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This thesis presents a study of two-dimensional electron systems in GaAs-(Al,Ga)As heterojunctions and quasi-two-dimensional electron and hole systems in graphite within the quantum Hall effect regime of low temperature and high magnetic field. This thesis covers three main sets of experimental work as well as details of the experimental methods (chapter 2) used and the background theory behind the observed results (chapter 1). The first experimental results presented in this thesis in chapter 3 focus on contactless measurement of the equilibrium magnetisation of sample A2268, a ten layer multiple quantum well sample. Fitting the shape of dHvA oscillations at various temperatures to different models for the density of states, various properties of the system can be estimated, such as the shape of the disorder-broadened density of states and the presence of a background density of states between the Landau levels. Chapter 4 focuses on measurements of the decay of induced circulating currents in the quasi-dissipationless quantum Hall regime in two samples, V0049 and T73. The induced current is measured via contactless measurement of the associated magnetic moment. The magnitude of the induced current is found to be affected by the sweep rate of the magnetic field and also the distance of approach. The decay of the induced currents is observed at several temperatures and for different magnetic field sweep rates and distances of approach. Decays are observed for up to several days at time, far longer than previously possible. Information about the rate of decay can be used to build a picture of the decay mechanisms present in the quantum Hall regime. The presence of a power-law decay regime indicates many decay mechanisms contribute to the decay of a circulating current in the quasi-dissipationless quantum Hall regime. Chapter 5 focuses on both contactless magnetometry and transport experiments carried out on a graphite sample. The experiments aim to confirm or dispute recent claims of Dirac fermions in graphite. Experiments are carried out at temperatures in the range 30 mK to ~4 K and at two different angles to the applied magnetic field. Phase analysis of both Shubnikov de Haas and de Haas van Alphen oscillations is used to distinguish between normal and Dirac fermions. Observation of quantum Hall effect displays the presence of a half-integer quantum Hall staircase similar to that observed in graphene.

Dans cette thèse le magnétotransport à basse température (T = 10 mK) et l'effet de Haas-van Alphen sont examinés pour le graphite naturel et le graphite HOPG. Dans la première partie, le magnétotransport au champ magnétique jusqu'à B = 11 T est présenté. Une analyse de Fourier du signal après soustraction du fond de magnétorésistance montre que le transport électrique dans le graphite est dominé par deux types de porteurs avec des fréquences et phases en accord avec le modèle SWM. Nous confirmons la validité du modèle SWM par des calculs détaillés de la structure de bande en champ magnétique. Le mouvement de l'énergie de Fermi pour B > 2 T est calculé d'une manière auto-cohérente en supposant que la somme des concentrations des électrons et des trous est constante. Des mesures sous champs magnétiques intenses (0 < B < 28 T) sont employées pour étudier l'effet Zeeman et la phase de l'onde de densité de charge. En ce qui concerne l'effet Zeeman, les calculs SWM incluant le mouvement de l'énergie de Fermi nécessitent un facteur de Lande g = 2.5 pour reproduire la séparation de spin des motifs dans les données. Les mesures de l'onde de charge de densité confirment que le champ magnétique auquel l'onde de charge de densité apparait est lie a la température par une formule de type Bardeen-Cooper-Schrieffer (BCS). Des mesures de l'effet de Haas-van Alphen confirment les résultats obtenus par de magnétotransport à bas champ.

Dans cette thèse le magnétotransport à basse température (T = 10 mK) et l'effet de Haas-van Alphen sont examinés pour le graphite naturel et le graphite HOPG. Dans la première partie, le magnétotransport au champ magnétique jusqu'à B = 11 T est présenté. Une analyse de Fourier du signal après soustraction du fond de magnétorésistance montre que le transport électrique dans le graphite est dominé par deux types de porteurs avec des fréquences et phases en accord avec le modèle SWM. Nous confirmons la validité du modèle SWM par des calculs détaillés de la structure de bande en champ magnétique. Le mouvement de l'énergie de Fermi pour B > 2 T est calculé d'une manière auto-cohérente en supposant que la somme des concentrations des électrons et des trous est constante. Des mesures sous champs magnétiques intenses (0 < B < 28 T) sont employées pour étudier l'effet Zeeman et la phase de l'onde de densité de charge. En ce qui concerne l'effet Zeeman, les calculs SWM incluant le mouvement de l'énergie de Fermi nécessitent un facteur de Lande g = 2.5 pour reproduire la séparation de spin des motifs dans les données. Les mesures de l'onde de charge de densité confirment que le champ magnétique auquel l'onde de charge de densité apparait est lie a la température par une formule de type Bardeen-Cooper-Schrieffer (BCS). Des mesures de l'effet de Haas-van Alphen confirment les résultats obtenus par de magnétotransport à bas champ.

La croissance exponentielle dans l’utilisation du réseau Internet ainsi que l’apparition de nouveaux types d’applications ont augmenté les contraintes du réseau en termes de sécurité. Depuis quelques années, les techniques biométriques ont prouvé une grande précision et fiabilité et ont été utilisées dans plusieurs domaines afin de sécuriser l’accès à différentes ressources. Des solutions intégrant des agents et des systèmes multi-agents (SMA) ont aussi prouvé leur efficacité pour la résolution de nombreux problèmes dans les réeaux. Nous proposons un modèle « IrisCrptoAgentSystem » (ICAS) basé sur la méthode biométrique pour l’authentification utilisant l’iris de l’œil et la méthode de cryptographie asymétrique utilisant l’algorithme « Rivest-Shamir-Adleman » (RSA), et en intégrant des agents. Ce modèle doit assurer un accès sécurisé aux informations et garantir la protection des informations confidentielles. Notre travail porte sur la mise en place de nouvelles méthodes dans le modèle d’authentification biométrique afin de donner plus d’efficacité à notre modèle ICAS. Nous introduisons des aspects prétopologiques dans l’élaboration de la hiérarchie indexée pour classer les gabarits DHVA. Notre approche consiste à améliorer les méthodes relatives à la localisation des contours externe et interne de l’iris. / The exponential growth in the use of the Internet as well as the emergence of new types of applications has increased the network’s constraints in terms f security. Fort the last several years, biometric techniques have proven their applicability and reliability in providing secure access to shared resources in different domains. Furthermore, software agents and multi-agent systems (MAS) have evidently been efficient in resolving several problems in network. Therefore, the aim of this research is to propose a model “IrisCryptoAgentSystem” (ICAS) that is based on a biometric method for authentication using the iris of the eyes and an asymmetric cryptography method using “Rivest-Shamir-Adleman” (RSA) in an agent-based architecture. This model should provide secure access to information and ensure the protection of confidential information. Therefore, our work focuses on the development of new methods in biometric autheitcation in order to provide greater efficiency in the ICAS model. We introduce pretopological aspects in the development of the indexed hierarchy to classify DHVA templates. Our approach aims to improve the existing methods for the localization of the external and the internal edges of the iris.

Title: Ground state investigations of Ce and U intermetallic compounds Author: Attila Bartha Department: Department of Condensed Matter Physics Supervisor: RNDr. Jan Prokleška, Ph.D., Department of Condensed Matter Physics Abstract: Rare earth and actinide intermetallic compounds offer a plethora of interesting physical properties due to the varied behavior of f -electrons together with numerous interactions these electrons are exposed to. In this thesis we address a broad spectrum of ground state investigations on CePd2X3 (X=Zn, Ga) and (Ce,U)nTIn3n+2 (T=Rh, Ir) compounds. Single crystals of CePd2Zn3 and CePd2Ga3 compounds were synthesized for the first time using Bridgman method. CePd2Ga3 revealed a ferromagnetic transition with TC = 6.7 K with a strong magnetocrystalline anisotropy. CePd2Zn3 orders antiferromagnetically below TN = 1.9 K. Results of magnetization measurements on Ce2IrIn8 revealed effective magnetic moment µeff = 2.45µB/Ce3+ and a paramagnetic Curie temperature θP = −31 K. Decomposition of Hall resistivity ρxy(B) into NHE and AHE revealed a predom- inance of AHE in the temperature range from 60 K up to 100 K. Ce2RhIn8 was studied by means of magnetic field and angle dependent magnetization and heat capacity measurements. The resulting phase diagram reveals a complete unfold- ing and...

Heavy fermion metals often exhibit novel electronic states at low temperatures, due to competing interactions and energy scales. In order to characterize these states, precise determination of material electronic properties, such as the Fermi surface topology, is necessary. Magnetic field is a particularly powerful tool, since it can be used as both a tuning parameter and probe of the fundamental physics of heavy fermion compounds. In CePb3, I measured magnetoresistance and torque for 23 mK ≤ T ≤ 400 mK, 0 T ≤ H ≤ 18 T, and magnetic field rotated between the (100), (110), and (111) directions. For H||(111), my magnetoresistance results show a decreasing Fermi liquid temperature range near Hc, and a T^2 coefficient that diverges as A(H) ∝ |H −Hc|^−α, with Hc ~ 6 T and α ~ 1. The torque exhibits a complicated dependence on magnetic field strength and angle. By comparison to numerical spin models, I find that the “spin-flop” scenario previously thought to describe the physics of CePb3 does not provide a good explanation of the experimental results. Using novel data acquisition software that exceeds the capabilities of a traditional measurement set-up, I measured de Haas–van Alphen oscillations in YbRh2Si2 for 30 mK ≤ T ≤ 600 mK, 8 T ≤ H ≤ 16 T, and magnetic field rotated between the (100), (110), and (001) directions. The measured frequencies smoothly increase as the field is decreased through H0 ≈ 10 T. I compared my measurements to 4f-itinerant and 4f-localized electronic structure calculations, using a new algorithm for extracting quantum oscillation information from calculated band energies, and conclude that the Yb 4f quasi-hole remains itinerant over the entire measured field range, with the behaviour at H0 caused by a Fermi surface Lifshitz transition. My measurements are the first to directly track the Fermi surface of YbRh2Si2 across this field range, and rule out the 4f localization transition/crossover that was previously proposed to occur at H0.

Heavy fermion metals often exhibit novel electronic states at low temperatures, due to competing interactions and energy scales. In order to characterize these states, precise determination of material electronic properties, such as the Fermi surface topology, is necessary. Magnetic field is a particularly powerful tool, since it can be used as both a tuning parameter and probe of the fundamental physics of heavy fermion compounds. In CePb3, I measured magnetoresistance and torque for 23 mK ≤ T ≤ 400 mK, 0 T ≤ H ≤ 18 T, and magnetic field rotated between the (100), (110), and (111) directions. For H||(111), my magnetoresistance results show a decreasing Fermi liquid temperature range near Hc, and a T^2 coefficient that diverges as A(H) ∝ |H −Hc|^−α, with Hc ~ 6 T and α ~ 1. The torque exhibits a complicated dependence on magnetic field strength and angle. By comparison to numerical spin models, I find that the “spin-flop” scenario previously thought to describe the physics of CePb3 does not provide a good explanation of the experimental results. Using novel data acquisition software that exceeds the capabilities of a traditional measurement set-up, I measured de Haas–van Alphen oscillations in YbRh2Si2 for 30 mK ≤ T ≤ 600 mK, 8 T ≤ H ≤ 16 T, and magnetic field rotated between the (100), (110), and (001) directions. The measured frequencies smoothly increase as the field is decreased through H0 ≈ 10 T. I compared my measurements to 4f-itinerant and 4f-localized electronic structure calculations, using a new algorithm for extracting quantum oscillation information from calculated band energies, and conclude that the Yb 4f quasi-hole remains itinerant over the entire measured field range, with the behaviour at H0 caused by a Fermi surface Lifshitz transition. My measurements are the first to directly track the Fermi surface of YbRh2Si2 across this field range, and rule out the 4f localization transition/crossover that was previously proposed to occur at H0.