Modern devices require fundamental length scales to be analysed in a maximum detail to enable research of new types of phenomena and design new materials. In this thesis, an advancement in Lorentz microscopy will be presented where the focus was placed not only onto resolution in spatial space but also onto resolution in reciprocal space. This allows greater sensitivity to measurements of the integrated magnetic induction within thin samples. This was achieved by a novel approach to the data acquisition, where instead of a segmented (annular) detector, a pixelated detector was used to measure the deflection of the scanning transmission microscopy (STEM) probe due to the in-plane integrated magnetic induction. Computer vision algorithms were researched to find an efficient, noise-robust way to register the deflection of the STEM probe. This enabled a novel approach to data analysis, where a scatter of the 2D integrated induction (a bivariate histogram) is used to show the distribution of the magnetic induction vector. The experimental results are supported by simulations, where a model of a thin polycrystalline sample causes a shift of the simulated beam due to phase modulations. The results of the detection in both the simulation and experiment showed that cross-correlation based processing can efficiently separate the low spatial frequencies (from the in-plane magnetic induction), and high spatial frequencies (from the structure of the polycrystalline sample). This work will enable quantitative analysis of a greater number of thin magnetic samples, for which the current methods are hampered by the diffraction contrast. This will be particularly helpful for the study low moment, out of plane, magnetised thin films. Currently such systems are of great interest due to the tunability of their magnetic properties and the novel magnetic structures present within them. This work also provides an important step for computational methods in transmission electron microscopy, as this is one of the first examples of 4D data acquisition of processing in STEM (where two dimensions represent the spatial scanning dimensions and other two the reciprocal space). Imaging methods developed in this thesis were applied to the topic of skyrmions in a thin layer of a FeGe cubic helimagnet, where the very fine detail of the structure of their in-plane integrated magnetic induction was shown to contain a distorted modulations of its profile. This was compared to a simple three harmonic frequency model, which was altered to fit some characteristics of the imaged magnetic skyrmions. In this work, for the first time, a direct comparison of differential phase contrast and electron holography will be shown for a simple experiment in which the integrated electric field between two needles was measured in free space in the same microscope. Although it was concluded that both methods are equivalent, some small discrepancies of measured values were present due to a long range electric field in electron holography and/or drift of the beam in between scans in STEM.

La synthèse du graphène reste une étape problématique mais la technique par dépôt de vapeur chimique sur les métaux a beaucoup progressé ces trois dernières années, c'est-à-dire depuis que l'on sait qu'il est possible de retirer sans le dénaturer le graphène de son support métallique. Cette étape de transfert appelle à l'étude de la force d'interaction qui a lieu entre graphène et métal. L'influence de cette liaison graphène-métal sur la structure électronique a déjà été largement étudiée et nous proposons plutôt ici de nous pencher sur la dispersion des phonons. Celle du graphène isolé est d'ailleurs remarquable pour les deux grandes anomalies de Kohn (KA) qu'elle présente sur sa branche de plus haute fréquence aux points de haute symétrie Gamma et K. La pente de la dispersion autour de ces KA est une mesure directe du couplage électron-phonon (EPC) entre ces modes de phonons et les électrons des points de Dirac. Nous avons montré que cet EPC, qui a par exemple beaucoup d'influence sur la mobilité électronique ou sur le processus de résonnance Raman, est fortement altéré suite à l'interaction avec certains substrats métalliques. Lors de notre étude ab initio du graphène sur nickel(111), nous avons en effet observé une disaparition complète des anomalies de Kohn. Ceci est dû à la forte hybridation entre les bandes pi du graphène et les bandes d du nickel. La spéctroscopie Raman et la diffraction électronique à basse énergie sont des moyens efficaces et courants pour caractériser le graphène sur les substrats métalliques. Nos résultats montrent comment tirer de ces mesures davantage d'informations concernant le caractère physisorbé ou chimisorbé du graphène sur le métal. / The fabrication of high quality large-area graphene films is one of the biggest problem. Recently, the synthesis by chemical vapor deposition of hydrocarbons over a metallic substrate have shown promising results. To manage as well as possible the transfer process of the graphene layer in an insulator, it is important to know the interaction strength between the graphene layer and the metallic substrate. We propose in this paper to calculate the phonon dispersion of an adsorbed graphene. The phonon dispersion of graphene is known to display two strong Kohn Anomalies (kinks) in the highest optical branch (HOB) at the high-symmetry points Gamma and K. The phonon slope around the Kohn anomalies is related to the electron-phonon-coupling (EPC) with the graphene pi bands. We show that this EPC, which has strong impact, for example, on Raman scattering and electron transport, can be strongly modified due to interaction with a metallic substrate. For graphene grown on a Ni(111) surface, our ab initio calculations show a total suppression of the Kohn anomaly ; the HOB around Gamma and K becomes completely flat. This is due to the strong hybridization of the graphene pi-bands with the nickel d-bands that lifts the linear crossing of the pi bands at K. Raman spectroscopy and low-energy electron diffraction are quick and reliable methods to characterize graphene on metallic substrates. Our results show how to obtain additional information, by means of vibrational and photoemission spectroscopy, on the chemisorption or physisorption of graphene layers on metal surfaces.

"Literature" at the end of each chapter. / Special Collections copy submitted together with 5 selected reprints as the author's thesis (D.Sc.)-- University of Adelaide, 1935. / xi, 324 p. : / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (D.Sc.)--University of Adelaide, 1935

"Literature" at the end of each chapter. / Special Collections copy submitted together with 5 selected reprints as the author's thesis (D.Sc.)-- University of Adelaide, 1935. / xi, 324 p. : / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (D.Sc.)--University of Adelaide, 1935

An overview of quantum phenomena associated with nanoelectronic structures is presented, including resonant tunnelling and mini-band formation in vertical transport devices and channel conductance quantization and interference in lateral devices. The method of construction of these structures is briefly described. Methods of calculating the transmission coefficient are reviewed. In one dimension the transfer matrix method is described and also two derivatives of the approach for circumventing the numerical instability encountered when calculating the wavefunction. In two dimensions an un-coupled matching states method and an asymptotic time dependent method are described. As an alternative to the above methods a coupled network theory is presented for the first time which genuinely represents the 2D time independent electronic wavefunction. Nodes on the network are described by a unitary scattering matrix from which a 2D transfer matrix is derived, connecting lines on the network. The scattering matrix for the whole system is created by combining the 2D scattering matrices for each line, themselves derived from the transfer matrices. The use of the scattering matrix is necessary to ensure numerical stability and current conservation. It is shown that the bandstructure of the network is essential to creating a genuine 2D model whilst at the same time introducing a perturbing influence on the manifestation of physical phenomena. The advantages over other models is the complete absence of restriction on the potential profile considered and no requirement to separate the scalar energy and potential quantities into x and y components. Also no problem with current continuity has been encountered. A major disadvantage is the large time required to calculate wavefunctions compared with the un-coupled matching states method. The network is shown to reproduce the channel conductance quantization recently observed experimentally and is in good agreement with both a 1D analytic model and a 2D un-coupled model. The network is applied to channels containing single and double barriers. In the latter case the resonances are found not to coincide with those predicted by a 1D model. Also the wavefunction on resonance resembles one of the quasi states of the well but with a phase shift. When applied to waveguides involving an interface between channels of different widths the network reveals a tendency for the wavefunction to relax to its original transverse state as it gets further from the interface. This tendency is most pronounced for a tapered junction at low energy (energy of the order of the first transverse eigenvalue). The transmission coefficient for an abrupt junction displays unusual dips above the quantization threshold of the narrow channel. Scattering into higher modes is reduced both by reducing the ratio of channel widths and by reducing the absolute lengths of the device. Finally circle and ring devices are studied, results displaying similarities with Finch's time dependent calculations. In particular scattering into the arms of the ring is observed to be mainly into the first mode if the energy is low and mainly into the third mode if the energy is of the same order as the third transverse eigenvalue of the channel. The tendency to relax into the original transverse state still operates over the whole device.

It is widely acknowledged that the world needs to reduce the level of greenhouse gas emissions. It is proposed to use potentially cleaner renewable energy sources to replace fossil fuels, and thus reduce greenhouse gas emissions. A significant challenge facing renewable energy sources, however, is that the power generation facilities are often located far from the load centres, meaning that new high capacity long-distance transmission systems would need to be built. This is a particular issue since there are increasing difficulties in obtaining approval to construct new overhead lines (OHL). An alternative is gas-insulated lines (GIL), a system for the transmission of electricity over long distance and is considered as a viable technical solution in places where OHL cannot be constructed. The currently adopted gas medium in GIL, however, is sulphur hexafluoride (SF₆), which is a potent greenhouse gas. Trifluoroiodomethane (CF₃I) has been proposed as an alternative insulation medium to SF6 in GIL, and this thesis investigates the potential of using a CF₃I gas mixture in GIL applications. It is hoped that the research can lead to a new form of environmentally friendly power transmission system that could cope with the increasing power demand in large metropolitan areas, and contribute to the reduction of SF6 usage in the high-voltage industry. The literature survey reviewed the research work on CF₃I gas and its mixtures to date. Several research gaps were identified, and these informed the investigations carried out in this research. Reduced-scale coaxial test systems with the electric field properties of a full-scale 400 kV GIL were designed, developed and fabricated. The designs were simulated using COMSOL to ensure that the highest field would be along the centre of the conductor. The effective ionisation coefficients of various CF3I gas mixtures were calculated using BOLSIG+, which provided estimated values for the critical reduced field strength of each gas mixture. Extensive laboratory tests using a standard lightning impulse (1.2/50) were conducted on the fabricated prototypes using various CF₃I/CO₂ and CF₃I/N₂ gas mixtures to determine the 50% breakdown voltage. The breakdown characteristics of CF3I gas mixtures were examined for pressure, geometric ratio, impulse polarity, buffer gas and mixture content. Based on the measured breakdown voltage and calculated critical reduced field strength of various CF3I gas mixtures, a two-stage streamer/leader mathematical model was developed to evaluate the reduction in field strength at higher pressures. A comparative study was carried out on CF3I gas mixtures in a rod-plane electrode configuration under standard lightning impulse and steep-front square impulse waveforms. This investigation focused on the V-t characteristics of CF3I gas mixtures in this particular configuration. A phase equilibrium experiment was also carried out to determine the boiling point of various CF3I gas mixtures.

Στην παρούσα διπλωματική εργασία γίνεται καταγραφή σημάτων τάσης, ρεύματος, ηλεκτρικού πεδίου και εκπεμπόμενου φωτός κατά την εφαρμογή κρουστικών τάσεων σε ένα διάκενο ανομοιογενούς πεδίου ράβδου-πλάκας. Σκοπός της εργασίας είναι η μελέτη φαινομένων που προηγούνται της διάσπασης σε ένα διάκενο ράβδου-πλάκας χρησιμοποιώντας ένα καθετήρα μέτρησης πεδίου τύπου Meek-Collins στο επίπεδο της πλάκας, και η ταυτόχρονη λήψη σημάτων ρεύματος και φωτός προς την επαλήθευση της ορθής λειτουργίας του καθετήρα. Στο πρώτο κεφάλαιο δίνεται το απαραίτητο θεωρητικό υπόβαθρο αναφορικά με τους μηχανισμούς διάσπασης σε διάκενα αερίων (ομοιογενή και ανομοιογενή). Γίνεται επίσης αναφορά στη στατιστική ανάλυση των πειραματικών αποτελεσμάτων για το χαρακτηρισμό των διακένων. Στο δεύτερο κεφάλαιο παρουσιάζεται η πειραματική διάταξη και ο εξοπλισμός που χρησιμοποιήθηκε. Ιδιαίτερη έμφαση δίνεται στην αρχή λειτουργίας και την κατασκευή του καθετήρα Meek-Collins. Στο τρίτο κεφάλαιο αναλύονται η πειραματική διαδικασία και τα αποτελέσματα των πειραματικών μετρήσεων. Τέλος, στο τέταρτο κεφάλαιο, παρουσιάζονται συνοπτικά τα συμπεράσματα που προέκυψαν από την εκπόνηση της παρούσας εργασίας. / The present diploma thesis is devoted to the acquisition of voltage, current, electric field, and emitted light waveforms during the application of impulse voltages to a non-uniform rod-to-plane gap. The main claim of this work is the study of the prebreakdown effects in a rod-to-plane gap by means of an electric field measuring probe (Meek-Collins), and the simultaneous acquisition of current and light signals for confirming the probe functionality. In the first chapter, the appropriate theoretical background related to the breakdown mechanisms of gas gaps (both uniform and non-uniform) is provided. The statistical analysis of experimental results for gap characterization is also mentioned. In the second chapter, the experimental setup and the equipment used are presented. The fundamental principle and the construction details of the Meek-Collins probe are emphasized. In the third chapter, the process followed and the experimental results are analyzed. Eventually, in the fourth chapter, the conclusions of this work are briefly presented. The present diploma thesis is inter-related to the one of Mr George Vagenas, whose work was realized in the High Voltage Laboratory on the construction and calibration of the Meek-Collins probe used herein. In practice, both theses were carried out in parallel.

Errata has been inserted on p. 136 and 162. / Bibliography: p. 263-280. / xxiii, 280 p. : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Presents the results of two investigations into the characteristics of electron beams for application in radiation therapy. / Thesis (Ph.D.)--University of Adelaide, Dept. of Physics and Mathematical Physics, 1997?

Depuis sa prédiction, le transfert de spin est devenu un sujet de recherche important et certaines applications ont déjà été commercialisées. Cependant, ce domaine laisse encore de nombreuses questions quant aux processus physiques mis en jeu. Notre travail s'est consacré à l'étude de la dynamique d'aimantation sous l'effet de transfert de spin dans des vannes de spin avec une aimantation perpendiculaire au plan, ces dernières présentant une meilleure efficacité. Pour cela, nous avons dans un premier temps développé un programme de simulation macrospin que nous avons par la suite confronté aux résultats expérimentaux. Notre étude s'est d'abord focalisée sur les diagrammes d'état champ-courant, qui donnent une perspective globale du comportement de l'aimantation. Nous avons pu montrer que le champ extérieur ou l'anisotropie sont capables de briser la symétrie du système et ainsi expliquer la différence entre prédictions et observations. Nous avons ensuite mis en évidence des états cantés, qui sont des états où l'aimantation est figée à un angle précis. La dernière partie de cette thèse fut consacrée à la dynamique d'aimantation à différentes échelles de temps de la seconde à la centaine de picosecondes. C'est ainsi que nous avons pu constater les limites du modèle macrospin, incapable d'expliquer les résultats expérimentaux. Nous nous sommes alors tournés vers un modèle micromagnétique, et avons pu montrer qu'il permet de mieux décrire les résultats expérimentaux / Ever since its prediction, spin transfer has spawned a lot of interest, and some applications have already been commercialized. However the spin transfer effect is still not fully understood. This thesis focused on magnetization dynamics and spin transfer effect in spin valves with out of plane magnetization, proven to be more efficient. With this in mind, we developped a software simulating magnetization dynamics in spin valve. We then confronted the software with experimental results to check our model and its hypotheses veracity. Another part of this thesis was to study current-field state diagrams, which offer a global perspective on magnetization behavior. We managed to show the applied field and the anisotropy could produce a symmetry breaking of the system and thus explain the experimental results. We were also able to highlight canted states of the magnetization. Those are frozen states of the magnetization for certain angles. The last part describes magnetization dynamics at different time scales from a second to a hundred picoseconds. We showed the macrospin model is not appropriate for short current pulses, although we could highlight the influence of some parameters. We finally the micromagnetics model allowed for a better description of the experimental results

Στην παρούσα εργασία ασχοληθήκαμε με τα ηλεκτροτεχνικά υγρά, τόσο θεωρητικά όσο και πειραματικά. Σε θεωρητικό επίπεδο περιγράφουμε το ρόλο των μονωτικών λαδιών, τις χρήσεις τους, τις ιδιότητές τους και τους παράγοντες που τα επηρεάζουν. Σε πειραματικό επίπεδο, πραγματοποιούμε πλήθος μετρήσεων με στόχο την εξακρίβωση και την εξαγωγή συμπερασμάτων από την εφαρμογή γραμμικώς αυξανόμενης a.c. τάσης πάνω σε δύο διαφορετικά είδη μονωτικών λαδιών. / As networks of High Voltage Alternate Power can be characterized those who function in voltage higher than 1000V. Therefore, the electrological equipment which is going to be settled and function in alternate power voltage higher than 1KV is characterized as a High Voltage Equipment. The insulation of the equipment and that of the high voltage establishments is indispensable in order to maintain the potential difference between the under high voltage conductible parts. Alongside its main destination, insulation may have other roles too such as the mechanic support of the wire way, the exchange of temperature e.c.t. In the present project we dealt with the electro – technical liquids in theoretical and also in experimental level. According to the theoretical level, we described the role of the insulating oils, their uses, their attributes and the factors that affect them. According to the experimental level we accomplished a number of measurements, having as intension the verification and the deduction of conclusions coming from the application of the linearly ongoing a.c. voltage on two different kinds of insulating oils. From the aforementioned measurements we noticed the impact that the length of the gap, the temperature, even the combination of the two different insulating oils have on the tendency in breaking not only quantitavely (change of size) but also qualitatively (change of statistical attributes). For carrying out the experimental procedure we utilized the device of the High Voltages Laboratory’s electrological equipment. All measurements and also the selection of the appropriate testing prescription were done by this device, taking each time the necessary safeguards for us and for the lab’s facilities. Finally, the results were gathered in billboards and were farther analyzed according to indispensable diagrams for the deduction of conclusions. In this point is very important to mention that on those billboards is indicated the average value which refers to the tendency in breaking and that of typical variations, just for practical reasons.