This thesis describes the characterisation of GaMnAs, related compounds and heterostructures. GaMnAs and other (III,Mn)V compounds have provided many interesting insights into fundamental physics, and are of considerable potential interest commercially in the field of spintronics. This study examines a set of samples grown by molecular beam epitaxy and characterised using several techniques: primarily this study makes use of the x-ray absorption techniques, x-ray magnetic circular dichroism(XMCD) and x-ray absorption spectroscopy (XAS). In addition, x-ray diffraction (XRD), transport measurements and super conducting quantum interference device (SQUID) magnetometry were used as complimentary techniques. GaMnAs layers with epitaxial Fe grown on top, are shown to have a sub-nanometre interfacial layer which remains polarised above room temperature. A detailed understanding of these systems is obtained by applying the element specific nature of XMCD in combination with two different probing depths to explore separately the nature of the coupling of the bulk and interfacial region. The coupling between the interfacial layer and the Fe is shown to be strongly antiferromagnetic (AF). A weaker coupling is also shown to exist between the Fe and the bulk of the \gamnas layer below the Curie temperature (Tc). This coupling is also AF at low fields, leading to an exchange bias for the entire layer. Doping of GaMnAs with P is shown to have several effects on the magnetic properties of the GaMnAs layer. Changes in the layer strain are observed using high resolution XRD. This strain also manifests itself in the Mn L_2,3 XMCD spectra and the relationship between the two is shown to be linear. A pronounced effect on the magnetic anisotropy is observed using SQUID measurements, with the easy axis switching from in-plane, in the compressively strained GaMnAs, to out-of-plane in the higher doped GaMnAsP layers. A decrease in total magnetic moment per Mn atom and Tc are observed with increased doping. This is inferred not to be due to a direct effect of the P on the local surrounding of the Mn ions, owing to the striking similarity of the XMCD spectra. This is instead attributed to reduced participation of Mn ions in the magnetic ordering. Finally, K edge XMCD is used to reveal the element specific nature of unoccupied states near the Fermi level in a set of GaMnAs and (In,Ga,Mn)As samples with differing Mn doping levels . The character of the holes in low-doped samples is shown to be markedly different than for those in the highly doped metallic samples. A transfer of orbital magnetic moment from the Mn to the As sites is observed on crossing the metal-insulator transition, with the large XMCD on Mn sites in low doped samples interpreted as a sign of hole localisation around the Mn ion.

This doctoral thesis presents density functionalcalculations applied in several domains of interest in solidstate physics and materials science. Non-collinear magnetismhas been studied both in an artificial multi-layer structure,which could have technological relevance as a magnetic sensordevice, and as excitations in 3d ferromagnets. The intricatebulk crystal structure of γ-alumina has been investigated.An improved embedded cluster method is developed and applied tostudy the geometric and electronic structures and opticalabsorption energies of neutral and positively charged oxygenvacancies in α-quartz. Ab initio total energycalculations, based on the EMTO theory, have been used todetermine the elastic properties of Al1-xLixrandom alloys in the face-centered cubiccrystallographic phase. The obtained overall good agreementwith experiment demonstrates the applicability of the quantummechanics formulated within the framework of the DensityFunctional Theory for mapping the structural and mechanicalproperties of random alloys against chemical composition.

Numerical simulation techniques play an important role due to their flexibility in dealing with a broad range of complex geometries and material responses. This flexibility requires substantial computational time and memory. Most numerical methods use structured grid for graphical discretization, although this approach is straightforward it is not ideal for smoothly curved boundaries. In this thesis the two-dimensional Transmission Line Modelling (TLM) method based on unstructured meshes is adopted. TLM is an established numerical simulation technique that has been employed in a variety of applications area. Using unstructured meshes to discretize the problem domain permits smooth boundary presentation which provides significant enhancement in the flexibility and accuracy of the TLM simulations. An algorithm is developed to implement Unstructured Transmission Line Modelling (UTLM) which is carefully designed for simplicity and scalability of model size. Several examples are employed to test the accuracy and efficiency of the UTLM simulations. Delaunay meshes, as a type of unstructured meshes, provide good quality triangles but have the disadvantage of providing close to zero transmission line length which has impact on the maximum permissible time step for stable operation. In this thesis, a simple perturbation method for relaxing the minimum link length and clustering triangles in pairs is presented, which permits substantial increase in time step and hence computational runtime to be made without compromising the simulation stability or accuracy. Also, a new model for relaxing the short link lines that fall on the boundaries is presented. UTLM method is based on temporal and spatial sampling of electromagnetic fields which results in dispersion. In this thesis, dispersion characteristics of the unstructured TLM mesh are investigated and compared against structured TLM results for different mesh sizes and shapes. Unlike the structured TLM mesh, the unstructured mesh gives rise to spatial mode coupling. Intermodal coupling behaviour is investigated in a statistical manner upon the change of the mesh local characteristics.

The thermally induced flux motion and the elemental pinning force, fp, for Nb thin films (1000-5000A) were measured for applied magnetic fields ranging from 0.3 to 7.5G, and temperatures from 4.22K to 5.72K. / The magnitude of f$\sb{\rm p}$(H,d,T) ranged from 10$\sp{-12}$ to 10$\sp{-11}$ N/m which is approximately 5 orders of magnitude smaller than Lorentz force depinning measurements made on Nb for the high field regime$\sp{16}$ (flux line lattice), as well as the low field regime$\sp{15}$ (isolated essentially non-interacting fluxoids). Some of these results are similar to the works of Huebener, et al, who first found a large discrepancy between the transport current method (J x B) and the thermal method, S$\nabla$T, when calculating the pinning force on a flux line lattice structure. A model was proposed to explain the discrepancy in terms of an electron scattering effect at or near the grain boundaries and extending into the grains which produces a channeling effect whereby the transport electrons flow away from the trapped fluxoids such that their local Lorentz force per unit length is less than the value calculated from J$\sb{\rm ave\Phi o}$. / The $\Delta\Phi$ vs $\Delta$T data for small $\Delta$T values, implied a spectrum of pinning force values where the fluxoids that were depinned and had substantially weaker pinning forces than the majority of the remaining trapped flux lines. Using statistical arguments, the qualitative features of the $\Delta\Phi$(H,d) data and f$\sb{\rm p}$(H,d) is explained. The data exhibited a magnetic field threshold, below which there is no flux motion for the temperature range studied. The value of the minimum required applied field necessary for flux motion increases with increasing thickness. / Source: Dissertation Abstracts International, Volume: 49-12, Section: B, page: 5358. / Major Professor: William G. Moulton. / Thesis (Ph.D.)--The Florida State University, 1988.

An Electromagnetic Pulse (EMP) resulting from, for example, the detonation of a nuclear weapon is characterised by a wave of electromagnetic activity able to couple with power lines and electro-sensitive equipment with the potential of rendering an establishment or on a greater scale, a whole city impotent. Protection against such occurrences is of paramount importance. It is now accepted that an important consideration when devising protective schemes against such phenomena is an accurate understanding of the effects on propagating waveforms such as those coupled to wires, when electrical breakdown of the dielectric material surrounding such wires occurs, otherwise known as electrical discharge. Such issues can occur around the affected wires if the electric fields generated exceed the dielectric strength of the surrounding medium, typically air or soil. Under these circumstances, the signature of the coupled waveform is known to change in characteristic ways. The form and degree of distortion needs to be understood if the harmful effects are to be prevented by protection systems put in place. The purpose of this thesis is to first describe the mechanisms that lead to the development of the Nuclear–Electromagnetic Pulse (NEMP) and the mechanisms of the discharge that can result once such pulses have coupled to a wire. Next, some of the previous corona-modelling approaches are discussed. Many of the modelling approaches have been applied to 1-D transmission-line simulations. When 3-D simulations have been performed, the Finite-Difference (Time Domain) or FD-TD approach seems to be the preferred method. At the time of writing, no 3-D Transmission Line simulations of discharge phenomena around wires were available. Hence, here, the 3-D Transmission Line Modelling Method (TLM) is described with a view to modelling such behaviour. In particular, the Embedded-Wire-Node (EWN) is used to model the discharge development around the wire. This is a fine-wire technique used to reduce computational fatigue. The node can be adapted to accept changes related to electrical discharge allowing for a real-time, self-consistent recreation of such effects. The 3-D TLM approach proves to be a decent candidate to the modelling of such behaviour. Both advantages and disadvantages of this method are discussed.

Combining stainless steel with polyester fibres adds an attractive conductive behaviour to the yarn. Once knitted in such a manner, fabrics develop sensing properties that make them suitable for wearable applications as a consequence of the variation of their conductivity when subject to structural deformations. The use of such textiles as accurate strain/motion sensors can be considered the new frontier in wearable sensor applications, in opposition to their use as simple strain/motion detectors as mainly done until now. This thesis aims to characterise the electrical properties of knitted conductive textiles made of 20% stainless steel and 80% polyester fibres in the context of their application as fabric strain sensors. However, the use of conductive textiles as sensors is challenging and requires the combined study of their electrical and mechanical behaviours. In the first part of this thesis, the fabric resistance variation is analysed with a purposely built measurement system during a considerable number of stretch-recovery cycles performed by an Instron tensile machine. From the resulting electrical characterisation, it is possible to select the best knit pattern candidates for the second part of the study, which regards their integration into a supporting garment and further investigation of their electrical and mechanical properties in view of their application as wearable motion sensors. Two acquisition systems capture both free and constrained motions of a participant's knee and the electrical resistance variation due to combined deformation of samples and supporting leggings. In both parts of this thesis, a detailed analysis in terms of linearity, stability, sensitivity and hysteresis confirms that the overall sensing performance progressively decreases during extended use as a consequence of the short-term irreversible fabric deformation and does not significantly recover after short rest periods. For the development and use of high performance fabric-based strain sensors, it is therefore evident the importance of the long-term maintaining of a reversible stretch-recovery behaviour not only of the sensing fabrics but also of the supporting garments they are integrated in.

This thesis investigates the electron-phonon interactions in fabricated Semiconductor Devices i.e. Schottky Diodes, Quantum Well (QW) embedded p-i-n photodiodes, two dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG) structures all on (311) GaAs substrates. Fast electrical measurements were performed on Schottky diodes fabricated on the (311B) GaAs surface. Optical excitation generates a compressive strain pulse in an Al film which is at the opposite side of the GaAs substrate. Two coherent acoustic phonon modes were generated in the (311B) GaAs substrate: quasi-longitudinal phonons (QLA) and quasi-transverse phonons (QTA). These two modes were detected successfully using the Schottky diode, the dependence of QLA and QTA signals on different applied bias, pump powers and temperatures were investigated. The dependence of signals on bias reveals that the temporal position of the signals for QLA and QTA moves slightly earlier times with increasing reverse bias because the width of the depletion layer increases with increasing negative bias. The first peak arrival time for QLA phonons in the measured signal moves to earlier times with increasing pump power. This was caused by the nonlinear acoustic properties of the GaAs substrate. Conversely, QTA phonons arrival time was not dependent on pump power which indicates the elastic nonlinear effects are not crucial for QTA signals. Investigation of the temperature dependence reveals that the arrival time changes to later times with increasing temperature because of the decreasing sound velocity with increasing temperature. The polarity of the experimental signals was compared to the simulated signals of electron-phonon coupling interactions concludes the predominant role of piezoelectric phonon-electric transformation mechanism over deformation potential. Detailed theoretical calculations show the dominance of the piezoelectric mechanism up to frequencies of 70GHz and 250GHz for longitudinal and transverse phonons respectively. Low dimensional structures i.e. Quantum well embedded p-i-n photodiode detector (QW-p-i-n), two-dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG), fabricated on the (311) GaAs surface, were also investigated as detectors for longitudinal and transverse acoustic signals. The first experimental evidence of high frequency tail in transverse signal is observed using GaAs based QW-p-i-n detector. The longitudinal signal confirms the existence of the GHz oscillations, with the most significant occurring 170 GHz, while in transverse acoustic signal we observed a high frequency tail from 300 ps to 800 ps. Fast Fourier transforms (FFT) analysis of this tail shows the existence of high frequency phonons up to 200 GHz. Photocurrent pump-probe measurements was also performed with three different Al film thicknesses (20nm, 30nm and 100nm). Experiments performed with 20nm film thickness clearly shows a weak signal at 250 GHz in the amplitude spectrum which is not observed for 30 nm and 100 nm aluminium films. This observation indicates that the thinner aluminium films can generate higher frequency phonons. The longitudinal and transverse signals were also detected by the 2DHG; for QLA predominantly the peak is negative while for QTA it is positive in contrast to the 2DEG experiments due to the reason that in 2DEG experiments the origin of the signal is due to the decrease in carrier density but for 2DHG the signal is due to the increase in carrier density. This thesis studies the interaction of subterahertz acoustic phonons with charge carriers with the aim of developing high speed acoustoelectric detectors for subterahertz compressional and shear acoustic waves, and it has been shown that the Schottky diodes are successful in this application.

Source: Dissertation Abstracts International, Volume: 24-01, page: 0337. / Thesis (Ph.D.)--The Florida State University, 1963.

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Photothermal energy conversion is important when designing optically active devices based on plasmonic nanoparticles. Many early applications of these nanoparticles, like photothermal tumor ablation, drug delivery, and microfluidic devices, depend on the conversion of light to heat. In this dissertation, we compare three nanoparticle species' theoretical absorption efficiency from electromagnetic calculations with their photothermal transduction efficiency from measurements of temperature in an illuminated system. Several mechanisms that may account for differences between the two efficiencies are suggested. With a view specifically toward clinical applications, our analysis assumes a random orientation of nanorods, as would occur naturally in the tumor vasculature. For the samples studied here, photothermal transduction efficiencies differed only by a factor of two or three, regardless of particle type and concentration. Both experiment and theory show that particle size plays a dominant role in determining transduction efficiency, with smaller particles more efficient for heating and larger particles for combined heating and imaging. Additionally, we evaluate the potential of mixtures of plasmonic nanoparticles for CO 2 scrubbing substrates that could be used in space applications. These measurements indicate possible dynamic nanoscale effects that need to be accounted for when modeling photothermal transduction.