• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 144
  • 105
  • Tagged with
  • 252
  • 252
  • 129
  • 34
  • 34
  • 30
  • 29
  • 25
  • 21
  • 20
  • 19
  • 17
  • 15
  • 14
  • 13
  • 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.
1

The density dependence of the refractivity of gases.

Burns, Robert Charles. January 1978 (has links)
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1978.
2

Towards practical quantum cryptography.

January 2009 (has links)
The information society that presides today is dependent on the communication industry to facilitate unintelligible data transfers between authenticated parties. Such requirements have, to date, taken advantage of security based on the mathematical complexities of certain algorithms. However, the advancement of computing power and the advent of the quantum computer together with the vulnerability of this scheme to mathematical progress have prompted the introduction of quantum cryptography. This process, through the laws of quantum physics, ensures provably secure data communication. Quantum cryptography provides physical protection to individual bits of information thus providing a hardware implemented solution. The implementation of this theoretical concept requires much practical innovation for transparent deployment into current cryptographic solutions. This thesis introduces the concept of quantum cryptography in a practical perspective. It raises a few core concerns with the present quantum cryptographic technology and provides some solutions towards the practical deployment of commercially feasible quantum cryptographic systems. The thesis commences with an introduction to classical cryptography focussing on key management protocols. This is followed by the presentation of the basic concepts of Quantum Key Distribution (QKD) together with an explanation of some QKD protocols and parameter required to classify such protocols. Chapter 2 discusses the theoretical and practical aspects of quantum channels in particular optical fibre. The primary challenges of transferring classical and quantum data along these channels are mentioned together with some solutions. A description of experimental usage with present QKD solutions is presented in Chapter 3. An investigation into highly efficient QKD protocols follows illustrating effective post-distribution processing for increasing the efficiency of the BB84 protocol. Chapter 4 begins with the limitations of present day QKD systems and explicates Quantum Networks as a possible solution. An introduction to classical networking theory is first presented after which some quantum network architectures based on passive optical networks are illustrated. Finally the proposed Quantum City project in conjunction with the eThekwini Municipality is explained. The realization of this project is intended to be complete by the third quarter of 2008 effectively making Durban into the first Quantum City in the world. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2009.
3

Invariant multipole theory of induced macroscopic fields in homogenous dielectrics.

Welter, Allard. January 2013 (has links)
A harmonic plane electromagnetic wave incident on a molecule distorts its charge distribution, thereby producing an infinite series of induced multipole moments expressed in terms of contributions that are due to the electric and magnetic fields E and B, and their space and time derivatives. For a linear dependence of an induced moment on a particular field property, as treated in this thesis, the constant of proportionality is essentially the corresponding molecular polarizability. Each polarizability is of a definite multipole order (electric dipole, electric quadrupole–magnetic dipole, electric octopole– magnetic quadrupole, etc.). The contribution of each multipole term to a physical property diminishes rapidly with increasing multipole order. In general, the moments and polarizabilities are dependent on an arbitrary choice of molecular coordinate origin, relative to which the positions of molecular constituents are referred. Electromagnetic observables are expressible, in part, in terms of contributions of the polarizabilities of the same multipole order. The aim of multipole theory is to explain effects to the lowest relevant multipole order, since higher-order contributions are negligible. A necessary criterion for such a theory is that it be independent of the choice of molecular coordinate origin. Van Vleck [1] introduced this condition, and Buckingham [2] and others [3, 4] have used it as a standard test of the theory. The macroscopic continuum theory of electromagnetics, as embodied in Maxwell’s macroscopic equations, involves molecular properties and electromagnetic fields averaged over a sampling volume of dimensions much smaller than the wavelength of the fields and much larger than molecular dimensions [5]. This averaging entails specifying a set of molecular coordinate origins. The multipole expressions for the macroscopic induced bound charge and current densities and the propagation equation are origin independent in part due to cancellation of their origin dependences among terms of the same multipole order — the so-called Van Vleck–Buckingham condition [6]. The multipole expressions for the dynamic response fields, D(E,B) and H(E,B), above electric dipole order depend on origin, and thus the theory is only partially invariant. To obtain a consistent invariant multipole theory of induced macroscopic fields up to electric octopole–magnetic quadrupole order, origin-independent expressions corresponding to the molecular polarizabilities are determined. When used in place of the molecular polarizabilities, these invariant expressions leave the originindependent aspects of the theory unchanged, and yield physically acceptable expressions for the macroscopic fields. The resulting theory is fully invariant for both transmission and reflection. The procedure to determine invariant polarizabilities requires manipulations of expressions involving Cartesian tensors up to rank four, contracted with isotropic tensors up to rank eight, at electric octopole–magnetic quadrupole order. The algebraic software package mathematica was used to facilitate the evaluation of these expressions. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.
4

Equilibrium properties of some high-temperature superconductors.

January 2007 (has links)
An important fundamental problem in the understanding of the high-Tc superconducting systems is the determination of their equilibrium magnetization behaviour, in particular their constitutive Brev(H) orMrev(H) behavior. Single crystal specimens of these materials are typically small (order of micron/millimeter), and are generally in the form of platelets. Their superconductivity properties are, moreover, highly anisotropic. The magnetization [M(H0)] curves in these systems also manifest a hysteresis due to vortex pinning, and, at fields below the lower critical field Hc1, due to a “geometry” effect, which results from a non-uniform internal field distribution in the platelet specimen geometry in a perpendicular applied magnetic field H0. In the present work a brief review of the field is given and a treatment (due to Doyle and Labusch) of the problem is described in some detail, and is used in the analysis of magnetization data [M(H0)] on single-crystal platelet specimens of the YBCO and BSCCO high-Tc superconducting systems. The treatment, which is based on a rigorous theoretical analysis of a quasi-static arbitrary distribution of vortices in a specimen of arbitrary shape (Labusch and Doyle ), predicts the quasi-static magnetization behavior M(H) of the specimen, and allows for the inclusion of explicit relations for the equilibrium “constitutive” Brev(H, T), and for the bulk vortex pinning force density Pv(B). An analytical formula for Brev(H, T) in terms of the fundamental characteristic properties ?ij(T) (the anisotropic Ginsburg -Landau parameter) and the critical field Hc(T) (or the lower critical field Hc1(T)) is obtained from an accurate model fit to a numerical solution of the non-linear Ginsburg-Landau equation (Labusch and Doyle ). For the determination of ? and Hc c1, (i.e. the G-L parameter and the lower critical field along the crystalline c- axis of platelet specimens) from M(H0, T) experimental isotherms (where H0 is the magnetic field applied along the c-axis -the thin dimension of the platelet specimens), a computer algorithm, which incorporates the above treatments, was used. In order to obtain a fit between theoretical model results (of the numerical algorithm for equilibrium behavior) and the experimental M(H0, T) data, experimentally obtained hysteresis curves were averaged by taking the mean values of M(H0) for H0 increasing and decreasing over the entire M(H0) loop. This data was then normalized by Hc1(T) for both M and H0, with Hc1(T) and ?(T) being used as fitting parameters. / Thesis (M.Sc.)-University of KwaZulu-Natal, 2007.
5

Beam generated instabilities in space plasma.

Misthry, Surversperi Suryakumari. January 1999 (has links)
Electrostatic instabilities associated with a model applicable to the auroral acceleration region consisting of an ion beam, precipitating electrons and stationary background electrons are theoretically investigated. The kinetic dispersion relation is solved numerically without approximations. It is shown that two low-frequency plasma instabilities are present and these may generate the low-frequency electric field fluctuations (LEFs) that have been observed in the acceleration region. A parameter variation study is carried out in order to reveal the features of the instabilities. The model is adapted to suit two possible regions of study: (1) drifting cool ions and (2) drifting cool ions and counter-streaming hot electrons. The slow ion-acoustic instability which dominates at low ion beam drift velocities is studied by varying plasma parameters such as the propagation angle, ky/k, the wavenumber, k, the cold background electron density, nco, the hot electron temperature and the cool ion beam temperature. The second mode, the modified two-stream instability, which dominates at larger ion beam drift velocities and at oblique angles of propagation is investigated in a similar manner. To complete the study of these two instabilities, the effect of drifting hot electrons is examined briefly through a similar parameter variation study. / Thesis (M.Sc.)-University of Durban-Westville, 1999.
6

Novel pulsed power applications.

Turner, Geoffrey Robert. January 2001 (has links)
The purpose of this dissertation is to examine the mechanisms of the glow (Townsend) discharge, the arc (streamer) discharge, the corona discharge, and the vacuum discharge. The physics of each discharge is presented and then investigated by way of mathematical model and experiment. Four novel pulsed power experiments constructed for the purpose of examining each discharge are presented. Namely a transverse electric atmospheric carbon-dioxide laser, a flashlamp, a surface corona apparatus, and a plasma opening switch. Methods for the measurement of short duration intense electric and electromagnetic events are included. Practical aspects of pulsed power experimentation are discussed. / Thesis (M.Sc.)-University of Natal, Durban, 2001.
7

Interferometric measurement of induced birefringence in polycrystalline ZnSe.

Govender, Patricia. January 2011 (has links)
The aim of this research project was to assemble an apparatus capable of measuring field-induced birefringence in polycrystalline ZnSe. A Michelson interferometric apparatus was assembled, and care was taken to actively stabilize the interferometer against the effects of environmental noise by using electronic feedback techniques. This greatly enhanced the sensitivity of the interferometric measurements. In addition, the applied electric field was modulated, allowing sophisticated phase-sensitive detection techniques to be used to extract the induced birefringences. Once assembled, the interferometer was tested using electric-field induced birefringence in a perspex sample, since there is Kerr-effect data in the literature against which to compare our measured quadratic electro-optic coefficients. The interferometer was then used to measure the quadratic electrooptic coefficients of polycrystalline ZnSe, these being, to the best of our knowledge, the first such measurements for this species. The theory of electro-optic and photoelastic phenomena is comprehensively reviewed. This has permitted a critical discussion of the measured Kerr coefficients obtained in this project. It is demonstrated how quadratic electro-optic coefficients measured using the traditional technique of static polarimetry might include contributions arising from the linear electro-optic effect, these data being rendered suspect. In addition, suggestions are made as regards to the future possibilities for extending the apparatus to allow for direct measurement of stress-induced birefringences. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2011.
8

An experimental investigation of the weak-link problem in granular high-Tc superconductors.

Reddy, Kevin Poobalan. January 2002 (has links)
Grain boundary weak-link behaviour in superconductors is investigated using a critical state model (CSM), which has been specifically formulated for the applied field range o ~ Ha ~ Hc1g , where Hclg is the intra-granular lower critical field. The CSM includes an expression for the inter-granular critical current density, Jc(H, T), that has been derived from a percolation model for a random 3D network of weak-links in which the current Bowing through the individual grain boundary junctions is assumed to be spatially random. This expression, namely: Jc(H,T) = Jco[exp(-H/Ho)+b], where JCO ) Ho , and b are characteristic weak-link parameters, includes a field-dependent component and a field-independent component both of which are microstructurally sensiti ve. Calculated magnetisation behaviour, obtained from the critical state model, are fitted to experimental isothermal D.C magnetisation data, j\1(Ha) , for 0 ~ Ha ~ Hclg , 71.5K~ T ~ Tc, obtained from the two control specimens, namely: Y IBa2Cu307-x and EUIBa2Cu307-x , x ~ I, and from a third specimen, which is a mixture of the two control compounds. In the CSM fits, the temperature dependent characteristic parameters Jco , Ho, and bare treated as free fitting parameters. These free fitting parameters are subsequently fitted to specific theoretical models for Jca(T), Ho(T), and b(T) and a comparison is made between the control specimens and mixed specimen parameters to establish the effect of mixing on the grain boundary weak-link behaviour. Jca and b are found to be significantly larger in the mixed specimen as compared with either of the two control specimens. / Thesis (M.Sc.)-University of Natal, Durban, 2002.
9

The effect of antisymmetrization in diquark models of baryons.

Mabuza, Boy Raymond. January 1997 (has links)
In this thesis the effect of antisymmetrization in diquark models of baryons composed of light (u and d) quarks is investigated. The diquark in this study is considered alternately as a pointlike and as a composite particle. The wave functions for both diquark and diquark-quark systems have been determined in a non-relativistic approximation by using the radial Schrodinger equation and a range ofcentral potentials. The ground state masses ofthe diquarkquark system have been calculated in three distinct ways: (1) The ground state energy eigenvalues and the wave functions for thediquark and diquarkquark, each being treated as a two-body system, have been calculated by using the generalized Runge-Kutta and search methods. (2) The expectation values for the potential energy and kinetic energy have been calculated by using the wave functions derived in (1) for the two-body system without antisymmetrization. These results have been checked by applying the virial theorem in parallel calculations. (3) The potential and kinetic energy expectation values have also been determined by taking antisymmetrization into account via operator kernels namely, norm, potential and kinetic energy which have been derived by using the non-local Generator Coordinate Method (GCM). The expectation values of these operator kernels have been calculated with respect to the wave functions produced in (1). For the purpose of performing the integrations the wave functions , expanded in terms of cubic splines, and Gaussian quadrature have been employed. Lastly the diquark and diquark-quark ground state masses were calculated for each approach, (1) - (3), and compared with (a) each other, (b) the results for a two-body system, (c) the results for a full three-body treatment and, (d) the average mass of N- b:. . The form factors and root mean square radii ofthe baryon for the four central potentials have been calculated with antisymmetrization for each approach (1) - (3) and compared with (a) each other, (b) the results for the baryon without antisymmetrization (c) the results for the baryon with antisymmetrization including the meson cloud (d) the experimental data. The trends found are striking and it can be concluded that there is a strong dynamical effect due to the presence of antisymmetrization in diquark models of baryons. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1997.
10

An experimental study of diamond and the nitrogen vacancy centre as a source of single photons.

Semonyo, Malehlohonolo. January 2009 (has links)
For applications in Quantum Information and Quantum Key Distribution an on-demand source of single photons is desirable because absolute security is of utmost importance. Photons are quantum systems; hence encoding information onto them offers a secure alternative to classical cryptography as a measurement cannot be performed on photons without altering their properties. The Nitrogen Vacancy (NV) centre in diamond is a good source of such photons. It is photo-stable and its location in diamond offers robustness. It has zero phonon line at 637 nm and its relative short luminescence life-time of about 12 ns makes it suitable for generating single photons. This thesis covers two aspects: Firstly the characterization of defects in diamonds and subsequent selection of diamonds suitable for use in the single photon setup and secondly, the development of the experimental setup for single photon generation. This thesis sets out to describe the development of a laboratory based single photon source using the NV centre in diamond. For this purpose a suite of diamond samples were selected and subjected to various spectroscopic tests in order to characterize and classify the samples, especially the presence of the NV centres and their concentrations. The characterization of the defects was done through the use of the following spectroscopic techniques: Ultraviolet-Visible-Near infrared spectroscopy, Infrared spectroscopy, Electron Spin Resonance and Photoluminescence. These techniques enabled us to understand the types and origins of crystal defects that were present in the diamond samples used in this study and to use this to select diamonds that are most suitable for use in generation of single photons. The experimental setup for single photon generation using the NV centre is based on a confocal microscope arrangement. Single NV centres were identified by measuring the second order autocorrelation function of the fluorescence light emitted by the sample when illuminated with a laser. This measurement was done using a Hanbury-Brown Twiss (HBT) interferometer. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2009.

Page generated in 0.0567 seconds