitting boundary to simulate the phenomenon. In the third part of this work, we focus on the application of the theoretical model we developed for the source-collector sheath system in the study of the charging of large dust grains. We use the formalism of the MOML (Modified Orbital Motion Limited) approach incorporating the conclusions of our theoretical studies. More specifically, we investigate how the use of the source-collector sheath model applied in MOML affects its results. The outcomes of this study are compared with numerical studies available in the literature.
37

A privileged quantum state from causal structure

Buck, Michel January 2014 (has links)
This thesis investigates a new proposal for a privileged ground state of a free scalar quantum field in arbitrary regions of spacetime. This Sorkin-Johnston (SJ) state, implicit in work by S. Johnston on quantum field theory on causal sets, is defined solely in terms of the spacetime causal structure and is unique in any globally hyperbolic spacetime region. The first part of the thesis contains an analysis of the simplest possible setting: a flat two-dimensional causal interval. The simplicity of the setup makes analytic calculations tractable and allows for some general features of the state to be better understood. The second part deals with an investigation of the SJ state in de Sitter space. It turns out to be possible to construct the state explicitly using limiting procedures, which provides further interesting insights. In particular, the state is found to depend on the spacetime dimension, field mass, and on the choice of subregion, differing in many cases from the usual 'Bunch-Davies' vacuum. The formalism does not select a unique state in spacetimes that are not globally hyperbolic, which include, among others, spacetimes exhibiting spatial topology change. These are relevant in the context of quantum gravity and in relation to the old question as to whether violent spacetime curvature fluctuations at Planckian scales can lead to changes in spatial topology, or whether such transitions are unphysical. Some efforts to understand the SJ state in the topology-changing two-dimensional 'trousers' spacetime are discussed in the final part of the thesis.
38

Advances in spacecraft magnetic cleanliness verifcation and magnetometer zero offset determination in anticipation of the solar orbiter mission

Pudney, Maxsim January 2014 (has links)
The Solar Orbiter mission aims to study the Sun by linking what the spacecraft visibly detects on the Sun's surface with the measurements it makes in-situ in the solar wind, which will advance our knowledge of how the Sun controls the inner heliosphere. The spacecraft has strict magnetic cleanliness requirements that demand deviation from standard testing, analysis and data correction practices. We present advances made towards the detection of magnetometer zero offsets and the verification of unit magnetic field emissions in anticipation of the Solar Orbiter mission. We demonstrate an improvement to the detection of magnetometer zero offsets through the automatic calculation of a key parameter vital to the removal of the spacecraft field from measured data. The existing technique uses pre-existing rotations in the solar wind to calculate the magnetometer zero offsets. Our improvement uses the measured solar wind data to automatically calculate the important minimum compressional standard deviation (MCS) parameter, demonstrating an improvement in the offset calculation probability of up to 10% at aphelion and 5% at perihelion. We also suggest an improvement to the extrapolation of source emission measurements made close to equipment under test (EUT), in order to verify field emissions against strict magnetic cleanliness requirements. We propose the use of magnetic field scaling that follows an inverse square law close to the EUT and an inverse cube law beyond a chosen break distance 3 times the scale size of the EUT. Due to the importance of the 1-100 kHz range to the search coil instrument on Solar Orbiter, we study a potential improvement to the shielding effectiveness of test facilities over this frequency range. We test a small prototype design using thin high permeability layers that demonstrates shielding by a factor of 90 at 8 kHz, however when scaled up to test facility size we find that aluminium shielding is more effective.
39

Stochastic dynamics of crystal defects

Swinburne, Thomas January 2014 (has links)
The state of a deformed crystal is highly heterogeneous, with plasticity localised into linear and point defects such as dislocations, vacancies and interstitial clusters. The motion of these defects dictate a crystal's mechanical behaviour, but defect dynamics are complicated and correlated by external applied stresses, internal elastic interactions and the fundamentally stochastic influence of thermal vibrations. This thesis is concerned with establishing a rigorous, modern theory of the stochastic and dissipative forces on crystal defects, which remain poorly understood despite their importance in any temperature dependent micro-structural process such as the ductile to brittle transition and irradiation damage. From novel molecular dynamics simulations we parametrise an efficient, stochastic and discrete dislocation model that allows access to experimental time and length scales. Simulated trajectories of thermally activated dislocation motion are in excellent agreement with those measured experimentally. Despite these successes in coarse graining, we find existing theories unable to explain stochastic defect dynamics. To resolve this, we define crystal defects through projection operators, without any recourse to elasticity. By rigorous dimensional reduction we derive explicit analytical forms for the stochastic forces acting on crystal defects, allowing new quantitative insight into the role of thermal fluctuations in crystal plasticity.
40

Nanopatterning and nanoscale characterisation of solution-processible electronics

Shaw, Joseph January 2014 (has links)
Solution-processible electronics represent an emerging technology that will revolutionise the field of inexpensive large-area/volume electronics. In this thesis two scanning probe-based methods; scanning thermal lithography (SThL) and conductive atomic force microscopy (CAFM), are used to firstly enhance the patterning resolution of organic semiconductors and secondly improve the electrical characterisation techniques used to study charge transport in solution-processed systems. By combining SThL with suitable organic precursors, on-demand patterning of semiconducting pentacene nanoribbons ~73 nm in width, is demonstrated. By using pentacene nanoribbons as the transistor semiconductor, the first fully functional nanostructured organic transistors via SThL were produced. Using finite element simulations and experimental data, the effects of simultaneously heating the substrate during SThL patterning were assessed. Substrate heating was found to be an economical and simple way to increase SThL 'writing' speed, and hence patterning throughput by ~2,000%. To demonstrate the applicability of SThL beyond direct patterning of electro-active compounds, an organic precursor was used as a positive etch mask for the patterning of various metal films. This approach enabled the patterning of metal electrodes with sub-500 nm resolution highlighting the potential of SThL as a rapid prototyping tool for nanoscale electronics. Finally, the origin of the enhanced electrical conductivity observed in solution-processed transparent electrodes composed of silver nanowires (AgNWs) and a conductive binder was studied using CAFM. Two different solution-processible binder materials; reduced graphene oxide and zinc oxide (ZnO), were employed. By analysing the lateral charge transport in these composite electrode systems, the impact of the binder material on the macroscopic conductivity was assessed. The formation of binder-composed conductive bridges between AgNWs was identified as a key feature responsible for the enhanced conductivity in the composite electrodes. The ZnO-AgNW hybrid had sheet resistance comparable to conventional indium tin oxide electrodes, with the added benefit of low temperature (~200 °C) solution-processing.

Page generated in 0.0384 seconds