Inspired by graphene, I investigate the properties of several different real and artificial Dirac materials. Firstly, I consider a two-dimensional honeycomb lattice of metallic nanoparticles, each supporting localised surface plasmons, and study the quantum properties of the collective plasmons resulting from the near field dipolar interaction between the nanoparticles. I analytically investigate the dispersion, the effective Hamiltonian and the eigenstates of the collective plasmons for an arbitrary orientation of the individual dipole moments. When the polarisation points close to normal to the plane the spectrum presents Dirac cones, similar to those present in the electronic band structure of graphene. I derive the effective Dirac Hamiltonian for the collective plasmons and show that the corresponding spinor eigenstates represent chiral Dirac-like massless bosonic excitations that present similar effects to those of electrons in graphene, such as a non-trivial Berry phase and the absence of backscattering from smooth inhomogeneities. I further discuss how one can manipulate the Dirac points in the Brillouin zone and open a gap in the collective plasmon dispersion by modifying the polarisation of the localized surface plasmons, paving the way for a fully tunable plasmonic analogue of graphene. I present a phase diagram of gapless and gapped phases in the collective plasmon dispersion depending on the dipole orientation. When the inversion symmetry of the honeycomb structure is broken, the collective plasmons become gapped chiral Dirac modes with an energy-dependent Berry phase. I show that this concept can be generalised to describe many real and artificial graphene-like systems, labeling them Dirac materials with a linear gapped spectrum. I also show that biased bilayer graphene is another Dirac material with an energy dependent Berry phase, but with a parabolic gapped spectrum. I analyse the relativistic phenomenon of Klein Tunneling in both types of system. The Klein paradox is one of the most counter-intuitive results from quantum electrodynamics but it has been seen experimentally to occur in both monolayer and bilayer graphene, due to the chiral nature of the Dirac quasiparticles in these materials. The non-trivial Berry phase of pi in monolayer graphene leads to remarkable effects in transmission through potential barriers, whereas there is always zero transmission at normal incidence in unbiased bilayer graphene in the npn regime. These, and many other 2D materials have attracted attention due to their possible usefulness for the next generation of nano-electronic devices, but some of their Klein tunneling results may be a hindrance to this application. I will highlight how breaking the inversion symmetry of the system allows for results that are not possible in these system's inversion symmetrical counterparts.

Ce manuscrit est divisé en trois parties distinctes. les deux<br />premières parties sont axées sur l'étude de quelques mécanismes<br />intrinsèques de l'effet Hall anormal. Nous montrons notamment que la<br />diffusion des porteurs de charge par des impuretés ou des défauts<br />donne une contribution intrinsèque non négligeable à l'effet Hall<br />anormal. Nous proposons ensuite une expérience, basée sur un réseau de<br />nanocylindres magnétiques, placé au dessus d'un gaz d'électrons<br />bidimensionnel, dans laquelle les contributions intrinsèques de<br />l'effet Hall anormal sont clairement identifiables et contrôlables.<br />Enfin, nous abordons le problème du mécanisme de chiralité de spin<br />proposé pour expliquer l'effet Hall dans le composé pyrochlore<br />Nd$_2$Mo$_2$O$_7$. En utilisant un modèle de gaz sur réseau ainsi<br />qu'une configuration magnétique non colinéaire (chirale), nous<br />montrons que la conductivité transverse a une dépendance complexe par<br />rapport à la chiralité de spin.<br /><br />La dernière partie de ce manuscrit est dédiée à l'étude de quelques<br />propriétés des gaz d'électrons soumis à un champ magnétique<br />inhomogène. Après un bref rappel sur la dynamique d'électron en<br />présence d'un gradient de champ magnétique constant, nous abordons le<br />problème d'un champ magnétique périodique spatialement. En calculant<br />quelques états de Bloch aux points de haute symétrie, nous montrons qu'il<br />existe des états pour lesquels les électrons sont localisés au<br />voisinage des lignes de champ nul. Le calcul des courants de<br />probabilité montre que ces états sont porteurs de courants permanents<br />dont l'origine est liée aux inhomogénéités du champ magnétique au<br />voisinages des lignes de champ nul.

Cette thèse en physique constitue une étude théorique des états de bord dans des cristaux bidimensionnels qui exhibent deux cônes de Dirac (dégénérés en spin) dans leur relation de dispersion. Les deux systèmes considérés sont le graphène d'une part, et le réseau carré traversé d'un demi quantum de flux magnétique d'autre part. L'accent est mis sur la description analytique des niveaux d'énergie dispersifs sous fort champ magnétique (régime de l'effet Hall quantique), à l'approche du bord. Selon la géométrie du réseau cristallin et la forme du bord considéré, différents types de couplage sont induits sur les composantes de la fonction d'onde, donnant lieu à des structures d'états de bord différentes mais qui peuvent néanmoins être décrites de façon communes. En l'absence de champ magnétique, des états de bord peuvent également exister dans ces systèmes, mais ceux-ci ont une origine différente et leur existence même dépend de la nature des bords. Dans le cas du graphène, on montre comment comprendre l'existence de tels états en terme d'une phase de Berry particulière, appelée phase de Zak. Cette approche permet entre autre de comprendre comment manipuler ces états de bord en induisant une transition topologique de la phase de Zak à partir des paramètres de volume. Un autre type de transition topologique est également étudié. Il s'agit de la fusion des cônes de Dirac dans le réseau carré à demi flux. On montre que le mécanisme donnant lieu à ce phénomène est totalement différent de celui connu dans le graphène, et que le voisinage de la transition peut toutefois être décrit avec le même Hamiltonien effectif. Une partie plus courte traite de la localisation faible sur un cylindre désordonné en présence d'interactions électroniques. Le but de cette étude est d'illustrer le rôle de la géométrie sur les mécanismes de décohérence dus aux interactions électron-électron dans les systèmes diffusifs. Les harmoniques de la correction de localisation faible alors calculées mettent en évidence différents régimes qui permettent de sonder les différentes échelles de longueur caractérisant la décohérence. Ces longueurs révèlent la sensibilité des processus cohérents à la géométrie, et sont caractérisées par des lois de puissance en température spécifiques.

The Hall resistivity in ferromagnetic materials has an anomalous contribution proportional to the magnetization, which is defined as the anomalous Hall effect (AHE). Being a central topic in the study of ferromagnetic materials for many decades, the AHE was revived in recent years by generating many new understandings and phenomena, e.g. spin-Hall effect, topological insulators. The phase diagram of the AHE was shown recently to exhibit three distinct regions: a skew scattering region in the high conductivity regime, a scattering-independent normal metal regime, and an insulating regime. While the origin of the metallic regime scaling has been understood for many decades through the expected dependence of each contribution, the origin of the surprising scaling in the insulating regime was completely unexplained, leaving the primary challenge to the last step to understand fully the AHE. In this dissertation work we developed a theory to study the AHE in the disordered insulating regime, whose scaling relation is observed to be omega_xy^AH is proportional to omega_xx^(1.40∼1.75) in a large range of materials. This scaling is qualitatively different from the ones observed in metals. In the metallic regime where kFl > > 1, the linear response theory predicts that omega_xx is proportional to the quasi-particle lifetime tau, while omega_xy^AH scales as alpha*tau beta*tau^0, indicating that the upper limit of the scaling exponent is 1.0. Basing our theory on the phonon-assisted hopping mechanism and percolation theory, we derived a general formula for the anomalous Hall conductivity (AHC), and showed that the AHC scales with the longitudinal conductivity as omega_xy^AH ~ omega_xx^gamma with gamma predicted to be 1.33 <= gamma <= 1.76, quantitatively in agreement with the experimental observations. This scaling remains similar regardless of whether the hopping process is long range type (varible range hopping) or short range type (activation E3 hopping), or is influenced by interactions, i.e. Efros-Shklovskii (E-S) regime. Our theory completes the understanding of the AHE phase diagram in the insulating regime.

Made available in DSpace on 2015-03-26T13:30:54Z (GMT). No. of bitstreams: 1 texto completo.pdf: 265242 bytes, checksum: ae9c791b949415eacc531f321dffc178 (MD5) Previous issue date: 2007-02-28 / Fundação de Amparo a Pesquisa do Estado de Minas Gerais / The coffee growing reality in Viçosa region is peculiar among producers, as in socioeconomic aspect as well as in production. It is known that population control of coffee berry borer is very important to this activity, due to the grain direct damage, although many producers did not consider it a real problem, despising its possible influence in the harvest. The objectives of this work were to verify if coffee berry borer is a problem in Viçosa region and if the altitude affects this plague infestation. For this, it was made a data survey of altitude and production in 25 large properties, 30 medium and 34 small, numbers considered representative of each producer extract in the region. By anamnesis, it was known the socioeconomic reality and coffee production in the referred properties. Besides, in the harvest, it was determined the real infestation of the coffee berry borer, by sampling, in the farms. This data were compared with those obtained in the interviews. It can be verified that the borer is a proven problem in Viçosa region and that the altitude did not influence the plague infestation percentile. / A realidade da cafeicultura na região de Viçosa é muito peculiar entre os produtores, tanto no aspecto socioeconômico quanto no de produção. Sabe-se que o controle populacional da broca do café é de suma importância para essa atividade devido aos danos diretos causados aos grãos, embora muitos cafeicultores não a considerem um problema real, menosprezando, assim, sua possível influência nas safras. Desse modo, o objetivo deste trabalho foi constatar se a broca do café realmente é problema na região de Viçosa e se a altitude afeta a porcentagem de infestação dessa praga. Para isso, levantaram-se dados de altitude e produção em 25 propriedades grandes, 30 médias e 34 pequenas, números esses considerados representativos de cada estrato de cafeicultor da região. Por meio de anamnese ativa, conheceu-se a realidade socioeconômica e da produção de café das referidas propriedades. Além disso, na colheita foi determinada a real infestação da broca, por amostragem, nas lavouras. Esses dados foram comparados com os obtidos nas entrevistas. Pôde-se constatar que a broca é comprovadamente um problema na região de Viçosa e que a altitude não influenciou a porcentagem de infestação da praga.

L'étude des phases de la matière est d'un intérêt fondamental en physique. La théorie de Landau, qui est le "modèle standard" des transitions de phases, caractérise les phases de la matière en termes des brisures de symétrie, décrites par un paramètre d'ordre local. Cette théorie a permis la description de phénomènes remarquables tels que la condensation de Bose-Einstein, la supraconductivité et la superfluidité.<p><p>Il existe cependant des phases qui échappent à la description de Landau. Il s'agit des phases quantiques topologiques. Celles-ci constituent un nouveau paradigme et sont caractérisées par un ordre global défini par un invariant topologique. Ce dernier classe les objets ou systèmes de la manière suivante: deux objets appartiennent à la même classe topologique s'il est possible de déformer continument le premier objet en le second. Cette propriété globale rend le système robuste contre des perturbations locales telles que le désordre. <p><p>Les atomes froids constituent une plateforme idéale pour simuler les phases quantiques topologiques. Depuis l'invention du laser, les progrès en physique atomique et moléculaire ont permis un contrôle de la dynamique et des états internes des atomes. La réalisation de gaz quantiques,tels que les condensats de Bose-Einstein et les gaz dégénérés de Fermi, ainsi que la réalisation de réseaux optiques à l'aide de faisceaux lasers, permettent d'étudier ces nouvelles phases de la matière et de simuler aussi la physique du solide cristallin.<p><p>Dans cette thèse, nous nous concentrons sur l'etude d'isolants topologiques avec des atomes froids. Ces derniers sont isolants de volume mais possèdent des états de surface qui sont conducteurs, protégés par un invariant topologique. Nous traitons trois sujets principaux. Le premier sujet concerne la génération dynamique d'un isolant topologique de Mott. Ici, les interactions engendrent l'isolant topologique et ce, sans champ de jauge de fond. Le second sujet concerne la détection des isolants topologiques dans les expériences d'atomes froids. Nous proposons deux méthodes complémentaires pour caractériser celles-ci. Finalement, le troisième sujet aborde des thèmes au-delà de la définition standard d'isolant topologique. Nous avons d'une part proposé un algorithme efficace pour calculer la conductivité de Berry, la contribution topologique à la conductivité transverse lorsque l'énergie de Fermi se trouve dans une bande d'énergie. D'autre part, nous avons utilisé des méthodes pour caractériser les propriétés quantiques topologiques de systèmes non-périodiques.<p><p>L'étude des isolants topologiques dans les expériences d'atomes froids est un sujet de recherche récent et en pleine expansion. Dans ce contexte, cette thèse apporte plusieurs contributions théoriques pour la simulation de systèmes quantiques sur réseau avec des atomes froids. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

In dieser Arbei wird, auf Grundlage der Figur des Sandmanns von E.T.A. Hoffmann, nach Gruselfiguren in der Moderne gesucht, die ebenfalls als Sandmänner ansehen kann. Die Figuren, die mit dem Original aus der schwarzen Romantik verglichen werden, kommen dabei aus unterschiedlichen Medien und sind nicht nur aus Büchern, sondern auch aus der Musik, der Graphic Novel, sowie dem Film. Es wird auf intermediale Weise untersucht, wie der jeweils gezeigte Sandmann aussieht, welche Eigenschaften er aufweist und wie er sich verhält. Die ausgewählten Sandmänner kommen teilweise aus dem europäischen und teilweise aus dem amerikanischen Raum.

<p>U radu su prikazani eksperimentalni rezultati natkritične ekstrakcije etarskog ulja bobica<br />kleke (<em>Juniperus communis</em> L.) i ulja iz semena uljane tikve golice (<em>Cucurbita pepo</em> L.). Ispitan je uticaj pritiska, temperature, stepena usitnjenosti čestica i protoka natkritičnog<br />ugljendioksida na promenu prinosa ulja sa vremenom. U cilju poređenja, usitnjeno seme uljane tikve ekstrahovano je i u ekstraktoru većih dimenzija, NOVA-SWISS, High<br />pressure extraction plant, kao i heksanom i petroletrom u ekstraktoru tipa Sokslet.<br />Praćena je i promena kvaliteta ekstrakata sa vremenom: u etarskom ulju kleke, GC-FID i GC-MS metodama, određen je relativni sadržaj 50 terpenskih jedinjenja i sve komponente ulja su svrstane u 5 osnovnih grupa (monoterpene, seskviterpene, oksidovane monoterpene, oksidovane seskviterpene i ostale komponente). U tikvinom ulju ekstrahovanom natkritičnim ugljendioksidom određen je masnokiselinski sastav GC-MS analizom, sadržaj tokoferola HPLC analizom, sterola i skvalena GC-MS metodom. Određeni su uslovi koji favorizuju ekstrakciju ispitanih jedinjenja za obe sirovine. Dat je dateljan prikaz matematičkih modela koji se koriste za opisivanje natkritične ekstrakcije etarskih ulja i masnih ulja, počev&scaron;i od najop&scaron;tijeg modela koji uključuje diferencijalne bilanse mase za rastvorak u masi natkritičnog fluida, u fluidu unutar pora čestica usitnjenog matrijala i u čvrstoj fazi, koji se uvođenjem određenih pretpostavki pojednostavljuje i svodi na modele koji su izabrani da budu ispitani u okviru ovoga rada. Ispitani su modeli kreireni po analogiji sa hlađenjem vrele kugle u masi fluida, tj. modeli tipa jedne sfere i to: Model jedne sfere-1 (MJS-1), koji pored uticaja koeficijenta efektivne difuzije ulja u materijalu na brzinu prenosa mase uzima u obzir uticaj koeficijenta prenosa mase kroz film natkritičnog fluida oko čestice, pri čemu je njegova vrednost procenjena preko postojećih korelacija; MJS-1 (2 par), u kome je spolja&scaron;nji koeficijent prenosa mase uzet kao drugi prilagodljiv parametar modela; MJS-2, gde je koeficijent efektivne difuzije jedini prilagođeni parametar, Model karakterističnog vremena i pro&scaron;ireni model klipnog toka koji je predložila Sovov&aacute;. Za modelovanje natkritične ekstrakcije ulja semena tikve kori&scaron;ćen je i kombinovani model Honga i sar. Softverskim paketima Mathcad 2001 Professional i Solver dodatka unutar Microsoft Excel 2003, određeni su parametri ispitanih modela u cilju najboljeg slaganja modela sa ekperimentalnim podacima. Za obe ispitane sirovine, među ispitanim modelima, izabrani su modeli koji najbolje opisuju njihovu ekstrakciju natkritičnim ugljendioksidom. Pro&scaron;ireni model klipnog toka koji je predložila Sovov&aacute; pokazao se podjednako dobrim za modelovanje natkritične ekstrakcije obe sirovine i ne&scaron;to bolji od ostalih primenjenih modela.</p> / <p>This study provides results of supercritical carbon dioxide (SCCO₂) extraction of juniper berries (<em>Juniperus communis</em> L.) and pumpkin seeds (<em>Cucurbita pepo</em> L. convar.&nbsp;<em>citrullina</em>) in a laboratorysupercritical fluid extraction apparatus. The influence<br />of pressure, temperature, particle size and carbon dioxide flow on the extraction kinetics of pumpkin seed oil and juniper berry essential oil was studied. Ground pumpkin seeds were also extracted with supercritical carbon dioxide in NOVA-SWISS, High Pressure Extraction Plant, and with hexane and petroleum ether in a laboratory Soxhlet extractor. This work was also aimed to investigate the evolution of the composition of juniper fruit supercritical CO₂ extracts with time, at different extraction pressures and to emphasize the most favorable condition for the extraction of different terpene hydrocarbon groups, reporting the qualitative differences among extracts collected during successive extraction time periods. Juniper berry extracts were analyzed by capillary gas chromatography, using flame ionization (GC-FID) and mass spectrometric detection (GC-MS). More than 200 constituents were detected in the extracts and the contents of 50 compounds were reported in the work. Dependence of the percentage yields of monoterpene, sesquiterpene, oxygenated monoterpene and oxygenated sesquiterpene hydrocarbon groups on extraction time was investigated and conditions that favored the yielding of each terpene groups were emphasized. GC-MS analysis of FAME, prepared by transesterification of pumpkin seed oil with KOH in methanol, was performed. Fatty acid compositions of supercritical CO₂ pumpkin seed extract fractions collected in successive time intervals over the course of the extraction were determined. The same fractions were analyzed by high pressure liquid chromatography (HPLC), using diode-array detector (DAD) in order to determine a- and g-tocopherol contents. Sterol and squalene contents were determined by GC-MS analysis, as well. Conditions that favored the yielding of tocopherols, squalene and sterols were emphasized. A general mass transfer model and its simlifications were analysed. Extraction curves were evaluated by &ldquo;hot sphere&rdquo; mathematical models SSM-1 (Single Sphere Model 1 &ndash; in which the external mass transfer coefficient also influences the extraction profile and film mass transfer coefficients were estimated by the correlations), SSM-1 (2 par) (film mass transfer coefficient is used as the second adjustable parameter), SSM-2 (only effective diffusivity influence is considered), Characteristic time model and by the extended Lack&rsquo;s plug-flow model given by Sovov&aacute;. A combined model of Hong et al. was also fitted to the experimental data for pumpkin seed oil SCCO_2&nbsp;extractions. Relative merits of the models are demonstrated. Good agreement between the extended Lack&rsquo;s plug-flow model and the experimental measurements was obtained.</p>

No description available.

Optical microcavities (OMs) are receiving increasing attention owing to their potential applications ranging from cavity quantum electrodynamics, optical detection to photonic devices. Recently, rolled-up structures have been demonstrated as OMs which have gained considerable attention owing to their excellent customizability. To fully exploit this customizability, asymmetric and topological rolled-up OMs are proposed and investigated in addition to conventional rolled-up OMs in this thesis. By doing so, novel phenomena and applications are demonstrated in OMs. The fabrication of conventional rolled-up OMs is presented in details. Then, dynamic mode tuning by a near-field probe is performed on a conventional rolled-up OM. Next, mode splitting in rolled-up OMs is investigated. The effect of single nanoparticles on mode splitting in a rolled-up OM is studied. Because of a non-synchronized oscillating shift for different azimuthal split modes induced by a single nanoparticle at different positions, the position of the nanoparticle can be determined on the rolled-up OM. Moreover, asymmetric rolled-up OMs are fabricated for the purpose of introducing coupling between spin and orbital angular momenta (SOC) of light into OMs. Elliptically polarized modes are observed due to the SOC of light. Modes with an elliptical polarization can also be modeled as coupling between the linearly polarized TE and TM mode in asymmetric rolled-up OMs. Furthermore, by adding a helical geometry to rolled-up structures, Berry phase of light is introduced into OMs. A -π Berry phase is generated for light in topological rolled-up OMs so that modes have a half-integer number of wavelengths. In order to obtain a deeper understanding for existing rolled-up OMs and to develop the new type of rolled-up OMs, complete theoretical models are also presented in this thesis.