Dilute magnetic semiconductors are an important family of materials that have many potential applications in spintronics; (Ga,Mn)As, (In,Ga,Mn)As and (Ga,Mn)N are of major interest. This thesis investigates dierent aspects of these, using the synchrotron radiation techniques of x-ray magnetic circular dichroism (XMCD) and x-ray magnetic linear dichroism (XMLD), supported by superconducting quantum interference device (SQUID) magnetometry and magnetotransport measurements. A large anisotropic XMLD signal is observed for the Mn L-edge in (Ga,Mn)As. In unannealed (Ga,Mn)As, an apparently reduced Mn magnetic moment is commonly observed. It is thought to be related to compensation of both carriers and magnetic moment, caused by interstitial Mn. This issue is investigated using combined data from XMCD, XMLD and SQUID magnetometry. The findings suggest that substitutional and interstitial Mn form `non-magnetic' pairs which do not have a preferred spin orientation. (Ga,Mn)N is studied by x-ray absorption and field-dependent XMCD at the Mn L-edge. Two distinct Mn congurations are identified: Mn2+ is prevalent towards the surface with nearly paramagnetic behaviour, while a weakly ferromagnetic Mn2+/Mn3+ mixed valence exists within the bulk. The weak ferromagnetism, often observed in (Ga,Mn)N, is attributed to coupling between the impurities by the double exchange mechanism. Finally, XMCD is used to measure the orbital polarization of As 4p states of (III,Mn)As materials. These states correspond to those of the holes involved in the itinerant exchange interaction in ferromagnetic semiconductors. The coupling between the localized d states of the magnetic impurities and the valence band p states of the host is demonstrated by an anisotropy in the orbital moment of these states. This is experimental confirmation of the origin of the magnetocrystalline anisotropy in dilute magnetic semiconductors.

This thesis presents an investigation into the influence of magnetic cohesion on the stability of granular slopes. We consider magnetic cohesion that results from the interaction between dipole moments induced in grains by a uniform magnetic field. The repose angle of spheres is known to increase much more slowly with magnetic cohesion than in experiments with liquid-bridge cohesion. To our knowledge, nowhere in the literature has anyone offered a satisfactory explanation of this discrepancy. Our two-dimensional molecular dynamics simulations of granular piles show that shear occurs deep in the pile. The addition of a magnetic field causes the motion to shift farther down into the pile, preventing the angle from increasing substantially. We investigate different models of wall friction, and discover that wall interactions have a significant influence on the rate of increase of the slope angle with magnetic cohesion. In three-dimensional simulations we observe an initial decrease in the repose angle as the cohesion is increased, contrary to expectations. We explain this effect by considering how the transverse magnetic force influences the particle distribution of the pile. In contrast, draining-crater experiments reveal that the angle of repose of diamagnetic bismuth grains increases dramatically with cohesion in a vertical field. We argue that this difference is due to the non-spherical shape of the grains, and investigate further the influence of grain shape by using non-magnetic `voids' of different shapes in a paramagnetic solution. We discover a strong positive correlation between the grain aspect ratio and the size of the effect of magnetic cohesion on the slope angle. This is because a non-spherical grain accumulates magnetic charge on sharp edges and corners, increasing the magnetic field in its immediate vicinity and leading to stronger interactions with neighbouring grains. Also, in piles of grains with larger aspect ratios, avalanches occur closer to the surface, thus increasing the stability of the pile.

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No description available.

In this thesis, the surface wave propagation along a periodically perforated conducting ground plane with dielectric coating has been studied for frequencies low enough for the ground plane to be approximated accurately by two bonded wire arrays. The field components have been obtained by using the z-directed electric and magnetic Hertz potentials which greatly simplify the application of the boundary conditions at the interfaces. The application of the appropriate boundary conditions on both sides of the plane of the wire mesh has resulted in a doubly infinite system of equations which, when truncated, can be solved for the wire currents and the propagation characteristics of the surface wave supported by the perforated ground plane. The plane is then modeled by a tensor impedance matrix relating the tangential components of the electric field to the components of the current density in the plane of the mesh. It has been shown that the surface wave propagation constant and the impedance matrix do not significantly depend on the direction of propagation for electrically small wire spacings. For such cases, it is shown that the components of the electric field can be directly related to the second order derivatives of the components of the current density flowing along the perforated ground plane.

The problem of electromagnetic scattering by a sphere on a layered substrate is treated and numerical results are presented. Initially, the Rayleigh limit, where the sphere radius is small compared to the wavelength, is considered. Closed-form expressions for the far-zone scattered fields are derived from the radiation of the induced dipoles in the sphere in the presence of the substrate, and these incorporate in a rather explicit manner the various parameters of the problem. The general case, where the ratio sphere radius/wavelength is arbitrary, is also considered. A rigorous formulation is used based on the Mie solution for the scattering by a sphere in a homogeneous medium and an extension of Weyl's method for dipole radiation in the presence of a flat surface. Numerical results obtained using the rigorous formulation for electrically small spheres are in very good agreement with those obtained using the Rayleigh approximate method.

A method for computing the per-unit-length capacitance matrix and the inductance matrix for multiconductor transmission lines in a multi-dielectric medium is presented. The multi-dielectric medium consists of both planar and non-planar dielectric regions. The formulation is based on an integral equation method for the free charge distribution on conductor surfaces and the polarization charge distribution on the non-planar dielectric interfaces. The kernel of the integral equation is a space domain Green's function for a layered medium. The numerical solution is obtained by the method of moments.

Bit error rate computation for optical communication systems incorporating equalizers and under both noise and intersymbol interference (ISI) is discussed. An accurate method based on a saddlepoint approximation is used for the computation. Previous work based on saddlepoint approximation has considered only the use of basic integration-and-dump detection. When ISI is strong, this simple detection method is unsatisfactory. Instead, a raised-cosine filtering is often used to achieve minimal ISI. This thesis considers both integration-and-dump and raised-cosine equalizers. The use of equalizers other than integration-and-dump complicates the computation because of the complexity of the moment generating function involved. Two different input pulses are considered to study the effect of ISI. Results show that when intersymbol interference is strong, the use of raised-cosine equalizers can reduce intersymbol interference and improve the performance of the system significantly.

<p> Terahertz (THz) heterodyne receiver systems are required by NASA to monitor gas concentrations related to the Earth's ozone depletion. To this end, NASA needs compact, solid state, tunable THz local oscillators. THz LOs have been developed using three means: 1) All-electronic LOs using mixers in combination with Gunn oscillators, 2) Hybrid Photo-electronic LOs using a cw analog of the Auston switch, and 3) All-photonic THz LOs using coherent sources, such as vapor lasers or solid-state Quantum Cascade Lasers, and down converting lasers using nonlinear crystals. In this dissertation, we began with two frequency stabilized Nd:YAG lasers, locked to a common reference cavity, as a starting point to having a stable input into a nonlinear optical frequency conversion system. Following this, we explored the nonlinear crystals useful for THz generation, and the phasematching schemes that could be employed by each. We concluded by settling on highly insulating III-V semiconductor crystals as the proper choice of nonlinear element, and put together a new phasematching method that is most useful for them.</p>

<p> Hemodialysis vascular access, the interface between a dialysis patient and a dialysis machine, is quite literally the lifeblood of a patient's health. Vascular access dysfunction is the leading cause of hospitalization in hemodialysis patients. The occlusive growth of neointimal hyperplasia (NH) in expanded polytetrafluoroethylene (ePTFE) ringed grafts is the primary cause of failure. To further develop a proposed thermal ultrasound treatment to reduce or prevent NH in arteriovenous vascular grafts, the acoustic properties of ePTFE were studied in water and alcohol solutions. Previous reports of ePTFE acoustic properties are critiqued. It was found that the acoustic transmission and attenuation through ePTFE, and therefore the potential for an ultrasound-based therapy for NH, are heavily dependent on the medium in which the graft is immersed, suggesting that the acoustic properties of implanted grafts will change as grafts mature in vivo. The acoustic impedance and attenuation of water-soaked ePTFE were 0.478 &plusmn; 1.43 &times; 10^-2 MRayl and 1.78 &plusmn; 0.111 Np/cm*MHz, respectively, while the acoustic impedance and attenuation of ePTFE in alcohol were 1.49 &plusmn; 0.149 MRayl and 0.77 &plusmn; 1.1 &times; 10^-2 Np/cm*MHz, respectively. The use of focused ultrasound to heat implanted ringed ePTFE grafts was numerically modeled from 1.35- and 1.443-MHz transducers for in vitro geometries. Power deposition and heating, in turn, differed by an order of magnitude between various graft acoustic properties. Graft rings were predicted to be substantial absorbing and scattering features. In vitro phantom models were constructed: one with and one without thermocouples. At 1 W of acoustic power, the maximum temperature rise was 8&ring; C. The thermocouple model containing a water-soaked graft did not experience heating in the far graft wall. The MRTI model confirmed that the graft rings are an absorbing/scattering feature. Heating was not prevented in the presence of water flow through the graft. Water was not heated significantly. Overall, results suggest ultrasound exposure can be used to generate temperature rises corresponding with the potential prevention or inhibition of NH in ringed ePTFE vascular grafts. A hybrid therapeutic/diagnostic transducer design with a therapeutic semi-annular array surrounding a diagnostic linear array is presented. Compared to a solid transducer of the same dimensions, there were only marginal aberrations in the focal plane. Numerical optimization of the element drive configuration indicated that the least distorted focal plane was produced by uniform phase and magnitude at each element.</p>